Understanding Animal Biology: Unit 9

Chordata

Diverse phylum Contains the most recognizable organisms in the animal kingdom - vertebrates

Endostyle

Hallmark Characteristic of chordates The endostyle was only recently recognized as a chordate characteristic Usually found on the pharyngeal floor Secretes mucus to trap small food particles coming in from the mouth

Agnathan Phylogenetic Relationships

---dotted lines - probable phylogenetic relationship, with inferred geologic range--- 1-chordate characteristics 2-cephalized brain, vertebrae 3-extensive dermal skeleton/ lateral line system in grooves 4-pectoral fins 5-jaws, pelvic fins

Conodonts

510 - 200 mya Characteristics: ---Small, thin, wormlike ---Tiny rows of teeth, like hagfish ------Teeth contained calcium carbonate ---Bilateral symmetry ---Some blocks of segmented muscle ---Large eyes with associated eye muscles ---Mineralized exoskeleton ---Bone cells present Observed characteristics place it as a near gnathostome vertebrate BUT no gill apparatus found ---Small size, could use body surface for gas exchange

Notochord

A flexible rod that supports a chordate's back

Agnathan

A member of a jawless class of vertebrates represented today by the lampreys and hagfishes.

Class Pteraspidomorphi (Diplorhina)

All extinct ---Also known as Heterostracans Oldest documented fossil record Considered most primitive ---monophyletic assemblage Primitive bone splinters, no true bone cells Most marine

Lamprey Reproduction

All lampreys ascend freshwater streams to breed ---marine forms are anadromous (leave the sea as adults to spawn) In North America, all lampreys spawn in winter or spring ---Males build nest by lifting stones with oral discs and using body vibrations to form oval depressions on the substrate to act as nests ---Female anchors to a rock near nest and male attaches to her head ---As the sticky eggs are shed into nest, the male fertilizes them and the adults die soon thereafter

Parasitic Lampreys

Attach to a fish by a sucker-like mouth and use sharp keratinized teeth to rasp through flesh as they suck body fluids Inject anticoagulant into a wound to promote blood flow Lamprey drops off when engorged ---wound may be fatal to fish Parasitic freshwater adults live 1-2 years before spawning and dying while anadromous forms live 2-3 years ---Nonparasitic lampreys do not feed after metamorphosis and die within a few months after spawning

Class Myxini (hagfish)

Body fluids are in osmotic equilibrium with seawater (unlike other vertebrates) Have low-pressure circulatory system ---Main heart behind gills and three accessory hearts

Adult Tunicate

Chordate characteristics? Only two! ---absence of notochord Drives blood in one direction for a few beats, then reverses direction for a few beats

Ostracoderms & Cyclostomes

Denote extinct & extant jawless fishes ---Considered terms of convenience, have no taxonomic status & do not denote evolutionary relationship Ostracoderms (meaning bone & skin) ---Earliest agnathans ---Very diverse group displaying varied appearance extensive body exoskeleton ---considered a paraphyletic assemblage Cyclostomes (meaning "round mouth") ---Hagfish & lamprey - living agnathans ------Both lack bone but have single median nostril

Subphylum Vertebrata

Digestive, respiratory, circulatory, and excretory systems reflect increased metabolic demands ---Muscles in pharynx pump water and enable gas exchanges ---Muscles in gut move food ---Digestion: liver and pancreas ---Circulatory: ventral, three-chambered heart ------Erythrocytes with hemoglobin ---Excretion: glomerular kidneys

Subphylum Cephalochordata (lancelets)

Displays swimming mechanism like that of fish ---"Swimming" performed by coordinated movement of serially arranged myomeres along notochord ---Contraction & relaxation produce side-by-side undulation that thrust body forward Sense organs are simple, including a simple photoreceptor "eye" Separate sexes—gametes are released via atriopore ---External fertilization, holoblastic cleavage, gastrula forms by invagination

Phylum Chordata Divisions

Divided into 3 subphyla: ---Urochordates ---Cephalochordates ---Vertebrata Urochordates & Cephalochordates are often termed protochordata and are represented by small, nonvertebrate marine animals.

