Chapter 30: An Introduction to Animals
Overview of Major Animal Phyla: Protostomes (possible sister group to Lophotrochozoa and Ecdysozoa)
(Group and Phylum - Common Name or Example Taxa - Number of Described Species) (1) Chaetognatha - Arrow worms, Pterobranchs - 120
Overview of Major Animal Phyla: Deuterostomes
(Group and Phylum - Common Name or Example Taxa - Number of Described Species) (1) Hemichordata* - Acorn worms - 108 (2) Echinodermata* - Echinoderms (sea stars, sea urchins, sea cucumbers) - 7000 (3) Chordata* - Chordates: tunicates, lancelets, sharks, bony fishes, amphibians, reptiles (including birds), mammals - 65,000
Overview of Major Animal Phyla: Protostomes: Lophotrochozoa
(Group and Phylum - Common Name or Example Taxa - Number of Described Species) (1) Phoronida - Horseshoe worms - 10 (2) Gnathostomulida - Gnathostomulids - 100 (3) Entoprocta - Entoprocts, Kamptozoans - 170 (4) Gastrotricha - Gastrostrichs - 400 (5) Brachiopoda* - Brachiopods (lamp shells) - 550 (6) Acanthocephala - Acanthocephalans - 1150 (7) Nemertea - Ribbon worms - 1200 (8) Rotifera* - Rotifers - 2100 (9) Bryozoa* - Bryozoans, ectoprocts, moss animals - 5700 (10) Annelida* - Segmented worms - 16,800 (11) Platyhelminthes* - Flatworms - 20,000 (12) Mollusca* - Mollusks (clams, snails, octopuses) - 85,000
Overview of Major Animal Phyla: Non-bilaterian Groups
(Group and Phylum - Common Name or Example Taxa - Number of Described Species) (1) Placozoa - Placozoans - 1 (2) Ctenophora - Comb Jellies - 190 (3) Acoela - Acoelomate worms (other than flatworms) - 350 (4) Porifera - Sponges - 8500 (5) Cnidaria - Jellyfish, Corals, Anemones, Hydroids, Sea Fans - 11,500
Overview of Major Animal Phyla: Protostomes: Ecdysozoa
(Group and Phylum - Common Name or Example Taxa - Number of Described Species) (1) Priapulida - Priapulids - 16 (2) Kinorhyncha - Kinorhynchs - 130 (3) Onychophora - Velvet worms - 165 (4) Nematomorpha - Hair worms - 330 (5) Tardigrada* - Water bears - 1045 (6) Nematoda* - Roundworms - 25,000 (7) Arthropoda* - Arthropods (spiders, insects, crustaceans) - 1,200,000
If ctenophores and cnidarians lack true mesodermal muscle, how do these shape changes take place?
(a) All animals share homologous genes for contractile proteins - Even sponges, which lack muscle and are generally sessile as adults, possess the genes for key contractile proteins, such as actin and myosin - These proteins and others are used in ctenophores and cnidarians in contractile cells derived from endoderm and/or ectoderm called epitheliomuscular cells, which are functionally similar to true mesodermal muscle cells, but evolved independently (b) The entire genetic tool kit for muscles did not evolve all at once in the early Cambrian before the diversification of triploblasts - Some protein components of the muscle contractile apparatus evolved earlier and are homologous among animals, but these components were also co-opted and elaborated upon in different ways in different lineages (c) The result we observe today across animal groups is functional similarity (movement by contractile cells or tissues) achieved by independent evolutionary paths - Another example of convergent evolution with deep homology
Ctenophores and Cnidarians muscle used to move
(a) Both larval and adult ctenophores swim primarily using cilia, but in some species, the adults also have contractile tissue that can change the body's shape and is used for swimming (b) Many cnidarians can also change the shapes of their bodies, seen in the rapid shortening of a sea anemone in response to a threat by a predator, and swimming by jet propulsion in jellyfish
Ctenophores body plan
(a) Has an opening to the environment that is used for ingesting food and releasing gametes (b) Expel waste through two tiny anal pores - Suggests that at least some materials pass through the gastrovascular cavity in one direction
Body symmetry
(a) Is a key morphological aspect of an animal's body plan - A body is said to be symmetrical if it can be divided by a plane so that the resulting pieces are nearly identical
Body Symmetry Type: Bilateral Symmetry
(a) Organisms with bilateral symmetry have only one plane of symmetry and tend to have a long, narrow body Ex. The annelid (polychaete) worm has one plane of symmetry running lengthwise down its middle (1) Bilateral symmetry occurs in triploblastic lineages
Body Symmetry Type: Radial Symmetry
(a) Radial symmetry: means organisms have at least two planes of symmetry - Ctenophores, many cnidarians, and some sponges Ex. Almost any plane sectioned through the center of a hydra (phylum Cnidaria) produces two symmetrical halves (1) Many ctenophores are often described as having a special kind of radial symmetry called biradial symmetry, meaning organisms have two planes of symmetry, not more - The two halves defined by the planes have rotation symmetry (as though spun 180 degrees) rather than resembling mirror images (2) Radial symmetry evolved independently in the phylum Echinodermata, a group that includes sea stars, sea urchins, feather stars, and brittle stars (3) Radial symmetry appears to have arisen earlier in the evolution of animals than bilateral symmetry
How are symmetry and nervous systems related?
