60
R. € I .
MrLLAR
serting its pharynx within the colony, extracts whole zooids which pass
intact into the gut (Jennings, 1957). Where the prey is larger, as in the
solitary species Corella willmeriana Herdman, the flatworm Eurylepta
leoparda Frceman enters the branchial sac before attacking the tissues,
rolling itself into a tubular shape to pass through the oral siphon
(Lanibert, 1968) ; after 3-7 days only the test remains. Experiments
made by Lambert suggest that this flatworm feeds only on Corellu
willmeriana and refuses other solitary and colonial species. This recalls
earlier work by Crozier ( 19 17), who found that specimens of Pseudoceros
crozieri would feed only on the species of ascidian with which they had
previously been associated.
Finally, amongst the mammals, the Grey Seal has been noted to
take ascidians (Rae, 1968), although these probably form a very small
part of a varied diet. Man can also be included amongst the predators
since some of the larger ascidians are collected for food in the Mediterranean and Chile (Harant, 1951) and in Japan (Tokioka, 1953).
If an ascidian is common and large it is sometimes used for bait, as
in the case of Pyura praeputialis (Heller) in Australia (Hedley, 1915))
where local populations may be depleted by fishermen (Pope and
McDonald, 1968). In South Africa a few species also serve as bait
(Harant, 1951). Apart from such deliberate gathering, large quantities
of ascidians must be destroyed accidentally in the course of commercial
fishing, and where they are regarded as pests, as on oyster beds, they
may be systematically dredged and destroyed (Cole, 1956).
3. Commensals, parasites and symbionts
It is generally a fairly simple matter to decide if one animal preys
upon another, but other kinds of association between species-phoresis,
commensalism, mutualism and parasitism-are often difficult to distinguish in practice. No doubt most cases of such association could be
classified if the physiology of the partners had been studied, but such
studies involving ascidians are rare.
Many instances of commensalism are known in which ascidians act
as the hosts, and their suitability is doubtless due to the large branchial
and atrial cavities, the filter-feeding habit which brings continuous
supplies of food and water, the relative ease of entry to the cavities,
and the absence of effective defensive mechanisms.
The Crustacea contain some of the best known commensals, of
which the notodelphyid copepods are particularly widespread. Species
of this family are almost exclusively associated with ascidians. Schellenberg (1922) listed 89 species, and others have since been added (Illg,
1958 ; Illg and Dudley, 1961, 1965). Little is known of the effect of these
R. € I .
MrLLAR
serting its pharynx within the colony, extracts whole zooids which pass
intact into the gut (Jennings, 1957). Where the prey is larger, as in the
solitary species Corella willmeriana Herdman, the flatworm Eurylepta
leoparda Frceman enters the branchial sac before attacking the tissues,
rolling itself into a tubular shape to pass through the oral siphon
(Lanibert, 1968) ; after 3-7 days only the test remains. Experiments
made by Lambert suggest that this flatworm feeds only on Corellu
willmeriana and refuses other solitary and colonial species. This recalls
earlier work by Crozier ( 19 17), who found that specimens of Pseudoceros
crozieri would feed only on the species of ascidian with which they had
previously been associated.
Finally, amongst the mammals, the Grey Seal has been noted to
take ascidians (Rae, 1968), although these probably form a very small
part of a varied diet. Man can also be included amongst the predators
since some of the larger ascidians are collected for food in the Mediterranean and Chile (Harant, 1951) and in Japan (Tokioka, 1953).
If an ascidian is common and large it is sometimes used for bait, as
in the case of Pyura praeputialis (Heller) in Australia (Hedley, 1915))
where local populations may be depleted by fishermen (Pope and
McDonald, 1968). In South Africa a few species also serve as bait
(Harant, 1951). Apart from such deliberate gathering, large quantities
of ascidians must be destroyed accidentally in the course of commercial
fishing, and where they are regarded as pests, as on oyster beds, they
may be systematically dredged and destroyed (Cole, 1956).
3. Commensals, parasites and symbionts
It is generally a fairly simple matter to decide if one animal preys
upon another, but other kinds of association between species-phoresis,
commensalism, mutualism and parasitism-are often difficult to distinguish in practice. No doubt most cases of such association could be
classified if the physiology of the partners had been studied, but such
studies involving ascidians are rare.
Many instances of commensalism are known in which ascidians act
as the hosts, and their suitability is doubtless due to the large branchial
and atrial cavities, the filter-feeding habit which brings continuous
supplies of food and water, the relative ease of entry to the cavities,
and the absence of effective defensive mechanisms.
The Crustacea contain some of the best known commensals, of
which the notodelphyid copepods are particularly widespread. Species
of this family are almost exclusively associated with ascidians. Schellenberg (1922) listed 89 species, and others have since been added (Illg,
1958 ; Illg and Dudley, 1961, 1965). Little is known of the effect of these