areas of the western Atlantic, and Pleurobrachia off California and Oregon, exhibit
population outbursts which nearly eradicate their predominantly copepod prey and
then subside. Many ctenophores, but not Pleurobrachia, are bioluminescent,
producing a faint blue light along the digestive canals under the comb rows. Some
deep-sea ctenophores can release bioluminescent mucus to distract approaching
predators. Observations and collections from deep submersibles are revealing a
previously unrealized diversity in this phylum, including some very delicate species
as large as a meter in size. New genera, families, and orders have been added based on
these deep collections (e.g. Madin & Harbison 1978).
Platyhelminthes
(^) Flatworms are not important members of the plankton, but there are planktonic
species. Specimens are often found swimming in the vicinity of tropical reefs. These
probably come and go from the bottom.
Lophophorate Phyla
(^) Brachiopods, bryozoa, entoprocts, and phoronids do not contribute to the
holoplankton, but some species in these groups release abundant planktonic larvae in
coastal areas, larvae of bizarre form compared to the adults. Resemblances to the
space cruisers of science fiction are unmistakable.
Nemertea
(^) Around 100 species of pelagic nemertean worms have been described, although the
systematics are far from complete (Roe & Norenburg 1999). Nemerteans are
unsegmented, predatory worms, the planktonic forms as large as 20 cm in length,
which capture prey with an eversible proboscis armed with hooks. Shallow-living
forms live closely associated with the bottom, but swim up off it. A number of
holoplanktonic species are found in mesopelagic and bathypelagic depths.
Annelida
(^) There are holoplanktonic polychaetes, fewer than 100 species, but in six distinct
families, suggesting that a pelagic lifestyle has evolved repeatedly. They usually have
transparent bodies, large flap-like parapodia, and very large, probably image-forming
eyes. All of these are predators, capturing prey either with strong jaws or protrusible
oral stylets. The most common genus is Tomopteris (Plate 6.5), worms bearing very
long setae on the ends of its elongate tubular parapodia. Tomopteris are never
abundant, but occur in modest numbers in most regions. According to Latz et al.