6
R. H. MILLAR
quantities of sediment, but whether the animals are captured alive and
selectively, or merely engulfed along with the sediment is not known.
The modifications to the oral siphon, which include enlargement and
great muscular development, suggest active capture, but alternatively
they could be adaptations for scooping up large quantities of bottom
deposit (Millar, 1970). Nevertheless, Monniot and Monniot (1968) regard
the structure of Gasterascidia sandersi Monniot and Monniot as adapted,
not only to a predatory habit, but to movement over the substratum,
an ability which may enable this ascidian to capture prey.
One further problem which awaits investigation concerns the feeding
habits of the curious interstitial ascidians described in a series of papers
by C. and F. Monniot (see Monniot, 1966). Some of these species,
which rarely exceed 3 mm in length, move amongst the sand grains of
the substratum, and presumably feed on organic particles or organisms
in the interstitial water. The branchial structure, although modified, is
essentially similar to that of filter-feeding ascidians.
111. BREEDING
Reproduction in ascidians takes three forms : asexual reproduction
by budding, which occurs only in some families ; fission of colonies, a
process recorded in few species ; and sexual reproduction or breeding,
which occurs universally throughout the group.
A. Breeding season
A number of methods can be used to investigate the timing and
duration of the breeding season.
(i) Macroscopic or microscopic examination of specimens collected
at intervals throughout the year shows the cyclic activity of ripening,
filling and emptying of the gonads, and in certain species or populations
only the presence of spent gonads in the samples indicates the onset of
spawning (Millar, 1964a, 1960; Diehl, 1957) ; in other cases information
of this kind complements studies by more direct methods. Establishing
the period when animals have full gonads with ripe gametes does not,
however, do more than define the period within which breeding may
occur, given the appropriate stimulus, and species are known which
have full gonads throughout the year, but which only breed successfully
(judged by the settlement of larvae) in a more restricted period (Raja,
1963). Some workers have used artificial fertilization or spontaneous
spawning of animals brought into the laboratory at intervals, to provide
a criterion of ripeness (Hirai and Tsubata, 1956; McDougall, 1943;
Levine, 1962).
R. H. MILLAR
quantities of sediment, but whether the animals are captured alive and
selectively, or merely engulfed along with the sediment is not known.
The modifications to the oral siphon, which include enlargement and
great muscular development, suggest active capture, but alternatively
they could be adaptations for scooping up large quantities of bottom
deposit (Millar, 1970). Nevertheless, Monniot and Monniot (1968) regard
the structure of Gasterascidia sandersi Monniot and Monniot as adapted,
not only to a predatory habit, but to movement over the substratum,
an ability which may enable this ascidian to capture prey.
One further problem which awaits investigation concerns the feeding
habits of the curious interstitial ascidians described in a series of papers
by C. and F. Monniot (see Monniot, 1966). Some of these species,
which rarely exceed 3 mm in length, move amongst the sand grains of
the substratum, and presumably feed on organic particles or organisms
in the interstitial water. The branchial structure, although modified, is
essentially similar to that of filter-feeding ascidians.
111. BREEDING
Reproduction in ascidians takes three forms : asexual reproduction
by budding, which occurs only in some families ; fission of colonies, a
process recorded in few species ; and sexual reproduction or breeding,
which occurs universally throughout the group.
A. Breeding season
A number of methods can be used to investigate the timing and
duration of the breeding season.
(i) Macroscopic or microscopic examination of specimens collected
at intervals throughout the year shows the cyclic activity of ripening,
filling and emptying of the gonads, and in certain species or populations
only the presence of spent gonads in the samples indicates the onset of
spawning (Millar, 1964a, 1960; Diehl, 1957) ; in other cases information
of this kind complements studies by more direct methods. Establishing
the period when animals have full gonads with ripe gametes does not,
however, do more than define the period within which breeding may
occur, given the appropriate stimulus, and species are known which
have full gonads throughout the year, but which only breed successfully
(judged by the settlement of larvae) in a more restricted period (Raja,
1963). Some workers have used artificial fertilization or spontaneous
spawning of animals brought into the laboratory at intervals, to provide
a criterion of ripeness (Hirai and Tsubata, 1956; McDougall, 1943;
Levine, 1962).