small, usually free-swimming crustaceans which
occur in both freshwater and the sea, where they
form a major part of the plankton upon which
larger animals feed. The word "copepod"
comes from Greek words meaning
"oar-feet." There are over 4,500
species of copepods, distributed among 10 orders.
The vast majority of copepods are less
than a millimeter long, but a few marine species
can be over a centimeter in length. All have
segmented bodies covered with stiff bristles
called setae, a hard exoskeleton, two
pairs of antennae (one long, one short), and many
pairs of legs (5 pairs being the most common).
Most have only one eye, colored black or red, on
top of the head. The eye, which in some species
is only present in the larval stage, can only
differentiate between light and dark.
Free-swimming copepods are
common in all freshwaters, particularly in
stagnant ponds, as well as all oceans and seas
except in the Arctic and Antarctic. The larvae
are generally attracted to sunlight and inhabit
the upper parts of water, whereas adults tend to
live at lower depths.
Some copepods use their legs to
swim about, while others swim by alternately
bending and straightening their bodies. These
latter copepods spend most of their time crawling
about over algae or other aquatic vegetation.
Larval copepods often move differently from the
adults, in jerky rushes followed by a period of
floating or free gliding.
exhibit a wide range of feeding behaviors. Some
eat tiny floating plants, such as diatoms and
algae. Others filter small particles of food from
the water with a complex sieving mechanism, the
limbs around the mouth setting up a continuous
pulsating movement that drives water through the
sieve. There are also copepods that capture and
eat relatively large animals, including other
copepods. A number of copepods, which are mainly
marine, are parasitic on fish. These are usually
found on the inside of the gill covers, where
they suck the fish's blood.
reproductive methods vary somewhat by species,
but in most the female, which is almost always
much larger than the male, lays her eggs in an
egg-sac (or sacs) attached to her body. A few lay
their eggs directly into the water. Some
freshwater copepods are able to avoid the effects
of unfavorable conditions, such as excessive cold
or drought, by producing two types of egg. One
type develops rapidly and hatches in a few days,
while the other, larger and more resistant, type
lies on the bottom of the pool or lake and takes
weeks to develop. It is probably through the
second egg type that copepods are dispersed, the
eggs becoming attached to the muddy feet of
aquatic birds and carried from one stretch of
water to another. Larvae are completely different
from adults and typically go through 11 moults
before reaching their final form and size. Some
copepods go through their entire life cycle in
about a week, while others may live as adults for
up to 6 months.
While copepods themselves do not tend to fall
prey to parasites, many species of parasites, including tapeworms and flukes, rely
on copepods for part of their life cycle.
Questions or comments about