Animal Characteristics and Evolution

• Lecture 8 concerns Animal Diversity Part 1
• The lecture uses Chapter 33 Section 33.1 "Animal Body Plans" (pp. 739-745) and Chapter 42 Section 42.1 (pp. 945 to 952) of Biology: How Life Works, Morris et al as references

Objectives

• Animal Body Plans (required reading)
• Sponges
• Radial Animals (Cnidarians and Ctenophorans)
• Placozoans

What is an Animal?

• The animal kingdom is very diverse, with 1.3 million living species of identified to date
• Animals are multicellular, heterotrophic eukaryotes with a specific mode of early development
• 95% of all known animal species are invertebrates (no backbone)

Animal Phylogeny

• Animals are closely related to choanoflagellates
• Animals are multicellular
• Animals form a gastrula during early development
• Animals synthesize collagen
• Sponges diverged from other animals early
• Cnidarians diverged later from animals and their descendants have complex organs

Phylogeny of Bilaterian Animals

• Blastopore becomes the anus in deuterostomes
• Blastopore becomes the mouth in lophotrochozoans

Phylum Porifera - Sponges

• Sponges are sessile, have a porous body, and have choanocytes
• Sponges can be found in both freshwater and marine environments
• Sponges are suspension feeders, capturing food particles suspended in the water that passes through their body

Phylum Porifera - Sponges Continued

• Choanocytes and choanoflagellates share similarities, supporting the theory that animals evolved from a choanoflagellate like ancestor
• Germ layers of sponges are loose groupings of cells and do not constitute actual tissues
• Sponges lack true nerves or muscles, with individual cells capable of sensing and reacting to environmental changes
• Sponges have no symmetry
• Sponges are basal animals

Phylum Porifera - Sponges Continued

• Mesohyl is a gel-like matrix supporting cells that form spicules and generate reproductive cells
• Epidermis is the outer layer of cells
• Pores (ostia) allow water to enter the sponge
• Spicules are skeletal elements made of calcium carbonate or silica, though some lack spicules and rely on protein
• Spongocoel is the central cavity of the sponge
• Osculum is the opening through which water exits the sponge
• Choanocytes are flagellated cells that line the interior and create currents
• Amoebocytes are mobile cells involved in nutrient transport and skeleton formation

Phylum Porifera - Sponges Nutrition and Reproduction

• Sponges digest food via intracellular digestion
• Some use symbiotic organisms to gain additional nutrition
• There is coordination of flagellar beating of choanocytes
• Gas exchange occurs via diffusion
• Gametes develop in mesohyl, sperm is released, and fuses with eggs in mesohyl of another sponge

Phylum Porifera - Sponges: Antibiotics and Toxins

• Some sponges produce toxins and antibiotics
• Cribrostatin can kill antibiotic resistant strains of Streptococcus, and ongoing research may reveal other anti-cancer agents

Phylum Cnidaria

• All animals except for sponges belong to the clade Eumetazoa (true tissues and symmetry)
• Cnidaria is one of the oldest groups in this clade
• They are the sister group to Bilateria
• Cnidarians have radial symmetry, gastrovascular cavities, and chidocytes

Phylum Cnidaria Continued

• Cnidaria is a diverse group including jellies, corals, and hydras
• Floating forms are called (medusa)
• Sessile forms are called (polyp)
• They are diploblastic
• Synapomorphies include radial symmetry and a mouth surrounded by tentacles armed with stinging cells (cnidocytes)

Phylum Cnidaria Digestion

• A closed internal gastric cavity is also called a gastrovascular cavity
• This cavity is the site of extracellular/intracellular digestion and excretion
• Digestive enzymes allow digestion of large food items.
• A single opening functions as both mouth and anus

Phylum Cnidaria Body Wall

• The body wall has two layers of cells
• The outer eipdermis layer is from ectoderm
• The inner gastrodermis layer is from endoderm
• Cells of the epidermis and gastrodermis are bundles of microfilaments arranged into contractile fibers, which are muscle-like cells
• Mesoglea is an acellular layer

Phylum Cnidaria Hunting

• Cnidaria are Carnivorous
• Use tentacles to capture prey
• Tentacles are armed with cnidocy with nematocysts, which provides function in defense and capture of prey
• They have Noncentralized nerve net, which are simple sensory receptors; radially distributed

Four Major Classes of Cnidarians

• Hydrozoans alternate between polyp and medusa
• Anthozoans occur only as polyp
• Cubozoans are box jellies and sea wasps with toxic cnidocytes living primarily in tropical environments
• Scyphozoans predominantly live as medusa amongst plankton

Phylum Ctenophora (Comb-Jellies)

• They resemble Cnidarians in body plan, and are thought to be close relatives but more complex
• Important differences between cnidarians and comb-jellies exist
• Comb-jellies have cilia
• Comb-jellies have a simple flow through gut with a mouth and anal pore
• This unique combination of traits makes placement on the tree of life difficult

Phylum Placozoa

• Very simple organisms lacking specialized tissues and differentiated cell types
• Placozoa have an upper and lower epithelia
• Interior fluid is crisscrossed by a network of multinucleate fiber cells
• They have Cilia on cell surfaces to allow movement
• Genome contains many genes for transcription factors and signaling molecules are present in cnidarians and bilaterian animals

Phylogenetic Tree of Placozoans and Ctenophores Added

• It is not clear where ctenophorans fit in phylogenic tree
• DNA sequences support Placozoans as a sister group to both Cnidaria and Bilaterians
• Cnidaria and Bilateria as sister groups are supported by molecular, anatomical and embryological data

Where Do The Ctenophora Fit?

• Traditional Hypothesis suggests that ctenophora branch off with Cnidaria and Bilaterians as a sister group to sponges. However the alternate hypothesis finds that DNA sequence comparisons show evidence that ctenophores are the sister group to all other animals.
• The traditional view is supported by molecular data

Explore the shared characteristics of animals, including unique features like collagen. Discover how sponges and placozoans obtain nutrients. Contrast traditional and alternative hypotheses on early metazoan evolution, focusing on Ctenophora's placement and Placozoan genomes.