(1988), T. nisseni can spray yellow, luminescent particles from its parapodia when
disturbed. Most benthic annelids have planktonic larvae with biconical shape and
ciliary locomotion, called trochophores, a larval type shared with mollusks and other
“Trochozoan” phyla.
Mollusca
(^) Mollusks are important as both holoplankton and as meroplankton. The larvae of all
major classes contribute to the meroplankton. The snails (gastropods) are the primary
contributors of holoplanktonic forms, which are well described and well illustrated by
Lalli and Gilmer (1989). There are planktonic representatives of two major gastropod
groups, the opisthobranchs and the prosobranchs.
Opisthobranch Groups
Euthecosomata
(^) These are shelled pteropods, an informal term meaning “wing foot”. The foot of these
snails modifies during development into paired wings, which act as swimming organs.
In the most common genus, Limacina (= Spiratella, Fig. 6.3), the wings extend from
the mouth of a fairly typical but left-coiled snail shell, usually about 3–5 mm diameter
in older adults. Left-coiled or sinistral means that when the shell is held with the top
of the whorl up and the opening facing you, the opening will be to your left. A vast
majority of benthic snails are right-coiled or dextral. In other common Euthecosome
genera, the shells are not coiled but are variations on a cone, e.g. Cavolinia (Plate
6.6), Clio, Creseis, Cuvierina, and Diacria. Head and wings emerge from the opening
of the cone, the tip of which usually points down. Euthecosome shells are formed of
aragonite (Fig. 6.4), a mineral form of calcium carbonate shared with true corals and
distinct from the mineral calcite of benthic snails, clams, foraminifera, and the red
algal family Corallinaceae. In a few relatively shallow parts of the Atlantic, sediments
termed pteropod ooze are largely composed of pteropod aragonite.
Fig. 6.3 Shell variations in the common euthecosome genus Limacina. The lines
connect different views of the same species.
(^) (After McGowan 1968.)