Wiegmann and Ruthe, 1832
Members of this family are small (less than 10 mm) and possess a dextrally-coiled shell into which the body can retract and close off with a chitinous operculum, which is attached to the dorsal side of the foot. A single gill and osphradium are positioned dorsally within the mantle cavity. A pair of well-developed tentacles arise ahead of the eyes. In all atlantids a large sucker is located on the posteroventral margin of the fin. It is present in both sexes and serves to hold prey tightly while the strong radula tears off and ingests pieces of prey tissue (Richter, 1968; Thiriot-Quiévreux, 1969). The radula is of two types (I and II), and can show distinct sexual dimorphism. Since males and females feed on the same prey, we know nothing about a possible functional basis for this dimorphism.
The identification of most heteropods, but especially of species in the genus Atlanta, can be very difficult. To facilitate identification, we have distinguished eight species groups in the Atlantidae, each of which is named after a representative species. (Similarly, van der Spoel, in his 1976 text, arranged the various species of Atlanta that he recognized at the time into a series of six species groups.) It is our hope that even the inexperienced student will be able to distinguish between these species groups. This accomplished, the probability of identifying the specimens at hand to the species level should be greatly increased. The list below includes the species of atlantids currently recognized from the worlds oceans, arranged by species groups. Those species that we consider to be present in the South Atlantic are characterized below. Papers containing descriptions (more or less complete) of all these species are found in the list of references. For synonymy lists see van der Spoel (1976).
Oxygyrus keraudreni species group (Protatlanta souleyeti, Oxygyrus keraudreni)
Shell with a glass-like, transparent keel, composed of conchiolin. Leading edge of keel truncate, sloping abruptly and ending at the shell opening. Although all or part of the adult shell is constructed of conchiolin, the larval shell is calcareous. Radula type I.
All remaining species of Atlantidae are members of the genus Atlanta Lesueur, 1817 in which the coiled adult shell is produced directly from the calcareous larval shell. Both the adult shell and keel are calcareous. Keel height varies from low to high, and the leading edge is not truncate, but slopes towards the shell mouth and ends in a narrow fissure. Those species whose distributions are either limited to the Pacific Ocean or the Indo-Pacific region are not described, but are listed under the species group in which they belong (and in the outline classification above).
Atlanta fusca species group (Atlanta fusca, Atlanta turriculata)
Shell small (less than 2.0 mm maximal diameter) and pigmented (yellowish-brown to brown, or reddish brown). Keel tall and rounded. Spire strongly sculptured and shaped either as a tall cone or a turret. Eyes type a, operculum type a. Radula type I, small and ribbon-shaped. Central tooth (like in all Atlanta species) monocuspid, with one posterio-lateral process on each side. Lateral teeth with a strong rectangular process on the inner side (like in all Atlanta species) and bicuspid, even in adult specimens. Marginal teeth monocuspid.
Atlanta inflata species group (Atlanta inflata, Atlanta helicinoides, Atlanta californiensis)
Shell small (diameter less than 2.0 mm) and inflated (width about 40% of shell diameter). Spire low, with shallow sutures and surface sculpture ranging from none (smooth) to low and more or less distinct, evenly-spaced spiral ridges. Inner walls of spire gradually decalcified after metamorphosis so that only a thin cartilaginous membrane separates the whorls. (Note that this membrane is visible using transmitted light or when the spire is broken). Operculum type c, radula type I (up to 120 tooth rows observed).
Atlanta lesueuri species group (Atlanta lesueuri, Atlanta oligogyra)
Shell small to medium size (less than 4-6 mm), with an extremely small spire of about 2 and 1/2 whorls. After metamorphosis whorl size increases rapidly, and shells of the largest adults consist of only 3 and 1/2 whorls. Operculum type b. Radula type I; number of tooth rows not limited (>100 observed).
Atlanta peroni group (Atlanta peroni, Atlanta fragilis, Atlanta frontieri)
Shells large (to 10 mm diameter) and flat, with smooth surfaces and a small spire. Colorless or faintly tinted in pink or yellow. Eyes type b, operculum type b. Type II radula, large but with a low growth angle. Number of tooth rows limited (up to 50-70). In full-grown specimens lateral teeth with a small but distinct accessory cusp on the posterior side close to the main cusp. Considerable confusion has surrounded the species comprising this group. Subsequent to Tesch (1949) and prior to Richter (1993) only a single species (A. peroni) was recognized. The latter study recognized four valid species, resulting in the resurrection of A. rosea (discussed below) and the description of two new species; A. frontieri, represented in the Indian Ocean by specimens previously identified by Richter (1974) as A. peroni, and A. fragilis, which appears to be limited to the Atlantic Ocean.
Atlanta gaudichaudi species group (Atlanta gaudichaudi, Atlanta plana, Atlanta echinogyra)
Shell small to intermediate size (maximal diameter 2.5-5 mm), with small, but distinctly helicoid spire. Radula type I, with unlimited numbers of tooth rows and monocuspid lateral teeth. These radular characters distinguish this species group from the A. peroni species group, in which the number of tooth rows is limited and the lateral teeth are bicuspid.
Atlanta inclinata species group (Atlanta inclinata, Atlanta tokiokai)
The two species comprising this group share a series of characters that are so striking that it seems extremely difficult to distinguish them. They are: 1) large, beehive-shaped, inclined spire; 2) decalcified inner walls of the spire (like in the A. inflata group); 3) internal wall structure of radially-arranged lines (visible only using transmitted light); 4) surface sculpture of small tubercula (or punctae) on the spire and, to a lesser degree, on the body whorl; 5) type b eyes; and 6) type c operculum. This species group and the next one include four species with large and inclined spires. Van der Spoel (1976) grouped all species with inclined spires in an A. inclinata species group. However, this distinctive character has concealed basic, but less striking, differences between the species (Richter, 1990). Comparisons of these four species indicate that an inclined spire has developed along two quite different lines in the genus Atlanta, and the species in each lineage appear to be more closely related to species with straight spires than they are with the species in the other lineage.
Atlanta gibbosa species group (Atlanta gibbosa, Atlanta meteori)
Shells of medium size (up to about 4 mm diameter), completely transparent and colorless, with thin walls and a high keel that penetrates between the last whorls. Eyes type b, with lenses that are conspicuously large (in adult specimens), thus suggesting that these species prefer deeper daytime depths (Richter, 1974). Operculum type b. Type II radula, with low growth angle. Lateral teeth straight (not curved as in most species) and with strong accessory cusps. The two species in this group appear to be found exclusively in the Indo-Pacific region, although A. gibbosa was reported by Eydoux and Souleyet (Souleyet, 1852b) from an Atlantic plankton sample. Thus, we have included the characterization of this species group and A. gibbosa below.
Unassigned species (Atlanta peresi)