Cnidaria and Ctenophora

Cnidaria and Ctenophora

• Both phyla diploblastic

• adult organisms of both groups exhibit radial or biradial symmetry and are not cephalized

• cnidarians are known as sea anemones and jellyfishes, and ctenophores as comb jellies

Diversity and Classification 

• 9,000 species 

• 5 classes

  • Anthozoa (Majority 60%)

  • Hydrozoa (2nd largest 36%)

  • Scyphozoa

  • Staurozoa

  • Cubozoa

Biology and Importance

• Widespread in marine waters (shallow,warm, tropical); few in freshwater

• mostly sessile; motile species are slow moving

• predators of much more motile animals

• may live symbiotically with other animals (ex: clown fish)

PHYLUM CNIDARIA

Form and Function (General Characteristics)

• diploblastic

• no coelomic cavity

• radial or biradial symmetry

• tissues, but no organs (cell-tissue grade)

• two types of individuals: polyps and medusae

• cnidocytes with nematocyst (stinging organelles)

• Body plan: dimorphism 

  • Polyp:

    • tubular body

    • mouth surrounded by tentacles, leads to gastrovascular cavity

    • attached to substrate by pedal disc

    • reproduce by budding

    • if buds detach = clones; if buds remain attached = colony

  • Medusa:

    • Bell-shaped (or umbrella-shaped) body

    • Mouth on concave side and surrounded by tentacles

    • usually free-swimming

    • sensory structures at margins of bell

    • generally, reproduce sexually 

• Body wall

  • Epidermis (outer layer)

    • epitheliomuscular cells, sensory cells, nerves and cnidocyte cells

  • Mesoglea (middle layer)

    • gelatinous (95-96% water), bridge between epidermis and gastrodermis; acts as an elastic skeleton

  • Gastrodermis (inner layer) 

    • cells for digestion and absorption (nutritive muscle cells), gland cells, interstitial cells

• Cnidocytes

  • Produce over 20 organelles; nematocysts are one type

  • Nematocysts are thread-like, may bear barbs,spines, and deliver toxins (some may not)

  • Operculum with cnidocil trigger (some mechanical, some chemical)

  • very fast firing

Homeostatic Functions and Structures

• no excretory system (accomplished by diffusion)

• no respiratory system (accomplished by diffusion)

• no circulatory system (accomplished by diffusion)

• no osmoregulatory system (accomplished by diffusion)

• no temperature regulation 

Nervous system

• Nerve net:

  • diffuse nervous system

  • lack concentrations of nerve cells (no brain or ganglia)

  • bidirectional travel of nerve impulses; lack insulating material (myelin)

Cnidaria: Class Hydrozoa 

• Mostly marine and colonial 

• typical life cycle with both asexual polyps and sexual medusa

Hydroid colonies

• contain base (hydrorhiza), stalk (hydrocauli), with one or more terminal zooids

• hydrocauli composed of living (coenosarc) and non-living (perisarc) elements

• feeding zooids are hydranths; reproductive zooids are gonangium 

Solitary hydroid

• basal disc for attachment

• dioecious and capable of both sexual and asexual reproduction

Hydroid medusa

• posses a velum 

  • concentrates water being expelled from under the medusas oral surface, acting as a jet-propulsion system

• mouth suspended on a manubrium 

Colonial Hydroid

• Pneumatophore

• Tentacles are zooids

Cnidaria: Class Scyphozoa

• most of the large jellies; drift or swim in seas and oceans

• lack velum

• manubrium forms four oral arms

• dioecious 

Typical scyphozoan

• reproduction 

  • zygote to planula to scyphistoma to strobila to ephyrae to mature jelly 

Atypical scyphozoan 

• the “upside-down jelly”

• symbiotic relationship with zooxanthellae - a yellowish-brown symbiotic dinoflagellate

• generate food for themselves and jelly via photosynthesis

Cnidaria: Class Anthozoa

• Anthozoa = flower animals

• polyp only; no medusa

• all marine; deep to shallow; cold to warm; solitary to colonial (warm shallow waters preferred)

• gastrovascular cavity is large in comparison to other cnidarians

  • mouth - pharynx - gastrovascular cavity

• Three subclasses

  • Hexacorallia :

    • sea anemones, hard corals

  • Ceriantipatharia :

    • tube anemones and thorny corals

  • Octocorallia:

    • soft and horny corals, sea pens, sea fans

Subclass Hexacorallia (sea anemones)

• large, heavy polyps

• attach to substrate with pedal disc; mouth on oral disc

• mouth to pharynx to gastrovascular cavity

• GV cavity divided by complete (primary) and incomplete (secondary) septa

Subclass Hexacorallia (true or stony corals)

• miniature anemones in calcareous cups

Subclass Ceriantipatharia (tube anemones, thorny corals)

• low diversity

• tropical 

• tube anemones solitary

• thorny corals colonial

Subclass Octocorallia (soft corals, sea pens, sea fans)

• all colonial

• gastrovascular cavities communicate with each other

• very colorful

Phylum Ctenophora

• Diversity

  • 150 species of comb jellies

• Biology

  • all are marine; all waters, but more abundant in warm waters 

  • only medusa

• Form and Function (General Characteristics)

  • eight equally-spaced rows of comb-like plates of cilia

  • comb rows used in locomotion

  • no head, but oral-aboral axis present

  • mouth to pharynx to digestive tract to anal pore

  • body is transparent with gelatinous layer form ectoderm and endoderm 

  • tentacles may be present and may have colloblasts (glue cells) for prey capture

• Homeostatic Functions and Structures

  • respiration and excretion by diffusion

  • no circulatory, osmoregulatory, or temperature regulation

• Nervous system

  • Nerve net

  • Statolith present at the aboral end (controls orientation).

  • Epidermis bears abundant chemo and tactile sensory cells

• Reproduction

  • most are monoecious

  • some larva are free-swimming, and transitions into an adult without metamorphosis

• Bioluminescence

  • Most ctenophores are bioluminescent (e.g. produce chemical light).