TEE BIOLOGY OF ASCIDIANS
6
1960). In other species the gut contents vary, perhaps according to the
local water conditions. Kott (1952, 1964) and Millar (1955a, 1963,
1966a) noted large quantities of mud in the gut of Ascidia sydneiensis
Stimpson, but the same species may contain algal cells, diatoms and
peridineans, with little inorganic matter (Millar, 1960).
Subtle differences appear to exist in the food of related species living
in the same area, as with Ascidia nigra (Savigny) and A . interrupta
Heller (Goodbddy, 1966). In this case, differences in the arrangement
of the oral tentacles may be responsible, although there might be some
variation in the food content of the water since the species occupy
somewhat different ecological niches.
Little experimental work has been done on the nature and quantity
of food required by ascidians, but Milkman (1967) maintained cultures
of Botryllus schlosseri (Pallas) in sea water containing the centric
diatom Cyclotellanana Hustedt at a concentration of 1-2 x 106 cells/ml.
Apart from filter-feeders which accept a wide range of material
suspended in the surrounding water, there is an ecological group of
ascidians which appear habitually to take in bottom deposits. Amongst
shallow-water species Styela coriacea Alder and Hancock is apparently
a deposit-feeder (Diehl, 1957), but it is the small-bodied deep water
species living on a soft muddy substratum which have most commonly
developed this habit. The gut contents of these animals are similar to
the surrounding sediment, and the small size of the body allows the oral
siphon to draw in sediment from the loose interface of the substratum
and water (Millar, 1970). In these animals the gut was found to contain,
in addition to inorganic material, small brown “cells ” and many
bacteria. Those abyssal ascidians with a long stalk, such as Culeolus
spp. may, however, live with the oral siphon some distance above the
sediment, and their gut contents have been found to lack the bottom
deposits common in sessile forms (Millar, 1959a).
Although ascidians probably originated in shallow seas with a rich
plankton and in consequence evolved the filter-feeding mechanism
which most of them still possess, a number of species penetrated into
deep water. Of these, a very few have abandoned the original feeding
mechanism in favour of a quite different kind, adapted to taking larger
organisms and bottom material. In Octacnemus Moseley, Hexacrobylus
Sluiter and Gasterascidia Monniot and Monniot the perforated branchial
aac is replaced by an unperforated tube or sac which is obviously incapable of filtering particles from a current of water. Instead, relatively
large animals such as ostracods, nematodes, copepods and other
crustaceans are taken (Ritter, 1906; Madsen, 1947; Millar, 1959a, 1970;
Monniot and Monniot, 1968). These occur in the gut together with
6
1960). In other species the gut contents vary, perhaps according to the
local water conditions. Kott (1952, 1964) and Millar (1955a, 1963,
1966a) noted large quantities of mud in the gut of Ascidia sydneiensis
Stimpson, but the same species may contain algal cells, diatoms and
peridineans, with little inorganic matter (Millar, 1960).
Subtle differences appear to exist in the food of related species living
in the same area, as with Ascidia nigra (Savigny) and A . interrupta
Heller (Goodbddy, 1966). In this case, differences in the arrangement
of the oral tentacles may be responsible, although there might be some
variation in the food content of the water since the species occupy
somewhat different ecological niches.
Little experimental work has been done on the nature and quantity
of food required by ascidians, but Milkman (1967) maintained cultures
of Botryllus schlosseri (Pallas) in sea water containing the centric
diatom Cyclotellanana Hustedt at a concentration of 1-2 x 106 cells/ml.
Apart from filter-feeders which accept a wide range of material
suspended in the surrounding water, there is an ecological group of
ascidians which appear habitually to take in bottom deposits. Amongst
shallow-water species Styela coriacea Alder and Hancock is apparently
a deposit-feeder (Diehl, 1957), but it is the small-bodied deep water
species living on a soft muddy substratum which have most commonly
developed this habit. The gut contents of these animals are similar to
the surrounding sediment, and the small size of the body allows the oral
siphon to draw in sediment from the loose interface of the substratum
and water (Millar, 1970). In these animals the gut was found to contain,
in addition to inorganic material, small brown “cells ” and many
bacteria. Those abyssal ascidians with a long stalk, such as Culeolus
spp. may, however, live with the oral siphon some distance above the
sediment, and their gut contents have been found to lack the bottom
deposits common in sessile forms (Millar, 1959a).
Although ascidians probably originated in shallow seas with a rich
plankton and in consequence evolved the filter-feeding mechanism
which most of them still possess, a number of species penetrated into
deep water. Of these, a very few have abandoned the original feeding
mechanism in favour of a quite different kind, adapted to taking larger
organisms and bottom material. In Octacnemus Moseley, Hexacrobylus
Sluiter and Gasterascidia Monniot and Monniot the perforated branchial
aac is replaced by an unperforated tube or sac which is obviously incapable of filtering particles from a current of water. Instead, relatively
large animals such as ostracods, nematodes, copepods and other
crustaceans are taken (Ritter, 1906; Madsen, 1947; Millar, 1959a, 1970;
Monniot and Monniot, 1968). These occur in the gut together with