THE BIOLOQY OF ASCIDIANY
37
pricked by a needle. Behaviour of this kind may allow the animal to
make adjustments of its position in response to local changes such as
the encroachment of other sessile organisms or displacement of the
substratum.
Most ascidians are fixed to a solid object, and although some seem
to have little preference for organic or inorganic surfaces, others appear
to be selective. Perhaps the success of Ciona intestinalis is partly due
to a catholic taste in Substrata, for it lives on rock, shells and other inorganic things and also on Zostera and algae (Dybern, 1963). Several
species of the family Pyuridae live on rocks and in addition on the hard
cuticle-like surfaces of large brown algae, on dead calcareous algae, and
on other ascidians with a hard superficial layer of test, but are not found
on living calcareous algae or on soft brown algae (C. Monniot, 1965a).
It is not clear whether this preference is determined solely by the hardness of the surface or by its chemical nature. In the Baltic Sea Styela
eorincea uses the shells of living Astarte borealis in preference to stones
(Dybern, 1969~). Many species of Ascidiidae, Botryllinae and
Didemnidae frequently occur on living algae, and some like Aplidium
pallidurn (Verrill) apparently prefer the surface of living plants.
(One of the synonyms of that species-Aplidium zostericola Giard-refers
to this fact). Of the species of the family Didemnidae occurring in
a small area, Lafargue (1 968) noted that two (Trididemnum delesseriae
Lafargue and Diplosoma singulare Lafargue) required a flexible
substratum, another (Diplosoma listerianum (Milne Edwards) ) occurred
more frequently on flexible than on rigid supports, and the remaining
eleven species lived on a rigid substratum or on both types. Amongst
species attached to either algae or inorganic surfaces, there may
be preferences for certain kinds of alga, and Dybern (1969b) found
Ascidiella scubra (Miiller) to be common on laminarians, fucaceans,
Halydris siliquosa and Delesseria sanguinea, but not on polysiphonians
and furcellarians. The abundance of some species is limited by local
shortage of suitable hard substrata, as on parts of the Scottish coast
where Ascidiella aspersa is moderately common but becomes very
numerous when additional surfaces are placed in the water (Millar,
1961b). But even in the presence of suitable objects, it has been observed that the numbers of certain pyurids may be limited if the objects
are far apart (C. Monniot, 1965a).
Soft loose substrata present particular problems to sessile animals,
but numerous ascidians, of several families, have adopted this habitat,
and three kinds of adaptation have evolved. The commonest is the
production of test filaments, which penetrate the deposit and become
coated with particles, thus effectively fixing the body in position
37
pricked by a needle. Behaviour of this kind may allow the animal to
make adjustments of its position in response to local changes such as
the encroachment of other sessile organisms or displacement of the
substratum.
Most ascidians are fixed to a solid object, and although some seem
to have little preference for organic or inorganic surfaces, others appear
to be selective. Perhaps the success of Ciona intestinalis is partly due
to a catholic taste in Substrata, for it lives on rock, shells and other inorganic things and also on Zostera and algae (Dybern, 1963). Several
species of the family Pyuridae live on rocks and in addition on the hard
cuticle-like surfaces of large brown algae, on dead calcareous algae, and
on other ascidians with a hard superficial layer of test, but are not found
on living calcareous algae or on soft brown algae (C. Monniot, 1965a).
It is not clear whether this preference is determined solely by the hardness of the surface or by its chemical nature. In the Baltic Sea Styela
eorincea uses the shells of living Astarte borealis in preference to stones
(Dybern, 1969~). Many species of Ascidiidae, Botryllinae and
Didemnidae frequently occur on living algae, and some like Aplidium
pallidurn (Verrill) apparently prefer the surface of living plants.
(One of the synonyms of that species-Aplidium zostericola Giard-refers
to this fact). Of the species of the family Didemnidae occurring in
a small area, Lafargue (1 968) noted that two (Trididemnum delesseriae
Lafargue and Diplosoma singulare Lafargue) required a flexible
substratum, another (Diplosoma listerianum (Milne Edwards) ) occurred
more frequently on flexible than on rigid supports, and the remaining
eleven species lived on a rigid substratum or on both types. Amongst
species attached to either algae or inorganic surfaces, there may
be preferences for certain kinds of alga, and Dybern (1969b) found
Ascidiella scubra (Miiller) to be common on laminarians, fucaceans,
Halydris siliquosa and Delesseria sanguinea, but not on polysiphonians
and furcellarians. The abundance of some species is limited by local
shortage of suitable hard substrata, as on parts of the Scottish coast
where Ascidiella aspersa is moderately common but becomes very
numerous when additional surfaces are placed in the water (Millar,
1961b). But even in the presence of suitable objects, it has been observed that the numbers of certain pyurids may be limited if the objects
are far apart (C. Monniot, 1965a).
Soft loose substrata present particular problems to sessile animals,
but numerous ascidians, of several families, have adopted this habitat,
and three kinds of adaptation have evolved. The commonest is the
production of test filaments, which penetrate the deposit and become
coated with particles, thus effectively fixing the body in position