19 – Hexacorals 1: Sea Anemones and Anemone-like Animals
207
studies on zoanthids from the Indo-Pacific (including
genetic studies) may increase this number, and clarify
the identity of species representatives of many genera.
Identification of genera is often possible from external
features but even this needs to be confirmed by internal
observation in many cases, for example, the genus Zoanthus has a divided marginal sphincter. Confirmation of
some identifications requires histological sectioning.
Much progress is being made in zoanthid taxonomy
and natural history, and many new publications on this
group can be expected over the coming years.
Anatomy
The column wall is without special external structures
as seen in Actiniaria. The oral disc has unbranched tentacles, which occur in an exococelic and an endocoelic
cycle (these cycles often appearing as a single marginal
ring). The mouth is usually slit-like and has a single
syphonoglyph, which is ventral. The septal arrangements are unique in living Anthozoa and new septa
form only to either side of the ventral septal pair. In
general, each pair of septa has one large septum (macroseptum, extending to the actinopharynx) and a small
septum (microseptum), except for the dorsal directives,
which are both microsepta, and the ventral directives,
which are both macrosepta. This is called the ‘brachynemous’ arrangement (as in Fig. 19.1H), but a few zoanthids also have another complete pair of macrosepta on
each side (the ‘macronemous’ arrangement).
Life history and ecology
Colonies of zoanthids may grow attached to the substratum by a broad colony base (e.g. in Palythoa species)
and many are epizoic, often using specific animal species, worm tubes (e.g. Acrozoanthus australis), sponges,
gorgonians and numerous other organisms as an attachment surface. Many species host symbiotic dinoflagellates (zooxanthellae) in the ectodermal tissues.
Sexual reproduction probably occurs in all species and
follows similar patterns to those of corals and sea
anemones. At least one species has been seen to spawn
during the coral mass spawning event on the GBR.
Zoanthids also form clones by division of colonies and
large areas of reef flat may contain representatives of
only a few genotypes because of this.
The intertidal reef flat often appears to be a preferred habitat for zoanthids, probably due to the rapid
growth and cloning ability of the genera Palythoa and
Protopalythoa that occur here and are particularly obvious a few months after damage to corals and other
organisms, for example by events such as cyclones.
Species of Protopalythoa and Zoanthus occur in calmer
sublittoral and back-reef areas. Zoanthids are very
toxic, their tissues containing a toxin known as palytoxine. For this reason they are not as popular as corallimorphs for aquarium culture.
N ORDER CORALLIMORPHARIA (‘JEWEL
ANEMONES’ OR ‘CORALLIMORPHS’)
Usually grouped with sea anemones, this relatively
small order of around 40 named species is sometimes
regarded as ‘skeleton-free corals’, although the close
relationship with Scleractinia is questioned by a recent
genetic study. They occur as clones of polyps connected
by stolons (Fig. 19.8G–J). The polyps have a broad, thin
oral disc and short, featureless column. The mouth protrudes from the centre of the oral disc. There are a variable number of short tentacles, usually occurring
throughout the oral disc, but sometimes in rows and
sometimes there are specialised tentacles (bearing
nematocyst batteries in rounded tips). The nematocyst
composition (cnidom) is less complex than in Actiniaria, but some corallimorph nematocysts are very large.
Corallimorphs are often very brightly coloured and a
single species may occur in many colour morphs. There
are probably around 20 species on the GBR, although
further study may discover more.
Corallimorphs may be non-zooxanthellate (family
Corallimorphidae) or zooxanthellate (Discosomatidae
and Ricordaeidae). Many are formidable predators.
The large discosomatid Amplexidiscus fenestrafer has
been observed to feed by enveloping small fish in its
oral disc. Other discosomatids may be able to form monocultures by defending territory using nematocysts
stored in the tips of the marginal tentacles. They are
mostly subtidal and many avoid bright light. They are
generally very tolerant of environmental conditions
and are popular as aquarium subjects for this reason
as well as their attractive appearance. Histological
207
studies on zoanthids from the Indo-Pacific (including
genetic studies) may increase this number, and clarify
the identity of species representatives of many genera.
