Heteropoda
Introduction

The Heteropoda comprise a group of holopelagic caenogastropod molluscs that are found in moderately low to low abundances throughout the world’s oceans, mostly at tropical to subtropical latitudes. All heteropods are carnivorous, utilizing image-forming eyes to locate prey and a protrusible radula with elongate, hook-shaped teeth to capture their prey.

Heteropods exhibit a series of striking adaptations to the pelagic environment. The foot (or parts of it), sole-like in bottom-dwelling gastropods, is laterally compressed and enlarged as a swimming fin. In normal position all heteropods swim with their ventral side (and the ventral fin) turned upward (consequently the dorsally-positioned shell or visceral nucleus hangs below the body). On the postero-ventral edge of the fin a sucker is situated, which is large in the Atlantidae and smaller in the Carinariidae and Pterotracheidae. The eyes are highly developed, with large, spherical lenses and narrow retinas. The taenioglossate radula (tooth formula = 2-1-R-1-2, see He. 5 Characters of taxonomic importance) is characterized by enlarged lateral and marginal teeth with elongated shafts and hook-shaped ends. Like all taenioglossate radulae, the marginal teeth are swivel-mounted and their long shafts spread out laterally when the radula is protruded from the mouth, forming a highly effective means of snaring prey. The bodies and shells of heteropods are largely transparent; only the eyes and viscera are opaque. All heteropods are dioecious and show sexual dimorphism (e.g., large penis and penial appendage in males, radular differences in some atlantids, absence of sucker in female pterotracheids). Internal fertilization follows sperm transfer by spermatophores during copulation. Fertilized eggs are layed singly or in egg-strings; the latter either free-floating or attached to the female (Firoloida). Larvae are free-swimming veligers, with right-coiled, calcareous shells. The larval velum is small and bilobed in newly-hatched larvae; later deeply fissured with six (Atlantidae, Carinariidae) or four (Pterotracheidae) long, slender lobes. The velum is bilaterally symmetrical, except for one species of pterotracheid in which one lobe is distinctly broadened. The larvae are planktotrophic, feeding on small phytoplankton and zooplankton; changing to carnivory after metamorphosis. Duration of the larval stage is unknown.

Three families (Atlantidae, Carinariidae and Pterotracheidae) comprise the superfamily Carinarioidea. The Atlantidae, which exceed by far the other two families in number of species and abundance, are generally considered to be the most primitive family. Their soft parts can be fully retracted into the dextrally (right-hand) coiled shell and sealed off at the shell aperture by a chitinous operculum. The shell of atlantids has undergone striking modifications, compared with the shell of their presumptive bottom-dwelling ancestors. The adult shell (teleoconch) develops directly, and without a sharp dividing line, from the larval shell (protoconch). The last body whorl is greatly enlarged and laterally flattened, bearing a keel along its outer edge. Both the flattened body whorl and keel function to stabilize the body in the water and counterbalance the skulling movements of the large, laterally-compressed swimming fin. Swimming movements appear awkward, being clearly hampered by the short body. The necessity to retract the animal completely into the shell leaves little room for alterations. Nonetheless, there are remarkable differences between the various species of Atlanta and the monotypic Oxygyrus. In the former genus the shell seems far too large for the animal. When completely retracted into the shell, almost one-half of the enlarged, outermost shell whorl remains unoccupied. This can be attributed, at least in part, to the highly-functional increase in shell diameter (increase in lateral surface area to increase stabilization during swimming and sinking). In Oxygyrus the shell lumen is almost completely filled with the retracted animal. The body is elongated and, in conjunction with the bilateralized shell, swimming motions are much swifter and more accurate than in Atlanta. In fact, the swimming behavior of Oxygrus resembles that of Carinaria much more than Atlanta.

The Carinariidae and Pterotracheidae consist of animals whose bodies are greatly enlarged, drawn out in the anterior-posterior axis and basically cylindrical. The elongated esophagus connects the buccal region with the viscera, which are compacted into a structure termed the visceral nucleus. At a location posterior to the eyes the esophagus is modified as a kind of maw which holds prey organisms during a preliminary phase of digestion (attributed to secretions of the salivary glands; Gabe, 1952). The relatively small visceral nucleus includes the true stomach, intestine, digestive gland, kidneys, heart, gonads and gills. In Carinariidae the calcareous shell is reduced to a cap-like covering over the stalked visceral nucleus (Carinaria and Pterosoma), or is represented by the protoconch plus a rudimentary teleoconch embedded in the mantle tissues of the visceral nucleus (Cardiapoda). The body of the carinarids is more rigid than that of the pterotracheids, resulting in propulsion exclusively by skulling movements of the fin (like in the atlantids). The Pterotracheidae lack a shell in the adults (larvae shed the shell at metamorphosis). The visceral nucleus has a distinctive pyriform shape that is largely enveloped by the cylindrical body. In the Pterotracheidae, with longer and slimer bodies, the swimming movements of the fin are reinforced by lateral undulations of the body, especially in stressful situations.

A number of evolutionary trends are suggested among the shell-bearing species (Atlantidae and Carinariidae) that involve reduction in shell weight: 1) a decrease in the number of larval and adult shell whorls (e.g., Atlanta lesueuri, Atlanta oligogyra and all species of Carinaria); 2) reduction in the thickness (range = 3-40 µm; Richter, 1973a; Batten and Dumont, 1976) of the shell walls (e.g., Atlanta lesueuri, A. fragilis, A. meteori, A. gibbosa and all species of Carinaria); 3) a secondary dissolution of the inner shell walls, so that only a thin organic membrane remains to divide the whorls (e.g., Atlanta inflata, A. helicinoides and A. californiensis); and 4) replacement of the aragonitic shell material in Atlanta by conchiolin (e.g., the keel in Protatlanta and both the keel and shell in Oxygyrus).

The literature on Heteropoda is diverse and wide-spread, dating from the first species description (Pterotrachea coronata) given in 1775 (Niebuhr, ms. Forskål). However, sound heteropod taxonomy can be dated from the year 1852, when Eydoux and Souleyet published their famous paper on the Heteropoda of the Bonite Expedition. Due to uncritical investigations based on damaged or badly-preserved specimens or the overvaluation of single morphological characters, the number of species rose to about 100 before Tesch (1906) started to revise the chaotic taxonomy of the group. For the complete list of references on Heteropoda up to 1974, we refer the interested reader to van der Spoel (1976). Reviews of the biology of the Heteropoda were given by Thiriot-Quiévreux (1973) and Lalli and Gilmer (1989).