(a) Sponges generally lack both nerve cells and symmetry; however, sponges do possess some of the essential tool-kit genes needed for the development and function of nerves (b) Cnidarians have nerve cells that are mostly organized into a diffuse arrangement called a nerve net. - These generally radially symmetric animals either float in water or live attached to a substrate - Radially symmetric organisms are more likely to encounter prey and other aspects of the environment in any direction; as a result, a diffuse nerve net can receive and send signals efficiently - In some species, clusters of nerves also occur adjacent to sensory cells, such as near the mouth (c) Ctenophores have a nerve net, nerves associated with the tentacles, and a specialized sensory organ used for sensing light and gravity - Some research shows that ctenophores also have clusters of nerves, called ganglia (d) The nervous systems of bilaterians are diverse, ranging from a nerve net to a more complex central nervous system, or CNS - In a CNS, some neurons are clustered into one or more large tracts or cords that project throughout the body; others are clustered into ganglia - Most of the bilaterally symmetric animals living today move through their environment - Bilaterally symmetric organisms tend to encounter prey and other aspects of the environment at the leading end - As a result, the animal benefits by having many neurons concentrated at that end, with nerve tracts that carry information down the length of the body Ex. How body symmetry is associated with the nervous system: (1) Radially symmetric animal (e.g., hydra) have a nerve net to diffuse neurons in the hydra (2) Bilaterally symmetric animals (e.g., earthworm) have a central nervous system (CNS) - Cerebral ganglion (brain) - Ganglia
Central Nervous System
(a) The evolution of the CNS coincides with cephalization: the evolution of a head, or anterior region, where structures for feeding, sensing the environment, and processing information are concentrated (b) The large mass of neurons that is located in the head, and that is responsible for processing information to and from the body, is called the cerebral ganglion, or brain (c) The exact timing of the origin of the CNS and brain are hotly debated - Recent evidence supports these structures not arising just once in a common ancestor. Rather, the genetic tool-kit for a nervous system evolved early and was co-opted multiple times in different lineages via convergent evolution (d) The origin of a bilaterally symmetric body plan with cephalization and a CNS enabled rapid, directed movement and hunting - Lineages with this body plan had the potential to diversify into an array of formidable eating and moving machines
Bilateral symmetry and the nervous system
- Over 99% of modern animals are overtly bilaterally symmetric (a) Hypothesis: the evolution of the nervous system and the evolution of the head are tightly linked to bilateral symmetry and that together, these characteristics contributed to the radiation of bilaterians (b) The function of the neurons and nervous systems is to transmit and process information in the form of electrical signals
Is the bilateral symmetry in sea anemones homologous to bilateral symmetry in triploblastic animals, or is it another example of convergent evolution?
