Digestive system, feeding and ecology of some New Zealand intertidal ascidians

Abstract

Some 130 species of ascidians have been recorded from New Zealand waters (Croxall, 1972 in press). A number of these are known only from off-shore collections, but about 70% have been recorded intertidally. Although they are perhaps most abundant immediately below low water, as members of what is termed (on wharves, buoys and ships) the fouling community, they are also important members of associations of sessile, filter-feeding animals existing around the low tide mark on all reasonably sheltered hard shores.

In these situations the number of ascidian species, both solitary and compound, may be particularly striking. Two other important elements, the sponges and bryzoa, usually show a similar variety. The likely similarity of the food in all these groups and, at least in ascidians, the relative uniformity of structure and pattern, raises some obvious problems. Why do so many kinds of ascidian occur commonly in an association along with many other sessile filter feeders? Why are there not one or two species in place of the ten or more solitary and compound ascidians usually common in any location of this type? It was in an attempt to provide information bearing upon these questions that this investigation was undertaken.