Marine Biology 3

The Open Ocean

A simple description of the open ocean is the region of the ocean that lies far out and away from land. However, to understand its precise location you first need to familiarize yourself with a few terms. Along the world's coasts, the land extends under the ocean, sloping gently downward to a depth of about 200 meters. This gentle slope is the continental shelf. Beyond the continental shelf the land slopes steeply downward and is called the continental slope. At a depth of 3000 to 4000 meters, the steep incline becomes much more gentle. It is called the continental rise. The continental rise is wider than the slope and eventually levels out into the ocean floor, called the abyssal plain. Scientists call the region of the ocean away from the shore and from the seafloor "pelagic." The pelagic region of the ocean lying over the continental shelf is the neritic zone. Scientists refer to the ocean waters that lie beyond the neritic zone as the open ocean. In the region that makes up the open ocean, the pelagic water column is so deep that scientists have divided it into layers on the basis of its depth. By contrast, scientists consider the pelagic water column over the neritic zone to be shallow and treat it as a single layer.

Selma is creating an aquarium-style coral reef model to teach her students about coral reefs. What should she include in the model?

Abarren sand and warm, clear, fresh water.

Blades

Atop the stipe are the leaves, called blades. Blades have a long, flat, leaf-like structure. They use nutrients from water and sunlight to produce food for the kelp plant.

Barrier Reefs

Barrier reefs are older than fringing reefs. These reefs run along the border of the shores at a distance. A lagoon is present in between the reef and the shore. The Great Barrier Reef is the largest coral reef in the world and is situated off the coast of Queensland, Australia.

Reproduction (deep sea adaptions)

Biologists believe that bioluminescence may be one method that deep-sea organisms use to locate a mate. Some may also use a well-developed sense of smell. The anglerfish has an interesting reproductive adaptation. The males are considerably smaller than the females. They attach themselves to the lower side of a female using hooked teeth. The attachment is often permanent and the blood vessels of the two fish fuse at this point. The male obtains its nutrition from the female. The female is assured of a reliable source of sperm.

Deap Sea

Biologists have also discovered communities of deep-sea organisms around deep-sea lakes called brine pools. Brine pools are areas of water near the sea floor that contain water with a much higher concentration of salt than the surrounding water. The high salt content makes the water so dense that it does not mix with the overlying water. At one such brine pool in the Gulf of Mexico, biologists have found a bed of mussels around its rim. The mussels form a relationship with bacteria that utilize methane gas seeping from the pool to obtain food. Biologists have not discovered any animals living in the brine pools. They have, however, found bacteria in these pools. Biologists have identified hundreds of species living around hydrothermal vents and cold seeps. Ninety-five percent of these organisms are new to science. The difference in vent and cold seep ecosystems is their temperature and the chemical used for energy. Another important difference is the rate of growth in these environments. Organisms at hydrothermal vents grow faster. The giant tube worms are one of the fastest growing animals on Earth. Tube worms at seep pools grow much more slowly.

Effects of shipping and fishing

Boat anchors and propellers can uproot or damage vast areas of seagrass meadows. Destructive methods of fishing such as dredging can clear vast meadows.

Marine Life

Both the temperate and tropical zones are dense and diverse with marine life. However, a vast majority of marine species is found in the temperate zone. The temperate zone contains species that are commercially important and a major source of fish for the fishing industry. Some commercial species caught exclusively in the temperate zone include Atlantic herring, halibut, haddock, mackerel, blue mussels, and northern lobsters. The temperate zone is also dominated by species of algae and seagrass, while the tropical zone is mostly dominated by coral.

Branching Corals

Branching corals are a type of hard coral with numerous branches. They may have secondary branches stemming off from a primary branch.

Which component forms major skeleton of reef building?

Calcium carbonate

Changes in seawater

Changes in seawater, such as temperature, can be harmful and kill off large amounts of seagrass. Changes in temperature can also make seagrass susceptible to natural disease.

South Carolina has a small river that flows into the sea. over time, sediments build up along the coastline and cause the body of water to be cut off from the sea. what type of estuary undergoes this kind of change?

Coastal plain

Cold Seeps

Cold seeps are areas of the seafloor where methane seeps out onto the seabed. The temperature at the seeps is the same as the surrounding seawater, around 4°C. Scientists refer to them as cold seeps to differentiate them from the hydrothermal vents. Chemosynthetic bacteria exist in these cold seeps. They use methane as a source of energy to combine the carbon from dissolved carbon dioxide and water to form organic molecules. The bacteria use these organic molecules to carry out life processes such as growth and reproduction. Many invertebrate deep-sea creatures, such as clams and tube worms, form a relationship with these chemosynthetic bacteria. They provide the bacteria a safe place in which to dwell. The bacteria produce organic molecules, which these organisms use as food. Other invertebrates, such as certain species of snails and worms, consume the bacteria directly. Shrimp and crabs then come to feed on the detritus (dead organic matter) from these organisms. The variety of these organisms attracts predators, such as fish and octopuses, which feed on the organisms present there.

Protection

Coral reefs protect the shoreline from erosion. They protect the coastal areas from damage by minimizing the wave intensity.

Tourism

Coral reefs provide a scenic view underwater. Tropical coral reefs have led to an increase in recreational tourism. Snorkeling and scuba diving in the coral reef has given rise to resorts and hotels near the coral reefs.

Elsa is authoring a book about coral reefs. She has to write about how coral reefs protect the shores. Which point should she mention?

Coral reefs reduce wave intensity and minimize shore erosion

Corals

Corals are marine invertebrate organisms. They are stationary and do not swim across in the ocean. They belong to the phylum Cnidaria and class Anthozoa. They are similar to jellyfish and sea anemones. The corals are cylinder-shaped organisms ranging from a few millimeters to a centimeter in diameter. The presence of tentacles covering a single opening is a characteristic feature of corals. Some corals secrete calcium carbonate and form an exoskeleton around them. These secretions are responsible for the coral reefs found in the waters near the tropical regions of the world. Huge coral reefs are present in the Indo-Pacific region, the Red Sea, and the Caribbean region. These coral reefs contribute to a thriving ecosystem, which sustains a number of marine plants and animals. Coral reefs provide a beautiful imagery for scuba divers and snorkelers for recreational purposes. The abiotic factors that corals require are shallow and saline waters, warm temperatures, and ample sunlight. However, scientists have found some corals deep in the oceans, too.

