Chapter 33: An Introduction to Animals
Chapter 33: An Introduction to Animals Biol 112
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Date Created: 03/28/16
Chapter 33: An Introduction to Animals Intro The radiation of animals began around 550 million years ago during the Cambrian explosion Biologists estimate that there are between 8 million and 50 million extant animal species -only 1.4 million have been described to date The modern rate of animal extinction is accelerating due to human activity What is an Animal? Animals are eukaryotes that share key traits: 1. Multicellularity, with cells that -lack cell walls -have an extensive extracellular matrix 2. Heterotrophy -they obtain necessary carbon compounds from other organisms -most ingest their food rather than absorbing it 3. Motility -they move under their own power at some point in their lifetime All animals except sponges also have 1. Nerve cells called neurons that transmit electrical signals to other cells 2. Muscle cells that can change the shape of the body by contracting Muscles and neurons are adaptations that allow a large, multicellular body to move efficiently Animals are similar to multicellular fungi in that -they are both multicellular heterotrophs -they both digest and absorb nutrients However, animals are the only multicellular heterotrophs on the tree of life that ingest food before digesting it What Key Innovations Occurred during Animal Evolution? Biologists consider these 3 types of data: Fossils, Comparative morphology, and Comparative genomics 1. Fossils -The fossil record is inconsistent, but it is the only direct evidence of o Animal morphology o Where they lived o When they existed 2. Comparative morphology o Distinguishes between shared characteristics o These data can be used to: -define the fundamental architecture, or body plan, of each lineage -infer which characteristics arose first -infer which animal groups are more closely related 3. Comparative genomics o Provides info about the relative similarity of genes or genomes of diverse organisms o Provides dramatic insight into phylogenetic relationship and evolutionary history Origin of Multicellularity Animals are a monophyletic group -all animals have a single common ancestor Sponges (phylum Porifera) include the 2 most basal animal lineages -multicellularity appears to have originated in a sponge-like animal Sponges are the earliest animals to appear in the fossil record -1 sponges appear more than 600 million years ago Morphological Evidence Sponges share key characteristics with the choanoflagellate outgroup -both are sessile: adults live permanently attached to a substrate -both feed in a similar way (filter feeding) Choanoflagellates sometimes form colonies: groups of attached individuals -sponges were once considered colonies of single-celled protists However, sponges contain many specialized cell types -these cells are dependent on each other -some occur in organized layers surrounded by extracellular matrix -some sponges also have an epithelial layer Molecular Evidence Comparative genomic studies -support the hypothesis that sponges are the most basal group of animals -suggest that despite the morphological simplicity of sponges, they possess a complex developmental tool kit of genes The tool kit contains the genes needed for all basic molecular processes required by animals, including: -cell specialization -regulation of cell cycling and growth -adhesion among cells, and between cells and ECM -Recognition of self and nonself (innate immunity) -developmental signaling and gene regulation -programmed cell death The Origin of Embryonic Tissue Layers While sponges have the genetic tool kit needed for cell-cell and cell-ECM adhesion, most don’t have complex tissues -tissues are groups of similar cells that are organized into tightly integrated structural and functional units Animals other than sponges are divided into two groups based on the number of embryonic tissue layers they have: Diploblasts are animals whose embryos have 2 types of tissues, or germ layers: 1. ectoderm “outside skin” 2. endoderm “inside skin” Triploblasts are animals whose embryos have 3 germ layers: 1. endoderm 2. ectoderm 3. mesoderm “middle skin” The Origin and Diversification of Tissues Germ layers develop into distinct adult tissues and organs In triploblasts: -ectoderm becomes the skin and nervous system -endoderm becomes the digestive tract -mesoderm becomes the circulatory system, muscle, and internal structures such as bone and most organs Most cnidarians (jellyfish, corals, sea anemones) and all ctenophores (comb jellies) are diploblastic All other animals are triploblastic The relatively simple diploblast body plan of ctenophores and cnidarians evolved from ancestral animals later than the sponges Origin of Bilateral Symmetry, Cephalization, and the Nervous System Body symmetry is a key morphological aspect of an animal’s body plan Animals with radial symmetry-such as cnidarians, ctenophores, and some sponges- have at least 2 planes of symmetry Radial symmetry evolved independently in the echinoderms Most other animals exhibit bilateral symmetry, with a single plane of symmetry and long, narrow bodies Homology or Convergent Evolution? While most cnidarians appear radially symmetric, the internal morphology of some species is actually bilaterally symmetric -This is especially true of many species of sea anemone Origin of the Nervous System Biologists hypothesize that the evolution of the head and nervous system are tightly linked to the evolution of bilateral symmetry Sponges lack nerve cells and symmetry Radially symmetrical cnidarians and ctenophores have nerve cells that are organized into a nerve net They are equally likely to encounter their environment in any direction -a diffuse nerve net can send and receive signals efficiently All other animals have a centralized nervous system (CNS) -some neurons are clustered into tracts or chords -other neurons are clustered in masses called ganglia Bilaterally symmetric organisms sense their environment at one end -it is advantageous to have many neurons concentrated at that end and nerve tracts carry information down the length of the body Evolution of the CNS coincided with cephalization -evolution of a head where structures for feeding, sensing the environment, and processing information are concentrated. Cerebral ganglion=brain a) Nerve Net diffuse neurons b) CNS Clustered neurons in ganglia Origin of the Coelom A coelom is an enclosed, fluid-filled body cavity between the tubes -provides a space for oxygen and nutrients to circulate -enables the internal