Heteropoda
Methods

Studies on heteropods have been based almost exclusively on animals collected using plankton nets and trawls or observations on animals surviving in aquaria for relatively short periods of time. Limited in-situ observations (Hamner, et al., 1975; Lalli and Gilmer, 1989) have provided our meager knowledge on the natural behavior of heteropods (e.g., swimming, feeding, mating, buoyancy, escape responses).

Because heteropods range widely in size a variety of different types and sizes of nets (ranging from small, fine-meshed plankton nets to large, coarse-meshed trawls) have been employed by researchers. Like many other planktonic animals, heteropods are able to avoid relatively slow-moving plankton nets with narrow openings (McGowan and Fraundorf, 1966; Seapy, 1990a); a factor that is rarely appreciated or taken into account.

Fixation and preservation of heteropods can best be accomplished using 2% formalin in sea water, buffered to pH 8.3 with sodium borate (borax). Use of a pH buffer is critical because the aragonitic shells of Atlantidae and Carinariidae are extremely sensitive to reduced pH and will dissolve rapidly. Shell dissolution is a significant problem for shelled molluscan zooplankton in general (Turner, 1976). If heteropods are maintained for long periods of time in liquid preservative, the pH should be checked every three months. Specimens in rich plankton samples are even more endangered because the gradual deterioration of proteins increases the acidity of the solution; even when buffered. Because of health hazards associated with formaldehyde exposure, many researchers transfer formalin-fixed specimens to 80-95% ethanol (Thiriot-Quiévreux, cited in Turner, 1976, recommends 90-95%). Below a concentration of 80% ethanol requires buffering to neutralize acids soluble in alcohol. A major drawback to the usage of ethanol, however, is that it causes dehydration and bleaching of tissues, in addition to shrinking the body; a particular problem in Pterotracheidae and Carinariidae. Although a variety of liquid fixation and preservation procedures have been developed, none offer the ideal solution.

By far the best method for preserving atlantid and carinarid shells is to dry them and store them in acid-free glass tubes or gelatine capsules. As a testament to this method, the invaluable type specimens of atlantids collected and described by Souleyet (1852a) are maintained dry in the British Museum and are still extant, whereas the type specimens are stored in ethanol in the Musée d'Histoire Naturelle de Paris and the shells are completely dissolved.