ANTARCTIC BENTHOS
95
fauna with the next highest percentage of small forms is that of South
Australia (38%). The percentage of infaunal species in the Antarctic
is extremely low (59%) compared with the average (about 73%, as in
the Arctic).
Nicol (1964) pointed out that in common with the Arctic, no bivalves
in Antarctic waters attach themselves to the substrate by cementing
the shell to other solid objects. He also showed that this habit was
extremely rare in abyssal mollusca, another essentially cold-water
habitat. He could bring forward no completely satisfying explanation
for this lack except that most cold-water bivalves have thin shells (in
marked contrast to most forms which cement their shells to the
bottom).
In a further consideration of the characteristics of cold-water marine
bivalves Nicol (1967) showed that representatives of this group shared
a number of characters, whether arctic, antarctic, or abyssal molluscs
were considered. Colour patterns are for example lacking in contrast
to the common browns, reds, purples, and yellows of tropical waters.
In the Antarctic the large thin-shelled pectinid Adamusium colbecki
(Smith) is a purplish-red. The only other coloured bivalves Nicol knew
from the Antarctic, Lasaea consanguinea (Smith) and Lissarca miliaris
(Philippi), were mainly Subantarctic in distribution, reaching the
Antarctic continent only along the Antarctic Peninsula.
The shells of cold-water bivalves are generally thin, lacking spines
or similar ornament, and the shell surface is often chalky. The growth
rings on Antarctic bivalve shells are closely spaced indicating that
growth is slow. Nicol believed that the cessation of growth probably
did not result from seasonal temperature fluctuations but rather because of lack of phytoplankton during the winter months.
A comparatively large number of Antarctic bivalves attach themselves to other organisms by means of a byssus. The attachment may
be to sedentary plants and animals, such as algae, hydroids, other
colonial coelenterates or Bryozoa, or even to the spines of echinoids.
Such a habit may be of advantage for dispersal when fragments of the
substrate are broken off to drift, but the success of forms with the habit
of attachment in the Antarctic undoubtedly stems from the advantages
gained by such species in obtaining adequate food. The plankton
shower in the Antarctic forms the major source of food for sedentary
animals and in the many areas where a dense deep growth of hydrozoans, bryozoans, sponges and gorgonaceans occurs the competition for
the available food must be strong. Small bivalves would be at a distinct
disadvantage in such an environment unless they could position themselves high in the canopy and maintain themselves there. For a bivalve
95
fauna with the next highest percentage of small forms is that of South
Australia (38%). The percentage of infaunal species in the Antarctic
is extremely low (59%) compared with the average (about 73%, as in
the Arctic).
Nicol (1964) pointed out that in common with the Arctic, no bivalves
in Antarctic waters attach themselves to the substrate by cementing
the shell to other solid objects. He also showed that this habit was
extremely rare in abyssal mollusca, another essentially cold-water
habitat. He could bring forward no completely satisfying explanation
for this lack except that most cold-water bivalves have thin shells (in
marked contrast to most forms which cement their shells to the
bottom).
In a further consideration of the characteristics of cold-water marine
bivalves Nicol (1967) showed that representatives of this group shared
a number of characters, whether arctic, antarctic, or abyssal molluscs
were considered. Colour patterns are for example lacking in contrast
to the common browns, reds, purples, and yellows of tropical waters.
In the Antarctic the large thin-shelled pectinid Adamusium colbecki
(Smith) is a purplish-red. The only other coloured bivalves Nicol knew
from the Antarctic, Lasaea consanguinea (Smith) and Lissarca miliaris
(Philippi), were mainly Subantarctic in distribution, reaching the
Antarctic continent only along the Antarctic Peninsula.
The shells of cold-water bivalves are generally thin, lacking spines
or similar ornament, and the shell surface is often chalky. The growth
rings on Antarctic bivalve shells are closely spaced indicating that
growth is slow. Nicol believed that the cessation of growth probably
did not result from seasonal temperature fluctuations but rather because of lack of phytoplankton during the winter months.
A comparatively large number of Antarctic bivalves attach themselves to other organisms by means of a byssus. The attachment may
be to sedentary plants and animals, such as algae, hydroids, other
colonial coelenterates or Bryozoa, or even to the spines of echinoids.
Such a habit may be of advantage for dispersal when fragments of the
substrate are broken off to drift, but the success of forms with the habit
of attachment in the Antarctic undoubtedly stems from the advantages
gained by such species in obtaining adequate food. The plankton
shower in the Antarctic forms the major source of food for sedentary
animals and in the many areas where a dense deep growth of hydrozoans, bryozoans, sponges and gorgonaceans occurs the competition for
the available food must be strong. Small bivalves would be at a distinct
disadvantage in such an environment unless they could position themselves high in the canopy and maintain themselves there. For a bivalve