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19. Hexacorals 1: Sea Anemones and
Anemone-like Animals (Actiniaria,
Zoanthidea, Corallimorpharia,
Ceriantharia and Antipatharia)
C. C. Wallace
N OVERVIEW
Because they have no skeleton and do not participate in
the building of reefs, sea anemones and their kin (Class
Anthozoa; Orders Actiniaria, Zoanthidea, Corallimorpharia Ceriantharia and Antipatharia) are not as intensely
studied as their relatives, the hard corals (Order Scleractinia in the same class). Nevertheless, they are prominent,
and in some cases well-known, members of coral reef
communities. The largest sea anemones, reaching almost a
metre in diameter, are hosts to anemone fish, crustaceans,
and various invertebrates that live among the anemones’
tentacles in fascinating symbiotic relationships. Other
anemones play guest rather than host and gain mobility
by attaching to molluscs, crabs or shells housing hermit
crabs. Zoanthids and corallimorphs can be abundant
occupiers of space, sometimes when this is made available
by bleaching or other damage to corals, and they too may
have symbiotic associations with many other reef organisms, thus securing space in the reef column, protection
from predators and/or access to food and energy sources.
Like hard and soft corals, the members of these groups
have life cycles involving a short larval stage and a
long polyp phase. The adult occurs as a single polyp in
Actiniaria (sea anemones) and Ceriantharia (tube anemones) and usually, but not invariably, as a colony or clone
of interconnected polyps in Corallimorpharia (jewel
anemones) and Zoanthidea (zoanthids). Antipatharia
(black corals) are exclusively colonial. The major external
characteristics of the polyp are a column, an oral disc
bearing the mouth, siphonoglyph(s) (flagellated furrows
that direct water currents into the gastrovascular cavity)
and tentacles, and (in solitary forms) the pedal disc or
bulb allowing attachment or anchorage. In colonial forms
polyps are joined by basal stolons or common coenenchyme. Main internal characters include the actinopharynx leading from the mouth and siphonoglyphs, and the
paired mesenteries that may be complete or incomplete
(reaching or not reaching the actinopharynx at their upper limit) and on which the gonads develop. (See Fig.
19.1 for examples of these characters in various orders.)
Animals from these orders contribute greatly to the
taxonomic and ecological diversity of the GBR and to
the dominant role of the Anthozoa in the coral reef environment. They are mostly carnivorous, feeding on tiny
planktonic or benthic animals or particulate matter,
and sometimes utilising symbiotic dinoflagellates in a
19. Hexacorals 1: Sea Anemones and
Anemone-like Animals (Actiniaria,
Zoanthidea, Corallimorpharia,
Ceriantharia and Antipatharia)
C. C. Wallace
N OVERVIEW
Because they have no skeleton and do not participate in
the building of reefs, sea anemones and their kin (Class
Anthozoa; Orders Actiniaria, Zoanthidea, Corallimorpharia Ceriantharia and Antipatharia) are not as intensely
studied as their relatives, the hard corals (Order Scleractinia in the same class). Nevertheless, they are prominent,
and in some cases well-known, members of coral reef
communities. The largest sea anemones, reaching almost a
metre in diameter, are hosts to anemone fish, crustaceans,
and various invertebrates that live among the anemones’
tentacles in fascinating symbiotic relationships. Other
anemones play guest rather than host and gain mobility
by attaching to molluscs, crabs or shells housing hermit
crabs. Zoanthids and corallimorphs can be abundant
occupiers of space, sometimes when this is made available
by bleaching or other damage to corals, and they too may
have symbiotic associations with many other reef organisms, thus securing space in the reef column, protection
from predators and/or access to food and energy sources.
Like hard and soft corals, the members of these groups
have life cycles involving a short larval stage and a
long polyp phase. The adult occurs as a single polyp in
Actiniaria (sea anemones) and Ceriantharia (tube anemones) and usually, but not invariably, as a colony or clone
of interconnected polyps in Corallimorpharia (jewel
anemones) and Zoanthidea (zoanthids). Antipatharia
(black corals) are exclusively colonial. The major external
characteristics of the polyp are a column, an oral disc
bearing the mouth, siphonoglyph(s) (flagellated furrows
that direct water currents into the gastrovascular cavity)
and tentacles, and (in solitary forms) the pedal disc or
bulb allowing attachment or anchorage. In colonial forms
polyps are joined by basal stolons or common coenenchyme. Main internal characters include the actinopharynx leading from the mouth and siphonoglyphs, and the
paired mesenteries that may be complete or incomplete
(reaching or not reaching the actinopharynx at their upper limit) and on which the gonads develop. (See Fig.
19.1 for examples of these characters in various orders.)
Animals from these orders contribute greatly to the
taxonomic and ecological diversity of the GBR and to
the dominant role of the Anthozoa in the coral reef environment. They are mostly carnivorous, feeding on tiny
planktonic or benthic animals or particulate matter,
and sometimes utilising symbiotic dinoflagellates in a