Z250 - Lec9 Outline. Cnidaria II: Diversity (2017)

Zoology 250 (2017)

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 ciliated planula larva with a "head" end; posterior mouth in some
- b) the planula larva may be the ancestral form of the Bilateria

2) Several characters distinguish the 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 + pharynx simple simple tubular + siphonoglyph

   - gastrovasc. septae no yes (some) yes

   - gastric filaments
      with cnidocytes no yes yes

   - tentacle form solid solid hollow

Medusa phase:

   - velum present absent - - -

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

   - manubrium simple
(no cnidocytes) complex
(mouth lobes
+ cnidocytes) - - -

   - gastrovascular canals unbranched branched - - -

   - gastric filaments
       with cnidocytes no yes - - -

   - prey capture mainly
tentacles mouth lobes
+ tentacles - - -

Gonads epidermal gastrodermal gastrodermal

Habitat mostly marine,
some freshwater marine
only marine
only

MtDNA form linear linear circular

	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 + pharynx	simple	simple	tubular + siphonoglyph
- gastrovasc. septae	no	yes (some)	yes
- gastric filaments with cnidocytes	no	yes	yes
- tentacle form	solid	solid	hollow
Medusa phase:
- velum	present	absent	- - -
- bell margin	simple	notched (rhopalia*)	- - -
- manubrium	simple (no cnidocytes)	complex (mouth lobes + cnidocytes)	- - -
- gastrovascular canals	unbranched	branched	- - -
- gastric filaments with cnidocytes	no	yes	- - -
- prey capture	mainly tentacles	mouth lobes + tentacles	- - -
Gonads	epidermal	gastrodermal	gastrodermal
Habitat	mostly marine, some freshwater	marine only	marine only
MtDNA form	linear	linear	circular

* combined photoreceptor & statocyst

3) Cl. HYDROZOA (~3000 sp.) exhibit a bewildering array of life-cycles; some emphasize polyp stage and others emphasize medusa stage; nearly all are marine (a few freshwater), examples include:
- 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
  - in some groups medusa phase is lost, in others polyp phase is lost
- b) Hydrocorals- polymorphic polyps form colonies with massive carbonate skeletons (exoskeleton); play an important role in coral reefs; medusa stage reduced; includes the "fire corals"
- c Or. Siphonophora- swimming or floating polymorphic colonies; may have multiple medusoid and polypoid forms in one colony!
- d) 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.), all marine, medusa form dominant; distinctive 8-lobed larval medusa (ephyra) arise from a benthic polyp by strobilation
5) Cl. ANTHOZOA (anemones, corals), all marine, medusa form absent
- a) SubCl. Octocorallia (~2000 sp.)- many colonial forms (soft corals, gorgonians and sea pens); typically have 8 pinnate tentacles and 8 gastrovascular septae
  - the tanned collagen skeleton of gorgonians is an endoskeleton (forms in mesoglea)
- b) SubCl.Hexacorallia (~4000 sp.)- a heterogeneous group (sea anemones and stony corals); more than 8 tentacles and gastrovascular 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, gastric filaments, gastrovascular septae in polyp, true myocytes, cellular mesoglea
- a) recent molecular data suggest a clade Medusozoa (Hyddrozoa + Scyphozoa), shared traits include: medusa, 4-fold radial symmetry of medusa, operculum on nematocyst, linear mtDNA; stay tuned!

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