An Introduction to Animal Diversity Chapter 32 An Introduction to Animal Diversity

Overview: Welcome to Your Kingdom The animal kingdom extends far beyond humans and other animals we may encounter Video: Coral Reef

Several characteristics, taken together, sufficiently define the group Concept 32.1: Animal are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers There are exceptions to nearly every criterion for distinguishing animals from other life forms Several characteristics, taken together, sufficiently define the group

Nutritional Mode Animals are heterotrophs that ingest their food

Cell Structure and Specialization Animals are multicellular eukaryotes Their cells lack cell walls

Their bodies are held together by structural proteins such as collagen Nervous tissue and muscle tissue are unique to animals

Reproduction and Development Most animals reproduce sexually, with the diploid stage usually dominating the life cycle

Video: Sea Urchin Embryonic Development After a sperm fertilizes an egg, the zygote undergoes cleavage, leading to formation of a blastula The blastula undergoes gastrulation, forming embryonic tissue layers and a gastrula Video: Sea Urchin Embryonic Development

LE 32-2_3 Blastocoel Cleavage Cleavage Zygote Eight-cell stage Blastula Cross section of blastula Blastocoel Endoderm Ectoderm Gastrula Gastrulation Blastopore

Many animals have at least one larval stage A larva is sexually immature and morphologically distinct from the adult; it eventually undergoes metamorphosis

All animals, and only animals, have Hox genes that regulate the development of body form Although the Hox family of genes has been highly conserved, it can produce a wide diversity of animal morphology

Concept 32.2: The history of animals may span more than a billion years The animal kingdom includes not only great diversity of living species but also the even greater diversity of extinct ones The common ancestor of living animals may have lived 1.2 billion–800 million years ago This ancestor may have resembled modern choanoflagellates, protists that are the closest living relatives of animals

LE 32-3 Single cell Stalk

LE 32-4 Somatic cells Digestive cavity Reproductive cells Hollow sphere of unspecialized cells (shown in cross section) Colonial protist, and aggregate of identical cells Beginning of cell specialization Infolding Gastrula-like “protoanimal”

Neoproterozoic Era (1 Billion–524 Million Years Ago) Early members of the animal fossil record include the Ediacaran fauna

Paleozoic Era (542–251 Million Years Ago) The Cambrian explosion marks the earliest fossil appearance of many major groups of living animals There are several hypotheses regarding the cause of the Cambrian explosion

Mesozoic Era (251–65.5 Million Years Ago) During the Mesozoic era, dinosaurs were the dominant terrestrial vertebrates Coral reefs emerged, becoming important marine ecological niches for other organisms

Cenozoic Era (65.5 Million Years Ago to the Present) The beginning of the Cenozoic era followed mass extinctions of both terrestrial and marine animals Modern mammal orders and insects diversified during the Cenozoic

Concept 32.3: Animals can be characterized by “body plans” Zoologists sometimes categorize animals according to morphology and development A grade is a group of animal species with the same level of organizational complexity A body plan is the set of traits defining a grade

Symmetry Animals can be categorized according to the symmetry of their bodies, or lack of it

Some animals have radial symmetry, the form found in a flower pot

LE 32-7a Radial symmetry

The two-sided symmetry seen in a shovel is an example of bilateral symmetry

LE 32-7b Bilateral symmetry

Bilaterally symmetrical animals have: A dorsal (top) side and a ventral (bottom) side A right and left side Anterior (head) and posterior (tail) ends Cephalization, the development of a head

Tissues Animal body plans also vary according to the organization of the animal’s tissues Tissues are collections of specialized cells isolated from other tissues by membranous layers

Animal embryos have concentric layers called germ layers that form tissues and organs Ectoderm is the germ layer covering the embryo’s surface Endoderm is the innermost germ layer Diploblastic animals have ectoderm and endoderm Triploblastic animals also have an intervening mesoderm layer

Body Cavities In triploblastic animals, a body cavity may be present or absent A true body cavity is called a coelom and is derived from mesoderm

Coelom Body covering (from ectoderm) Tissue layer lining coelom LE 32-8a Coelom Body covering (from ectoderm) Tissue layer lining coelom and suspending internal organs (from mesoderm) Digestive tract (from endoderm) Coelomate