Lamprey Development

Dramatic changes to an adult involve: eruption of eyes, keratinized teeth, enlargement of fins, maturation of gonads, and modification of gill openings Marine, parasitic lampreys migrate to the sea while others remain in freshwater

Class Myxini (hagfish)

Eel-like marine scavengers ---Predates on annelids, molluscs, dead and dying fish Nearly blind—locate food by an acute sense of smell and touch

Lamprey Development

Eggs hatch in two weeks and release ammoceotes ---Larvae are very unlike their parents in form and structure ---Initially live on yolk supply Drift downstream to burrow into sandy, low-current areas ---Suspension-feeding for several years until metamorphosis

Class Myxini (hagfish)

Enter animal through orifice or by digging inside using keratinized plates on the tongue that fold together like pincers Can be a nuisance to fishermen -devour catch before it can be pulled to the surface

Ostracoderms

Fishlike animal ---Appeared more than 500 mya ---3-10 inches in length Cartilaginous internal skeleton Bony external shields ---Covered head and most of trunk Thick flattened bodies with pair of side flaps for steering Clumsy swimmers Mouth served to obtain oxygen and retain bits of food

Pharyngeal Pouches and Slits

Hallmark Characteristic of chordates Formed by the inpocketing of the ectoderm and the invagination of endoderm of pharynx ---Aquatic chordates: two pockets break through to form pharyngeal slit ---Amniotes: pockets may not break through and only pouches are formed instead of slits

Notochord

Hallmark Characteristic of chordates In all vertebrates (except hagfishes) a series of cartilaginous or bony vertebrae form from mesenchymal cells ---Derived from blocks of mesodermal cells (somites) lateral to notochord In most vertebrates, the notochord is replaced by vertebrae but some remnants may persist between or within vertebrae

Notochord

Hallmark Characteristic of chordates Longitudinal flexible rod ---Located between the gut and nerve cord ---Originates from the mesoderm First part of the endoskeleton to appear in embryo

Dorsal Tubular Nerve Cord

Hallmark Characteristic of chordates Most invertebrates: nerve cord is solid and ventral to digestive tract Chordates: cord is tubular and dorsal to digestive tract ---Hollow cord is produced in the embryo by infolding of ectodermal cells on the dorsal side of body above the notochord ---Vertebrates: ------Anterior end enlarges to form the brain (surrounded by bony or cartilaginous cranium) ------Nerve cord passes through the protective neural arches

Postanal Tail

Hallmark Characteristic of chordates Postanal tail, somatic musculature, and stiffened notochord can provide motility for larval tunicates and Amphioxus during their free-swimming stages Efficiency increased in fishes due to addition of fins Smaller and vestigial in groups like mammals ---Mammals may have waggable tails ---Humans have a coccyx

Pharyngeal Pouches and Slits

Hallmark Characteristic of chordates Terrestrial tetrapods: pharyngeal pouches give rise to a variety of structures: Eustachian tube, middle ear cavity, tonsils, and parathyroid glands ---Feeding: ------Protochordates: perforated pharynx functions as filter-feeding apparatus -water is pulled in through the mouth by ciliary action and pushed out through the pharyngeal slits ------Vertebrates use muscular pharyngeal contractions to move water through the pharynx ---Blood flow through pharynx: ------Protochordates have simple aortic vessels surrounded by connective tissue ------Fishes have a capillary network with thin gas-permeable walls that improved gas exchange

Thyroid Gland

Hallmark Characteristic of chordates Thyroid gland is a derivative of the endostyle ---Some cells in the endostyle secrete iodinated proteins which are homologous to iodinated hormone secreted by the thyroid gland of adult lampreys (all other vertebrates)

Tunicate Larvae

Has all chordate characteristics Notochord only in tail-(urochordates) - disappears during metamorphosis. Dorsal nerve chord becomes reduced to single ganglion

Agnathan (characteristics)

Include Hagishes and lampreys ---Lack jaws, internal ossification (ribs or vertebrae), scales, or paired fins ---Share pore-like gill openings and an eel-like body In many respects, are primitive vertebrates ---Possess vertebrate synapomorphies but have not progressed much beyond protovertebrates

Subphylum Urochordata

Includes about 1600 species (may be up to 3000) Sessile marine animal as adults (some are free-living) ---adhere to rocks, docks, boats In most species, only the larval form bears all the chordate hallmarks ---adults lose many of the chordate characteristics and retain the notochord only in the tail region

Hagfish (Myxini) Physiology

Instead of vertically articulating jaws like gnathostomes, they have a pair of horizontally moving structures with toothlike projections for pulling off food Discussions in the scientific literature about the hagfish being non-vertebrate ---no evidence of vertebrae but have cranium so placed in Craniata

