48
R. H. MILLAR
A. Predators
Predators are often recognized as such by the remains of their prey
found during the examination of stomach contents. Only a few
ascidians, however, have parts-the calcareous spicules and in some
the test-which sufficiently withstand digestion to be identified. It is
not surprising, then, that there are few records of ascidians forming
part of the diet of commercial fish, and it may be largely for this reason
that they are commonly regarded as distasteful. Thompson (1930,
1931) stated that ascidians are not of much general importance in the
diet of fish, although Ascidiella scabra (Muller) sometimes is the chief
item of food of the haddock in areas where both species are common.
Rae (1956, 1967) also records the occasional presence of ascidians in the
stomach of cod and lemon sole, the latter taking both solitary and
colonial forms. Some West Indian fish frequenting coral reefs eat
" tunicates '' (presumably ascidinns), which constitute 10-28% of the
stomach contents according to the species of fish (Randall and Hartman,
1968).
Amongst animals with a very varied diet, crabs are known to take
ascidians. Bancroft (1903) found that colonies of Botryllus schlosseri in
aquarium tanks at Naples were eagerly attacked by crabs, but Botrylloides leachi was soon rejected and the crabs quickly recognized it as
distasteful. The relatively constant appearance of Botrylloides leachi
may help would-be predators in learning t o avoid the species ; the colour
pattern of Botryllus schlosseri by comparison is very variable. Beaven
(1956) described how a single Blue Crab (Callinectes sapidus) accidentally
confined in a cage along with young oysters and Molgula mnhattensis
apparently ate all the ascidians in preference to the oysters. The conditions were, admittedly, unnatural as they are in most feeding experiments, but the observation at least indicates that ascidians may be
taken in nature on quite a large scale.
According to Goodbody (1963a), polychaetes probably fed on young
specimens of Ascidia nigra which he had under observation. Starfish
have rarely been recorded as predators of ascidians, but are known to be
the chief enemies of Pyura chilensis Molina (Gutihez and Lay, 1965).
It is amongst the molluscs that some of the main enemies are to be
found, or at least they are more frequently recorded, but in part this
results from their being less active, spending more time in attacking
their prey, and being therefore more readily observed. Of the prosobranch gastropods Erato voluta (Montagu) feeds on Botryllus schlosseri
and Botrylloides leachi, inserting its proboscis through the oral siphon
of the zooid (Fig. 19) (Fretter, 1951), and several other species of the
R. H. MILLAR
A. Predators
Predators are often recognized as such by the remains of their prey
found during the examination of stomach contents. Only a few
ascidians, however, have parts-the calcareous spicules and in some
the test-which sufficiently withstand digestion to be identified. It is
not surprising, then, that there are few records of ascidians forming
part of the diet of commercial fish, and it may be largely for this reason
that they are commonly regarded as distasteful. Thompson (1930,
1931) stated that ascidians are not of much general importance in the
diet of fish, although Ascidiella scabra (Muller) sometimes is the chief
item of food of the haddock in areas where both species are common.
Rae (1956, 1967) also records the occasional presence of ascidians in the
stomach of cod and lemon sole, the latter taking both solitary and
colonial forms. Some West Indian fish frequenting coral reefs eat
" tunicates '' (presumably ascidinns), which constitute 10-28% of the
stomach contents according to the species of fish (Randall and Hartman,
1968).
Amongst animals with a very varied diet, crabs are known to take
ascidians. Bancroft (1903) found that colonies of Botryllus schlosseri in
aquarium tanks at Naples were eagerly attacked by crabs, but Botrylloides leachi was soon rejected and the crabs quickly recognized it as
distasteful. The relatively constant appearance of Botrylloides leachi
may help would-be predators in learning t o avoid the species ; the colour
pattern of Botryllus schlosseri by comparison is very variable. Beaven
(1956) described how a single Blue Crab (Callinectes sapidus) accidentally
confined in a cage along with young oysters and Molgula mnhattensis
apparently ate all the ascidians in preference to the oysters. The conditions were, admittedly, unnatural as they are in most feeding experiments, but the observation at least indicates that ascidians may be
taken in nature on quite a large scale.
According to Goodbody (1963a), polychaetes probably fed on young
specimens of Ascidia nigra which he had under observation. Starfish
have rarely been recorded as predators of ascidians, but are known to be
the chief enemies of Pyura chilensis Molina (Gutihez and Lay, 1965).
It is amongst the molluscs that some of the main enemies are to be
found, or at least they are more frequently recorded, but in part this
results from their being less active, spending more time in attacking
their prey, and being therefore more readily observed. Of the prosobranch gastropods Erato voluta (Montagu) feeds on Botryllus schlosseri
and Botrylloides leachi, inserting its proboscis through the oral siphon
of the zooid (Fig. 19) (Fretter, 1951), and several other species of the