Hard substrate

Corals are stationary organisms. Therefore, they require a firm, hard substrate onto which they can attach. The ocean floor provides an ideal substrate for corals.

Tentacular feeding

Corals feed on drifting marine phytoplankton, zooplankton, and small fish, which waves bring near them. Corals have tentacles near the oral opening. The tentacles comprise cells called nematocysts, which contain venom. Corals sting the prey and stun them using the tentacles. Then they direct this prey toward their oral opening, so they can digest the prey. Later, after digestion, the corals spread their tentacles again in search of new prey. Most of the corals use this form of feeding during night. Corals that do not have zooxanthellae interdependence exclusively use this mode of feeding.

Ashley is preparing for a speech about the tentacles feeding pattern of the corals.

Corals have tentacles near the oral opening. The tentacles comprise cellls called nematocysts. These cells contain poison used by corals to sting the prey and stun them using tentacles.

Wave intensity

Corals need good wave intensity to bring in food and oxygen for survival. Waves also ensure that sedimentation does not occur and settle on the reefs. Similarly, waves also play an important role in coral reproduction.

Salinity

Corals need saline environments, and they do not survive in places near freshwater runoffs. Salinity boosts calcium to precipitate into calcium carbonate forming the exoskeleton. Similarly, low carbon dioxide aids in coral proliferation.

Sexual reproduction

Corals produce male and female gametes. They are stationary, so they release these gametes into water in large numbers. This process is called spawning. Spawning occurs as a synchronized process when both males and females release the gametes at the same time. They use environmental cues such as the lunar cycle, sunsets, and so on to synchronize the time of release. The gametes fuse together to give birth to coral larvae called planulae. Planulae are free floating and fragile, and are most likely to fall prey to many predators. Therefore, synchronization increases the chances of the gametes to meet and form large numbers of planulae. These planulae swim up toward the surface. The surface currents carry the floating planulae over a distance. Once the conditions are favorable, the planulae swim downward and settle to form polyps.

Marine creatures

Large marine animals may disturb and uproot seagrass while hunting or feeding. Detaching a seagrass plant from the root system is harmful to the plant.

Diatoms

Diatoms are one of the most prominent groups of phytoplankton. The organism's cell wall consists of two glasslike halves or valves. The cell wall has a number of pore-like structures. They help to reduce the weight of the organism and enable access to sea water (and nutrients). Flagellates are phytoplankton that have a whip-like structure, called a flagellum, attached to the cell. These phytoplankton use the flagella to propel the cell. The flagellum enables the organism to move up toward the sunlight. Dinoflagellates are larger flagellates that whirl through the water. (The Greek word dinos means whirling). Cyanobacteria, which were formally referred to as blue-green algae, are phytoplankton that belong to the group of photosynthetic bacteria. They play an important role in the nutrient-depleted waters of the open ocean. They convert atmospheric nitrogen into useable nitrates and ammonia. These forms of nitrogen are useable for other life forms in the open ocean. They obtain their requirement of this nutrient by consuming the cyanobacteria.

Digitate Corals

Digitate corals are also known as pillar corals. Unlike branching corals, these corals appear like cylindrical pillars without any branches.

Hard Corals with zooxanthellae

Dinoflagellates, called zooxanthellae, reside inside hard corals. These zooxanthellae impart a distinct color to the coral reefs. Moreover, some corals also produce proteins, which contain pigments that impart color to the reefs. Hard corals containing zooxanthellae require waters shallower than 60 meters in depth. Negligible sedimentation, sparse nutrition, and clear waters boost their proliferation. Some common hard corals are tabletop coral, brain coral, staghorn coral, and pillar coral.

Arrange the estuarine zones in decreasing order of salinity

Euhaline, Polyhaline, Mesohaline, Oligohaline

Which species can live in both salt water and fresh water?

Euryhaline organisms

Protecting Seagrass Meadows

Even though seagrass meadows play an important role in the marine ecosystem, their population faces constant threat. Seagrass meadows in temperate marine ecosystems generally consist of a single species. For this reason they are more sensitive to changes in sea temperature and salinity. They are also easily affected by human activities. No international legislation exists for their protection, so it has to be done locally.

Economic uses

Fishing over the coral reefs has supported humans over ages. Many fisheries rely on the coral reefs for their abundant fish supply. In developing countries, coral reefs provide almost a quarter of the total marine catch.

Formation of coral reefs

Hard corals form colonies consisting of millions of corals. The base of each coral secretes an exoskeleton of calcium carbonate. This creates a cup-like structure where the coral rests. The walls surrounding the coral are called theca. The basal plate lies at the base of the coral. The coral creates a new basal plate at regular intervals. This new basal plate elevates the coral upward. The gap between the old and the new basal plate creates chambers. This process continues and forms coral reefs.

Hard corals

Hard corals secrete calcium carbonate, and form an exoskeleton around themselves. This exoskeleton gives a stony appearance to these corals. Millions of corals form colonies, and together secrete calcium carbonate in a crystalline form. Calcium carbonate builds up over the years from these coral colonies to form coral reefs. Hard corals are known as hermatypic or reef-building corals. Some solitary hard corals may not be hermatypic due to their inability to build reefs. Hard corals show a six-fold symmetry with a tapering head of tentacles. They belong to the order Scleractinia.

Marine Life forms

In comparison to the area it covers, the open ocean is not as biologically rich as the neritic zone. The reason is that the water in the open ocean lies far away from land—the source of many essential nutrients brought into the sea by rivers. The unattached marine organisms that live in the pelagic column of water are called pelagos. Scientists divide the pelagos creatures into two broad categories on the basis of how they move in the water. The organisms that float freely in the water and are carried along by currents are called plankton. Plant-like plankton that contain chlorophyll and are capable of photosynthesis are called phytoplankton. Plankton that do not produce their own food through photosynthesis are called zooplankton. The other group of organisms that swim and move freely in water without the help of currents is called nekton. Examples of nekton include fish, mollusks, and crustaceans.

Photosynthetic life

In the open ocean, the photic zone in which photosynthesis is possible is a habitat surrounded by water. The only substrate is the seafloor where no light penetrates. Therefore, in the open ocean, photosynthetic life exists as unicellular phytoplankton. The phytoplankton easily stay afloat because of their size and structure. Some species have spines and appendages that increase the water's drag force on their body surface. The increased drag helps to keep them afloat. Therefore, they remain within the surface, sunlit waters where they can utilize energy from the sun. Another advantage of being small is that it increases their surface area in comparison to their volume. In the absence of roots, phytoplankton take up nutrients through their body surface. A larger surface area is, therefore, an advantage. It maximizes their ability to absorb nutrients.

