Molluscs
165
and hydrocoral Millepora, and Magi/us antiguus the corals Goniastrea and
Leptoria (Demond 1957; Gohar and Soliman 1963; Robertson 1970; Ott and
Lewis 1972; Harris 1975). The ovulid pro sob ranch gastropod, Ovula ovum
feeds mainly on alcyonacean octo corals Lobophytum, Sarcophyton and
Sinularia, and the Cyphoma gibbosum on gorgonaceans.
The ability to feed on soft octo corals and on zoantharians is especially
peculiar to the opistobranch predatory gastropods Nudibranchia devoid of
shells. The fauna of the reef nudibranchs is extremely rich and varied
(Ostergaard 1955). Its members live mostly on the surface of soft corals and
sponges, which they also feed on. The soft corals possess strong chemical
defence substances and are very poisonous. The nudibranchs feeding on
them are able to neutralize their poisonous terpenoid toxins, which they
consume together with the food, and even accumulate them in their body,
becoming poisonous, and thus preserving themselves from predation.
Among them there are only some eolid species of Pinufius and Phestilla
genera, which are feeding on scleractinian corals. The eolid Aeolidopsis
rausonii feeds only on the zoantharians Palythoa, this being their specific
ectoparasite. They graze the polyps with the aid of a long proboscis
(Robertson 1980). Some species of opistobranchs from the order Nataspidea
feed on sponges and ascidians using also the proboscis armed with strong
toothed radula. Several species of eolid and glaucid nudibranchs even
learned to use, as secondary symbionts, the zooxanthellae, which they
consume when feeding on hermatypic soft corals. Along their body they
have a line of special accessory sinuses formed by intestines. These sinuses
are filled with the undigested zooxanthellae. During several days these
zooxanthellae remain alive and feed their new host with the translocated
products of photosynthesis. Gradually they are digested and replaced by a
new portion of algae. The same procedure is used by some omnivorous
opistobranchs from the genera Hermaea, Oxynoe and Elysia, employing as
secondary symbionts the nondigested chloroplasts from the seaweed tissues
they consume (Clark and Busacca 1978). The nudibranchs also learn to use
another cellular element of corals - the nematocysts. Swallowing the tissues
of corals they do not digest the larger nematocyst cell spyrocysts, but insert
them into tissues of their own epithelium, employing them for defence, the
same way as they are used by their original host corals (Edmunds 1966;
Robertson 1970).
The nonpoisonous nudibranchs have no shell, but they are well
camouflaged by their coloration and body forms. This makes them actually
undistinguishable from the substratum, e.g., from the branches of colonies
of their host corals. While experimenting with the alcyonacean Lemnalia the
author was astounded at seeing how one of its "branches" started to crawl
by the wall of the aquarium. This "branch" appeared to be a nudibranch,
which was later estimated as a new species. This species is living on
Lemnalia and is absolutely undestinguishable from the branch of this coral.
The poisonous nudibranchs have usually bright coloration as a warning
165
and hydrocoral Millepora, and Magi/us antiguus the corals Goniastrea and
Leptoria (Demond 1957; Gohar and Soliman 1963; Robertson 1970; Ott and
Lewis 1972; Harris 1975). The ovulid pro sob ranch gastropod, Ovula ovum
feeds mainly on alcyonacean octo corals Lobophytum, Sarcophyton and
Sinularia, and the Cyphoma gibbosum on gorgonaceans.
The ability to feed on soft octo corals and on zoantharians is especially
peculiar to the opistobranch predatory gastropods Nudibranchia devoid of
shells. The fauna of the reef nudibranchs is extremely rich and varied
(Ostergaard 1955). Its members live mostly on the surface of soft corals and
sponges, which they also feed on. The soft corals possess strong chemical
defence substances and are very poisonous. The nudibranchs feeding on
them are able to neutralize their poisonous terpenoid toxins, which they
consume together with the food, and even accumulate them in their body,
becoming poisonous, and thus preserving themselves from predation.
Among them there are only some eolid species of Pinufius and Phestilla
genera, which are feeding on scleractinian corals. The eolid Aeolidopsis
rausonii feeds only on the zoantharians Palythoa, this being their specific
ectoparasite. They graze the polyps with the aid of a long proboscis
(Robertson 1980). Some species of opistobranchs from the order Nataspidea
feed on sponges and ascidians using also the proboscis armed with strong
toothed radula. Several species of eolid and glaucid nudibranchs even
learned to use, as secondary symbionts, the zooxanthellae, which they
consume when feeding on hermatypic soft corals. Along their body they
have a line of special accessory sinuses formed by intestines. These sinuses
are filled with the undigested zooxanthellae. During several days these
zooxanthellae remain alive and feed their new host with the translocated
products of photosynthesis. Gradually they are digested and replaced by a
new portion of algae. The same procedure is used by some omnivorous
opistobranchs from the genera Hermaea, Oxynoe and Elysia, employing as
secondary symbionts the nondigested chloroplasts from the seaweed tissues
they consume (Clark and Busacca 1978). The nudibranchs also learn to use
another cellular element of corals - the nematocysts. Swallowing the tissues
of corals they do not digest the larger nematocyst cell spyrocysts, but insert
them into tissues of their own epithelium, employing them for defence, the
same way as they are used by their original host corals (Edmunds 1966;
Robertson 1970).
The nonpoisonous nudibranchs have no shell, but they are well
camouflaged by their coloration and body forms. This makes them actually
undistinguishable from the substratum, e.g., from the branches of colonies
of their host corals. While experimenting with the alcyonacean Lemnalia the
author was astounded at seeing how one of its "branches" started to crawl
by the wall of the aquarium. This "branch" appeared to be a nudibranch,
which was later estimated as a new species. This species is living on
Lemnalia and is absolutely undestinguishable from the branch of this coral.
The poisonous nudibranchs have usually bright coloration as a warning