Identification of genera is often possible from external
features but even this needs to be confirmed by internal
observation in many cases, for example, the genus Zoanthus has a divided marginal sphincter. Confirmation of
some identifications requires histological sectioning.
Much progress is being made in zoanthid taxonomy
and natural history, and many new publications on this
group can be expected over the coming years.
Anatomy
The column wall is without special external structures
as seen in Actiniaria. The oral disc has unbranched tentacles, which occur in an exococelic and an endocoelic
cycle (these cycles often appearing as a single marginal
ring). The mouth is usually slit-like and has a single
syphonoglyph, which is ventral. The septal arrangements are unique in living Anthozoa and new septa
form only to either side of the ventral septal pair. In
general, each pair of septa has one large septum (macroseptum, extending to the actinopharynx) and a small
septum (microseptum), except for the dorsal directives,
which are both microsepta, and the ventral directives,
which are both macrosepta. This is called the ‘brachynemous’ arrangement (as in Fig. 19.1H), but a few zoanthids also have another complete pair of macrosepta on
each side (the ‘macronemous’ arrangement).
Life history and ecology
Colonies of zoanthids may grow attached to the substratum by a broad colony base (e.g. in Palythoa species)
and many are epizoic, often using specific animal species, worm tubes (e.g. Acrozoanthus australis), sponges,
gorgonians and numerous other organisms as an attachment surface. Many species host symbiotic dinoflagellates (zooxanthellae) in the ectodermal tissues.
Sexual reproduction probably occurs in all species and
follows similar patterns to those of corals and sea
anemones. At least one species has been seen to spawn
during the coral mass spawning event on the GBR.
Zoanthids also form clones by division of colonies and
large areas of reef flat may contain representatives of
only a few genotypes because of this.
The intertidal reef flat often appears to be a preferred habitat for zoanthids, probably due to the rapid
growth and cloning ability of the genera Palythoa and
Protopalythoa that occur here and are particularly obvious a few months after damage to corals and other
organisms, for example by events such as cyclones.
Species of Protopalythoa and Zoanthus occur in calmer
sublittoral and back-reef areas. Zoanthids are very
toxic, their tissues containing a toxin known as palytoxine. For this reason they are not as popular as corallimorphs for aquarium culture.
N ORDER CORALLIMORPHARIA (‘JEWEL
ANEMONES’ OR ‘CORALLIMORPHS’)
Usually grouped with sea anemones, this relatively
small order of around 40 named species is sometimes
regarded as ‘skeleton-free corals’, although the close
relationship with Scleractinia is questioned by a recent
genetic study. They occur as clones of polyps connected
by stolons (Fig. 19.8G–J). The polyps have a broad, thin
oral disc and short, featureless column. The mouth protrudes from the centre of the oral disc. There are a variable number of short tentacles, usually occurring
throughout the oral disc, but sometimes in rows and
sometimes there are specialised tentacles (bearing
nematocyst batteries in rounded tips). The nematocyst
composition (cnidom) is less complex than in Actiniaria, but some corallimorph nematocysts are very large.
Corallimorphs are often very brightly coloured and a
single species may occur in many colour morphs. There
are probably around 20 species on the GBR, although
further study may discover more.
Corallimorphs may be non-zooxanthellate (family
Corallimorphidae) or zooxanthellate (Discosomatidae
and Ricordaeidae). Many are formidable predators.
The large discosomatid Amplexidiscus fenestrafer has
been observed to feed by enveloping small fish in its
oral disc. Other discosomatids may be able to form monocultures by defending territory using nematocysts
stored in the tips of the marginal tentacles. They are
mostly subtidal and many avoid bright light. They are
generally very tolerant of environmental conditions
and are popular as aquarium subjects for this reason
as well as their attractive appearance. Histological