Invariably overlooked, always misunderstood,
seldom studied and always there……heteropods, a bizarre group
of pelagic molluscs. They are amongst the most widespread
and successful groups of peculiar planktonic predators. They
are found from the surface waters to the abyssal depths, from
the tropics to the poles. Their group name refers to their
"different foot" and when compared to other swimming molluscs
this is certainly the case. Some species progress at great
speed by a sculling action of the single median, expanded
and flattened foot and can fly forwards or backwards, right
way up an upside down. The lower part of the foot is, in some
species, equipped with a drogue parachute used, it is said,
to slow sinking.
The typical molluscan shell may be dominant, diminutive
or disappeared! The paired and very dominant eyes have cylindrical
retinas, powerful plano-concave lenses and an optical design that
no-one can understand.
All heteropods are voraciously predacious, some
possibly even parasitic. They particularly feed upon other planktonic
molluscs like pteropods. Large deep sea heteropods can rasp nasty
holes in unwary fingers and their owners can be 35 cm long. Their
teeth are in the form of a radula - a conveyor belt of wickedly
sharp plates that act a little like an oil exploration rock drill!
Off British coasts, on warmer Gulf Stream currents,
the dominant genus is Atlanta, which we illustrate here - amongst
the smartest of humble heteropods.
This article was written
by Peter Parks
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