392
Destruction and Recovery of Reef Systems, Reef and Man
chronic infestation of reefs by this plaque, endangering their very existence
in large reef regions (Chesher 1969). So the attacks by this sea star on reefs
could be considered as an ecocatastrophe on a large scale, triggered mainly
by human activity, which has increased in the postwar period (Kuhlmann
1971; Endean 1974). The alarming descriptions in papers of the awful
pictures preseated by the degradation of formerly beautiful reefs to a state
of rubble heaps covered with ooze, attracted the attention of, government
agencies, the public and researchers to the Acanthaster problem (Endean
1973). Special studies were organized to evaluate the scale of the disabitrous
infestations and to find ways to combat them (Endean and Chesher 1973).
The genus Acanthaster, besides the "crown of thorn" star A. planci,
includes also two more species: A. ellisi and A. brevispinnus. A. ellisi is a
very rare spiny star found on Philippine reefs, which feeds on corals. A.
brevispinnus is detritovore. All three species are genetically very close and
easily form hybrids with each other (Lucas and Nash 1985). A. planci is
supposed to be a species which appeared recently, descended from A.
brevispinnus and acquired the ability for predation on corals. A. ellisi could
be an intermediate form between them on the way to these evolutionary
developments. The Acanthaster stars have very flexible motile spiny rays. By
their use they are able to crawl easily upon colonies of corals and hold onto
them. This lends them an advantage in competition with the other large
corallovore asteroid Culcita, a bulky star though devoid of rays, which is
able to graze only on small coral colonies or crusts. As a consequence, when
the Acanthaster species acquired the ability to digest the protective cetylpalmitate substances, soaking up the coenosarc of scleractinian corals, and
to feed on their tissues, it dropped out of competition and was free of food
limitation (Benson et al. 1978). In spite of this ability, these stars were
among the rare species of large reef invertebrate predators, just like the star
Culcita. The average size of A. planci is 30-35 cm, and some specimens can
be up to 50-60 cm large. The dorsal side of its body is covered by large
poisonous thorns 2-4 cm long. The tissues of the stars is saturated with
saponin, repelling potential predators by the bad taste. Acanthaster has
many rays: between 9 and 21. At the tip of the rays light-sensory organs and
chemoreceptors are situated. With the aid of its long ambulacral legs the
star moves very swiftly, which is unusual in other asteroids - up to 20 m h -1 ,
and may migrate several kilometers a month (Chesher 1969; Endean 1973).
When attacking the coral head, the star crawls onto it with the aid of its
rays, hiding the ambulacral legs which are sensitive to the coral's
nematocysts. After fastening its body on the colony the star turns its
stomach inside out. The stomach, as a white foam, engulfs an area of
20-30cm2 on the surface of the coral colony and digests tissues there for
4-5 h. Then the star pulls its stomach in, moves to a neighboring part of the
colony and repeats the procedure. When the number of stars on the reef is
normal (3-4sp. km- 2 ), they behave as nocturnal predators, being active
only at night and hiding in daytime. During the night, the star "processess"
Destruction and Recovery of Reef Systems, Reef and Man
chronic infestation of reefs by this plaque, endangering their very existence
in large reef regions (Chesher 1969). So the attacks by this sea star on reefs
could be considered as an ecocatastrophe on a large scale, triggered mainly
by human activity, which has increased in the postwar period (Kuhlmann
1971; Endean 1974). The alarming descriptions in papers of the awful
pictures preseated by the degradation of formerly beautiful reefs to a state
of rubble heaps covered with ooze, attracted the attention of, government
agencies, the public and researchers to the Acanthaster problem (Endean
1973). Special studies were organized to evaluate the scale of the disabitrous
infestations and to find ways to combat them (Endean and Chesher 1973).
The genus Acanthaster, besides the "crown of thorn" star A. planci,
includes also two more species: A. ellisi and A. brevispinnus. A. ellisi is a
very rare spiny star found on Philippine reefs, which feeds on corals. A.
brevispinnus is detritovore. All three species are genetically very close and
easily form hybrids with each other (Lucas and Nash 1985). A. planci is
supposed to be a species which appeared recently, descended from A.
brevispinnus and acquired the ability for predation on corals. A. ellisi could
be an intermediate form between them on the way to these evolutionary
developments. The Acanthaster stars have very flexible motile spiny rays. By
their use they are able to crawl easily upon colonies of corals and hold onto
them. This lends them an advantage in competition with the other large
corallovore asteroid Culcita, a bulky star though devoid of rays, which is
able to graze only on small coral colonies or crusts. As a consequence, when
the Acanthaster species acquired the ability to digest the protective cetylpalmitate substances, soaking up the coenosarc of scleractinian corals, and
to feed on their tissues, it dropped out of competition and was free of food
limitation (Benson et al. 1978). In spite of this ability, these stars were
among the rare species of large reef invertebrate predators, just like the star
Culcita. The average size of A. planci is 30-35 cm, and some specimens can
be up to 50-60 cm large. The dorsal side of its body is covered by large
poisonous thorns 2-4 cm long. The tissues of the stars is saturated with
saponin, repelling potential predators by the bad taste. Acanthaster has
many rays: between 9 and 21. At the tip of the rays light-sensory organs and
chemoreceptors are situated. With the aid of its long ambulacral legs the
star moves very swiftly, which is unusual in other asteroids - up to 20 m h -1 ,
and may migrate several kilometers a month (Chesher 1969; Endean 1973).
When attacking the coral head, the star crawls onto it with the aid of its
rays, hiding the ambulacral legs which are sensitive to the coral's
nematocysts. After fastening its body on the colony the star turns its
stomach inside out. The stomach, as a white foam, engulfs an area of
20-30cm2 on the surface of the coral colony and digests tissues there for
4-5 h. Then the star pulls its stomach in, moves to a neighboring part of the
colony and repeats the procedure. When the number of stars on the reef is
normal (3-4sp. km- 2 ), they behave as nocturnal predators, being active
only at night and hiding in daytime. During the night, the star "processess"