- Some corals and sea anemones have subtle bilateral symmetry despite the outward appearance of radial symmetry Ex. Nematostella is a small sea anemone that burrows in soft marine sediments - Only one plane of division results in mirror images on both sides of the plane (a) Used developmental regulatory genes as a tool to address this evolutionary question (b) In triploblastic, bilaterally symmetric animals, called bilaterians, bilateral symmetry is achieved by the combination of anterior-posterior ("head-tail") axis formation and dorsal-ventral ("back-belly") axis formation (c) Hox genes are regulatory genes that are important in the development of the anterior-posterior axis - Different Hox genes are expressed in different regions along the axis, which regulate the expression of other genes, and thus establish regional identities - Decapentaplegic (dpp) genes are important in the development of the dorsal-ventral axis (d) Genetic Evidence for Homology in Bilateral Symmetry in Cnidaria and Bilateria (1) Experimental setup: - Stain gene products (proteins) of Hox gene in developing Nematostella embryos and larvae to reveal location of expression - Dark blue stain showed where one Hox gene is expressed - Repeated for other Hox genes and dpp products (2) Results: - Hox genes are expressed sequentially along the anterior-posterior axis in Nematostella larva (seen in the longitudinal section) - Reference from prior research: Hox genes are expressed sequentially along the anterior-posterior axis in fruit fly larva and other bilaterians - Dpp genes are expressed aymmetrically along the dorsal-ventral axis in Nematostella- as in bilaterians (3) Conclusion: - The genetic determination of bilaterial symmetry has deep homology in Nematostella and bilaterians - Supports the hypothesis that bilateral symmetry in the sea anemone is homologous to bilateral symmetry in triploblastic animals, meaning that parts of the genetic tool kit that determine bilateral symmetry arose before the evolutionary split of cnidarians and bilaterians (e) Hox and dpp gene expression in Nematostella is not identical to that of bilaterians, which further studies on other cnidarians showed varying expression patterns of Hox and dpp genes - The entire genetic tool kit for bilateral symmetry did not evolve all at once in the early Cambrian before the diversification of bilaterians - Some components of the tool kit evolved earlier in the ancestor to cnidarians and bilaterians, and others evolved or were later co-opted differently, after the split of these two lineages (f) The presence of Hox, dpp, and other developmental regulatory genes in cnidarians supports the hypothesis that cnidarians are a sister group to the bilaterians - Such genes are absent in sponges and ctenophores
Fossil Evidence for Sponges-First Hypothesis
1) Fossil Evidence: - Sponges are the earliest animals to appear in the fossil record, more than 700 mya. - Sponges today can survive in oxygen concentrations as low as 0.5-0.4 percent of the present level, supporting that sponges could survive the low-oxygen conditions in ancient seas.
What Key Innovations Occurred During the Origin of Animal Phyla?
1) Fossils: provide direct evidence of what ancient animals looked like, when they existed, and where they lived - Likely to occur only for animals that were abundant, had hard parts, lived in areas where sedimentation was occurring, and/or lived recently 2) Comparative morphology: provides information about which larval or adult morphological characteristics can be used to define the fundamental architecture, or body plan, of each animal lineage - Data can be used to infer which characteristics arose first during the evolution of animals, and which animal groups are more closely related 3) Comparative development (or evolutionary developmental biology; evo-devo): provides information about patterns of gene expression and morphological change during development - Can reveal when diverse structures are determined by similar genes and developmental processes (genetic and developmental homology), OR when similar structures are determined by different genes and developmental processes (convergent evolution) 4) Comparative genomics: provides information about the relative similarity of genes or whole genomes of diverse organisms. - Providing dramatic insights into phylogenetic relationships and evolutionary history
What major groups occur within the bilaterian coelomates?
1) Protostomes ("first-mouth"), named for the embryonic development of the mouth before the anus 2) Deuterostomes ("second-mouth"), named for the embryonic development of the anus before the mouth
Insights from the Origin-of-Animals Debate
1) The evolution of animals is more complicated than a smooth transition from simple to complex. - Many animal genes evolved early and we co-opted, rearranged, diversified, and sometimes lost in subsequent lineages. 2) Many key innovations did not arise all at once. - Traits often evolve in a sequence of "tinkering" steps Ex. Deep homology: a morphological trait that evolved independently in two lineages (convergent evolution), which is built by the same (homologous) tool-kit genes and developmental processes inherited from a common ancestor. 3) Evolution did not stop within any of the lineages. - Sponges and Comb jellies alive today represent the product of millions of years of evolution following their descent from a common ancestor.
