component of the total biomass at most times, places, and depths. Members of the
following subclasses are important in the marine holoplankton.
Branchiopoda
(^) The Cladocera are the only order in the subclass Branchiopoda, and are familiar to
freshwater biologists as water fleas, such as Daphnia. The cladoceran body of 1–2
mm is dominated by the head and wrapped in a posteriorly pointed, bivalve carapace
resembling a clam shell. However, the first few segments, which bear large,
hemispherical, compound eyes, are outside this shell. Swimming is by rowing
movements of the antennae (the second limbs) that extend outside the carapace behind
the eyes. Feeding is predatory in marine Cladocera, which grab prey with limbs
surrounding the mouth. Those limbs are exposed, the valves being almost
incorporated into the posterior body. There are 10 marine species, with most
individuals belonging to the genera Evadne and Podon. While these genera are found
all across the oceans, they are never abundant or dominant. In estuaries, however, they
can be numerous in some seasons. Like the daphnids, Evadne and Podon can
reproduce parthenogenetically, occasionally alternating a sexual generation including
males. The genus Bosmina is often an abundant component of the plankton in the
brackish water of upper estuaries.
Ostracoda
(^) Ostracods have a bivalve carapace over the whole body, including the head. This
closely resembles a clam shell, with a distinct dorsal hinge and closure by a transverse
muscle. The body is strongly dominated by the head, with the trunk reduced to two
appendages and the abdomen to a posterior coil. Propulsion is by rowing with the
antennae, which bear long sweeps of setae and extend out of the shell through
oarlock-like notches. The number of described species is increasing rapidly, due to
attention from Martin Angel, Kasia Blachowiak–Samolyk, Vladimir Chavtur, and
others, but the total remains a few hundred, a majority in the family Halocypridoidea
of the order Myodocopa (many more ostracod species, not all of them Myodocopa,
are epibenthic, have calcified shells and, hence, a fossil record). The vast majority of
halocyprid individuals used to belong to species lumped in the planktonic genus
Conchoecia (Fig. 6.5), which is now divided by the small group of experts into 30 or
so genera.
Fig. 6.5 Conchoecia borealis, sketch of the body layout and limbs. The right valve is
in the background, the left valve is removed. The adductor muscle passes through the
isthmus. The teardrop structure on the left is the antennal basis housing the propulsive
muscles.
(After Iles 1961.)