Atlanta

Distribution

All the 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

The Atlanta, whose discovery dates back to the La Pérouse expedition and which were so named by Mr. Lesueur—though Mr. Rang was the first to make their true characteristics known—complete the series of Heteropods. They link these mollusks to the Gastropods, whose entire conformation they share, as we have previously seen.

The Atlanta do not differ as much from the Carinaria Lamarck, 1801 as one might believe at first glance; one can even form a very accurate idea of them by imagining them as Carinaria whose bodies, while retaining their shape, have been reduced in proportion, and whose shells have simultaneously achieved a development large enough for the animal to retract into them entirely. The branchial cavity with which they are provided, and the slightly different termination of the intestine and the oviduct within this cavity, are evidently nothing more than the result of this shell development, which has brought about an analogous development in the corresponding part of the mantle.

Finally, the ability of the Atlanta to shelter themselves completely within their shells further explains the presence in these mollusks of an operculum, arranged exactly as it is in Gastropods with turbinate shells.

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 ariejansseni Wall-Palmer, Burridge & Peijnenburg, 2016

Atlanta brunnea J. E. Gray, 1850

Atlanta californiensis Seapy & Richter, 1993

† Atlanta diamesa Woodring, 1928

Atlanta echinogyra Richter, 1972

† Atlanta eocenica K. van W. Palmer, 1937

Atlanta fragilis Richter, 1993

Atlanta frontieri Richter, 1993

† Atlanta funicularis Garvie, 2013

Atlanta gaudichaudi Souleyet, 1852

Atlanta gibbosa Souleyet, 1852

Atlanta helicinoidea J. E. Gray, 1850

Atlanta inclinata J. E. Gray, 1850

† Atlanta indoceratoides Beets, 1942

Atlanta inflata J. E. Gray, 1850

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 cordiformis Gabb, 1873: synonym of Atlanta selvagensis de Vera & Seapy, 2006

Atlanta depressa Gray, 1850: synonym of Atlanta helicinoidea J. E. Gray, 1850 (doubtful synonym)

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 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 keraudrenii Lesueur, 1817: synonym of Atlanta peronii Lesueur, 1817

Atlanta lamanoni Gray, 1850: synonym of Protatlanta souleyeti (E. A. Smith, 1888)

Atlanta lamanonii Souleyet, 1852: synonym of Protatlanta souleyeti (E. A. Smith, 1888)