A pseudocoelom is a body cavity derived from the blastocoel, rather than from mesoderm

Body covering (from ectoderm) Pseudocoelom Muscle layer (from LE 32-8b Body covering (from ectoderm) Pseudocoelom Muscle layer (from mesoderm) Digestive tract (from endoderm) Pseudocoelomate

Acoelomates are organisms without body cavities

Wall of digestive cavity (from endoderm) LE 32-8c Body covering (from ectoderm) Tissue- filled region (from mesoderm) Wall of digestive cavity (from endoderm) Acoelomate

Protostome and Deuterostome Development Based on early development, many animals can be categorized as having protostome or deuterostome development

Cleavage In protostome development, cleavage is spiral and determinate In deuterostome development, cleavage is radial and indeterminate

Protostome development (examples: molluscs, annnelids, arthropods) LE 32-9a Protostome development (examples: molluscs, annnelids, arthropods) Deuterostome development (examples: echinoderms, chordates) Cleavage Eight-cell stage Eight-cell stage Spiral and determinate Radial and indeterminate

Coelom Formation In protostome development, the splitting of solid masses of mesoderm to form the coelomic cavity is called schizocoelous development In deuterostome development, formation of the body cavity is described as enterocoelous development

Protostome development (examples: molluscs, annnelids, arthropods) LE 32-9b Protostome development (examples: molluscs, annnelids, arthropods) Deuterostome development (examples: echinoderms, chordates) Coelom formation Coelom Archenteron Coelom Mesoderm Blastopore Blastopore Mesoderm Schizocoelous: solid masses of mesoderm split and form coelom Enterocoelous: folds of archenteron form coelom

Fate of the Blastopore In protostome development, the blastopore becomes the mouth In deuterostome development, the blastopore becomes the anus

Protostome development (examples: molluscs, annnelids, arthropods) LE 32-9c Protostome development (examples: molluscs, annnelids, arthropods) Deuterostome development (examples: echinoderms, chordates) Fate of the blastopore Anus Mouth Digestive tube Mouth Anus Mouth develops from blastopore Anus develops from blastopore

Concept 32.4: Leading hypotheses agree on major features of the animal phylogenetic tree Zoologists recognize about 35 animal phyla Current debate in animal systematics has led to the development of two phylogenetic hypotheses, but others exist as well

One hypothesis of animal phylogeny based mainly on morphological and developmental comparisons

LE 32-10 “Radiata” Deuterostomia Protostomia Bilateria Eumetazoa Porifera Cnidaria Ctenophora Phoronida Ectoprocta Brachiopoda Echinodermata Chordata Platyhelminthes Mollusca Annelida Arthropoda Rotifera Nemertea Nematoda “Radiata” Deuterostomia Protostomia Bilateria Eumetazoa Metazoa Ancestral colonial flagellate

One hypothesis of animal phylogeny is based mainly on molecular data

LE 32-11 “Radiata” “Porifera” Deuterostomia Lophotrochozoa Bilateria Calcarea Silicarea Ctenophora Cnidaria Echinodermata Chordata Brachiopoda Phoronida Ectoprocta Platyhelminthes Nemertea Mollusca Annelida Rotifera Nematoda Arthropoda “Radiata” “Porifera” Deuterostomia Lophotrochozoa Ecdysozoa Bilateria Eumetazoa Metazoa Ancestral colonial flagellate

Points of Agreement All animals share a common ancestor Sponges are basal animals Eumetazoa is a clade of animals with true tissues Most animal phyla belong to the clade Bilateria Vertebrates and some other phyla belong to the clade Deuterostomia

Disagreement over the Bilaterians The morphology-based tree divides bilaterians into two clades: deuterostomes and protostomes In contrast, recent molecular studies assign two sister taxa to protostomes: the ecdysozoans and the lophotrochozoans

Ecdysozoans shed their exoskeletons through a process called ecdysis

Lophotrochozoans have a feeding structure called a lophophore Other phyla go through a distinct larval stage called a trochophore larva

An ectoproct, a lophophorate Structure of trochophore larva LE 32-13 Apical tuft of cilia Mouth 100 µm Anus An ectoproct, a lophophorate Structure of trochophore larva

Future Directions in Animal Systematics Phylogenetic studies based on larger databases will likely provide further insights into animal evolutionary history