Ancestral Vertebrate

Many different hypotheses Several groups proposed ---Placement within vertebrate groups uncertain ---Significance to vertebrate origins unclear Living groups can be investigated for clues, but ancestors are long extinct and fossils are few

Ancestral Vertebrate

Many hypotheses exist concerning the origins of vertebrates ---Incomplete fossil record ---Reconstructions from studying living organisms' development ---Unsurprising that there is disagreement A more accurate pictures is developing: ---Modern methods of phylogenetic reconstruction ---New fossil evidence ---Molecular probes to assess taxonomic relationships

Agnathan (characteristics)

Monophyletic or paraphyletic? ---More analysis needed to clarify phylogenetic relationships Hagfishes and lampreys are grouped with Vertebrata ---Have cranium and other homologies ---Look similar but are quite different from one another

Relationship between Extant Non-Vertebrate Chordates & Vertebrates

Morphology supports a vertebrate/cephalochordate association ---features shared may simply be ancestral chordate features

Subphylum Vertebrata

Most posses endoskeleton of cartilage and/or bone ---Permits almost unlimited body size Endoskeleton forms a jointed scaffold for segmented muscles to attach ---Vertebrae and intervertebral discs in place of notochord ---Myomeres "w"-shaped to allow for more control Endoskeleton initially composed of cartilage, which gives way to bone ---Cartilage ideal for attachment in areas of high mechanical stress ---Bone has superior structural strength ------Important for mineral regulation

Class Pteraspidomorphi (Diplorhina)

Observed characteristics: ---paired nasal openings (diplorhiny) ---head shield ---fusion of several bony plates with occasional lateral & dorsal spines ------function? ---exoskeleton ------body plates & scales ---no paired fins ---lateral eyes - small ---head not flattened ---single opening from gill chamber Pteraspids were slow swimmers but not bottom feeders

Class Petromyzontida (lampreys)

Only living member of class 24 species of lampreys in North America ---half are nonparasitic brook-dwelling species, rest are parasitic Marine lamprey Petromyzon marinus is destructive to native fishes; occurs on both Atlantic coastlines and can grow to a length of 1 m Lampreys also found in Eurasian regions

Modern Ancestral Vertebrate Hypotheses

Origin & evolution of early vertebrates probably occurred where? ---Evidence suggests marine waters Evolution of early vertebrates ---Characterized by an increasingly active lifestyle ---Proceeded in three major steps ------Leads early vertebrates away from suspension feeding typically observed in ancestors (1) ------Depend on muscularized pharynx, creating food-bearing water current (2) ------Lead then to selection & capture of prey - muscularized mouth & jaws (3

Modern Ancestral Vertebrate Hypotheses

Phylogenetic relationships ---Tunicates are vertebrates sister group ---Evidence for body inversion ------Dorsoventrally inverted body plan ---------Major evolutionary event ---------Reason unknown

Garstang's Hypothesis

Postulated that ancestral chordates were sedentary filter feeders ---Free-swimming larvae evolved with sessile adult ------Larvae failed to metamorphose, developed functional gonads ------Adult form was "lost", creating new free-swimming species ---Paedomorphosis: Retention of embryonic or juvenile characteristics in the adult

Traditional Chordate Classification

Protochordata vs Vertebrata (Craniata), which have a skull ---Cladistics excluded hagfish from Vertebrata due to lack of bones but retained them under Craniata due to presence of cranium

Ostracoderms & Cyclostomes

Proxies for most primitive of vertebrates Highly modified, adapted to specialized lifestyles Departed in many ways from general ancestral state

Cephalochordates

Relationship between extant-non-vertebrate chordates and vertebrates several anatomical features shared with vertebrates but absent in tunicates ---most closely related? HOWEVER... molecular analysis tells a different story ---tunicates are sister group of vertebrates sessile adult stage is a derived character

Notochord

Semi-rigid body of fluid-filled cells enclosed in a fibrous sheath -extends the length of body Usually found above the gut and just below the nervous system Primary purpose is to stiffen the body, provide scaffolding for the attachment of swimming muscles

Class Petromyzontida characteristics (lampreys)

Single nasal opening, similar brain and CNS unite lampreys with fossil cephalaspidomorphs and serve as a primary basis for uniting lampreys with similar ostracoderms within cephalaspidomorphs.

