2/15/00: Review from last lecture and first lecture:

Synapomorphies (shared primitive features) of the Metazoa

• Multicellularity
• Cell adhesion
• Blastula (hollow[bilateria], solid ["stereoblastula"/planula in cnidarians], or half-solid ["amphiblastula" in sponges] ball of cells)
• Molecular data
• Collagen

Early Animal Evolution

• The "Cambrian Explosion"
• Whole animal fossils fairly rare prior to 540 mya
• Trace fossils
• Poriferans
• Cnidarians
• "Ediacaran" fauna (strange and weird Pre-Cambrian types)

(Introductory Slides, lecture 2/15/00:)

Diploblastic Diversity
Cnidaria and Ctenophora

Diploblastic Diversity
Cnidaria and Ctenophora

Eumetazoan Synapomorphies

(Text, lecture 2/15/00:)

Eumetazoan Synapomorphies (shared primitive features of the Eumetazoa)

• Tissue level organization
• Or higher
• Two cell layers
• Endoderm
• Ectoderm
• not homologous with layers of porifera
• Gastrula (know how develops from blastula)
• Nervous system
• True sense organs

Phylum Cnidaria

• Four living classes
• "Hydrozoa"
• Scyphozoa
• Cubozoa
• Anthozoa

Cnidarian Characteristics

• Tentacles
• Nematocysts
• Stinging organelles that reside inside cells

Cnidarian Characteristics

• Tentacles
• Nematocysts
• Stinging organelles
• Extremely high internal osmotic pressure
• Trigger mechanism

Cnidarian Characteristics

• Tentacles
• Nematocysts
• Two body forms
• Polyp/medusa
• Called "dimorphism"
• Not in all cnidarians

Cnidarian Characteristics

• Tentacles
• Nematocysts
• Polyp and Medusa
• Planula larva

Class "Hydrozoa"

• Paraphyletic
• Acellular mesoglea
• No gastrodermal nematocysts
• Epidermal gonads
• Often colonial
• Polyp and/or medusa
• Medusa has velum

Class "Hydrozoa"

• Hydra, an unusual Hydrozoan
• No medusa stage
• Asexual reproduction by budding
• Sexual reproduction in polyp

Hydrozoa: Polyp and Medusa

• Complex life cyle ("metagenesis")
• Alternation of asexual and sexual generations
• Asexual polyp
• Sexual medusa
• Polyp stage frequently colonial and polymorphic

Hydrozoa: "Polymorphic" polyps

• Obelia, a typical hydrozoan
• Several types of polyps (= polymorphic)
• Gastrozooid = Hydranth
  • feeding
• Gonozooid = Gonangia (singular = gonangium)
  • produce medusae (singular = medusa)

Hydrozoa: Polymorphic polyps

Siphonophores - Floating hydroid colonies

• Physalia: Portugese Man-of-War (a polymorphic colony)
• Pneumatophore
  • Flotation
• Gastrozooids
  • Feeding
• Gonozooids
  • Reproduction
• Dactylozoids
  • Defense, capture

Scyphozoa, Cubozoa, and Anthozoa united by:

• Gonads relocated to gastrodermis

Scyphozoa and Cubozoa united by:

• Reduced polyp

Class Scyphozoa

• Classic "jellyfish"
• Medusa dominant
• Distinctive characters
• Fibrous/cellular mesoglea
• Complex "manubrium" (oral arms)
• Strobilation

Class Scyphozoa: Reproduction

Class Cubozoa

• Extremely potent nematocysts
• Distinctive characters
• Cuboid body
• Velarium
• 4 tentacles or clusters
• No strobilation
• Fast!
• Excellent "eyes"

   

Class Anthozoa

• Distinctive characters
• No medusa stage
• More next time

Evolution of Cnidarian Life Histories

• Primitive cnidarian life cycle probably lacked attached polyp stage - E.g., Aglaura

Evolution of Cnidarian Life Histories

• Primitive medusoid stage with planula/actinula larval stage
• Origin of Polyp (originally no strobilation)
• Appearance of polyp allowed:
• Utilization of new foods
• Extended larval life (polyp as extension of planula stage), longer time for growth
• Colonies, specialized functions

Basic life cycle changes

"Hydrozoan"

• Polyp capable of asexual reproduction
• Colonial from incomplete separation
• Individual specialization - polymorphism
• Scyphozoan
• Addition of strobilation (new form of asexual reproduction)
• Increased number of larval stages
• Anthozoan
• Loss of medusa, polyp reproduces sexually

Evolution of Adult Motility

• Cnidarians first group to evolve advances in adult motility
• Why be motile?
• Seek out better feeding areas
• Capture larger, more active prey
• Scatter offspring over larger area
• Two types of locomotion - passive and active

Passive Locomotion

• Common among floating hydrozoan colonies. E.g., Physalia
• Pneumatophore
• Gastrozooids
• Gonozooids
• Dactylozoids

Active Locomotion

• 1. Hydrozoan medusae relatively inefficient
• 2. Scyphozoan medusae - several advances
• More fibrous mesoglea -> more force
• Strong radial fibers -> narrow opening
• Stronger coronal muscle
• Septate gut with canals -> higher metabolism
• Manubrium with oral arms -> prey capture

3. Cubozoan medusa

Most efficient

• Scyphozoan characters plus:

Manubrium (oral arms) replaced by tentacles

Velarium further narrows bell opening

 

 

Phylum Ctenophora

• Previously allied with Cnidarians due to primitive similarities
• Radial symmetry
• Diploblastic
• Gelatinous mesoglea
• Diffuse nerve net
• Nematocysts, occasionally present, are stolen from Cnidarians

Ctenophores and Bilaterians

• Ctenophores are the sister-group of
• Bilateria (general view)
• All other Eumetazoa (your text)
• Deuterostomes (a few people)
• True muscle
• Like Bilateria

Ctenophore characteristics

• Ctenes in 8 comb rows
• Colloblasts for prey capture
• Complex gut with anal canals

Ctenophore locomotion and feeding

• Ctenes beat in "metachronal" wave
• Mouth forward
• Sophisticated relative to Cnidarians

Summary

• Cnidaria
• Successful: number of species and ecologically
• Diploblastic
• Dimorphic - Polyp and Medusa
  • Variatons on this theme: de-emphasis/loss of one or other, polyp may be polymorphic and colonial
• Ctenophora
• Fewer species; can be important ecologically
• Closer to Bilateria
• Ciliary combs - primary mode of locomotion