Origin of animals - Two theories

• Syncytial theory
  • Animals evolved from a multinucleate protociliate
• Colonial theory
  • Animals evolved by ingression from hollow, spherical, colonial flagellates

Syncytial Theory

• Metazoa evolved from multinucleate ciliates
• Based on
  • Multinucleate ciliates
  • Bilaterality in ciliates
• Cellularization -> Differentiation

Syncytial Theory

• Problems:
  • Radial symmetry in "Radiates"
  • No evidence of cellularization in basal metazoans
  • Doesn't explain metazoan flagella

Colonial Theory

• Metazoa evolved from colonial flagellates
• Based on
  • Colonial tendency in flagellates
  • Presence of flagella in multicellular organisms
• How?

Colonial Theory

• Metazoa evolved from colonial flagellates
• Based on
  • Colonial tendency in flagellates
  • Presence of flagella
• How?
  • Specialization of cells within a colony

Support for Colonial Theory

• Flagellated sperm widespread in animals
• Colonial tendency in flagellates
• Shape of mitochondrial christae (Sarcomastigophora flat, Ciliophora tubular)
• Molecular evidence -- close relationship between flagellates and metazoans
• Similarity between choanoflagellates and choanocytes of sponges.

Choanoflagellates and Choanocytes

Synapomorphies (shared charateristics) of the Metazoa

• Multicellularity
• Cell adhesion
• Some type of blastula
• Molecular data
• Collagen

Synapomorphies of the Metazoa

• Multicellularity
• Cell adhesion
• Some type of blastula (solid --stereoblastula-planula (placozoa, cnidaria, some sponges) or amphiblastula [sponges], hollow blastula, triploblastic phyla)
• Molecular data
• Collagen

Parazoans

• Very simple multicellular animals
• No tissue layers
• Least animal-like of all animals
• Consists of
  • One minor living phylum - Placozoa
  • One major living phylum - Porifera
  • Several extinct phyla

Phylum Placozoa

• Most primitive animals? Probably not
  • Sister group of Cnidaria? Eumetazoa?
  • Similar to planula larvae
• Flattened, amoeba-like, but multicellular
• Composed of
  • Upper/lower surfaces of flagellated cells
  • Inner region of loosely packed cells
• Both asexual and sexual reproduction

Phylum Porifera

• Cellular level organization
• Highly successful
• About 5,000 species

Phylum Porifera

• Cellular level organization
• Highly successful
• Skeleton
  • Organic - spongin
  • Mineral - spicules

Phylum Porifera

• Cellular level organization
• Highly successful
• Skeleton with spongin or spicules
• Filter Feeding
  • Channels and choanocytes
• Some totipotent cells

Additional aspects of sponge biology

• Cells can reaggregate
• Flexible body form
• Respiration and waste disposal by diffusion
• Allelochemicals (defensive toxins)
• Reproduction
  • Asexual - budding and gemmules
  • Sexual
    • Choanocytes - Sperm and/or ova
    • Archaeocytes - ova

General Poriferan Body Plan

• Ostia (or dermal pores)
• Incurrent openings
• Interior water system
• Variously contructed or subdivided
  • Asconoid, syconoid, leuconoid
• Oscula
• Excurrent openings

General Poriferan Body Plan

• Four primary cell types:
• Choanocyte
  • Line interior chambers
  • Generate water current
• Porocyte
  • Form ostia
• Amoebocyte = Archaeocyte
  • Amoeboid cells crawl among layers
  • Totipotent
  • Secrete skeleton

Poriferan feeding

• Choanocytes create current
• Water enters ostia
• Archaeocyte phagocytosis in incurrent canals. 2-5 mm

Poriferan feeding

• Choanocytes create current
• Water enters ostia
• Archaeocyte phagocytosis
• Water passes choanocytes
• Review corkscrew motion of flagellum -- pushes water out rear -- pulls through collar
• Particles trapped by collar 0.1-1.5 mm

Poriferan feeding

• Choaocytes create current
• Water enters ostia
• Archaeocyte phagocytosis (incurrent canals)
• Water passes choanocytes
• Trapped particles passed to choanocyte cell body
• Phagocytosis
• Passed to archaeocyte
• Digests, transports nutrients
• Digestion intracellular!

Poriferan feeding: summary

• Several sites of filtering
  • Ostia - dermal pores (> 50 mm)
  • Incurrent canals (5-50 mm)
  • Choanocytes (< 5 mm)
• Nested set of sieves
• Most food (80%) captured by choanocytes
• One family of carnivorous sponges

Three Grades of Poriferan Organization

Asconoid body plan

• Simplest body plan
• Most primitive?
• Small, tube like body
• Large spongocoel

Asconoid water flow

• Ostia --> Spongocoel --> Osculum
• Velocity = 1/(cross sectional area)
• Ostia
• Small, numerous --> moderate velocity
• Spongocoel
• Large cross section --> low velocity
• Osculum
• Single, small cross section --> high velocity

Limits to Asconoid design

• Size restrictions
• Surface area increases as L²,volume as L³
• Doubling length
• Four times the surface area
• Eight times the spongocoel volume
• Too few choanocytes to move the water!
• Little support for the walls

Syconoid body plan

• Incurrent canal --> flagellated canal --> spongocoel --> osculum
• Folded body wall results in
• Increased area of choanocytes
• Strengthened body wall
• Decreased volume of spongocoel (faster water exit)
• Larger body size now possible

Leuconoid body plan

• Largest, most successful sponge body plan
• Evolved several times
• Spongocoel lost, replaced by excurrent canals (narrow, fast)
• Ostium--> incurrent canal --> flagellated chamber --> excurrent canal --> osculum
• Huge number of choanocytes

Leuconoid water flow

• Velocity differences greater
• Absence of spongocoel, thicker more complex body walls, allows larger sizes

Poriferan Classes

• Three clades
• Hexactinellida
• Demospongiae
• Calcarea
• One grade
• "Sclerospongiae"

Poriferan Fossils

Class Calcarea

• CaCO3 spicules
• Shallow (solubility)
• May be asconoid, syconoid, or leuconoid
• Reef builders

Class Hexactinellida

• Glass Sponges
• Six-ray siliceous spicules
• Usually Deep Sea
• Silica solubility
• Syconoid or Leuconoid

Class Demospongiae

• Most diverse sponges (80 % of all species)
• Spongin fibers; may have siliceous spicules or may lack minerals - comfy!
• Leuconid only
• Largest body size

"Class Sclerospongiae"

• Coralline sponges (silicious spiculues, calcareous base)
• Not monophyletic
• Ancient -- now restricted to crevices and deep sea
• Possibly outcompeted by corals

External body form and ecology

• Body form reflects habitat
• Four types
  • Erect
  • Mound (massive)
  • Encrusting
  • Boring

Erect sponges

• Quiet water, can't withstand waves
• Toxins deter predators
• Gets clean water well above substrate
• Usually tubular shape

Mound sponges

• More resistant to wave action
• Toxins deter predators
• Still can get some clean water
• Solid or tubular

Encrusting sponges

• Very resistant to wave action
• Chemical toxins deter predators
• May be cryptic or on open surface
• Controls large area of substrate
• Solid body

Boring sponges

• Inside calcareous substrates (especially coral; capable excavating hundreds kg/m2/yr--lagoons, beach sand)
• Protected from waves and predators, have lost chemical toxins
• Usually solid
• 3 dimensional microhabitat--responsible for high reef diversity

Porifera: summary

• Filter feeding design
• Sessile (swimming larva)
• Flexible cells (totipotency) and body form
• Protist like in some respects
  • Respiration
  • Excretion
  • Prey capture/digestion