Subphylum Cephalochordata (lancelets)

Small marine animals (~3-7 cm long) Represents an idealized chordate ---display all chordate characteristics in adult Approximately 29 species (five in North America)

Subphylum Vertebrata

Specialized structures help for location, capture & digestion of food ---Increased speed and mobility ---Adaptations to support high metabolic rate Anterior end of the nerve chord enlarges as tripartite brain ---Forebrain, midbrain, hindbrain ---Protected by bony cranium Paired eyes with lenses and inverted retinas Pressure receptors - paired inner ears for equilibrium and sound perception Chemical receptors (taste and olfactory) Lateral lines for detecting water vibrations Electroreceptors for detecting prey signals

Prevertebrate

Step 1 in evolution of early vertebrates Suspension-feeding prevertebrates ---Used cilia to create food-bearing current Shift from prevertebrate to vertebrate involved two mechanical changes in pharynx ---Development of an encircling muscular band in pharynx ------Squeezed water out of pharyngeal slits ---Cartilage replaced collagen in support bars in pharynx ------Restore original shape after muscle contraction ---**Initially, muscular pump supplemented ciliary pumps Advantages? ---Removed size limit of organisms imposed by surface ciliary pumps ---Gills: complex, folded respiratory membranes ------Support increased respiratory demands of active prechordates

Agnathan

Step 2 in evolution of early vertebrates Muscular pharyngeal pump brought early vertebrate evolution to agnathan stage What is an agnathan? ---Early vertebrate lacking jaws but possess muscular pump to produce food-bearing current ---Diversification observed as organism exploited pharyngeal pump ---Deposit feeders-draw in sediment rich in organic particles ---Still utilize cilia & mucus of branchial basket ------Collect particles from suspension & transport to esophagus Function of muscular pharynx (pump) ---Moves material through mouth ---Push water across newly evolving gills

Gnathostome

Step 3 in evolution of early vertebrates Transition involved change in feeding method-raptorial feeders ---Grab individual, selected food particles from surroundings ---Raptorial feeders favored sudden & forceful expansion of pharyngeal pump, along with ability to close mouth firmly to prevent escape/loss of captured food Mechanism & problems? ---Springy cartilage recoiled, allowing production of suction that drew food into mouth, HOWEVER, too weak of a system for forceful capture & ingestion Jaw development ---When powered by quick muscle action, eliminated problems & removed prey limits ---Active predation

Calcichordates

The earliest vertebrates 600 - 400 mya Characteristics: ---Echinoderm-like skeleton, plates made of calcium carbonate, bilateral, fish-like shape Calcichordate hypothesis ---Each lineage of chordates evolved from its own lineage ---Not much support

Calcichordates

The earliest vertebrates Beginning bone, with other vertebrate-like characteristics: ---Side surface pore - gill slits? ---Internal head chamber - brachial basket? stomach? ---Whip-like tail - postanal tail? ---Expanded anterior neutral tube- brain? However! ---no evidence of a notochord, nerve cord or segmented muscle

Paedomorphosis

The retention in an adult organism of the juvenile features of its evolutionary ancestors.

Garstang's Hypothesis

This is less supported - rejected by some zoologists - due to recent information from gene expression and phylogenies based on gene sequences

Hagfish Reproduction

This is mostly a mystery ---Hatch as small, fully formed hagfish ------believed to have direct development without metamorphosis; no known larval stage ---In some species, females produce small numbers of large, yolky eggs that take up to five months to hatch ---Spawning times, places, and age at maturity are unknown

Subphylum Urochordata (Tunicate: Sea Squirt)

Tunic is a tough, non-living cellulose that surrounds the animal Some are planktonic, some are colonial Adults are filter feeders-highly specialized construction

Class Myxini (hagfish)

Unique trait: ---tie themselves in knots use a knot to pull themselves out the places where they feed ---scrape the slime off themselves to clean themselves

Traditional Chordate Classification

Vertebrates ---Amniota (having an amnion) and Anamniota (lacking an amnion) ---Agnatha (jawless) and Gnathostomata (having jaws) -------Gnathostomata: Pisces (with fins) and Tetrapoda (usually with two pairs of limbs)

Chordates

an animal phylum that has a notochord, a dorsal hollow nerve cord, and gill slits at some time in its life cycle

Chordate shared features with non-chordate invertebrates

bilateral symmetry, anteroposterior axis, coelom, tube-within-a-tube body plan, metamerism (linear series of body segments) & cephalization

