Atlanta (gastropod)

Atlanta is a genus of pelagic marine gastropod molluscs in the family Atlantidae. They are sometimes called heteropods.

Distribution
All of the nineteen species but one, Atlanta californiensis, dwell in tropical and subtropical waters. The majority of species (ten) are cosmopolitan and, among the remaining nine species, five are Indo-Pacific, two are restricted to the Pacific Ocean, one is Indo-Atlantic, and one is limited to the Atlantic Ocean.

They are floating or swimming snails in tropical and subtropical seas. Most have a cosmopolitan distribution, but A. brunnea, A. pulchella and A. quoyi are only found in American waters. A. fusca, A. pacifica and A. rosea are restricted the seas around Japan.

Description
It has been recognized by several authors that identification of species in this genus is difficult and is dependent on their morphology of eyes, radula and operculum.

Main diagnostic features include: the shell and keel are calcareous; larval shell becomes the spire in the adult shell.

Snails of this genus are very small. Their coiled, calcareous shell has a diameter of less than 1 cm. The protoconch of the larval shell is retained after metamorphosis and becomes the spire of the adult shell. The number of spire whirls varies from 2½ (in the A. lesueuri- group) to 6 (A. gibbosa) and is thus also helpful in the identification of a species. The spire shape differs between the species groups, from very small (A. lesueuri- group), to inflated or flat (A. inflata- group ) to large (A.inclinata- group and A. gibbosa- group).

They can retract into their shell and close it off with an operculum. This operculum is cartilaginous and flexible. In 1961 Richter distinguished three types of the operculum in which the larval gyre of the operculum is apical. This gyre can be relatively somewhat larger (macro-oligogyre), smaller (micro-oligogyre) or a single gyre (monogyre).

The eye morphology also consists of three types with differences in pigmented region between the lens and the retina.

The radula is typically taenioglossate with one central (rachidian) tooth, with on each side one lateral tooth and two marginal teeth. In 13 species the number of tooth rows increases during growth (Type I), while in 8 species the radula has a limited number of tooth rows (Type II).

Identification
Many authors (e.g., Thiriot-Quiévreux, 1973, p. 240; Richter, 1974, p. 60; Seapy, 1990, p. 107) admit that identification of Atlanta species is difficult and including soft-part features (eyes, radula, operculum) or application of transmitted light to observe inner shell structures (Richter, 1987, p. 178) are very helpful in distinguishing species with similar shells. However, such methods are unavailable for fossil material. This makes identifying fossil species of Atlanta quite difficult and even well-preserved specimens occasionally can only be related to existing taxa with a query (e.g., Atlanta sp. in Janssen, 2004, p. 108; Atlanta cf. echinogyra in Jansen 2007). Advantageous in this study of fossil atlantids, however, is the fact that all specimens are preserved as opaque aragonitic shells as a result of recrystallisation, which facilitates assessing protoconch shape and ornament with a normal 25 or 50× binocular magnification, they are thus much easier studied than in the usually very transparent and shiny Recent specimens. Still, here, too, study of the larval shell shape and micro-ornamentation by SEM is highly desirable or even indispensable.

Species
Atlanta includes a large number of Recent species. Lalli & Gilmer (1989) listed 14 species, but Richter & Seapy (1999) recognised 21 extant species, provisionally subdivided into seven 'species groups' (and one species unassigned). A further Recent species was described since; Atlanta selvagensis de Vera & Seapy, 2006.

Species in the genus Atlanta include:


 * † Atlanta arenularia Gougerot & Braillon, 1965 - this is the oldest known Atlanta from the Bartonian of the Paris Basin
 * Atlanta brunnea J. E. Gray, 1850 - synonym: Atlanta fusca Eydoux & F. L. A. Souleyet, 1852
 * Atlanta californiensis Seapy & Richter, 1993
 * Atlanta echinogyra Richter, 1972
 * Atlanta fragilis Richter, 1993
 * Atlanta frontieri Richter, 1993
 * Atlanta gaudichaudi Souleyet, 1852
 * Atlanta helicinoidea J. E. Gray, 1850
 * Atlanta inclinata J. E. Gray, 1850
 * Atlanta inflata J. E. Gray, 1850
 * Atlanta lesueurii J. E. Gray, 1850
 * † Atlanta lingayanensis Janssen, 2007 - from Pliocene of Philippines
 * Atlanta meteori Richter, 1972
 * Atlanta oligogyra Tesch, 1906
 * Atlanta peronii Lesueur, 1817
 *  Atlanta plana Richter, 1972
 * Atlanta pulchella Verrill, 1884
 * Atlanta quoyii Gray, 1850
 * † Atlanta richteri Janssen, 2007 - from Pliocene of Philippines
 * Atlanta rosea Souleyet, 1852
 * † Atlanta seapyi Janssen, 2007 - from Pliocene of Philippines
 * Atlanta selvagensis de Vera & Seapy, 2006
 * Atlanta tokiokai van der Spoel & Troost, 1972
 * Atlanta turriculata d'Orbigny, 1836
 * Species brought into synonymy:
 * Subgenus Atlanta (Heliconoides) d'Orbigny, 1835: synonym of Heliconoides d'Orbigny, 1835
 * Atlanta bulimoides d'Orbigny, 1834: synonym of Limacina bulimoides (d'Orbigny, 1834)
 * Atlanta fusca Souleyet, 1852: synonym of Atlanta brunnea J. E. Gray, 1850
 * Atlanta gaudichaudii Souleyet, 1852: synonym of Atlanta gaudichaudi J. E. Gray, 1850
 * Atlanta gibbosa Souleyet, 1852: synonym of Atlanta inclinata J. E. Gray, 1850
 * Atlanta helicinoides Souleyet, 1852: synonym of Atlanta helicinoidea J. E. Gray, 1850
 * Atlanta inclinata Souleyet, 1852: synonym of Atlanta inclinata J. E. Gray, 1850
 * Atlanta inflata Souleyet, 1852: synonym of Atlanta inflata J. E. Gray, 1850
 * Atlanta inflata d'Orbigny, 1834: synonym of Heliconoides inflata (d'Orbigny, 1834)
 * Atlanta lamanoni Gray, 1850: synonym of Protatlanta souleyeti (E. A. Smith, 1888)
 * Atlanta lesueurii Souleyet, 1852: synonym of Atlanta lesueurii J. E. Gray, 1850
 * Atlanta lesueurii d'Orbigny, 1835: synonym of Limacina lesueurii (d'Orbigny, 1835)
 * Atlanta quoyana Smith, 1888: synonym of Atlanta quoyii J. E. Gray, 1850
 * Atlanta steindachneri Oberwimmer, 1898: synonym of Atlanta peronii Lesueur, 1817
 * Atlanta trochiformis d'Orbigny, 1834: synonym of Limacina trochiformis (d'Orbigny, 1834)

Based on similar morphologies, these species have been placed in seven species groups:

Tesch (1908) was the first to group together the species of Atlanta sharing similar morphologies. He recognized four species groups; the Atlanta peronii-, Atlanta inflata-, Atlanta turriculata-, and Atlanta inclinata-groups. In addition to these four, three additional ones are currently recognized; the Atlanta lesueurii-, Atlanta gaudichaudi- and Atlanta gibbosa groups. Except for Tesch's Atlanta turriculata-group, the composition of Tesch's species groups has changed by species invalidations, the addition of new species over time, and addition of three new species groups. The main changes in Tesch's species groups have occurred in the Atlanta peronii-group (with Atlanta gaudichaudi and Atlanta lesueurii now forming their own species groups) and the Atlanta inclinata-group (the Atlanta gibbosa now forming its own group).