Heteropoda
Characters of taxonomic importance

Four morphological characters (shell, eyes, operculum and radula) are of great importance in the identification of heteropods, particularly Atlantidae, and are discussed below.

—Shell
The shells of heteropods are calcareous (composed of aragonite) with the exception of the keel in Protatlanta and the keel and shell in Oxygyrus (composed of a proteinaceous, cartilaginous-like material, conchiolin). Because the adult shell (teleoconch) of atlantids and carinarids is produced directly from the larval shell (the protoconch), the larval shell is present as the inner, helical part of the adult shell in atlantids and at the apex of the adult shell in carinarids. The first shell material secreted forms the embryonic shell (referred to as protoconch I), which is dome-shaped and is produced by the mantle of the developing embryo while it is still inside the egg capsule. Although the embryonic shell is uncalcified in some groups of gastropods, it is always calcified in the heteropods. The remaining portion is the true larval shell, referred to as protoconch II. In atlantids, carinarids and pterotracheids, larval shells are often distinctive in size, shape and whorl ornamentation.

In Atlantidae the overall shape and surface ornamentation on the larval shell and the spire of the adult shell are of great taxonomic importance (Thiriot-Quiévreux, 1973, 1975; Richter, 1986, 1987, 1990, 1993; Seapy, 1990b; Seapy and Thiriot-Quiévreux, 1994). Presence and expression of surface ornamentation, depth of the suture (separating the shell whorls) and depth of the umbilicus can best be observed using a scanning electron microscope. General shape of the spire and spire whorls are best investigated using transmitted light.

The number of shell whorls comprising the spire is an important taxonomic character in Atlantidae. Unlike most bottom-dwelling gastropods, however, the exact number of spire whorls can be difficult to determine because the transition between the larval shell (protoconch) and adult shell (teleoconch) is often difficult to determine. Nonetheless, even an approximation of the number of spire whorls is infinitely more reliable for taxonomic purposes than the total number of shell whorls, since the latter increases with shell growth. Here we have approximated the number of spire whorls by determining the location where whorl width begins to increase rapidly, to accomodate the increased size of the post-metamorphic animal.

(He. table Atlantidae)
(He. table Carinariidae)
(He. table Pterotracheidae)

In Carinariidae the protoconch is compact, thick-walled and helicoid. Unlike the atlantids, the border between the protoconch and teleoconch is quite distinct. Larval shells of carinarids are transparent, but some species have a yellow, brown or pink coloration, which is visible even on the protoconchs at the apex of the adult shells. The teleoconch is transparent and colorless, with thin and brittle, undulating walls. The adult shell ranges in size from a large, cap-shaped or flattened structure covering the visceral nucleus (in Carinaria and Pterosoma, respectively) to a greatly-reduced shell, only slightly larger than the larval shell, which is retained at the apex of the visceral nucleus (in Cardiapoda).

In Pterotracheidae only a larval shell develops, and it is thrown off during metamorphosis. Three Pterotrachea larvae have been distinguished from the Mediterranean (Krohn, 1860; Richter, 1968; Thiriot-Quiévreux, 1973). Here we give short descriptions of the shells of these species. 1) Shell transparent and smooth, with a perfectly round diameter and strong walls. Whorls (about 1 and 1/2) enlarge slowly, and the last half detaches from the first, leaving a wide gap between the first and second whorls. 2) Shell extremely thin, so that it collapses when dried. Whorls (also about 1 and 1/2) enlarge rapidly and are not separated by a deep, wide suture. Entire shell wall undulated parallel to growth lines. 3) Shell flat and helicoid, consisting of about two whorls that enlarge rapidly. The walls are thinner than in the first species, but are still strong and completely calcified.

—Eyes
The eyes are enclosed in a gelatinous capsule and consist of a distal spherical lens, intermediate region that is partially to completely pigmented, and a proximal, strip-like retina. The pigmented layers shield the space between the lens and retina from extraneous light. The eyes of different species can vary in shape (from triangular to cuboidal to rectangular) and the pigment layers can differ in coverage and disposition in the eye. In the Atlantidae, three basic eye types were characterized by Richter (1961), which he subsequently (1974) refered to as types a, b and c. (He 2a , He 2b , He 2c).