Morphological Evidence for Sponges-First Hypothesis
2) Morphological Evidence: Sponges share key characteristics with choanoflagellates (a) Both are: - Benthic, meaning they live at the bottom of aquatic environments, AND - Sessile, meaning that adults live permanently attached to a substrate rather than moving freely. (b) Both are suspension feeders, using cells with similar morphology: - The beating flagella of choanoflagellates, AND - Specialized cells in sponges called choanocytes create a water current, trapping bacteria and other organic debris from the water flow. The trapped food particles are then digested inside the choanoflagellate cell body or within other cells of the sponge. - Feeding occurs at the cellular level, unlike feeding in all other animals. (c) Choanoflagellates sometimes form colonies, which are groups of individuals that are attached to each other. - Sponges are considered by some biologists to be colonies of single-celled protists due to the ability of sponge cells to re-aggregate (reform as a whole) after being dissociated. - Sponges contain many specialized cell types that are dependent on each other, some of which occur in organized layers surrounded by extracellular matrix (ECM). Ex. The specialized feeding cells of sponges, choanocytes, are specialized, flagellated cells that line the internal chambers of sponges (colony).
Molecular Evidence for Sponges-First Hypothesis
3) Molecular Evidence: (a) Many molecular phylogenies show sponges branching off first during the diversification of animals. - Sponges are the sister group to all other animals. (b) Comparative genomic studies suggest that sponges possess a complex set of developmental tool-kit genes necessary for all the basic molecular processes required by animals: - Cell specialization - Regulate cell cycling - Adhesion among cells - Adhesion between cells and extracellular matrix - Recognize self vs. non-self - Developmental signaling and gene regulation - Apoptosis (programmed cell death)
What did research show one group of sponges have?
A true epithelium, which is a layer of tightly joined cells that covers the interior and/or exterior surface of the animal. - Epithelium is essential to animal form and function
What are muscles and neurons for animals?
Adaptations that allow a large, multicellular body to move efficiently
Key traits of animals
Animals form a monophyletic group, meaning all animals share a common ancestor that was multicellular Key traits: (1) All animals are multicellular eukaryotes - Cells lack cell walls but have an extensive extracellular matrix (ECM) - ECM includes proteins specialized for cell-cell adhesion and communication (2) All animals are heterotrophs, meaning they obtain carbon compounds from other organisms - Most ingest (or "eat") their food, rather than absorbing it across their exterior body surfaces (3) All animals move under their own power at some point in their life cycle (4) All animals other than sponges have: (a) neurons (nerve cells) that transmit electrical signals to other cells; AND (b) muscle cells that can change the shape of the body by contracting
Triploblasts
Animals whose embryos have three types of tissue. (1) Germ layers are called ectoderm ("outer skin"), mesoderm ("middle-skin"), and endoderm ("inner-skin"). (a) Ectoderm: gives rise to skin and the nervous system. - Produces the covering of the animal (b) Endoderm: gives rise to the lining of the digestive tract and organs that connect the digestive tract, such as the liver. - Produces the digestive tract and associated structures (c) Mesoderm: gives rise to the circulatory system, muscle, and internal structures such as bone and most organs. - Produces the tissues in between
Diploblasts
Animals whose embryos have two types of tissue. (1) Germ layers are called ectoderm ("outer skin") and endoderm ("inner-skin"). (2) In most cases the outer and inner "skins" of diploblast embryos are connected by a gelatinous material called mesoglea that may contain some cells. (3) Two groups of animals were recognized as diploblasts: (a) Ctenophora (comb jellies) (b) Cnidaria (include jellyfish, corals, sea pens, anemones, and hydra) Ex. Tube-shaped portion of a hydra's body (Cross-section) - Tissues derived from ectoderm (outside) - Tissues derived from endoderm (inside)
Anterior vs. Posterior
Anterior = oral (mouth) side Posterior = tail end
Tissues
Are groups of similar cells that are organized into tightly integrated structural and functional units. - Sponges do not have complex tissues
When did the radiation of animals begin?
Began around 550 million years ago during the Cambrian explosion - Diverse marine animals appeared in the fossil record with a variety of adaptations (muscles and nerves, shells, exoskeletons, internal skeletons, legs, heads, tails, brains, eyes, antennae, jaw-like mandibles, and segmented bodies). - Today, descendants of these marine animals are key consumers in virtually every ecosystem
Alternative Paths to the First Animal
Cell differentiation before multicellularity 1) The Traditional View: after a cell resembling a choanoflagellate began to form colonies, differentiation arose and eventually led to the first animal. (a) Ancestral cell like a choanoflagellate (b) Colony of identical cells (c) Colony with differentiated, specialized cells (d) First animal (sponge) SUPPORT: Choanocyte cells found in sponges look and act like choanoflagellates 2) The Alternative (Updated) View: The ancestral cell differentiated at various stages in its life cycle even before the emergence of multicellularity. (a) Ancestral cell with a complex life cycle (b) Colony of cells that have complex life cycles (c) Cells stay in one part of the life cycle, leading to different cell types (d) First animal (sponge) SUPPORT: Archaeocyte cells in sponges, which can differentiate into various cell types, have a gene expression profile similar to those of single-cell relatives with complex life cycles.
