The Great Barrier Reef
202
types). Much remains to be resolved in the classification of sea anemones, and relationships between and
among genera and families are not always clear. This is
an active research area and new developments can be
seen as updates in the ‘Hexacorallia of the World’ database (see Additional reading).
External anatomy
The sea anemone column may have up to three recognisable regions (from the base: scapus, scapulus and
capitulum: see Fig. 19.1) and above the column is a flat
or domed oral disc bearing the mouth and tentacles in
two to many cycles. The mouth is oval or slit-like and
has one, two or more siphonoglyphs. The margin of
the oral disc is often folded into a groove or fosse. Tentacles in Actiniaria are mostly hollow and simple but
may be branched, sometimes elaborately so, or carry
structures containing numerous nematocysts. Other
projections with various names such as verrucae (adhesive projections from the column), acroragi (projections bearing nematocysts) and marginal spherules
(projections without nematocysts) may occur on the
margin, in the fosse and on the column. A large group
of families (known as ‘acontiate’ anemones) have
nematocyst-bearing structures called acontia (fine hairlike projections from the base of the mesenteries), which
can be extruded through various parts of the body, and
sometimes through permanent holes in the column
called cinclides. All these external features are involved
in the identification process, but internal features such
as the structure of the marginal sphincter between the
top of the column and the oral disc, the number and
extent of mesenteries and the position of gonads on
them are all used in the complete description and identification of a species.
Internal anatomy
The mouth leads to an actinopharynx, below which is
the gastrovascular cavity. The body cavity is divided
by paired mesenteries, some complete (reaching the
actinopharynx) and others incomplete, in typical arrangements. Gonads (oocytes and testes) develop from
gastrodermal interstitial cells that ripen within the
mesenteries. The arrangement of gonads is used in the
diagnostic process, with either hermaphroditic (both
sexes occurring in each polyp) and gonochoric (sexes
separate) states occurring in different species. As might
be expected in soft-bodied animals without skeletons,
anemones are equipped with a variety of muscles,
which serve to maintain body shape, control tentacles
and maintain the position of the animal on or in the
substratum. Often the presence/absence, position, size
and shape of the muscles (especially of the marginal
sphincter muscle responsible for closing off the oral
disc) are diagnostic.
Life history and ecology
All sea anemones are capable of sexual reproduction
and many are also capable of asexual reproduction by
division of the polyp and separation of the two resulting polyps. The bulb-tentacle anemone, Antacmaea
quadricolor, and the magnificent sea anemone, Heteractis magnifica, are well known for forming huge clones of
individuals, occupying many square metres, by this
method. Other species, most notably the aquarium pest
Aiptasia pulchella, produce numerous cloned individuals by a process known as pedal laceration, and the
swimming anemone, Boloceroides mcmurrichi, is able to
reproduce by shedding tentacles, which then bud a
complete new anemone. Sexual reproduction involves
the development of eggs and sperm in separate gonads,
followed by release of the gametes into the water column for external fertilisation (in hermaphroditic and
possibly some gonochoric species) or fertilisation of
eggs in situ by sperm released from other individuals
and brooding of larvae inside or close to the female
parent (in gonochoric species).
Diversity
Of about 150 species of anemones known from Australian waters (including Antarctica), around 40 are known
so far from the GBR (Figs 19.3–19.7, Box 19.1). This is
likely to be a gross underestimate, particularly in regard to smaller and cryptic species and sand-dwelling
species, especially those from the family Edwardsiidae,
which is under review worldwide at present. As with
corals, many of the anemone species found on the GBR
have broad ranges through the Indo-Pacific.
202
types). Much remains to be resolved in the classification of sea anemones, and relationships between and
among genera and families are not always clear. This is
an active research area and new developments can be
seen as updates in the ‘Hexacorallia of the World’ database (see Additional reading).
External anatomy
The sea anemone column may have up to three recognisable regions (from the base: scapus, scapulus and
capitulum: see Fig. 19.1) and above the column is a flat
or domed oral disc bearing the mouth and tentacles in
two to many cycles. The mouth is oval or slit-like and
has one, two or more siphonoglyphs. The margin of
the oral disc is often folded into a groove or fosse. Tentacles in Actiniaria are mostly hollow and simple but
may be branched, sometimes elaborately so, or carry
structures containing numerous nematocysts. Other
projections with various names such as verrucae (adhesive projections from the column), acroragi (projections bearing nematocysts) and marginal spherules
(projections without nematocysts) may occur on the
margin, in the fosse and on the column. A large group
of families (known as ‘acontiate’ anemones) have
nematocyst-bearing structures called acontia (fine hairlike projections from the base of the mesenteries), which
can be extruded through various parts of the body, and
sometimes through permanent holes in the column
called cinclides. All these external features are involved
in the identification process, but internal features such
as the structure of the marginal sphincter between the
top of the column and the oral disc, the number and
extent of mesenteries and the position of gonads on
them are all used in the complete description and identification of a species.
Internal anatomy
The mouth leads to an actinopharynx, below which is
the gastrovascular cavity. The body cavity is divided
by paired mesenteries, some complete (reaching the
actinopharynx) and others incomplete, in typical arrangements. Gonads (oocytes and testes) develop from
gastrodermal interstitial cells that ripen within the
mesenteries. The arrangement of gonads is used in the
diagnostic process, with either hermaphroditic (both
sexes occurring in each polyp) and gonochoric (sexes
separate) states occurring in different species. As might
be expected in soft-bodied animals without skeletons,
anemones are equipped with a variety of muscles,
which serve to maintain body shape, control tentacles
and maintain the position of the animal on or in the
substratum. Often the presence/absence, position, size
and shape of the muscles (especially of the marginal
sphincter muscle responsible for closing off the oral
disc) are diagnostic.
Life history and ecology
All sea anemones are capable of sexual reproduction
and many are also capable of asexual reproduction by
division of the polyp and separation of the two resulting polyps. The bulb-tentacle anemone, Antacmaea
quadricolor, and the magnificent sea anemone, Heteractis magnifica, are well known for forming huge clones of
individuals, occupying many square metres, by this
method. Other species, most notably the aquarium pest
Aiptasia pulchella, produce numerous cloned individuals by a process known as pedal laceration, and the
swimming anemone, Boloceroides mcmurrichi, is able to
reproduce by shedding tentacles, which then bud a
complete new anemone. Sexual reproduction involves
the development of eggs and sperm in separate gonads,
followed by release of the gametes into the water column for external fertilisation (in hermaphroditic and
possibly some gonochoric species) or fertilisation of
eggs in situ by sperm released from other individuals
and brooding of larvae inside or close to the female
parent (in gonochoric species).
Diversity
Of about 150 species of anemones known from Australian waters (including Antarctica), around 40 are known
so far from the GBR (Figs 19.3–19.7, Box 19.1). This is
likely to be a gross underestimate, particularly in regard to smaller and cryptic species and sand-dwelling
species, especially those from the family Edwardsiidae,
which is under review worldwide at present. As with
corals, many of the anemone species found on the GBR
have broad ranges through the Indo-Pacific.