Heterotrophic Feeding of Corals
359
symbiotic genera of the families Alcyonacea (Lobophytum, Sarcophyton,
Alcyonium, Lithophyton, Sinilaria, Xenia), of Gorgonacea (Hicksoniella,
Plexaura, Pseudopterogorgia, Isis, Rumphella) , and of Stolonifera
(Tubipora). The first attempts to investigate the food ingested by polyps of
asymbiotic temperate-water species of Alcyonium - A. digitatum - by
microscopic analysis of their gut contents shewed that it fed on small
crustaceans, but much less efficiently when compared with scleractinian
corals. Most of their polyps did not contain any traces of food rests (Pratt
1906). Subsequent studies have proved the opinion that the octocorals are
very poor predators (Gohar 1940, 1948; Kinzie 1970). In Xenia for example,
Gohar did not find any signs of predatory feeding at all. The guts of Xenia
were permanently empty. This alcyonacean did not even display any "food
reaction" in the presence of food, which is a most important feature of
feeding behavior in Scleractinia (see above). As for the nutrition of another
symbiotic octocoral, Clavularia, its polyps extruded periodically an excess of
zooxanthellae into their gastral cavity, where these were digested and
assimilated.
The conclusion by Gohar (1948) was that the symbiotic alcyonaceans did
not need any predation or other sources of heterotrophic food, being
supplied with the necessary nutrition by their algal symbionts. This
conclusion was then supported by Goreau (1961) and Goreau et al. (1971).
But there still remained the reasonable question: why have these
alcyonaceans refused during evolution to use the sources of heterotrophic
feeding, with the aid of which other corals got not only energy, but, what is
maybe even more important, nutrients. Further research should supply the
answer. But even recent studies conducted in the 1970s, produced again
only somewhat indefinite results. The analyses of gut contents in polyps of
Alcyonium diditatum confirmed early data about their ability to catch small
slowly moving zooplankters, like young Oicopleura, larvaceans of ascidians
and foraminifera (Sebens and Koehl 1984). In the guts of gorgonacean's
polyps the veligers of Mollusca have been found (Grigg 1972). According to
Kinzie (1970), most of the Atlantic gorgonians investigated did not readily
feed on zooplankton, while in polyps of Eunicella clavigera he found
swallowed planktonic crustaceans. The gorgonian Leptogorgia was observed
catching the nauplii of artemia (Leversee 1976) and the Pseudoplexaura the
pieces of rat liver (Murdock 1978). Nevertheless, most authors were inclined
to accept the view that octocorals in general are weak predators and may
ingest preferably passively floating particles of detritus of pseudoplankton,
as well as foraminifera, eggs or slowly moving larvae of invertebrates. In
experiments, the polyps of gorgonians caught the sephadex beads soaked
with broth, or the cysts of artemia, but hardly caught the artemia nauplii.
Even those which had been caught and even swallowed, most often were
then again released by the polyps (Mariscal and Bigger 1977; Lasker 1981;
Lasker et al. 1983). According to Roushdy and Hansen (1961), some
alcyonaceans were able to feed on the phytoplankton. The alcyonacean
359
symbiotic genera of the families Alcyonacea (Lobophytum, Sarcophyton,
Alcyonium, Lithophyton, Sinilaria, Xenia), of Gorgonacea (Hicksoniella,
Plexaura, Pseudopterogorgia, Isis, Rumphella) , and of Stolonifera
(Tubipora). The first attempts to investigate the food ingested by polyps of
asymbiotic temperate-water species of Alcyonium - A. digitatum - by
microscopic analysis of their gut contents shewed that it fed on small
crustaceans, but much less efficiently when compared with scleractinian
corals. Most of their polyps did not contain any traces of food rests (Pratt
1906). Subsequent studies have proved the opinion that the octocorals are
very poor predators (Gohar 1940, 1948; Kinzie 1970). In Xenia for example,
Gohar did not find any signs of predatory feeding at all. The guts of Xenia
were permanently empty. This alcyonacean did not even display any "food
reaction" in the presence of food, which is a most important feature of
feeding behavior in Scleractinia (see above). As for the nutrition of another
symbiotic octocoral, Clavularia, its polyps extruded periodically an excess of
zooxanthellae into their gastral cavity, where these were digested and
assimilated.
The conclusion by Gohar (1948) was that the symbiotic alcyonaceans did
not need any predation or other sources of heterotrophic food, being
supplied with the necessary nutrition by their algal symbionts. This
conclusion was then supported by Goreau (1961) and Goreau et al. (1971).
But there still remained the reasonable question: why have these
alcyonaceans refused during evolution to use the sources of heterotrophic
feeding, with the aid of which other corals got not only energy, but, what is
maybe even more important, nutrients. Further research should supply the
answer. But even recent studies conducted in the 1970s, produced again
only somewhat indefinite results. The analyses of gut contents in polyps of
Alcyonium diditatum confirmed early data about their ability to catch small
slowly moving zooplankters, like young Oicopleura, larvaceans of ascidians
and foraminifera (Sebens and Koehl 1984). In the guts of gorgonacean's
polyps the veligers of Mollusca have been found (Grigg 1972). According to
Kinzie (1970), most of the Atlantic gorgonians investigated did not readily
feed on zooplankton, while in polyps of Eunicella clavigera he found
swallowed planktonic crustaceans. The gorgonian Leptogorgia was observed
catching the nauplii of artemia (Leversee 1976) and the Pseudoplexaura the
pieces of rat liver (Murdock 1978). Nevertheless, most authors were inclined
to accept the view that octocorals in general are weak predators and may
ingest preferably passively floating particles of detritus of pseudoplankton,
as well as foraminifera, eggs or slowly moving larvae of invertebrates. In
experiments, the polyps of gorgonians caught the sephadex beads soaked
with broth, or the cysts of artemia, but hardly caught the artemia nauplii.
Even those which had been caught and even swallowed, most often were
then again released by the polyps (Mariscal and Bigger 1977; Lasker 1981;
Lasker et al. 1983). According to Roushdy and Hansen (1961), some
alcyonaceans were able to feed on the phytoplankton. The alcyonacean