ammonium replacing sodium ion). Many acantharians, rads, and forams (not
Phaeodaria) contain zooxanthellae, autotrophic dinoflagellate cells as symbionts in
their cortical cytoplasm. Thus, they acquire nutrition by a combination of farming the
zooxanthellae internally and contact capture of various animals ranging upward in
size to small copepods like Oncaea. Among Foraminifera, there are also deep-living
pelagic species without symbionts and a distinctive group of benthic forms. Many
planktonic foraminifera range up to 1 cm in overall cell diameter (some benthic forms
are much larger). Gowing (e.g. 1989) has shown that Phaeodaria are generalist
particle feeders, finding bacteria, diatoms, dinoflagellates and both protozoan and
crustacean remains in their food vacuoles. They can be collected from the surface
down to mesopelagic depths, tending to be most abundant just below ∼100 m.
Cnidaria
(^) These are animals bearing nematocysts or stinging cells on their outer surfaces,
particularly along their tentacles; all the planktonic forms are gelatinous. Cnidaria is
usually considered one of the “lower” phyla of metazoa, or multicellular animals,
because their level of organization is relatively simple, with no centralized nervous or
circulatory systems. There are two basic tissue layers: an outer covering and a gut
lining. Between those is firm, jelly-like matter called mesoglea. There are moderately
sophisticated sensory organs in many forms, particularly in medusae, including eyes
and statocysts (gravity and acceleration sensors). Three cnidarian groups are
represented in the plankton: Hydrozoa, Scyphozoa, and Cubozoa.
Hydrozoa
(^) The hydromedusa stages of this group can be an important component of the
zooplankton. Medusae (Plate 6.2) are bell-shaped, with the mouth at the end of a
stalk, the manubrium, hanging from the top inner surface like a clapper. They are
predators, capturing prey with tentacles on the bell edge and then transferring them to
the mouth. This stage alternates with a sessile, usually colonial, polyp stage.
Hydromedusae are mostly small jellyfish, the gamete-producing life-phase of these
dimorphic animals. Zygotes develop into small, ciliated larvae called planulae that
settle to become polyps, then colonies. The bell has a flexible band of tissue, a velum,
extending into the bell opening from its edge, which serves to narrow the aperture and
concentrate the propulsion jet when the bell contracts. Some of the oceanic genera,
particularly among the narcomedusae, do not have alternating generations, and their
zygotes develop directly into medusae.
(^) An abundant tropical–subtropical “neustonic” (living right on the sea surface)
hydrozoan group are the Porpitidae (e.g. Porpita and Vellela), the hydroids of which
form colonies suspended downward from a stout, gas-filled float 3 to 10 cm across. In