Distribution of the eye types among the species of atlantids is summarized in He. table 4 .

In Pterotracheidae eye shape is effected by the width of the retinal base, resulting in eyes that range in shape from rectangular to triangular when viewed dorsally. Bonnevie (1920) and subsequent workers have emphasized the importance of eye shape in distinguishing between the species of Pterotrachea. For the most part, eye morphology has not been used as a taxonomic character in Carinariidae.

—Operculum
The operculum of all Heteropoda is cartilaginous and flexible. It is present in all larval stages. In Atlantidae the larval operculum grows spirally, but after metamorphosis it grows in a straight line. Thus, the nucleus (or gyre) is situated excentrically in the narrower upper portion of the operculum. Richter (1961) distinguished three different types of opercula in the atlantids, which he characterized as macro-oligogyre ,micro-oligogyre andmonogyre; he subsequently (1974) designated these as types a, b and c, respectively.

Only Oxygyrus does not fit into one of these three types. Its operculum is broadly triangular (almost trapezoidal), lacks a spiral part and shows growth lines somewhat like those on the shell of Mytilus (Bonnevie, 1920). In three species of Indo-Pacific atlantids (Atlanta turriculata, A. echinogyra and A. plana) the nucleus is decorated by spines or hooks, arranged in a specific pattern (Tokioka, 1961; Richter, 1972). Opercular types for the species of Atlantidae are summarized inHe. table 4 .

In Carinariidae and Pterotracheidae the operculum is present only in the larvae, since it is thrown off during metamorphosis. The carinarid operculum consists of two layers, connected only in the central part. The outer layer is larger than the inner layer and is larger than the shell aperture. When the larva withdraws into its shell, the inner layer enters the aperture while the fringe of the outer layer bends outward, forming a perfect closure. The same type of operculum is present in the pterotracheid Firoloida desmaresti, a species with a larval shell quite similar to that of carinarids. All other species of pterotracheids (in the genus Pterotrachea) have an extremely thin operculum, which looks from the outside like a set of stacked dishes of different sizes (Richter, 1968).

—Radula
Radula shape and radular tooth morphology were rejected as taxonomic characters by Buchmann (1924) and Tesch (1949) as a result of their lack of understanding of radula morphogenesis. Shapes of the radular teeth change dramatically during ontogenesis and depend on the position on the radula of the tooth row investigated. This morphogenesis of teeth continues in Atlantidae for a great part of radula growth, whereas in Carinariidae and Pterotracheidae (as well as in many other prosobranch gastropods) it is restricted to the late larval and first postlarval stages. For radula growth and tooth morphogenesis and function see Richter (1961, 1963).

Here we distinguish two types of radulae. In type I radulae the teeth that are produced first are never cast off from the anterior end of the radula as they outwear their use; the number of tooth rows increases continuously as the animal grows and all morphogenetic stages of teeth are present in mature animals (i.e., unlimited number of tooth rows). In type II radulae teeth are cast off the anterior end of the radula as soon as the radula has reached its specific number of tooth rows (i.e., limited number of tooth rows). In the latter case the number of tooth rows may be of taxonomic value. The radula types of atlantid species are summarized in He. table 4. All Carinariidae and Pterotracheidae have type II radulae.

In each tooth row the central, rachidian tooth is flanked on either side by a lateral and two marginal teeth (He 4a). The outward spreading of the swivel-mounted marginal teeth mentioned in the Introduction can be seen in this photograph as those teeth that flare outward at the turning point of the radula. The shapes of the central, lateral and marginal teeth are taxonomically important. At the family level the central tooth differs noticeably. Among the Atlantidae this tooth has a single median cusp in Atlanta (He 4b) and Protatlanta, but is tricuspid in Oxygyrus. In Carinariidae three long and prominent cusps are present (He 4c), while in Pterotracheidae a well-developed median cusp is flanked by numerous short cusps that decrease in size laterally (He 4d).