116
Plankton in Coral-Reef Waters
The swarms form not only endemics of reefs such as mysids, but also the
typical pelagic crustaceans living also in the open sea, such as the abovementioned Oithona and Acartia (Emery 1968; von Hentig 1971; Hamner
and Carleton 1979). For example, the typical pelagic copepods Acartia
spinosa, A. tonsa and Oithona nana, randomly distributed when living in
open sea, in reef environment form during the day dense swarms near the
edge of the thickets of sea-grass or in the holes amid them. Their swarms
could contain several thousands of specimens (von Hentig 1971). The
behavior of these swarms resembles that of the typical demersal reef
endemics, such as mysids or pelagic planktonovorous fish. The swarm of
cope pods behave as a single body, opposing the currents. When dispersed
by a diver, the swarm soon collects again in one place. The swarms were
observed to collect each day near the same shelters at the bottom. The
copepod swarms were observed in caves, perforating the patch reefs and the
outer reef slopes. Acartia spinata formed sometimes a large (2 m in
diameter) dense swarms between the patch reefs over piles of rubble and
also on outer slopes in grooves and caves of the buttress zone. Up the outer
slope there were observed also schools of some other copepod species like
Oithona oculata and the harpacticoid Metis sp. These pelagic copepods
behave in schools like typical shoaling animals. At night, their swarms
dispersed but in the morning they re-formed the schools at the same place.
Thus, these typical pelagic copepods, when living in reef waters, adopt the
behavior features peculiar to the endemic reef residents like mysids or the
other demersal zooplankters.
The reef mysids usually form small schools close to their shelters - holes
or crevices. The mysid Mysidium gracile is shoaling at daytime near the
aggregations of the urchin Diadema, which has long poisonous needles. On
being endangered, the school will hide instantly inside the thickets of
needles. Some mysid shoals were observed to keep also near the holes, in
which the pomacentrid fish live, and hiding in them when endangered. The
host fish did not hunt the mysids but attacked the intruder (Emery 1968).
The reasons of migrations of the demersal zooplankton might be different
in different taxa. Most of them migrate at night from the bottom to the
water column for feeding since there they are less endangered by the pelagic
predators. Another reason for their emergence from the bottom at night,
which may be not less important, is to avoid the grazing of bottom predators - molluscs, crustaceans, polychaetes, most of which feed at night, as
well as many benthic fish. For demersal predatory zooplankteJ;s, such as the
demersal mysids, cumaceans, cyclopoids and harpacticoids, the reason
for migrating at night up to the water column could be the hunting for
holoplankters and other demersal plankters, which are abundant at night
coming out from their shelters. Some demersal species go up to the water
column only for propagation, like some amphipods and a variety of polychaetes (Allen 1957). The benthoplanktonic young gammarids periodically
go to the water column from the bottom and swim to occupy new bottom
Plankton in Coral-Reef Waters
The swarms form not only endemics of reefs such as mysids, but also the
typical pelagic crustaceans living also in the open sea, such as the abovementioned Oithona and Acartia (Emery 1968; von Hentig 1971; Hamner
and Carleton 1979). For example, the typical pelagic copepods Acartia
spinosa, A. tonsa and Oithona nana, randomly distributed when living in
open sea, in reef environment form during the day dense swarms near the
edge of the thickets of sea-grass or in the holes amid them. Their swarms
could contain several thousands of specimens (von Hentig 1971). The
behavior of these swarms resembles that of the typical demersal reef
endemics, such as mysids or pelagic planktonovorous fish. The swarm of
cope pods behave as a single body, opposing the currents. When dispersed
by a diver, the swarm soon collects again in one place. The swarms were
observed to collect each day near the same shelters at the bottom. The
copepod swarms were observed in caves, perforating the patch reefs and the
outer reef slopes. Acartia spinata formed sometimes a large (2 m in
diameter) dense swarms between the patch reefs over piles of rubble and
also on outer slopes in grooves and caves of the buttress zone. Up the outer
slope there were observed also schools of some other copepod species like
Oithona oculata and the harpacticoid Metis sp. These pelagic copepods
behave in schools like typical shoaling animals. At night, their swarms
dispersed but in the morning they re-formed the schools at the same place.
Thus, these typical pelagic copepods, when living in reef waters, adopt the
behavior features peculiar to the endemic reef residents like mysids or the
other demersal zooplankters.
The reef mysids usually form small schools close to their shelters - holes
or crevices. The mysid Mysidium gracile is shoaling at daytime near the
aggregations of the urchin Diadema, which has long poisonous needles. On
being endangered, the school will hide instantly inside the thickets of
needles. Some mysid shoals were observed to keep also near the holes, in
which the pomacentrid fish live, and hiding in them when endangered. The
host fish did not hunt the mysids but attacked the intruder (Emery 1968).
The reasons of migrations of the demersal zooplankton might be different
in different taxa. Most of them migrate at night from the bottom to the
water column for feeding since there they are less endangered by the pelagic
predators. Another reason for their emergence from the bottom at night,
which may be not less important, is to avoid the grazing of bottom predators - molluscs, crustaceans, polychaetes, most of which feed at night, as
well as many benthic fish. For demersal predatory zooplankteJ;s, such as the
demersal mysids, cumaceans, cyclopoids and harpacticoids, the reason
for migrating at night up to the water column could be the hunting for
holoplankters and other demersal plankters, which are abundant at night
coming out from their shelters. Some demersal species go up to the water
column only for propagation, like some amphipods and a variety of polychaetes (Allen 1957). The benthoplanktonic young gammarids periodically
go to the water column from the bottom and swim to occupy new bottom