organs to move independently of each other The coelom likely evolved in the common ancestor of protostomes and deuterostomes a) coelomates have an enclosed body cavity completely lined with mesoderm b) acoelomates have no enclosed body cavity c) pseudocoelomates have an enclosed body cavity partially lined with mesoderm The coelom creates a container for circulation of oxygen and nutrients and acts as an efficient hydrostatic skeleton -allows soft-bodied animals to move without fins or limbs Origin of Protostomes and Deuterostomes The bilaterally symmetric common ancestor with a CNS, cephalization, and a coelom gave rise to many diverse lineages 2 subgroups based on embryonic development: -protostomes -deuterostomes 1. protostomes -the mouth develops first during gastrulation -blocks of mesoderm hollow out to form the coelom -includes arthropods, mollusks, and segmented worms 2. Deuterostomes -the mouth develops second during gastrulation -pockets of mesoderm pinch off to form the coelom -includes chordates and echinoderms protostomes deuterostomes blocks of solid mesoderm mesoderm pockets pinch split to form coelom off of gut to form coelom Origin of Segmentation Segmentation is the presence of repeated body structures A segmented backbone is a defining characteristic of the vertebrates -vertebrates are a monophyletic lineage within the Chordata (includes fish, reptiles, birds, amphibians, and mammals) Invertebrates lack a segmented backbone -segmentation occurs in annelids and arthropods What Themes Occur in the Diversification of Animals? Higher oxygen levels -may have enabled the evolution of big, mobile animals due to the efficiency of aerobic respiration The evolution of predation -predators exert selection pressure on their prey for characteristics that enable them to escape capture New niches beget more new niches New genes, new bodies -as the animal genetic tool kit evolved, the potential for morphological diversity increased Specialized Sensory Abilities --evolved as animals diversified Magnetic field Some animals can detect these and use them as a navigation aid Electric field -some aquatic animal can detect electrical activity in the muscles of passing prey Barometric pressure -some birds can avoid storms by sensing changes in air pressure --more examples— Some male moths have elaborate antennae to detect pheromones in the air Sea anemones detect and capture prey using their sense of touch What Animals Eat: Diversification of Ecological Roles Animals can be classified as: -detritivores: dead organic matter ex) millipedes -herbivores: ex) pandas -carnivores: ex) owls -omnivores: ex) humans Parasites -endoparasites live inside host, simple, worm-like bodies -ectoparasites live outside host, limbs or mouthparts allow them to grasp the host How Animals Feed: 4 General Strategies Suspension feeders (filter feeders) Capture food by filtering out or concentrating particles floating in water or drifting through the air Ex) barnacles use specialized legs to capture plankton Fluid feeders Suck or mop up liquids like nectar, plant sap, blood, or fruit juice Ex) butterflies and moths drink nectar through their extensible, hollow proboscis Deposit feeders Ingest organic material that has been deposited within a substrate or on its surface Ex) sea cucumbers use tentacles to mop up detritus from the seafloor Mass feeders biting into/taking chunks of food through the mouth Movement Functions of animal locomotion: -finding food -finding mates -escaping from predators -dispersing to new habitats Types of Limbs Lobe-like limbs little nodes used to crawl Tube feet tube-like structures used to crawl ex) starfish Jointed limbs Tentacles Parapodia little bristled legs Reproduction Animals are grouped according to where the eggs and embryos develop following internal fertilization -viviparous “live-bearing” species -oviparous “egg-bearing” species -ovoviviparous “egg-live-bearing” species Viviparous species -retain the embryos in the female’s body during development -give birth to live young -includes most mammals Oviparous species -lay eggs outside to develop independently of mother -embryos are nourished by yolk within the egg -includes majority of animal species Ovoviviparous species -female retains eggs inside her body during early development -growing embryos are nourished by yolk inside egg, not nutrients directly from the mother -females give birth to well-developed young -includes many fish, insects, and reptiles Life Cycles Metamorphosis – a drastic change from one developmental stage to another Development can be either: -direct, where the newborn young looks similar to adults -indirect, where the individuals undergo metamorphosis during their life cycle During indirect development: -embryogenesis produces larvae, which a. look radically different from adults b. live in different habitats and eat different foods -metamorphosis transforms larvae into juveniles, which: a. look like adults b. live in the same habitats and eat the same foods c. are still sexually immature -growth and maturation transform the juveniles into adults, which are the reproductive stage in the life cycle One hypothesis proposed to explain why natural selection favored metamorphosis emphasizes dispersal -in species that are sessile (immobile) as adults, the larvae function as a dispersal stage-they allow individuals to move to new habitats Another hypothesis is feeding efficiency -larvae and adults eat different foods and therefore, do not compete with each other for resources Porifera (sponges) About 8,500 species of sponges have been described Sponges are: -benthic, living at the bottom of aquatic environments -mostly suspension feeders -adults are sessile, but larvae swim using cilia -reproduce sexually or asexually Ctenophora (comb jellies) Transparent, ciliated, gelatinous diploblasts (meaning they only have 2 germ layers: the endoderm and ectoderm) Most are planktonic predators that feed in several ways: -instead of toxins, their tentacles have cells containing adhesive, which traps the prey Only about 190 species of ctenophores have been described Cnidaria (jellyfish, corals, anemones, hydroids) Most of 11,500 species are marine 4 main lineages: -anthozoa: anemones, corals, sea pens -hydrozoa: hydrozoans -cubozoa: box jellies -scyphozoa: jellyfish Reef-building corals secrete skeletons of calcium carbonate that create coral reef structure Specialized cells called cnidocytes are used to capture prey -usually near the mouth or tentacles -cnidocytes contain nematocysts: barbed thread that delivers a dose of venom
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