What does the combination of multicellularity, heterotrophy, and efficient movement produce?
Eating machines - Animals are the largest detritivores, herbivores, and carnivores on Earth
Basic Bilaterian body shape
Is a tube within a tube where materials quickly move in one direction (a) The inner tube (endoderm) is the individual's gut with a mouth at one end and an anus at the other - Gut is lined by endodermally derived cells (b) The outer tube (ectoderm) forms the skin and a nervous system (c) The middle tube (mesoderm) in between forms muscles and organs (d) The tube-like body plan with 2 openings enables specializations along the length of the gut and separates food and wastes (e) Some bilaterians have a fluid-filled cavity associated with the mesodermally derived tissues - Cavity is called the coelom, which provides a space for the circulation of oxygen and nutrients - Also enables the internal organs to move independently of each other and independently of the inner and outer tubes (1) Bilaterians whose coelom is completely lined with mesoderm derived tissues are considered to have a true coelom and are known as coelomates (2) Bilaterians that have no coelom, such as the flatworms (phylum Platyhelminthes), are called acoelomates ("no-cavity-form") (3) Bilaterians whose coelom is only partially lined with mesoderm-derived tissues, such as roundworms (phlyum Nematoda), and rotifers (phylum Rotifera), are known as pseudocoelomates ("false-cavity-form") (f) The coelom arose in the ancestral bilaterian and was subsequently modified, reduced, or lost in many lineages Ex. Flatworm Ex. Arthropods have a highly reduced coelom called a hemocoel (g) The evolutionary flexibility of the coelom has reduced its usefulness as a synapomorphy, or diagnostic character, for bilaterian animals - However, the absence or presence of a coelom is still an interesting component of the body plans of individual phyla
Cnidarians body plan
Looks like a sac (a) Has only one opening to the environment that is used for ingesting food, releasing gametes, and eliminating wastes (b) The space within the sac is called a gastrovascular cavity
Origin of animals
Originated from single-celled eukaryotes 1) Occur in the lineage Opisthokonta, containing: (a) Fungi (b) Choanoflagellates (protist) (c) Other protists 2) Choanoflagellates are the closest living relatives, or sister group, to animals - Shared a common ancestor about 900 mya
Apoptosis
Programmed cell death 1) Caspase-8: key cell death initiator molecule for apoptosis - First identified in humans - Conserved in all animals - Needed for multicellularity? - Allows cell pruning - Arose more than 500 mya
Are Ctenophores and Cnidarians triploblasts?
Recent data suggest that some ctenophores and cnidarians have mesodermal cells in their mesoglea - Some ctenophores and cnidarians contain genes coding for the structural components of mesodermal cells, but not the mesodermal specification genes that are present in true triploblasts. - Ctenophores and cnidarians share some components of the genetic tool kit for triploblasty, but not others. This suggests that the similarities among mesodermal cells in diploblasts and the mesoderm of triploblasts are the product of convergent morphological evolution based on a foundation of homologous genes, an example of deep homology.
Compare and Contrast: Multicellular fungi and animals
Similarities: - They are both multicellular heterotrophs that digest (break down) and absorb nutrients Differences: - Animals are the only multicellular heterotrophs on the tree of life that usually ingest their food first, before they digest it - As a result, digestion in animals typically occurs within the space, or lumen, of the digestive tract, rather than in the open exterior as occurs with fungi
Sponges-First Hypothesis
Sponges (phylum Porifera) are the most ancient lineage of animals
Animals other than sponges are typically distinguished by what?
The number of embryonic tissue layers they have. - Embryonic tissues are organized in layers called germ layers ("sprout" layers that differentiate during development). - The embryonic tissues found in animals develop into distinct adult tissues, organs, and organ systems.