Physical Properties

In the temperate regions sea temperatures seasonally fluctuate in a range between 5 and 25°C. The temperate zone is at an intermediate temperature due to cold currents moving from the polar zone and warm currents moving from the tropical zone. These currents mix and give rise to the temperate zone. The continental landmasses are responsible for forcing cold subpolar currents (rotating currents that flow between the polar and temperate region) toward the equator. Continental landmasses also move warm subtropical currents (rotating currents that flow between the tropical and temperate region) towards the poles. When these two currents meet they form circular currents in the ocean. The currents flow clockwise in the Northern hemisphere and counterclockwise in the southern hemisphere.

Seafloor

It is the bottom part of the forest. Creatures can live among the holdfasts, protected by the kelp and shaded from most sunlight. This region is home to substrate and benthic creatures. Brittle stars, sponges, and anemones are commonly found here. This zone is also home to other algae and so provides nourishment for herbivores such as sea urchins and abalone.

Epipelagic zone

It is the zone from the ocean surface to a depth of 200 meters. It has the greatest amount of sunlight and is therefore the most productive zone. It is also called the photic zone and is the limit of the open ocean's depth.

Abyssopelagic zone

It stretches from the bottom of the bathypelagic zone to the seafloor (abyssal plain) at a depth of 6000 meters. In comparison with the other zones, fewer organisms inhabit this region.

The caprella californica is a species of shrimp that inhabits kelp forest. Their main food source is found in areas with lots of available sunlight. Which habitat is most suited to this species?

Kelp Canopy

Body color (deep sea adaptions)

Many animals use color to avoid detection. Several Deep-Sea animals, such as squid and jellyfish, have transparent bodies. Many species of fish are black, such as Deep-Sea smelt and hidden bristlemouth. Some species of shrimp and squid are red. The absence of red light in these depths makes this color look dark. Such coloration helps them blend into the surrounding darkness and remain invisible. In the upper twilight zone of the deep sea, some fish have silvery sides. They reflect the faint light, which makes it difficult to detect them. Several fish, such as lantern fish, have rows of bioluminescent light-producing spots called photophores on their bellies. They produce a blue light that matches the faint sunlight penetrating the ocean from above. This bioluminescent light conceals the presence of the fish when viewed from below.

Gigantism (deep sea adaptions)

Many deep-sea creatures tend to grow very large. Examples include the giant squid and the giant isopod. The oarfish is another large fish. Giant tubeworms are also found near hydrothermal vents. Scientists are not certain about why this gigantism occurs. One possible reason is that many of these creatures live very long lives. The long life spans may be a result of delayed sexual activity because of scarce food conditions. They therefore have much longer to grow and so attain these gigantic sizes.

Animal Life

Marine animals consist of both large nekton species and unicellular plankton. Heterotrophic plankton, which do not produce their own food, are called zooplankton. Let's first look at some of the different types of zooplankton. Chaetognaths are a commonly encountered group of plankton. They are also known as arrow-worms because of their arrow-shaped body. They are transparent, which helps in camouflage. They have large jaws with bristles, which they use to capture their prey. Copepods are crustaceans. Most species are planktonic. Copepods are the most numerous form of plankton in the oceans. They are an important source of prey for fish larvae and are essential for maintenance of good fish stock. Amphipods are also planktonic crustaceans. They have pincer-like feeding appendages and large, well-developed eyes, which they use to locate and capture their prey. Some deep-water amphipods are scavengers. Euphausids, or krill, are another abundant group of zooplankton. They are also crustaceans. They are found in abundance in polar regions. They feed on phytoplankton and other small zooplankton. They are an important source of food for nekton species such as fish and whales. Ciliates are a group of zooplankton that have hair-like structures called cilia. They use their cilia to capture prey, as sensory structures, and for movement. Jellyfish are also plankton. Their size varies from a few millimeters to a few meters in length. They have soft, translucent bodies. They belong to a group call

Gas float

Most kelp plants have a gas float between the blades and stipe, called a pneumatocyst. It is a hollow bulb-like structure filled with gas that helps to keep the plant afloat.

Deep-Sea Coral

Most people think of coral as only existing in shallow sunlit waters off the coast. However, corals also exist in the aphotic zone. They exist in all the world's oceans along the continental slope and even in the abyssal zone in waters as deep as 6000 meters and as cold as 4°C. These corals belong to the same taxonomic group Cnidaria and all have stinging cells. However, they do not form associations with the algae zooxanthellae like the tropical shallow corals. In the absence of sunlight, deep-sea coral feed on other organisms. They use their tentacles to catch plankton and bits of organic debris. Like all coral, deep-sea coral polyps also secrete a hard skeleton of limestone. The individual skeletons merge together and form reefs that rise from the floor. Under the extreme conditions of low temperature, scarce food, and high pressure, deep-sea corals grow slowly, but live long lives of several thousand years. Deep-sea corals may be colonial and form coral thickets such as reefs. These reefs provide a habitat for other deep-sea creatures. Lophelia is one of the most important genera of deep-sea corals. Other deep-sea corals may grow independently, forming tree- and fan-like structures.

which example depicts the adaption of an intertidal organisms to the force of crashing waves and strong currents?

Mussels grow byssal threads that they attach to hard surfaces

Nekton

Nekton consists of all types of marine fish, such as cod, herring, sharks, and eels. Marine mammals, such as whales, dolphins, seals, manatees, and sea otters, are also components of nekton. The only invertebrate nektons are the cephalopod mollusks. Examples include the octopus, squid, and cuttlefish. Nektonic reptiles include turtles and snakes. Marine animals show a number of adaptations to survive in the open ocean. Let's look at a few of these adaptations. All marine animals have adapted body parts to assist in locomotion in water. They include fins and flippers. Fish have air bladders and marine mammals have lungs to control buoyancy. Several species of zooplankton undertake a vertical migration called diurnal vertical migration. They move to the surface at night where they feed on phytoplankton and other smaller species of plankton. In the day, they move to the darker depths of the ocean. Scientists believe that this migration in the day helps them to avoid predators that use vision to capture their prey. Many crustaceans adopt a red color as camouflage. Red light does not penetrate deep into the ocean. Animals with this color are harder to see because they will appear black and invisible in the dark waters of the aphotic (lightless) zone.

