19 – Hexacorals 1: Sea Anemones and Anemone-like Animals
205
BOX 19.1 SEA ANEMONES AS HOSTS AND GUESTS
Hosts
One of the best known and loved associations on coral reefs is that between large tropical sea anemones and the small, brightly coloured fish known as ‘clownfish’ or ‘anemonefish’. Fish and sea anemones live together in a close association known as ‘symbiosis’,
where both species benefit from the arrangement and concessions are made by the partners to make it possible for the two to coexist. Ten species of sea anemone are known to
play host to anemonefishes. These come from the families Stychodactylidae (species of
Heteranthus and Stichodactyla), Actiniidae (Entacmaea quadricolor and Macrodactyla doreensis) and Thalassianthidae (Cryptodendrum adhaesivum) (see Fig. 19.3 for some of these
species). Like most sea anemones, these hosts have a well-developed defensive system
based on nematocysts within the ectoderm of the tentacles. The fish swim around among
the tentacles despite this hazardous feature and benefit from the protection from larger
predators that would be harmed by the tentacles. The anemones themselves have also
been found to benefit by protection from predators due to the defensive activities of the
anemonefishes and a number of other ecological advantages have been documented.
At least 28 fish species are involved in anemone associations throughout the IndoPacific, all from the family Pomacentridae and most from the genus Amphiprion (see
Figs 19.3B, 19.4). Some anemonefish are quite host specific, but most can occur in many
hosts. A hierarchy exists among the fish species themselves, which means that certain
species are much more likely than others to successfully colonise and monopolise the
available hosts. Other factors too, such as habitat and geography, play a role in the availability and likelihood of settlement in a host of a particular fish species. The fish species
differ in the way they approach new hosts, but all have some mechanism for developing
a tolerance to the sting of the anemone via a mucous surface coating developed following gradual contact with the tentacles.
Guests
There are also numerous associations of sea anemones with other host organisms, where
again the anemone’s stinging capabilities provide some protection for the host and the
host gives the anemone access to resources. One thing that is difficult for anemones (at
least after the larval phase) is moving around and some of them avoid this problem by
living in association with motile animals such as slow-moving molluscs or mollusc shells
inhabited by hermit crabs.
of the animal), until the animal is unresponsive to touch.
Permits are required to collect, see Chapter 12.
N ORDER ZOANTHIDEA (‘ZOANTHIDS’)
These are mostly colonial forms (except for species of
the large tropical free living zoanthid genus Sphenopus),
in which the polyps lack a pedal disc and are joined by
coenenchyme, which often contains a thick mesoglea
(Fig. 19.8A–F). In one group of tropical zoanthids, including Palythoa and Parapalythoa, this coenenchyme
incorporates sand and other material within a mesogleal
matrix, so that the columns of the polyps are not visible.
The number of valid zoanthid species known to be
present on the GBR is around 13, but revisions and new
205
BOX 19.1 SEA ANEMONES AS HOSTS AND GUESTS
Hosts
One of the best known and loved associations on coral reefs is that between large tropical sea anemones and the small, brightly coloured fish known as ‘clownfish’ or ‘anemonefish’. Fish and sea anemones live together in a close association known as ‘symbiosis’,
where both species benefit from the arrangement and concessions are made by the partners to make it possible for the two to coexist. Ten species of sea anemone are known to
play host to anemonefishes. These come from the families Stychodactylidae (species of
Heteranthus and Stichodactyla), Actiniidae (Entacmaea quadricolor and Macrodactyla doreensis) and Thalassianthidae (Cryptodendrum adhaesivum) (see Fig. 19.3 for some of these
species). Like most sea anemones, these hosts have a well-developed defensive system
based on nematocysts within the ectoderm of the tentacles. The fish swim around among
the tentacles despite this hazardous feature and benefit from the protection from larger
predators that would be harmed by the tentacles. The anemones themselves have also
been found to benefit by protection from predators due to the defensive activities of the
anemonefishes and a number of other ecological advantages have been documented.
At least 28 fish species are involved in anemone associations throughout the IndoPacific, all from the family Pomacentridae and most from the genus Amphiprion (see
Figs 19.3B, 19.4). Some anemonefish are quite host specific, but most can occur in many
hosts. A hierarchy exists among the fish species themselves, which means that certain
species are much more likely than others to successfully colonise and monopolise the
available hosts. Other factors too, such as habitat and geography, play a role in the availability and likelihood of settlement in a host of a particular fish species. The fish species
differ in the way they approach new hosts, but all have some mechanism for developing
a tolerance to the sting of the anemone via a mucous surface coating developed following gradual contact with the tentacles.
Guests
There are also numerous associations of sea anemones with other host organisms, where
again the anemone’s stinging capabilities provide some protection for the host and the
host gives the anemone access to resources. One thing that is difficult for anemones (at
least after the larval phase) is moving around and some of them avoid this problem by
living in association with motile animals such as slow-moving molluscs or mollusc shells
inhabited by hermit crabs.
of the animal), until the animal is unresponsive to touch.
Permits are required to collect, see Chapter 12.
N ORDER ZOANTHIDEA (‘ZOANTHIDS’)
These are mostly colonial forms (except for species of
the large tropical free living zoanthid genus Sphenopus),
in which the polyps lack a pedal disc and are joined by
coenenchyme, which often contains a thick mesoglea
(Fig. 19.8A–F). In one group of tropical zoanthids, including Palythoa and Parapalythoa, this coenenchyme
incorporates sand and other material within a mesogleal
matrix, so that the columns of the polyps are not visible.
The number of valid zoanthid species known to be
present on the GBR is around 13, but revisions and new