Guide to the identification of marine plants and invertebrate animals ...

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PLANKTON
Pl-ankton is the term applied to al-I the organisms
that l-ive in suspension in the water and tend to drift
with water cuments. It comes f rom the Greek word
trwandererrr and sets such organisms apart from the
animal-s capable of efficient, self-directed movement.
These are cal-l-ed nekton.
Pl-ankton includes both plants and animal-s and
therefore may be sub-divided into these two giroups:
phytoplankton, the plants; zooplankton, the animal-s.
These organi-sms are usually microscopic or relatively
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organisms, but al-so the most widely dispersed.
The phytoplankton, a]l- the passj.vely floating
plants of the sea, are the basic producers that combine
simple inorganic compounds in the water to form complex
organic substances by photosynthesis. These then become
the food of the smal-l-er anj.mals of the zooplankton which
in turn form the food supply of larger animal-s. Therefore,
the importance of the phytoplankton lies in its reproduction
as the primary food supply of the sea. The chief
pl-ants of the phytoplankton are the dj.atoms and
dinof lagel-l-ates (Plate l-) .
The zoopl-ankton has considerabty greater variety
in its members. It is composed of two types of anj.mal-s.
First, those that spend ai-l- their l-ives as microscopic
plankton drifting in the sea are cal-l-ed holoplankton.
Along the Virgi-ni-a coast this group is best represented
by the protozoans cal-l-ed tintinnids, f oraminif era, and
radiol-arians, and by crustaceans call-ed copepods. Second,
the early life stages (eggs and floating larvae) of
microscopic animals that wil-l- develop l-ater into freeswimming
or bottorn-dwel-ling types are cal-l-ed meropl-ankton.
This includes mainly the developmental stages of invertebrates,
but al-so the young of many fishes. Some of
the most cornmon meroplankton found in .June and .Iuly were
the larvae of crabs, snails, barnacles, annel-ids and
oysters (Pl-ate 2). Meroplankton are abundant during
and shortly after the annual reproductive period of the
adul-ts, especially in spring and summer.