“Insects” of the Sea
|A female copepod bearing eggsacs.
Subphylum Crustacea (L. crusta
, shell) gets its name from
the hard shell that most crustaceans bear. Over 30,000
species have been described, and several times that number
probably exist. Most familiar to people are the edible ones,
for example, lobsters, crayfishes, shrimps, and crabs. In
addition to these “crusty” crustaceans, there is an astonishing
array of less familiar forms such as copepods, ostracods,
water fleas, whale lice, tadpole shrimp, and krill. They fill a
wide variety of ecological roles and show enormous variation
in morphological characteristics, making a satisfactory
description of the group singularly hard to frame.
We live in the age of arthropods, notwithstanding our
anthropocentric attachment to our tradition of calling the current era the age of mammals. Together, insects and crustaceans
compose more than 80% of all named animal
species. Just as insects pervade the terrestrial habitat (more
than a million named species and countless billions of individuals),
crustaceans abound in oceans, lakes, and rivers.
Some walk or creep on the bottom, some burrow, and some
(such as barnacles) are sessile. Some swim upright, others
swim upside down, and many are delicate microscopic
forms that drift as plankton in the oceans or in lakes.
Indeed, it is probable that the most abundant animals in the
are members of the copepod genus Calanus
recognition of their dominance of marine habitats, it is
understandable that crustaceans have been called “insects”
of the sea.
Arthropods that possess mandibles
(jawlike appendages) are known as
mandibulates and traditionally have
been united in subphylum Mandibulata.
As we noted in the previous section,
some authors think that phylum
Arthropoda is polyphyletic and that
arthropodization occurred more than
once. In addition, many investigators
now believe that there are sufficient
differences between crustaceans and
uniramians (insects, millipedes, centipedes,
pauropods, and symphylans)
to justify separation at least to subphylum
level. Both Crustacea and Uniramia
have, at least, a pair of antennae
a pair of mandibles
, and a pair of maxillae
on the head. These appendages
perform sensory, masticatory,
and food-handling functions, respectively.
The body may consist of a head
and trunk, but in the more derived
forms, a high degree of tagmatization has occurred so that there is a
well-defined head, thorax, and abdomen.
In most Crustacea one or more
thoracic segments are fused with the
head to form a cephalothorax
and abdominal appendages are
mainly for walking or swimming, but
in some groups they are highly specialized
in function. Crustacea are mainly
marine; however, there are many freshwater
and a few terrestrial species,
whereas uniramians are mainly terrestrial.
There are numerous species of
insects in freshwater habitats, but only
a few in marine.