Z250 - Lec9 Outline. Cnidaria II (2006)

Zoology 250 (2006)

Phylum CNIDARIA (II):
Diversity

(Cnidaria study images;
click on tree to see full cladogram)

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1) Life cycle: asexual reproduction and coloniality are widespread; most have separate sexes and show alternation of phases (polyp=asexual <<->> medusa=sexual); external fertilization; radial cleavage; both blastula and gastrula stages are present
- a) a distinctive ciliated planula larva forms after gastrulation
- b) the planula larva may be the ancestral form of the Bilateria

2) Comparisons among classes:

Cl. Hydrozoa Cl. Scyphozoa Cl. Anthozoa

Both polyp & medusa phase? yes (most) yes (most) no (polyp only)

   - medusa by budding strobilation - - -

Polyp phase:

   - polyp form cylindrical funnel shaped cylindrical

   - mouth simple simple tubular pharynx

   - gastrovasc. septae no yes (some) yes

   - gastric filaments
      with cnidocytes no yes yes

Medusa phase:

   - velum present absent - - -

   - bell margin simple notched
(rhopalia*) - - -

   - manubrium simple
(no cnidocytes) complex
(with cnidocytes) - - -

   - gastrovascular canals unbranched branched - - -

   - gastric filaments
       with cnidocytes no yes - - -

   - prey capture mainly
tentacles manubrium
+ tentacles - - -

Gonads epidermal gastrodermal gastrodermal

Habitat mostly marine,
some freshwater marine
only marine
only

	Cl. Hydrozoa	Cl. Scyphozoa	Cl. Anthozoa
Both polyp & medusa phase?	yes (most)	yes (most)	no (polyp only)
- medusa by	budding	strobilation	- - -
Polyp phase:
- polyp form	cylindrical	funnel shaped	cylindrical
- mouth	simple	simple	tubular pharynx
- gastrovasc. septae	no	yes (some)	yes
- gastric filaments with cnidocytes	no	yes	yes
Medusa phase:
- velum	present	absent	- - -
- bell margin	simple	notched (rhopalia*)	- - -
- manubrium	simple (no cnidocytes)	complex (with cnidocytes)	- - -
- gastrovascular canals	unbranched	branched	- - -
- gastric filaments with cnidocytes	no	yes	- - -
- prey capture	mainly tentacles	manubrium + tentacles	- - -
Gonads	epidermal	gastrodermal	gastrodermal
Habitat	mostly marine, some freshwater	marine only	marine only

* combined photoreceptor & statocyst

3) Cl. HYDROZOA (~3000 sp.) exhibit a bewildering array of life-cycles; some emphasize the polyp and others emphasize the medusa stage; all are marine except some hydroids and trachyline medusae:
- a) Hydroids- a solitary or colonial polyp form predominates
  - two orders: polyps thecate (e.g., Obelia) or naked (e.g., Hydra)
  - polyps show division of labor in colonial forms
  - most (but not all) possess a medusa stage
- b) Trachyline medusae- the polyp stage is reduced or absent
- c) Hydrocorals- polymorphic polyps form colonies with massive carbonate skeletons (exoskeleton); play an important role in coral reefs; medusa stage reduced; includes the "fire corals"
- d) Or. Siphonophora- swimming or floating polymorphic colonies; may have multiple medusoid and polypoid forms in one colony!
- e) Or. Chondrophora- entirely pelagic, float on the ocean surface; includes Velella, the "by-the-wind-sailor"; has multiple mouths; mainly polypoid forms; medusae are budded off for reproduction
4) Cl. SCYPHOZOA ("true" jellyfish; ~200 sp.), medusa form dominant
- a) new medusae are produced from a polyp by strobilation
- b) some polyp stages possess septae in the gastrovascular cavity
5) Cl. ANTHOZOA (anemones, corals), the medusa form is absent
- a) Octocorals (~2000 sp.)- diverse colonial forms (soft corals, gorgonians and sea pens); typically have 8 pinnate tentacles and 8 septae
  - the tanned collagen skeleton of gorgonians is an endoskeleton (forms in mesoglea)
- b) Hexacorals (~4000 sp.)- a heterogeneous group (sea anemones and stony corals); more than 8 tentacles and septae (typically multiples of 12)
  - the massive carbonate skeleton of corals is an exoskeleton (secreted by the epidermis)
6) Scyphozoa and Anthozoa united by: gastrodermal cnidocytes and gonads, gastrovascular septae in polyp, cellular mesoglea

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