THE BIOLOQY OF ASCIDIANS
55
Eldredge (1967), in which the larva of Diplosoma virens (Hartmeyer)
was seen to have special pouches filled with conspicuous green algae ;
the anatomical modification of the larva suggests that the relationship
here is regular and possibly of mutual benefit.
VII. GEOGRAPHICAL DISTRIBUTION
Many factors must have played a part in determining the present
pattern of geographical distribution of species. Changes in the positions
of land and sea masses during geological times were undoubtedly important, and have been used for instance to explain the widespread
occurrence of certain species in the West Indies, Mediterranean and
Indo-Pacific (Huus, 1927). Hydrographic conditions, too, have varied
and may have influenced speciation and the isolation of species (Knox,
1963). The biological characteristics of the group have also to be taken
into account. Amongst these is the short pelagic life of the larva which
restricts dispersal, and Huus (1927) has shown that few ascidian larvae
could be expected to survive oceanic transport over distances greater
than 500 km. Transport on driftwood and algae, and in recent times on
ships, is a more likely means of dispersal across wide and deep seas. The
spread of species along coastlines, however, is less subject to restrictions
imposed by larval biology and will be limited chiefly by the occurrence
of unfavourable features in the environment, such as unsuitable substrata, temperatures which do not permit breeding, and other factors
discussed elsewhere in this review.
Studies of the zoogeography of ascidians are rather few. One of the
most detailed was made by Hartmeyer (1923, 1924) who analysed the
ascidian faunas of the Arctic and Boreal and interpreted them in relation
to hydrographic conditions. The papers by Thompson (1930,1931,1933,
1934) were in part a translation of Hartmeyer’s work to which were
added records from Scottish and adjacent waters. Huus (1937) included
a review of world distribution in his major work on ascidians, PQrks
(1958) has dealt with Mediterranean species, Oka (1935) and Tokioka
(1963) with those of the Japanese seas and Kott (1969) with the
Antarctic species. In addition, many taxonomic papers describing
collections from diverse areas give some consideration to questions of
distribution and faunistic affinities.
One of the principal obstacles to resolving questions of geographical
distribution, faunistic relationship, and the origin of faunas is the unequal effort which has been made in different regions. The ascidians of
several areas are now quite well known, but for certain large and important regions information is scanty. This is particularly SO in the
waters of the Indian peninsula, and much of the Pacific and Atlantic
55
Eldredge (1967), in which the larva of Diplosoma virens (Hartmeyer)
was seen to have special pouches filled with conspicuous green algae ;
the anatomical modification of the larva suggests that the relationship
here is regular and possibly of mutual benefit.
VII. GEOGRAPHICAL DISTRIBUTION
Many factors must have played a part in determining the present
pattern of geographical distribution of species. Changes in the positions
of land and sea masses during geological times were undoubtedly important, and have been used for instance to explain the widespread
occurrence of certain species in the West Indies, Mediterranean and
Indo-Pacific (Huus, 1927). Hydrographic conditions, too, have varied
and may have influenced speciation and the isolation of species (Knox,
1963). The biological characteristics of the group have also to be taken
into account. Amongst these is the short pelagic life of the larva which
restricts dispersal, and Huus (1927) has shown that few ascidian larvae
could be expected to survive oceanic transport over distances greater
than 500 km. Transport on driftwood and algae, and in recent times on
ships, is a more likely means of dispersal across wide and deep seas. The
spread of species along coastlines, however, is less subject to restrictions
imposed by larval biology and will be limited chiefly by the occurrence
of unfavourable features in the environment, such as unsuitable substrata, temperatures which do not permit breeding, and other factors
discussed elsewhere in this review.
Studies of the zoogeography of ascidians are rather few. One of the
most detailed was made by Hartmeyer (1923, 1924) who analysed the
ascidian faunas of the Arctic and Boreal and interpreted them in relation
to hydrographic conditions. The papers by Thompson (1930,1931,1933,
1934) were in part a translation of Hartmeyer’s work to which were
added records from Scottish and adjacent waters. Huus (1937) included
a review of world distribution in his major work on ascidians, PQrks
(1958) has dealt with Mediterranean species, Oka (1935) and Tokioka
(1963) with those of the Japanese seas and Kott (1969) with the
Antarctic species. In addition, many taxonomic papers describing
collections from diverse areas give some consideration to questions of
distribution and faunistic affinities.
One of the principal obstacles to resolving questions of geographical
distribution, faunistic relationship, and the origin of faunas is the unequal effort which has been made in different regions. The ascidians of
several areas are now quite well known, but for certain large and important regions information is scanty. This is particularly SO in the
waters of the Indian peninsula, and much of the Pacific and Atlantic