Illustrated here are two
deep sea squids, class Cephalopoda, phylum Mollusca.
Cephalopods are a group of
exclusively marine molluscs that include squids, octopods, cuttlefish
and the chambered Nautilus. These animals are important
predators in the marine food chain and are generally adapted for
a nectic lifestyle, with the exception of octopods
which have taken up a benthic existance. Present day there are
about 600 living species of cephalopods, but over ten times that
many are known from the fossil record, where the group flourished
in the Palaeozoic era.
Millions of years ago all
cephalopods supposedly used the mantle to secrete an external
shell for protection. Today this habit is continued only by two
different 'living fossils' found in the deep sea realm, the several
species of Nautilus in the south-west Pacific and the small
squid Spirula sp. found at the upper edges of the abysses
in the Atlantic and Indian Oceans. Spirula, the
'fag-end fish', rarely exceeds four cm in length and is the only
extant cephalopod with a coiled internal shell (see
The deep sea squids are a
very important group ecologically, they are widespread and abundant
and provide nutrition for many cetaceans, seals, fish and birds,
aswell as being dominant predators themselves upon fish and crustacea.
Squids and cuttlefish catch prey with their 8 arms and 2 prehensile
tentacles, that are often covered with adhesive suckers, they
then guide food into the beak-like jaws of the oral cavity.
Squids move rapidly through
the water column by jet propulsion. They expel a jet of water
from the mantle cavity through a funnel which can be directed
to control the movement of the animal in the opposite direction.
The streamlined squids can swim faster than any other invertebrate,
reaching up to 25 miles per hour. Some squids have larger fins
for swimming and several retain ammonium salts in their body which
give them buoyancy. Shell bearing cephalopods, like Spirula
achieve a neutral buoyancy by sucking water out of the cavities
within their chambered shells.
Light producing organs are
found in many species of deep sea squid and may be spread over
the ventral surface, on the under side of the eyes and liver,
on the tips of tentacles to lure prey and in some cases a luminous
shower of sparks may be produced to temporarily blind attackers.
Lycoteuthis sp. (above right) is a small squid, 3.5cm in
length, with light organs on its eyes and within its body, as
seen through the transparent body wall.
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