Light Propagation

Of the light that reaches the ocean, some reflects off the surface, while most penetrates into the water. Water that is turbulent and has waves reflects more light. The ocean absorbs about 65 percent of visible light within the first meters. Very little light penetrates below a depth of 150 meters even in very clear water. The ocean readily absorbs light of longer wavelengths, such as red, orange, and yellow. They penetrate only to a shallow depth of a maximum of 50 meters. These colors appear black beyond this depth. Shorter wavelengths can penetrate further into the ocean. Blue light penetrates the deepest, which gives the ocean its characteristic blue color.Near the shore, the presence of suspended solid particles affects the transmission of light through the water. These particles decrease the depth of light penetration. They also scatter and absorb light wavelengths differently. Therefore, coastal waters appear greenish and brown.

Vision (deep sea adaptions)

Organisms have special sensory adaptations to survive in these dark waters. Some have very large eyes to capture the little light available. Others may be blind, but have enhanced senses of smell, touch, and vibration.

Bioluminescence (deep sea adaptions)

Organisms use bioluminescence for different purposes. Some, such as the anglerfish, have a fishing rod-like bioluminescent lure that dangles in front of them. The lure attracts prey that they eat. Some use bioluminescence to avoid predators. The bomber worm, for example, releases sacs from its body that emit a bright glow, which distracts the predator.

Correctly characterize life in the intertidal zone

Organisms use suckers to anchor themselves to rocks. The spray zone is the most exposed to the elements outside the sea.

Plankton

Plankton are an important source of food for nekton. The abundance and distribution of plankton is dependent on several factors. They include the availability of light and nutrients. In temperate regions, light and temperatures increase in spring. These conditions are ideal for phytoplankton to rapidly multiply. This sudden abundance in numbers is called a bloom. The phytoplankton bloom provides an abundant food source for zooplankton. It leads to a bloom in zooplankton. In summer, the dead organisms settle to the bottom of the ocean. Decomposers break them down and release the nutrients. The warm surface waters do not mix with the water below and so the nutrients remain in the deeper colder waters. The lack of nutrients results in a decrease in photosynthetic activity. Therefore, in summer there is a reduction in the number of phytoplankton. In fall, the surface waters cool and sink. The result is an upwelling of nutrient-rich water from the deep. The upwelling of nutrients results in another bloom. In the tropics, despite the availability of light, productivity is low. Nutrient levels are low because the constant high temperatures cause the water to remain stratified. There is very little mixing of waters. Therefore, tropical waters have low productivity in comparison to temperate waters.

The eel is an example of a fish that is suited to both fresh water and sea water. Which zone of an estuary would you expect to find eels in abundance?

Polyhaline

Which statement about the physical properties of deep sea water is true?

Pressure in the deep sea is several hundred times the pressure near the surface

Changes in biodiversity

Removal of certain species by excessive fishing can also affect seagrass. Removal of large predators can increase the number of small herbivores, which then eat larger quantities of sea grass.

Pollution

Runoff from sewage and fertilizers adds nutrients and chemicals to the sea. They cause algal blooms that block out sunlight essential to seagrass. Sediment can also muddy seawater and decrease the amount of available light.

Because of the vastness of the ocean, scientists divide the ocean into different zones. Which statements correctly describe the characteristics of these zones?

Scientists divide the ocean into different zones. All parts of the ocean, from the sandy beaches and rocky shores where the water meets land to the deepest sea floor, is called the pelagic region. This pelagic region is divided into a number of zones on the basis of the intensity of light that penetrates through the water. {The uppermost epipelagic zone, which extends from the ocean surface to a depth of 200 meters, receives the most sunlight and is suitable for photosynthesis.} The mesopelagic zone lies below the epipelagic zone and is the region of the ocean that lies adjacent to the continental slope. {The bathypelagic zone is the first layer of the aphotic zone.} The abyssopelagic zone exists in those regions of the ocean where deep-sea trenches occur.

Hydrothermal Vents

Scientists first discovered these vents in 1977 at the bottom of the Pacific Ocean near the Galapagos Islands. They were investigating unusual data that indicated temperature spikes in the water at some sites near the seafloor. At these sites, the temperature changed from near freezing temperatures of about 4°C to around 400°C. While exploring these areas, they discovered that there were vents at these sites through which hot water spewed out from the interior of the crust. The vents were near an ocean ridge where hot magma rises from the interior and cools to form new ocean crust. Seawater penetrates the crust through cracks near the ridge and gets superheated by the magma below. The heated water begins to dissolve minerals from the rocks as it rises. On emerging through cracks on the seafloor, it mixes with the cold seawater. This mixing results in chemical reactions that cause sulfur and other dissolved minerals to precipitate as solid particles. The precipitating solids cool to form chimney-like structures near the vents. Scientists have discovered chimneys that are several meters high. The water emerging through the vents is black because of the deposits of iron sulfide, which is black. Scientists therefore call these chimneys black smokers.

Medicine

Scientists have been conducting research to develop drugs using coral components. Some drugs developed from coral plants and animals may cure various diseases such as cancer, arthritis, and so on.

Limiting damaging fishing practices

These destructive practices include dredging, bottom trawling, and dynamite fishing. These methods destroy vast areas of seagrass, so limiting their activities can help conserve seagrass meadows.

Hydrothermal vents

Scientists were also surprised to find abundant sea life at the site of these vents. In the absence of light, organisms cannot photosynthesize food for energy. Later, scientists realized that bacteria use the energy from chemicals, such as hydrogen sulfide, released at hydrothermal vents to produce food. They use the energy to create organic molecules from water and dissolved carbon dioxide. The process is called chemosynthesis. Bacteria therefore thrive at these vents and form the base of food for other deep-sea animals. Biologists discovered a new species at these hydrothermal vents, the giant tube worm Riftia pachyptila. Like other tube worms, it anchors its tail to the substrate and secretes a mineralized tube into which it can withdraw when threatened. Riftia grow in dense clusters to lengths of up to two meters long. The worm has a red plume that it uses to absorb hydrogen sulfide and dissolved gases, such as oxygen and carbon dioxide. It has no digestive tract. Instead it has a specialized organ called a trophosome that lies within its body cavity. The trophosome contains bacteria that produce organic molecules through chemosynthesis, which the tube worm utilizes as a food source. Riftia is one of the first species to colonize a new hydrothermal vent and is usually the dominant life form at a vent.

