ANTAROTIO BENTHOS
163
most outstanding examples are probably the isopods, amphipods and
cirripedes, and the philobryid bivalves.
An investigation of the adaptational advantages to be gained by
animals reproducing viviparously could be a valuable study. It is
extremely likely that the relatively restricted period of phytoplankton
production in Antarctic waters gives advantages to animals that can
adapt themselves to a period of abundant plankton food followed by a
longer period when food must be sought elsewhere, or where reserves
can be utilized. There is evidence that a number of the life histories
already investigated do in fact conform to this type of rhythm. A viviparous life cycle is obviously easily adapted to such a cycle.
It is sometimes intimated that by evolving a viviparous life cycle,
or a brooding period in the life cycle, the animal concerned sacrifices
the ability to disperse itself widely. This is another example of human
Iogic to which most of the animals concerned do not subscribe. Many
of the animals which protect their eggs and developing young most
effectively and for the longest time have achieved the widest distribution pattern, often including some of the most isolated islands of the
Subantarctic, such as Kerguelen, within their ranges. Against all logic,
therefore, animals which brood their young have often achieved the
widest distribution patterns. Such dispersal success is easily enough
understood when the animal concerned is a bivalve like Philobrya or
Gaimardia which normally attaches itself to algae and can achieve its
dispersal by passive means. The significance of such a bivalve arriving
in a new area with several hundred well-developed young ready for
release is obvious enough. When the animal concerned lives buried in
the substrate or fixed to rocks, it is only the method of dispersal which
has become difficult to envisage.
One of the best examples known to the writer is the small chiton,
Hemiarthrum setulosum, which has been discussed by the writer (Dell,
1964). This chiton has been recorded from the Crozets, Kerguelen;
Heard, and Macquarie Islands, southern Chile, Magellan Strait, South
Georgia, the South Orkneys and the Antarctic Peninsula, a familiar
enough Antarctic-Subantarctic distribution pattern. Hemiarthrum
lives firmly adhering to rocks and broods its eggs and developing young
under the protection of the adult shell until they are small replicas of
the adult. It normally lives attached to rock in shallow water and has
never been reported attached to algae, or even to other molluscs such
as mussels which could in turn be attached to algae. Such a species
must be distributed as an adult or subadult, by a combination of circumstances which must be moderately rare, when suitable rocks to
which the chiton is adhering are torn away by attached algae and
163
most outstanding examples are probably the isopods, amphipods and
cirripedes, and the philobryid bivalves.
An investigation of the adaptational advantages to be gained by
animals reproducing viviparously could be a valuable study. It is
extremely likely that the relatively restricted period of phytoplankton
production in Antarctic waters gives advantages to animals that can
adapt themselves to a period of abundant plankton food followed by a
longer period when food must be sought elsewhere, or where reserves
can be utilized. There is evidence that a number of the life histories
already investigated do in fact conform to this type of rhythm. A viviparous life cycle is obviously easily adapted to such a cycle.
It is sometimes intimated that by evolving a viviparous life cycle,
or a brooding period in the life cycle, the animal concerned sacrifices
the ability to disperse itself widely. This is another example of human
Iogic to which most of the animals concerned do not subscribe. Many
of the animals which protect their eggs and developing young most
effectively and for the longest time have achieved the widest distribution pattern, often including some of the most isolated islands of the
Subantarctic, such as Kerguelen, within their ranges. Against all logic,
therefore, animals which brood their young have often achieved the
widest distribution patterns. Such dispersal success is easily enough
understood when the animal concerned is a bivalve like Philobrya or
Gaimardia which normally attaches itself to algae and can achieve its
dispersal by passive means. The significance of such a bivalve arriving
in a new area with several hundred well-developed young ready for
release is obvious enough. When the animal concerned lives buried in
the substrate or fixed to rocks, it is only the method of dispersal which
has become difficult to envisage.
One of the best examples known to the writer is the small chiton,
Hemiarthrum setulosum, which has been discussed by the writer (Dell,
1964). This chiton has been recorded from the Crozets, Kerguelen;
Heard, and Macquarie Islands, southern Chile, Magellan Strait, South
Georgia, the South Orkneys and the Antarctic Peninsula, a familiar
enough Antarctic-Subantarctic distribution pattern. Hemiarthrum
lives firmly adhering to rocks and broods its eggs and developing young
under the protection of the adult shell until they are small replicas of
the adult. It normally lives attached to rock in shallow water and has
never been reported attached to algae, or even to other molluscs such
as mussels which could in turn be attached to algae. Such a species
must be distributed as an adult or subadult, by a combination of circumstances which must be moderately rare, when suitable rocks to
which the chiton is adhering are torn away by attached algae and