Heteractis magnifica

Class Anthozoa-Corals and Sea Anemones, Sea Fans, Sea PansiesDiploria labyrinthoformis Paragorgia

Largest Class (> 6000 spp.), 3 Subclasses: 1) Hexacorallia Anemones and true corals2) Octocorallia Soft corals and sea fans3) Ceriantipatharia Black Corals and Tube

Anemones

Exclusively marine, solitary or colonial polypoids with no medusa stage

Polyps have internal cavity divided by septa

Pharynx penetrates well into G-V cavity and bears 1 or more ciliated grooves (siphonoglyphs)

Cnidae epidermal and gastrodermalMargins of mesenteries or septa have cnidocytes

Gonads gastrodermal

8 tentacles usually, or multiples of 6

tube anemone

Gorgoniaventalina

Diploria spp.

Plexaura homomallia

Cirrhipathesanguina

Sea Anemones

Large, solitary polyps, brightly colored

Oral disk may bear many tentacles

Slit shaped mouth with ciliated groove (siphonoglyph) at one end•Allows water to enter G-V cavity to maintain hydrostatic pressure

Polyp can contract and withdraw under muscular collar when threatened or inactive

Septa increase internal surface area for digestion, gas exchange

Mesoglea is much thicker than in Hydrozoans, partly cellular

Spirocysts in gastrodermis-sticky cnidocytes

Cnidocytes may be used aggressively against other anemones

ReproductionAsexual fission of various forms

Hermaphroditic but production of male and female gametes asynchronous so little self fertilization

Fertilization often inside with variable development before release of larva

Usually planktonic planula which settles and grows tentacles

Metridium giganteum

Longitutidal (A) and cross section (B)of a sea anemone

Anthozoan Reproduction

Habitat utilization of reef-dwelling sea anemones

Stony Corals (Madreporarian)

Grow in calcareous cups with sclerosepta-skeleton completely outside polyp (for some species)

Colonial and usually interconnected

Polyps are usually quite small

Foundations of coral reefs

Most have zooxanthellae whichprovide nutrients

Polyps are similar to anemones except lack siphonoglyphs

Carbonate cups have sclerosepta thathelp to anchor polyps

Carbonate is laid down in bands because of variation in rate of deposition•Allows for aging of the skeleton

Coral polyps feed in very similar way to anemones except that continuous bed of polyps provides a wide mesh for prey capture

Coral polyps of different genotypes compete with each other for limited attachment substrate•Secrete allelochemicals to kill neighbors

Octocorals - Sea Pens, sea Pansies, Sea Fans, Pipe Corals

Colonial polyps

Always with 8 tentacles, each of which is pinnate or with side branches

Polyps joined by connective tissue called coenenchyme with gastrodermal tubes

Skeleton is internal-individual spicules or fused mass- may be rigid or flexible

Sea Fans oriented perpendicular to current-can bend

Few cnidocytes so probably feed on small particles - not zooplankton

Reproduction is similar to other anthozoans

Cnidarian fossils from the Ediacaran fauna

Fossils are both medusoid and polypoid, and probably represent all cnidarianclasses (Scrutton 1979).

From Brusca and Brusca (2002)

Note: The extinct class Conulatais probably related to Scyphozoa

Morphological, mtDNA, and 18S rDNA data separately and combined support the basal position of Anthozoa but do not resolve the relationships among Scyphozoa, Cubozoa and Hydrozoa (Bridge et al. 1995).

H1: Anthozoans are the most basal(Polyp is the ancestral andmedusa is the derived condition)

H2: Anthozoans are the most derived(medusa is the ancestral condition but has been lost in Anthozoa. Polyps evolved secondarily)

Competing hypotheses (Figs 1A,B) of cnidarian life-cycle evolution and systematic relationships (Bridge et al. 1995.)

Cnidarian synapomorphies: radial, medusoid body; cnidae; planula larva; cnidarian coelenteron.

1. Polypoid body form and “alteration of generations”2. Gonads relocated in gastrodermis3. Partitions (mesenteries) appear, subdividing the coelenteron4. Polyp stage secondarily lost or reduced5. Evolution of the rhopalium6. Evolution of strobilation7. Evolution of “boxlike” medusa body8. Complex lensed rhopalial eyes9. Velarium10. Suppression of the medusoid stage11. Hexaradial and octaradial symmetry12. Actinopharynx13. Siphonoglyph14. Coelenteron acquires mesenterial filaments15. Loss of operculum16. Loss of cnidocil17. Evolution of tripartite series of flaps on cnidae18. Evolution of special ciliary cones associated with cnidae

Phylogeny of Cnidaria according to the medusa theory

Gordon Medusa

1. Cnidocil + loss of ancestral anthozoan traits2. Polyp stage lost or reduced3. Acraspedote medusa4. Evolution of the rhopalium5. Evolution of strobilation6. Evolution of “boxlike” medusa body7. Complex lensed rhopalial eyes 8. Velarium9. Relocation of gonad-forming tissue to epidermis10. Loss of gut mesenteries11. Simplification of the middle layer to an acellular mesoglea12. Evolution of the craspedote medusa form13. Loss of gastrodermal nematocysts

Phylogeny of Cnidaria according to the polyp theory (Brusca and Brusca 2002)