ABSTRACT
There is no sharp dividing line between the pelagic animals
that are regarded as plankton and those that constitute the
nekton. The distinguishing feature is that zooplankton drift
with the current systems whereas nekton are active swim-
mers, and are thus not at the mercy of the currents. Some
species that are regarded as part of the zooplankton, such as
some crustaceans, chaetognaths, and others, may have
considerable swimming speeds, equal to some that are
usually treated as part of the nekton. In the previous
chapter we have seen that krill have the ability to swim for
considerable distances. Many members of the nekton
(e.g., larval fish) also exist as plankton during the early
stages of their life cycle. There is considerable size overlap
between the zooplankton and the nekton. Some species that
are regarded as megazooplankton, (e.g., siphonophores,
scyphozoans [jellyfish] and colonial salps) can be very
large, greatly exceeding in size many nektonic species.
Such species, however, generally drift with the currents
and do not actively swim.
