THE BIOLOGY OF ASCLDIANS
77
My own estimates for the ascidian species, based on the published
records, are: Atlantic Ocean 72%, Pacific Ocean 68% and Indian
Ocean 43% (Millar, 1970), and the group appears to support the view
that abyssal faunas show zoogeographical divisions. Certain features of
the ascidians, however, such as the plasticity of body form, seasonal
changes in structure, and a high degree of variability, have led to uncertainty in the recognition of species, well illustrated by the synonymy
proposed by Kott (1969) for Cnemidocarpa nordenskjoldi and for
Culeolus murrayi Herdman. In the presence of such uncertainties, which
are common in abyssal species where identification often rests on few
specimens, reliable conclusions on specific endemism are difficult to
reach, and it may be useful to consider small distinctive genera, and
pairs or groups of similar species whether or not these ultimately prove
to represent a single species. By choosing taxa with highly unusual
characters-and these are frequent in abyssal forms-we may be confident that we are not dealing with species merely having a similar
appearance through adaptive convergence.
Styela loculosa Monniot and Monniot and Minostyela clavata Kott,
which are characterized by a peculiar form of gonad quite unlike that
of any other known ascidian, show a kind of bipolar distribution (Fig.
25). Another group of highly aberrant abyssal ascidians comprises
Hexacrobylus indicus Oka (synonym H . arcticus Hartmeyer) and H .
psammutodes Sluiter (Fig. 26). Oligotremapsammites Bourne is evidently
of the same group of species, but is known only from a depth of 92 m.
Kott (1969) believed that all of these represent one species, and although
I do not accept this opinion in view of the structural differences in the
pharynx, oral tentacles and gut, there is no doubt that the species form
a closely related group with a marked systematic separation from other
ascidians. The distribution is wide (Fig. 26) and cuts across the
boundaries of several regions. The genus Culeolus presents another
interesting case, and whether or not Kott (1969) is right in taking 11 of
the described species as synonyms of C. murrayi Herdman the great
difficulty in separating species underlines the advantage of grouping all
together for the present purpose. The wide distribution (Fig. 26)
lacks records principally in the Indian Ocean and north polar seas. The
last example is the genus Corynascidia (Fig. 25). C . suhmi Herdman is
known from widely separated areas and although not recorded from low
northern or southern latitudes, it may be expected to occur in most of
the deep oceans (Kott, 1969). As known at present its distribution
suggests some degree of bipolarity. C. sedem Sluiter and C . herdmuni
Ritter have been taken in depths less than 2 000 m from Indonesia and
the North Pacific respectively.
77
My own estimates for the ascidian species, based on the published
records, are: Atlantic Ocean 72%, Pacific Ocean 68% and Indian
Ocean 43% (Millar, 1970), and the group appears to support the view
that abyssal faunas show zoogeographical divisions. Certain features of
the ascidians, however, such as the plasticity of body form, seasonal
changes in structure, and a high degree of variability, have led to uncertainty in the recognition of species, well illustrated by the synonymy
proposed by Kott (1969) for Cnemidocarpa nordenskjoldi and for
Culeolus murrayi Herdman. In the presence of such uncertainties, which
are common in abyssal species where identification often rests on few
specimens, reliable conclusions on specific endemism are difficult to
reach, and it may be useful to consider small distinctive genera, and
pairs or groups of similar species whether or not these ultimately prove
to represent a single species. By choosing taxa with highly unusual
characters-and these are frequent in abyssal forms-we may be confident that we are not dealing with species merely having a similar
appearance through adaptive convergence.
Styela loculosa Monniot and Monniot and Minostyela clavata Kott,
which are characterized by a peculiar form of gonad quite unlike that
of any other known ascidian, show a kind of bipolar distribution (Fig.
25). Another group of highly aberrant abyssal ascidians comprises
Hexacrobylus indicus Oka (synonym H . arcticus Hartmeyer) and H .
psammutodes Sluiter (Fig. 26). Oligotremapsammites Bourne is evidently
of the same group of species, but is known only from a depth of 92 m.
Kott (1969) believed that all of these represent one species, and although
I do not accept this opinion in view of the structural differences in the
pharynx, oral tentacles and gut, there is no doubt that the species form
a closely related group with a marked systematic separation from other
ascidians. The distribution is wide (Fig. 26) and cuts across the
boundaries of several regions. The genus Culeolus presents another
interesting case, and whether or not Kott (1969) is right in taking 11 of
the described species as synonyms of C. murrayi Herdman the great
difficulty in separating species underlines the advantage of grouping all
together for the present purpose. The wide distribution (Fig. 26)
lacks records principally in the Indian Ocean and north polar seas. The
last example is the genus Corynascidia (Fig. 25). C . suhmi Herdman is
known from widely separated areas and although not recorded from low
northern or southern latitudes, it may be expected to occur in most of
the deep oceans (Kott, 1969). As known at present its distribution
suggests some degree of bipolarity. C. sedem Sluiter and C . herdmuni
Ritter have been taken in depths less than 2 000 m from Indonesia and
the North Pacific respectively.