Seagrasses provide several essential services

Seagrass act as feeding grounds and support thousands of marine species. They directly provide nutrients to grazing creatures ranging from small fish to large marine mammals, such as manatees. Seagrass also provide nourishment to other creatures indirectly. They are home to many bacteria that are fed on by the small species that reside in the seagrass. Even after seagrass leaves die, they decay and provide nourishment for many creatures that survive on decomposing matter. Seagrass also act as carbon repositories. They capture and store large amounts of carbon similarly to trees on land. When parts of a seagrass die, they become buried and the carbon returns to the sediments. Seagrass meadows are estimated to capture over 27 million tons of carbon a year. Because this carbon is stored in the sea, it's referred to as blue carbon.

Cutting down the sources of pollution

Seagrass can be affected by chemicals and other pollutants. Efforts should be made to limit the amount of pollutants that enter the sea, including sewage, fertilizer runoff, and marine debris.

Rebuilding seagrass meadows

Seagrass grows at a very slow rate and damaged areas may take a long time to regrow in the wild. Healthy populations can be grown in sanctuaries under controlled conditions. Healthy plants can then be transplanted to revive badly damaged areas.

The Seagrass Meadow

Seagrass is a marine flowering plant. Even though it looks like seaweed, it is more closely related to land grass, having evolved from land plants. Seagrass has grass-like leaves with stems, roots, flowers, and seeds. Seagrass can form large meadows underwater that resemble grassy meadows on land. Most species of seagrass live in the shallow temperate and tropical zone. They live here because they require adequate sunlight in order to carry out photosynthesis. Seagrass meadows support some of the most productive ecosystems. They act as shelter to small fish, invertebrates, and other marine animals. Due to their ability to provide shelter to many creatures, seagrass meadows are usually called nursery habitats. Some of the commonly found invertebrates include the snapping shrimp, purple-spined sea urchin, angelwing clam, bay scallop, egg cockle, blue crab, and so on. Some of the commonly found vertebrates include the anchovy, sea turtle, scaled sardine, seahorse, parrotfish, manatee, dolphin, and so on.

Which marine plant forms meadows in the coral reef?

Seagrasses

Rowan is doing a project on soft corals. Which is the correct way for Rowan to describe soft corals?

Soft corals are a species of coral that have no internal calcium carbonate skeleton

Clear water

Some corals show a mutual interdependence with zooxanthellae. Therefore, clear and shallow water ensures penetration of sunlight necessary for photosynthesis. Sunlight also warms the water. Corals thrive at a temperature ranging from 64° F to 95° F.

Storms and waves

Strong currents and waves caused by storms can cause physical damage to seagrass.

Abiotic factors

Temperature, sunlight, minerals, dissolved gases, and so on are the abiotic factors in the neritic zone. Abundant sunlight in the neritic zone keeps the temperature warm and stable as compared to the oceanic zone. Therefore, the neritic zone is the warmest zone in the ocean. However, the neritic zones near the poles have cooler water as compared to the tropical neritic zones. Similarly, proximity to the atmosphere makes the water rich in gases such as oxygen and carbon dioxide. This helps the marine plants to carry out photosynthesis and the organisms to respire. Tides carry the soil on the shore into the ocean. This adds minerals such as iron, copper, zinc, magnesium, and so on in the neritic zone, making it rich in nutrients.

what seaweed did Jane find

The ball-like structure jane observes is a pneumatocyst, and she presumably found a species of kelp

Bright canopy

The canopy is closest to the surface of the sea. It consists of the parts of the kelp blades that extend and float on the surface. This region sees the highest availability of sunlight. Species that live here require sunlight and higher temperatures. Creatures such as nudibranchs, crabs, skeleton shrimp, and young fish are common in the upper canopy. Most air-breathing life is found in the canopy, including birds, fish, otters, and other mammals.

Asexual reproduction

The corals also reproduce asexually through budding. After a considerable growth in size, a coral develops a bud. The bud separates from the parent polyp, and may start a separate colony in close proximity to the parent polyp. This process occurs throughout the life of the coral.

Salinity (deep sea physical adaptions)

The salinity in the deep oceans is fairly uniform. The average salinity of the oceans is about 35 parts per thousand (ppt). In the deepest parts of the ocean the salinity is 34.65 ppt.

Euhaline

The salinity of this zone ranges from 30 to 35 ppt.

Deep-Sea Discoveries

The deep sea is the largest habitat on our planet, yet humans know less about it than they do about the surface of the moon. Only at around the mid-nineteenth century did scientists begin exploring the deep sea. Even now, scientists have explored less than five percent of this habitat. Yet, this extremely small area of exploration has resulted in some startling discoveries that have amazed marine biologists. The discoveries have proved previously held beliefs to be false and offered scientists a new perspective of what could lie in the deep. Let's look at some unusual environments and organisms that marine biologists have discovered in the deep sea.

Oxygen (deep sea physical adaptions)

The deep-sea water is oxygen rich. Cold water in polar regions dissolves oxygen readily. In these regions, the water cools enough so that it sinks. Differences in temperature and salt concentration in different parts of the ocean cause deep ocean currents called thermohaline currents. These currents transport the oxygen-rich water through the ocean.

Temperature (deep sea physical adaptions)

The deep-sea water temperature remains fairly uniform between +4°C and -1°C. In polar regions, the difference between the surface temperature and deep-sea temperatures is minimal. In the tropics, the difference in temperatures between the surface and the deep sea may be more than 20°C.

Elkhorn Coral

The elkhorn coral shows typical structure like the antlers of the stag. This type of coral withstands vigorous wave motion due to its solid structure.

Encrusting Coral

The encrusting corals attach to a hard stony substrate. They spread in along the ocean floor in width rather than branching or growing upward. Therefore, they withstand vigorous storms and wave motions and are usually impervious to damage.

Foliase Coral

The foliase corals form flower-like whorls. They resemble the petals of a flower. These corals have spaces between the whorls. The space provides shelter to numerous small animals.

Fore reef

The fore reef, also known as the outer reef or reef front, is a reef zone toward the ocean (away from the shore). The fore reef rises deep from the ocean slope and is close to the water surface. The reef slope may be so steep that it may form a vertical wall known as drop off. The fore reef most often has grooves filled with sand. These grooves protect the corals from vigorous wave intensity. Massive and digitate corals are found on the fore reef. Foliose corals are present on the slopes.

