What is a Metazoan In other words what

What is a Metazoan? In other words what are the inherent Characteristics of the animal body plan that are different from that of a colonial heterotrophic protists? What are some characteristics of sponges that help us place them directly in the metazoan

Phylum Placozoa -- 2 -3 mm, 25 um- thick, resembling a large ameba -- Lacks anterior posterior polarity -- Asexual reproduction is prevalent --The most primitive animal? Trichoplax adhaerens

Phylum Placozoa Dorsal epithelium-like layer Fiber synctium thick glandular layer Flagellated cells Intercellular junctions -- Feed ventrally by absorption of digested material -- Lack organs but tissue-like outer walls -- A bit more than 2000 cells -- Only 4 different cell types - (20 in sponges; > 220 in mammals) -- Smallest genome of all animals cilium

Three competing Scenarios Functional Morph. Epitheliozoa Eumetazoa A. Earliest view of them as the basal metazoan B. Special cellular junctions consisting of two opposing dense plaques (desmosomes) not found in most sponges C. 16 S r. RNA data…maybe secondarily simplified from more complex ancestors?

Mitochondrial genome of Trichoplax adhaerens supports Placozoa as the basal lower metazoan phylum Dellaporta, Stephen L. et al. (2006) Proc. Natl. Acad. Sci. USA 103, 8751 -8756 Largest known mt genome 46 kbp, 2 x that of most metazoa with introns and other intrageneic spacers and large protein coding regions that are usually lacking in other animals Blue: known mt proteins Gray: ribosomal genes Green: unknown open reading frames Red lines: introns

(a sponge) Conclusions?

Phylum Cnidaria More than 9, 000 species, including jellyfish, corals, anemones, hydroids and much more.

Themes General characteristics of Cnidaria Why are they successful? Cnidaria Diversity and Taxonomy Classes, Sub. Classes of Anthozoans Similarity of Polyp and Medusa The Cnidaria Body Plan Class Scyphozoa, the jellyfish Complexity, Sensory & Nervous System

What Characteristics Account for the Great Success of Cnidarians? 1. Two distinct adult forms 2. Diphasic life cycle in some species 3. Ability to proliferate by cloning 4. And to form polymorphic colonies 5. Formidable prey capture device 6. Low Energy Demands

What Characteristics Account for the Great Success of Cnidarians? 1. Two Very Different Adult Forms Polyp Medusa

What Characteristics Account for the Great Success of Cnidarians? 2. Dimorphic Life Cycle in Many Species Life cycle Of the Hydroid Obelia

What Characteristics Account for the Great Success of Cnidarians? 3. Ability to proliferate by cloning

What Characteristics Account for the Great Success of Cnidarians? 4. And to form polymorphic colonies Hydractinea

What Characteristics Account for the Great Success of Cnidarians? 4. And to form polymorphic colonies Physalia Portuguese Man-O-War

What Characteristics Account for the Great Success of Cnidarians? 5. Formidable prey-capture device Nematocyst Prey tissue Empty Cnidocyte

What Characteristics Account for the Great Success of Cnidarians? 5. Formidable prey-capture device

What Characteristics Account for the Great Success of Cnidarians? 6. Low Energy Demands

Diversity of Cnidarians 3 Main Taxonomic classes Class Scyphozoa Anthozoa Hydrozoa Class Cubozoa contains a few jellyfish spp

How is it that organisms as different as jellyfish, hydroids and anemones can be classified in the same phylum? Giant anemone

Similarities of Polyp and Medusa

Epitheleomuscular cells cnidocytes mesoglea Nutritive muscular cells Gland cells

Nerve Nets and Muscular System Mouth

Epitheliomuscular Cells and gastromuscular cells Lacking mesoderm, but muscles derived from epithelial endodermal and ectodermal cells

Characteristics of the Class Scyphozoa (Adaptations to a drifting, carnivorous lifestyle) About 200 species

Hydromedusa for comparison • Small a few cm • Velum to aid in locomotion • Radial canals unbranched • Simple epidermal and endodermal nerve net • Nematocysts usually small without toxins

Characteristics of the Scyphozoa mesoglea gastric pouch gonad radial canal stomach gastrodermis Radial canal mouth

Characteristics of the Scyphozoa (Adaptations to a drifting, carnivorous lifestyle) Stomach G-V cavity more complex, adapted to subdue and digest larger and active prey G-V Canals MOUTH Gastric Pouches MANUBRIUM

Characteristics of the Scyphozoa (Adaptations to a drifting, carnivorous lifestyle) G-V cavity more complex, adapted to subdue and digest larger and active prey

Characteristics of the Scyphozoa mesoglea gastric pouch gonad radial canal stomach gastrodermis Radial canal mouth

Coronal Muscles and swimming How is swimming pace, direction coordinated?

How are the coronal muscles connected to the nervous system that includes: - Sensory neurons, motor neurons - Nerve net - Inner and outer nerve rings - Marginal ganglia associated with rhopalia that constitute a pacemaker

Characteristics of the Scyphozoa (Adaptations to a drifting, carnivorous lifestyle) The nervous system is more complex; unique sensory structures (than nerve net shared by all Cnidaria) To ganglia

Rhopalium (Digestive)

Rhopalia are Intergrated with Nerve Rings in Box Jellies (Cubomedusa) Garm et al. , 2006 Cell Tissue Research #325 EN: epidermal nerve GN: gastrodermal nerve RC: Ring Canal EN joins the nerve ring NCB: nerve cluster RN: ring nerve

24 eyes but no brain? Marginal ganglia Epidermal nerve of rhopalia Coronal muscles Inner and outer nerve rings Nerve net Epiheliomuscular system

When did a Centralized Nervous System Evolve? “Conventional” view Marlow et al. , 2009, Developmental Neurobiology

Cnidarian nerve net e. g. Hydra

“Sub-functionalized” N. S. Sea Anemone Nematostella

Nerve rings are Core of CNS in medusae -- Concentrations of hundreds of axons in parallel form coupled nerve rings. Having an annular form rather than a large ganglion does not make it less centralized. -- The annular configuration is a function of radial symmetry Mackie, 2004. Neurosignals #13