Cephalopods take air down
to the depths
Shell-bearing cephalopods such as the
pearly nautilus (Nautilus spp.) and paper
nautilus (Argonauta spp.) have to descend
into the depths by day for protection. This
requires them to be able to regulate their
buoyancy.
Australian marine biologists have
recently discovered that the Giant Paper Nautilus (Argonauta arga) achieves
this with the aid of air that it takes in at
the water’s surface. While the tiny, only
around ¾-inch (2-cm) males of this species do not produce a shell, the females
have a thin shell made of a papery substance, and this can be up to 17½ inches
( 45 cm) long. A certain amount of air can
be sealed into this shell with the aid of the
soft body tissue. Initially, the nautilus has
to actively dive by virtue of its inherent
buoyancy, but as it does so the water pressure on the air increases and compresses
it. Once the bubble of air reaches a certain volume, the nautilus is able to swim
without difficulty at the relevant depth,
moving horizontally with the minimum
expenditure of energy. In the opinion of
the scientists, the air bubble, stored up in
the narrowest part of the shell, also acts
as a keel, keeping the cephalopod’s shell
upright as it swims and providing stability so that it can swim faster and is better
able to escape its enemies.
RefeRences
Finn, J. K. & M. D. Norman. 2010. The argonaut
shell: gas-mediated buoyancy control in a
pelagic octopus. Proceedings of the Royal
Society, B, Biological Sciences; doi: 10.1098/
rspb.2010.0155.
