enthusiasts since William Hamner (1974) and colleagues initiated studies by blue-
water diving in the 1970s. There is no commonly accepted name for the other
plankton, which are typically ∼70% water (as are land animals). “Hard-bodied”
zooplankton isn’t quite right, and non-gelatinous defines them by what they are not.
Groups will be characterized here as gelatinous when they are.
An Introductory Description of the Forms of
Planktonic Animals
(^) There are planktonic representatives of every major (but not every “minor”) phylum,
and there are important phyla largely restricted to the plankton. We depart now on a
Cook’s Tour, visiting each phylum briefly. If no picture of an organism is provided, it
is because space and budget for figures are limited in textbooks. Good pictures can be
found by typing almost any genus name into a suitable worldwide-web search-engine
(e.g. Google). Try for the “heteropod” snail Carinaria, for example, and you will find
lovely pictures by Roger Seapy, Steven Haddock, and others.
Protista
(^) These are organisms complete in a single, eukaryotic cell. “Protozoa” are now not
considered “animals”, and the once formal term “animal” is lately supposed to be
used only for multicellular beasts. Protists are not one, monophyletic “phylum”, but
an agglomeration of forms variously related. Many marine phytoplankton share
animal-like characteristics: they are motile, have sensory organelles like those of
heterotrophic protists, and respond actively to environmental stimuli. Several groups
of flagellated phytoplankton, many prymnesiophytes for example, are “mixotrophs”
that both photosynthesize and graze on particles. Quite likely such mixotrophic
nanophytoplankton, and similar but unpigmented flagellates, account for much of the
grazing on pelagic bacteria, including cyanobacteria. The most obviously animal-like
algae are the dinoflagellates, many of which are strictly heterotrophic. Some
dinoflagellates (Chapter 2) capture prey, often phytoplankton, by encapsulating them
in a mucous sac (a pallium). Immobilized prey are ingested, forming food vacuoles.
Other dinoflagellates insert a tubular peduncle into prey, harvesting nutrition without
engulfing them.
(^) Diverse heterotrophic protozoa reside in the plankton, and they have received
attention in Chapter 5. Those of the sorts common in freshwater are present in the sea
as well: ciliates, flagellates and amoebae. Ciliates have a major role in pelagic food
webs, which remains under evaluation. Naked ciliates are mostly members of the
subgroup Choreotricha, which are abundant in all ocean areas. They belong to a small