THE BIOLOGY OF ASUIDIANS
47
plankton from the sea and also contribute significant amounts of
metabolites.
A further influence which ascidians may have on their environment
is in stabilizing a loose substratum, and many sand and mud-dwelling
species have abundant test fibrils which bind particles of the surrounding deposit. The molgulid Bostrichobranchus pilularis is an example, in
which the body is coated with mud. Moreover, specimens are loosely
attached to one another (Van Name, 1945), and evidently constitute 8
living mat on the surface of the sea-bed. The abundance of this species
in some areas (Abbott, 1951) suggests the important r d e which such
ascidians may play in fixing soft deposits. It is more surprising to find
ascidians playing a part in the formation of reefs, but Renouf (1937)
has described how, together with sponges, they assist in cementing slabs
and boulders on steep rocky banks of the shore.
New techniques may assist advances in unexpected directions, and
the increasing use of free diving methods has not only clarified the
relationships of ascidians to their substratum but also helped in
taxonomic studies. For example, Lafargue (1968) examined the didemnids of a limited area off the coast of Brittany, France, and her separation of a number of closely related species, using anatomical characters, is supported by differences of microhabitat in respect to depth,
illumination, rigidity of the substratum, and proximity to sediment.
Another aspect of local distribution which until recently has received
little attention is the extent to which a single species may be divided
into sub-populations. It is now known, however (Sabbadin and Graziani, 1967), that Botryllus schlosseri in the Lagoon of Venice has genetically controlled sub-populations existing under the same ecological
conditions but a few miles apart. Similarly, sub-populations exist under
different ecological conditions at the same location. It is possible that
variations in the colonies of species of Didemnidae presbnt a parallel
case, for the different forms tend to live in different microhabitats
(Lafargue, 196 8).
VI. PREDATORS, PARASITES, COMMENSALS
AND SYMBIONTS
c
Organisms are associated in various ways, which have been discussed and defined by Cheng (1967). Predation, parasitism and commensalism are important aspects of ascidian biology and I have used
these terms in accordance with Cheng’s definitions, but symbiosis, implying mutual metabolic benefit by associated organisms (which Cheng
recognizes as mutualism) is apparently of little significance within the
group.
47
plankton from the sea and also contribute significant amounts of
metabolites.
A further influence which ascidians may have on their environment
is in stabilizing a loose substratum, and many sand and mud-dwelling
species have abundant test fibrils which bind particles of the surrounding deposit. The molgulid Bostrichobranchus pilularis is an example, in
which the body is coated with mud. Moreover, specimens are loosely
attached to one another (Van Name, 1945), and evidently constitute 8
living mat on the surface of the sea-bed. The abundance of this species
in some areas (Abbott, 1951) suggests the important r d e which such
ascidians may play in fixing soft deposits. It is more surprising to find
ascidians playing a part in the formation of reefs, but Renouf (1937)
has described how, together with sponges, they assist in cementing slabs
and boulders on steep rocky banks of the shore.
New techniques may assist advances in unexpected directions, and
the increasing use of free diving methods has not only clarified the
relationships of ascidians to their substratum but also helped in
taxonomic studies. For example, Lafargue (1968) examined the didemnids of a limited area off the coast of Brittany, France, and her separation of a number of closely related species, using anatomical characters, is supported by differences of microhabitat in respect to depth,
illumination, rigidity of the substratum, and proximity to sediment.
Another aspect of local distribution which until recently has received
little attention is the extent to which a single species may be divided
into sub-populations. It is now known, however (Sabbadin and Graziani, 1967), that Botryllus schlosseri in the Lagoon of Venice has genetically controlled sub-populations existing under the same ecological
conditions but a few miles apart. Similarly, sub-populations exist under
different ecological conditions at the same location. It is possible that
variations in the colonies of species of Didemnidae presbnt a parallel
case, for the different forms tend to live in different microhabitats
(Lafargue, 196 8).
VI. PREDATORS, PARASITES, COMMENSALS
AND SYMBIONTS
c
Organisms are associated in various ways, which have been discussed and defined by Cheng (1967). Predation, parasitism and commensalism are important aspects of ascidian biology and I have used
these terms in accordance with Cheng’s definitions, but symbiosis, implying mutual metabolic benefit by associated organisms (which Cheng
recognizes as mutualism) is apparently of little significance within the
group.