Fringing Reefs

The fringing reefs are the youngest and the most common of all the coral reefs. This type of reef forms when the larval corals attach themselves to the edge of the submerged land surface. As the corals grow and develop, over a period, they form a fringe around the shoreline. Fringing reefs are present in many tropical ocean areas such as the Caribbean Sea, Red Sea, and so on.

The Deep Sea

The intertidal zone and the epipelagic zone of the open ocean are the areas of the ocean that people are familiar with. However, they form only a small fraction of the ocean. Over 90 percent of Earth's living space consists of the deep sea. The deep sea begins where the photic zone ends, at a depth of around 200 meters. It extends down to the sea floor. Let's look at the physical characteristics of the deep sea.

Massive Coral

The massive corals are boulder-shaped coral of large size. They grow slowly and withstand the vigorous wave motions.

Neritic zone

The neritic zone includes the coastal waters lying near to the shore. It is part of the epipelagic zone, which is the topmost zone of the ocean. The neritic zone lies between the low-tide water level and the upper edge of the continental shelf. It spans from the surface of the ocean to a depth of around 200 meters. The neritic zone receives an ample amount of sunlight. The waters are shallow enough to allow sunlight to reach the ocean floor. The continental runoffs mix with the coastal waters, making the neritic zone rich in nutrients. Therefore, this zone consists of diverse marine life. Snorkelers and scuba divers dive in the upper part of the neritic zone

Polyhaline

The next zone has a salinity range between 18 and 30 ppt.

Reef crest

The reef crest lies behind the fore reef toward the shore. It is the highest zone on the reef and closest to the water surface. The incoming waves crash against the reef crest. Therefore, elkhorn corals that withstand intense wave motion are present on the crest.

Reef flat

The reef flat is also known as the inner reef or the back reef. It is a zone closest to the shore or the lagoon. Water around this zone is relatively calmer. Therefore, fragile corals, like branching corals, are present in this zone.

Stipe

The stipe grows upward and is similar in appearance to the stem or stalk found in land plants. However, unlike land plants, stipes do not transport nutrients. They only provide support to kelp blades.

Table Corals

The table corals are shaped like tables. The top of the surface is broad and flattened. This exposes the zooxanthellae to sunlight.

Physical Properties

The temperate zone is more productive than the tropical zone. Though the tropical zone is rich in coral reefs and other marine life, it is deficient in nutrients. The temperate zones are rich in plankton and by extension rich in nutrients. Plankton forms the base of food for all organisms in the ocean and is responsible for sustaining many marine creatures. Due to the large amount of plankton, water in the temperate zone appears murky as opposed to the clear, plankton-deficient water of the tropical zone. The tropical zone consists of warm water with consistent temperature throughout the year. The temperate zone contains cooler water. The temperature of the water in this zone fluctuates seasonally.

Understory

The understory is the part below the canopy. It is shaded from sunlight by the canopy. Algae of medium height, such as oarweed and featherboa, are common in the understory. Species in this zone are shaded from direct sunlight. They live among the tall stalks of the kelp. Some creatures that live in this zone include the kelp perch, snail, and rockfish.

Light (deep sea physical adaptions)

The upper region of the deep sea contains some amount of sunlight that penetrates through, but it's insufficient for photosynthesis. This upper region is called the twilight zone. No light penetrates below 1000 meters and the ocean is almost completely dark. There is some light at these depths that originates from bioluminescence. This type of light is produced by chemical reactions within an organism's body without the generation of heat.Between a depth of 200 and 1000 meters, in temperate and tropical regions, scientists have found oxygen-minimum zones. Scientists believe that decomposers feeding on the abundance of decaying food particles that descend from the surface use up the oxygen. This consumption results in oxygen concentrations dipping to very low levels and in some cases even to zero.

because of the vastness of the ocean, scientist divide the ocean into different zone. Which statements correctly describe the characteristics of these zone?

The uppermost epipelagic zone, which extends from the ocean surface to a depth of 200 meters, receives the most sunlight and is suitable for photosynthesis. The bathypelagic zone is the first layer of the aphotic zone.

Pressure (deep sea physical adaptions)

The weight of the water above causes the pressure in the deep sea to increase the deeper you descend. For every 10 meters of increase in depth, the pressure increases by one atmosphere. Depending on the depth of the sea floor, the pressure can be as high as 1000 times the pressure on the surface.

Hadopelagic zone

The zone exists only in those regions on the ocean floor where deep-sea trenches occur. These trenches are found at subduction zones where one tectonic plate slides under another. The deepest known trench is the Mariana Trench near Guam in the Pacific Ocean. Scientists have recorded depths in this trench in excess of 10900 meters.

Characterize seagrass

They act as habitats for many marine species. They are a source of food for manatees and other herbivores.

Seagrasses provide several essential services

They modify the environment to create habitats suitable for a thriving marine ecosystem. A single square meter of seagrass can generate up to ten liters of oxygen a day. They also help stabilize the flow of water and sediments, thereby improving the quality of water. They also reduce erosion. They provide habitats and breeding grounds for hundreds of species of small creatures and invertebrates. They also provide these creatures protection (as a means of hiding) from larger predators. They can act as both permanent and temporary shelters to many marine creatures. Many marine species that reside in seagrass meadows are commercially viable and useful to humans.

Mesohaline

This zone has a higher salinity between 5 and 18 ppt.

Low-intertidal zone

This zone is exposed to air only during extreme low tide. As a result of this, this zone is mostly submerged. The zone is bordered by the low tide level and extreme low-tide height. Species in this zone are well adapted to sea water conditions, but are poorly adapted to extreme conditions. This zone is shallow and has high sunlight availability; because of this, vegetation is highest in this zone. This zone is also teeming with marine life that is protected from large predators by the shallow water and wave action. These species include snails, crabs, kelp, nudibranch (a shell-less mollusk), and sponges.

Spray zone

This zone is normally not submerged under water. The zone extends from the high tide height to the highest point on land affected by sea spray and storm waves. The spray zone gets submerged occasionally during the highest tide. This zone experiences regular spray from the sea. Availability of dissolved nutrients and oxygen is very low in this zone. Therefore, the zone is sparsely populated. Organisms in this zone are exposed to air, direct sunlight, cold, rainwater, and predators. The organisms that live in this zone require protection from drying out and from predators on land.

