292
P.W. Glynn
1982-1983 ENSO disturbance. Other common herbivores are damselfish,
which can effect coral zonation by protecting corals from grazers and
corallivores. Damselfish typically cultivate turf algae, which they defend
in patches roughly 25-50 em in diameter. In the Pearl Islands, Panama,
Stegastes acapulcoensis (Fowler) establishes territories in shallow reef
areas (1-3m depth) where it finds shelter in stands of Pocillopora spp.
(Wellington 1982). In such areas, Pocillopora survivorship is enhanced by
the damselfish, which exclude potential grazers and corallivores. As the
damselfish expand their territories they selectively bite and kill massive
coral species, such as Pavona gigantea, thus promoting the survival and
growth of Pocillopora. In deeper (4-8m) reef areas of low topographic
complexity, damselfish abundance declines and massive corals are favored.
Stegastes acapulcoensis, therefore, plays a pivotal role in the maintenance
of coral zonation, favoring the presence of branching species at shallow
depths and massive species in deeper waters.
Numerous corallivores are found on eastern Pacific reefs. About 20 species of mollusks, crustaceans, a sea star, sea urchins and fishes are known to
feed on live corals. ]enneria pustulata (Lightfoot), an ovulid gastropod, is
the most important mollusk predator. It is commonly found grazing on the
soft tissues of Pocillopora spp., leaving behind a bone-white, unblemished
skeleton. Usually just a few individuals graze on coral colonies, resulting in
only partial mortality. Occasionally large aggregations of ]enneria, up to
100 individuals, will attack and completely denude a large coral colony. Two
species of hermit crabs, Trizopagurus magnificus (Bouvier) and Aniculus
elegans (Stimpson), feed effectively on pocilloporid corals by scraping
branches with their chelipeds, thereby removing both soft tissues and skeletal fragments. While T. magnificus can attain high mean population densities of 28 in d. m -z on some reefs in Panama, they have not been seen to cause
the death of whole coral colonies. Earlier regarded as micropredators or parasites, xanthid crabs in the genus Trapezia (three species) and a shrimp in
the genus Alpheus (one species) are now regarded as mutualistic symbionts
of pocilloporid corals. These crustaceans are obligate symbionts, taking up
shelter among the coral host's branches where they feed on mucus, entrapped detritus and plankton, and perhaps occasionally on coral tissues
themselves. Recent studies have demonstrated that the movements of the
crabs around the basal branches of corals have a beneficial effect in promoting increased circulation and a higher survivorship of polyps in potentially
stagnant parts of the colony. Additionally, both crabs and shrimp symbionts
vigorously defend their coral hosts from attack by the crown-of-thorns
sea star Acanthaster planci. Stimson ( 1990) has shown that Hawaiian corals
hosting crabs are stimulated by their guests to produce fat-bodies, lipidfilled globules that serve as a supplemental energy-rich food source.
P.W. Glynn
1982-1983 ENSO disturbance. Other common herbivores are damselfish,
which can effect coral zonation by protecting corals from grazers and
corallivores. Damselfish typically cultivate turf algae, which they defend
in patches roughly 25-50 em in diameter. In the Pearl Islands, Panama,
Stegastes acapulcoensis (Fowler) establishes territories in shallow reef
areas (1-3m depth) where it finds shelter in stands of Pocillopora spp.
(Wellington 1982). In such areas, Pocillopora survivorship is enhanced by
the damselfish, which exclude potential grazers and corallivores. As the
damselfish expand their territories they selectively bite and kill massive
coral species, such as Pavona gigantea, thus promoting the survival and
growth of Pocillopora. In deeper (4-8m) reef areas of low topographic
complexity, damselfish abundance declines and massive corals are favored.
Stegastes acapulcoensis, therefore, plays a pivotal role in the maintenance
of coral zonation, favoring the presence of branching species at shallow
depths and massive species in deeper waters.
Numerous corallivores are found on eastern Pacific reefs. About 20 species of mollusks, crustaceans, a sea star, sea urchins and fishes are known to
feed on live corals. ]enneria pustulata (Lightfoot), an ovulid gastropod, is
the most important mollusk predator. It is commonly found grazing on the
soft tissues of Pocillopora spp., leaving behind a bone-white, unblemished
skeleton. Usually just a few individuals graze on coral colonies, resulting in
only partial mortality. Occasionally large aggregations of ]enneria, up to
100 individuals, will attack and completely denude a large coral colony. Two
species of hermit crabs, Trizopagurus magnificus (Bouvier) and Aniculus
elegans (Stimpson), feed effectively on pocilloporid corals by scraping
branches with their chelipeds, thereby removing both soft tissues and skeletal fragments. While T. magnificus can attain high mean population densities of 28 in d. m -z on some reefs in Panama, they have not been seen to cause
the death of whole coral colonies. Earlier regarded as micropredators or parasites, xanthid crabs in the genus Trapezia (three species) and a shrimp in
the genus Alpheus (one species) are now regarded as mutualistic symbionts
of pocilloporid corals. These crustaceans are obligate symbionts, taking up
shelter among the coral host's branches where they feed on mucus, entrapped detritus and plankton, and perhaps occasionally on coral tissues
themselves. Recent studies have demonstrated that the movements of the
crabs around the basal branches of corals have a beneficial effect in promoting increased circulation and a higher survivorship of polyps in potentially
stagnant parts of the colony. Additionally, both crabs and shrimp symbionts
vigorously defend their coral hosts from attack by the crown-of-thorns
sea star Acanthaster planci. Stimson ( 1990) has shown that Hawaiian corals
hosting crabs are stimulated by their guests to produce fat-bodies, lipidfilled globules that serve as a supplemental energy-rich food source.