THE BIOLOQY OF ASCIDIANS
79
If we can rely on these few cases where it is reasonably sure that we
are dealing with one species or at most a few closely related species,
there is less evidence to support the view that abyssal ascidians conform to the zoogeographic divisions proposed by Ekman and by
Vinogradova. Rather they appear to be widely distributed in the
abyssal regions and a t most tend to have either an equatorial distribution or t o occur in higher latitudes of both the northern and
southern hemispheres. There is therefore conflict between these conclusions based on the one hand on all recorded species and on the other
on aberrant or very distinctive species. Only if these groups of distinctive ascidians proved t o consist of separate, although similar,
species, would the contradiction be removed, with the conclusion that
a considerable degree of specific endemism exists.
The distribution of abyssal ascidians as a whole is by no mcanR
uniform, even within a single region, as Sokolska (1969) shows for the
Pacific Ocean. An analysis of all occurrences in the abyssal Pacific
recorded by the main deep-sea expeditions shows the ascidians to be
much more restricted than, for example sponges, which might be expected to have rather similar requirements.
VIII. ECONOMIC IMPORTANCE
A. Fouling
In comparison with some other groups of marine animals thc
ascidians cannot be said to have great economic importance. As fouling
organisms, however, they are significant and contribute largely t o the
problem of growth on the hulls of ships, on buoys and floating structures and on fixed harbour installations. The list of over 100 ascidian
species in “ Marine Fouling and its Prevention ” (Woods Hole Oceanographic Institute, 1952) indicates their potential importance, although
many of the species recorded are not sufficiently abundant to be harmful. Almost any shallow-water species in which the adult is normally
attached to a firm substratum may appear in a fouling community.
Ascidians have been studied as they occur on ships’ hulls (Berner,
1944 ; Skerman, 1960), but more often workers have used test panels or
blocks which are examined periodically to follow the course of fouling
(Weiss, 1948 ; Sentz-Braconnot, 1966 ; Relini, 1964 ; Stubbings and
Houghton, 1964 ; Allen and Wood, 195.0 ; Elroi and Komarovsky, 1961 ;
Kawahara, 1962; Nair, 1962 ; Raja, 1963 ; Skernian, 1959). Whatever
methods of asscssment are used, it is evident that ascidians form a
major part of the fouling community in many regions of the world. Of
79
If we can rely on these few cases where it is reasonably sure that we
are dealing with one species or at most a few closely related species,
there is less evidence to support the view that abyssal ascidians conform to the zoogeographic divisions proposed by Ekman and by
Vinogradova. Rather they appear to be widely distributed in the
abyssal regions and a t most tend to have either an equatorial distribution or t o occur in higher latitudes of both the northern and
southern hemispheres. There is therefore conflict between these conclusions based on the one hand on all recorded species and on the other
on aberrant or very distinctive species. Only if these groups of distinctive ascidians proved t o consist of separate, although similar,
species, would the contradiction be removed, with the conclusion that
a considerable degree of specific endemism exists.
The distribution of abyssal ascidians as a whole is by no mcanR
uniform, even within a single region, as Sokolska (1969) shows for the
Pacific Ocean. An analysis of all occurrences in the abyssal Pacific
recorded by the main deep-sea expeditions shows the ascidians to be
much more restricted than, for example sponges, which might be expected to have rather similar requirements.
VIII. ECONOMIC IMPORTANCE
A. Fouling
In comparison with some other groups of marine animals thc
ascidians cannot be said to have great economic importance. As fouling
organisms, however, they are significant and contribute largely t o the
problem of growth on the hulls of ships, on buoys and floating structures and on fixed harbour installations. The list of over 100 ascidian
species in “ Marine Fouling and its Prevention ” (Woods Hole Oceanographic Institute, 1952) indicates their potential importance, although
many of the species recorded are not sufficiently abundant to be harmful. Almost any shallow-water species in which the adult is normally
attached to a firm substratum may appear in a fouling community.
Ascidians have been studied as they occur on ships’ hulls (Berner,
1944 ; Skerman, 1960), but more often workers have used test panels or
blocks which are examined periodically to follow the course of fouling
(Weiss, 1948 ; Sentz-Braconnot, 1966 ; Relini, 1964 ; Stubbings and
Houghton, 1964 ; Allen and Wood, 195.0 ; Elroi and Komarovsky, 1961 ;
Kawahara, 1962; Nair, 1962 ; Raja, 1963 ; Skernian, 1959). Whatever
methods of asscssment are used, it is evident that ascidians form a
major part of the fouling community in many regions of the world. Of