Mesopelagic zone

This zone lies below the epipelagic and extends to a depth of 1000 meters. There is some light, but not enough for photosynthesis. This zone is called the twilight zone.

Bathypelagic zone

This zone lies between 1000 and 4000 meters. It is the uppermost part of the aphotic zone, where no light penetrates. It is also called the midnight zone.

Holdfast

Unlike other plants, kelp does not have roots. It has root-like structures called holdfasts that anchor the plant to the substrate. Unlike roots, holdfasts do not draw nutrients from the ground.

The Temperate Ocean

When you look at images of the Earth, the oceans appear as a continuous body of water. It gives the impression that the ocean is a uniformly mixed body of water. However, the marine system of the Earth is divided into five distinct climate regions. These regions are based on the climate classification. From warmest to coldest they are tropical, arid temperate, cold, and polar zones. The tropical zone lies between the Tropic of Cancer and the Tropic of Capricorn. It includes the central parts of the Pacific, Atlantic, and Indian oceans. The temperate zone, as depicted in the figure, is actually two distinct regions that lie on either side of the tropical zone. The Arctic Circle borders the northern zone and the Antarctic Circle borders the southern zone.

Feeding (deep sea adaptions)

With autotrophs absent in the deep sea, food is scarce. Organism must feed on prey or the decaying remains of dead organisms sinking from the surface water. Many fish, such as bristlemouths, have evolved large mouths with elastic jaws and rows of sharp teeth. Gulper eels have hinged jaws and expandable stomachs.

Seasonal Cycles

With the beginning of the fall season, wind directions change and nutrients drawn up by the upwell stops. The growth of the kelp forest slows. The plants begin to thin out and die. Holdfasts weaken and kelp plants get dislodged due to strong water currents. As winter approaches, the kelp forests thin out. This thinning allows more sunlight to penetrate to the bottom layers. Fresh algae and kelp species begin life in the lower layers, as both nutrients and sunlight is now available. These plants grow till the next spring cycle begins.The growth and abundance of kelp forests closely follows seasonal changes. The reason is that the amount of available sunlight and nutrients changes. During spring, surface water currents create upwells that draw nutrients from the depths. The spring season also sees longer daytimes, which mean more available sunlight. Both these factors lead to increased kelp growth. As the upper canopy grows it blocks sunlight from reaching the understory. The understory sees a decrease in growth of medium-height algae.

Zooxanthellae interdependence

Zooxanthellae are unicellular dinoflagellates that carry out photosynthesis. These zooxanthellae reside in some corals. Zooxanthellae and corals mutually benefit from this association. The corals produce carbon dioxide as a byproduct of cellular respiration. The zooxanthellae utilize this carbon dioxide to carry out photosynthesis, and provide organic molecules as food to the corals. The corals provide space and proximity to the sunlight in return to the zooxanthellae. Corals use this form of nutrition during the day. Most of the hard corals and some soft corals show the presence of zooxanthellae. Most of the hermatypic corals present in the neritic zone contain zooxanthellae. Zooxanthellae occupy up to 30 percent of the coral tissue.

rita wants to go snorkeling to see the beautiful coral reefs. Which place should she choose to snorkel to see the reefs?

a place near to the shore with ample of sunlight and saline waters

Tectonic Estuary

are formed by tectonic movement of the Earth's crust. This movement causes the crust to sink or fold, forming a depression. These depressions are then filled by fresh water from nearby rivers that drain outward to the sea. These types of estuaries tend to be very deep and are surrounded by mountainous areas.

Barrier-built Estuary

are formed when sandbars or barrier islands build up due to waves and currents. These estuaries are usually fed by low-volume rivers. The barriers at the mouth protect the estuary from waves. Sometimes these barriers can build up and completely block the river, forming lagoons. The barriers can be washed away over time by wave action or heavy rains.

Atoll Reefs

are the oldest type of reefs. They form when coral larvae attach to a volcanic island. As the island continues to submerge, the coral exoskeleton continues to build up. Finally, when the island completely submerges under water, the coral reef builds on top of it, encircling a lagoon of water within it. Atoll reefs in the Maldives are famous for their diversity.

Kelp forests

are vast structures and support thriving ecosystems. Kelp forests are similar to land forests in that they support vast numbers of creatures. Different species of kelp can coexist in the same forest and foster different species. Species also differ based on the part of the kelp forest in which they exist. are found along the west coast of the United States. They can be found all along the coast from Alaska to California. Kelp forests can also be found along the Atlantic coast, but this variety is smaller. Kelp forests grow in dense groups and provide food and shelter for a large number of marine species. Biologists refer to kelp as ecosystem engineers because they influence the species diversity of the area. Many species use the thick kelp blades as protection from currents and predators. Kelp forests are also used as shelter by marine birds and mammals, including gulls, terns, whales, seals, otters, and so on. The growth and abundance of kelp forests closely follows seasonal changes. The reason is that the amount of available sunlight and nutrients changes. During spring, surface water currents create upwells that draw nutrients from the depths. The spring season also sees longer daytimes, which mean more available sunlight. Both these factors lead to increased kelp growth. As the upper canopy grows it blocks sunlight from reaching the understory. The understory sees a decrease in growth of medium-height algae.

what is the temperature of the water at a cold seep?

around 4

How do organisms living in the intertidal zone use byssal hair?

as an anchor against tidal action

which pelagic zone of the ocean exists only in some regions...

hadopelagic

Selma is creating an aquarium-style coral reef model to teach...

barren sand, no plants, only fish and warm, clear, fresh water

why is the neritic zone rich in nutrients?

because continental runoffs provide nutrients to the ocean

Why is the neritic zone rich in nutrients?

because continental runoffs provide nutrients to the oceans

On a drive in the midnight zone, scientist see specks of light that flash on and off. What is the origin of this light?

bioluminesence

match the parts of the kelp plant to the given structure

blade, float, stipe, holdfast

What is formed by the mixing of salty sea water and fresh river water?

brackish water

which component forms the major skeleton of reef building?

calcium carbonate

which of these is a productive environment for coral growth?

clear and shallow waters, with temperatures around 64.4 to 95 degrees

continental order

continental shelf, continental slope, continental rise, abyssal plain, trench

Mid-intertidal zone

covered with water for approximately equal amounts of time per cycle. This zone extends from standard tide height to the low-tide level. Due to less direct exposure to the sun, the temperature variations are moderate. The mid-tide zone is also slightly more saline than sea water. However, this zone experiences more wave action than the high-tide zone. This zone has a higher population of vegetation, especially seaweed, compared to the upper zones. Organisms in this zone are also larger, more complex, and more diverse than those found in the high-tide or splash zone.