Jawless Fishes Muscle System

body muscle segmentally arranged - myomeres ---alternate waves of contractions down sides, provide locomotion muscular pharynx

Hallmark Characteristics of Phylum Chordata

dorsal, hollow nerve cord notochord pharyngeal slits or pharyngeal pouches - not unique to chordates, also in hemichordates (depending on species can form a variety of structures) postanal tail others ---endostyle, or thyroid gland ---segmented muscles ------Muscles attach to notochord and can bend without shortening to permit undulation

Jawless Fishes Respiratory System

external gill openings lamprey - parasitic ---Modification: during metamorphosis, pharynx differentiates dorsally into esophagus and ventrally into a blind terminal pharynx

Jawless Fishes Reproduction

hagfish: ---hatch as small, fully formed hagfish ---believed to have direct development without metamorphosis lamprey: ---have an extended larval period (ammocoetes) ------3-7 years, acting as burrowing suspension feeders ---in general, parasitic feeders have shorter larval period and longer parasitic adult life ---die after breeding once

Jawless Fishes Urogenital System

hagfish: ---isotonic to seawater ---pronephric kidney (adult) lamprey: ---hypotonic to seawater & hypertonic to freshwater ---opisthonephric kidney posses single gonad, ovary or testis; no oviducts or sperm ducts; gametes released into abdominal cavity

Jawless Fishes Reproduction

hagfish: ---little is known embryo undergoes meroblastic cleavage reproductive rate low, may breed continuously lamprey: ---spawn in freshwater embryo undergoes holoblastic cleavage

Jawless Fishes Sense Organs

lamprey: ---only living jawless vertebrate with a lateral line system (neuromasts); 2 lateral eyes, 2 semicircular canals, well developed olfactory organs hagfish: ---rudimentary eyes, no eyeball forms; well developed olfactory organs; sensory barbels; 2 semicircular canals, positioned to appear as one

Jawless Fishes Digestive System

lampreys: ---buccal funnel with oral disk; tongue with horny, epidermal teeth; secretes an anticoagulant; no swim bladder hagfish: ---scavenger; lack oral disk-feeding apparatus - 2 dental plates with 2 curved rows of horny cusps; low metabolic rate, changes with activity Observe: no stomach ---food→mouth →pharynx/region → esophagus →intestine →cloaca

Jawless Fishes Cardiovascular System

low blood pressure system heart found in pericardial cavity ---comprised of 4 regions - sinous venosus, atrium, ventricle & conus arteriosus ------order in which blood flows

Jawless Fishes Skeletal System

made of cartilage ---skull ------trough-like plate upon which brain sits ---lamprey - paired lateral neural cartilage bars, lie atop notochord lateral to spinal cord**, whereas only observed in hagfish tail ------rudiment of a backbone or vertebral column ---lamprey & hagfish possess caudal fin

Cephalaspid (order Osteostraci)

member of Class Cephalospidomorphi (Monorhina) also called osteostracans heavily armored with head shield, small scales covered rest of body body shape was fusiform or flattened dorsal eyes ---character - indicates slow-moving, along stream bottoms

Anaspid (order Anaspida)

member of Class Cephalospidomorphi (Monorhina) only jawless fish with streamline form active swimmers paired fins - long thin may have undulated for accurate maneuvering

Chordate shared deuterostome Characteristics

radial cleavage, anus from blastopore, mouth from secondary embryonic opening, coelom by enterocoely More structural unity in body plan (organs & organ systems) than is found in many other phyla

Jawless Fishes Nervous System

similar in design to vertebrate brain (similar basic design) ---anterior portion ------cerebral hemispheres & olfactory bulbs; lamprey have pineal glands dorsoventral flattened spinal cord, dorsal to notochord paired spinal nerves ---lamprey unique arrangement - separate dorsal & ventral portions

Class Cephalospidomorphi (Monorhina)

sister group to jawed fishes (?) body shape variable, suggests varied lifestyles nasohypophyseal opening ---result of single nasal opening merges with hypophysis ------dorsally placed characteristic, unites all member of this diverse group (synapomorphic trait)

Jawless Fishes Integumentary System

smooth & scaleless ---thin epidermis (living cells) with complex dermis (collagen fibers) epidermal secretions ---lamprey - cuticle ---hagfish - mucus, or "slime" coat & cuticle