how does density affect water circulation?

denser (more saline) water moves down and less dense (less saline) water moves up until it neutralizes

Which is the correct sequence of pelagic zones in increasing order of depth?

e, m, b, a, h

Mangrove tree can only survive in moderately saline water. What are such organisms called?

euryhaline

mangrove trees can only survive in moderately saline water...

euryhaline

trevor examines some water from the open ocean and finds that it contains several...

flagella

Coastal Plain Estuary

formed tens of thousands of years ago, at the end of the ice age. Receding glaciers led to the rise in sea levels. This flooded low-lying river valleys.

which type of coral reef has a shallow backreef zone?

fringing

Corals are

heterotrophs. They do not produce their own food and rely on other organisms for nutrition. Corals gain nutrition in two ways. Let's discuss these feeding patterns in corals.

in which zone would you expect the organisms to be the most adapted to surviving in harsh sunlight...

high tide

Which of these regions is the high-intertidal zone?

high tide zone

Oligohaline

his is the least saline zone of the estuary, having a salinity of about 0.5 to 5 ppt (parts of salt per thousand parts of water).

High-intertidal zone

his zone is covered with water only during the peaks of high tide. The zone extends from the high tide level to the average sea level (standard tide height). The environment is highly saline due to the low levels of water and high evaporation rates during low tides. Vegetation is also sparse because of the low levels of water. This zone may also have small rock pools containing sea water. Small fish and seaweed inhabit these isolated rock pools. Barnacles, crabs, algae, snails, and mussels mostly dominate this zone.

From which substance do microbes at hydrothermal vents obtain their energy?

hydrogen sulfide

From which substance do microbes at hydrothermal vents obtain their energy?

hydrogyen sulfide

estuary

is a body of water formed at the meeting point of a river (or multiple rivers) and the sea. Estuaries consist of brackish water. It's not as fresh as river water and not as saline as sea water. Estuaries may be made up of either river water entering the sea or tidal waves entering the river at its mouth. Estuaries are found at the meeting point of rivers and seas, so they produce an environment with features from both. They contain high levels of nutrients from both the sea and the river. Estuaries are one of the most productive marine habitats.

Mushroom Coral

is a mushroom, dome-shaped coral. It is a solitary coral. It does not attach itself to any surface.

Kelp

is a species of large seaweed belonging to the algae group. A typical kelp plant has four main parts from bottom to top.

Fjords

is created by glacial action. They are characterized by long and narrow valleys with steep sides. Glacial movement leaves deep channels with a barrier near the mouth called a sill. When the glacier retreats, the valley fills up with sea water and forms an estuary. Compared to other estuaries, fjords experience very little mixing of sea and river water. The sill prevents deep-sea water from mixing, resulting in low oxygen levels at the bottom. Fjords are found in areas formerly covered by glaciers.

while gutting fish, nathan encountered an air-filled sac-like structure. What is the purpose of this structure in fish?

it provides buoyancy to the fish

what is the function of the pneumatocyst in kelp?

keeps the kelp buoyant

an estuary in texas gets cut off from the sea by the accumulation...

lagoon

Brian is a marine biologist researching photosynthetic activity in the ocean. He travels in a submersible to an area that dimly lit. However, despite the light, he finds no photosynthetic organisms. Which zone of the ocean is he in?

mesopelagic

what is the ebb of the sea defined as?

movement of sea water away from the shore

Euryhaline

organisms can live in both salt water and fresh water

Water Chemistry

refers to the property of the water of an estuary.

a marine research team studies an estuary where they find that the water forms two layers of varying density...

salinity stratification

which of the following is a major repository of carbon in the sea?

seagrass meadow

Elkhorn coral

shaped like antlers

Digital corals

shaped like cylindrical structures with no branching

Foliose coral

shaped like whorls of petals

which marine creatures show diurnal vertical migration..

species of zooplankton

Nekton Species

squid

In which zone are you most likely to find kelp forests?

temperate zone ???

Terry is teaching his class about atoll reefs. Which of these points should Terry mention in his explanation?

that atoll reefs are the oldest

Argo is explaining coral feeding patterns to his daughter, specifically zooxanthellae interdependence. Which point should he explain to his daughter?

that zooxanthellae provide food to the corals via photosynthesis

Intertidal Zone

the area that exists between high and low tide. During high tide this area is submerged, while at low tide it is exposed to air. The intertidal zone is a rather dynamic environment because of the changes it experiences. Because of this only simple marine life creatures live here.

How does the color adaption help fish residing in the coral reefs?

the bright body color of the fish in the coral reefs help in Camouflaging to hide from the predator or ambush prey

A particular estuary shows periodic changes in the coloration of water. During the summer, water in the estuary appears greener than in winter. What can be the cause of this cyclic change in water color?

the change in color is caused by the decreases the growth of phytoplankton during winter

What is responsible for the temperate zone?

the continental landmass forcing warm and cold currents to meet

Moshe is a writer. He is creating content for an informative chart to be placed near the coral reefs for the visitors to the reefs. How should he describe the reef crest?

the highest area with high wave intensity

A group of students is trying to grow a kelp plant in a tank without any others animals. They use a temperature regulation system to keep the water temperature at about 12 degrees celcius. They bury the holdfast below the soil that has been erinched with nutrients and place the plant near a window in direct sunlight. However, over a few weeks the kelp plant withers and dies. Why did this happen?

the kelp draws nutrients from the water rather that the soil, which it does not have available

What is the ebb of the sea defined as?

the tidal phase during which the water level is falling and flood the tidal phase during which the water level is rising

What are the functions of the parts of a kelp plant in the understory?

the understory contains a long stem like stalk that gives stuctural support to the kelp. at the bottom in the holdfast that anchors the plant to the gound

What significance does the temperate zone have on commercial fishing?

the zone has a greater abundance of commercially viable

what are the calcium carbonate walls around the coral called?

theca

why are kelp and seagrass not found in deep oceanic water

there no availability of sunlight

What is the function of the pneumatocyst in kelp?

they float the blades for better exposure to sunlight

why are hydrothermal vents capable of nurturing...

they provide nutrients from earths surface, which the deep-sea creatures can feed on