All the species but one, Atlanta californiensis, dwell in tropical and subtropical waters.[3] 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.[3]
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;[3] larval shell becomes the spire in the adult shell.[3]
Snails of this genus are very small. Their coiled, calcareous[3] shell has a diameter of less than 1cm. The protoconch of the larval shell is retained after metamorphosis and becomes the spire of the adult shell.[3] 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[4] 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).[4]
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 CarinariaLamarck, 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. [5]
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.[1] However, such methods are unavailable for fossil material.[1] 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).[1] 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.[1] Still, here, too, study of the larval shell shape and micro-ornamentation by SEM is highly desirable or even indispensable.[1]
Species
Atlanta includes a large number of Recent species. Lalli & Gilmer (1989)[6] listed 14 species, but Richter & Seapy (1999)[7] recognised 21 extant species, provisionally subdivided into seven 'species groups' (and one species unassigned).[1] A further Recent species was described since; Atlanta selvagensis de Vera & Seapy, 2006.[1]
Species in the genus Atlanta include:
† Atlanta arenulariaGougerot & Braillon, 1965 - this is the oldest known Atlanta from the Bartonian of the Paris Basin[1]
Tesch (1908) was the first to group together the species of Atlanta sharing similar morphologies.[3] He recognized four species groups; the Atlanta peronii-, Atlanta inflata-, Atlanta turriculata-, and Atlanta inclinata-groups.[3] In addition to these four, three additional ones are currently recognized; the Atlanta lesueurii-, Atlanta gaudichaudi- and Atlanta gibbosa groups.[3] 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.[3] 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).[3]
References
This article incorporates CC BY-3.0 text from references.[1][3]
12Richter G. (1961). "Die Radula der Atlantiden (Heteropoda, Prosobranchia) und ihre Bedeutung für die Systematik und Evolution der Famiie". Morphol. Okol. Tiere. 50: 163–238. doi:10.1007/bf00408284.
↑Lalli C. M. & Gilmer R. W. (1989). Pelagic Snails. The Biology of Holoplanktonic Gastropod Mollusks. Stanford University Press, Stanford, California: viii + 259 pp. page 52.
↑Richter G. & Seapy R. R. (1999). "Heteropoda". In: D. Boltovskoy (ed.) South Atlantic zooplankton, 1. Backhuys, Leiden: 621–647. page 631.
↑WoRMS (2010). Atlanta brunnea J. E. Gray, 1850. In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2010) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=238118 on 2010-08-19
↑Gofas, S. (2009). Atlanta fragilis Richter, 1993. In: Bouchet, P.; Gofas, S.; Rosenberg, G. World Marine Mollusca database. Accessed through the World Register of Marine Species at http://marinespecies.org/aphia.php?p=taxdetails&id=244556 on 2010-08-19
↑WoRMS (2009). Atlanta oligogyra Tesch, 1806. In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2009) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=244554 on 2010-08-19
↑Gofas, S. (2009). Atlanta peronii Lesueur, 1817. In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2009) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=138838 on 2010-08-19
↑Gofas, S. (2009). Atlanta rosea Souleyet, 1852. In: Bouchet, P.; Gofas, S.; Rosenberg, G. World Marine Mollusca database. Accessed through the World Register of Marine Species at http://marinespecies.org/aphia.php?p=taxdetails&id=238120 on 2010-08-19
↑Vera, A de & Seapy, R.R. (2006). "Atlanta selvagensis, a new species of heteropod mollusc from the northeastern Atlantic Ocean (Gastropoda: Carinarioidea)". Vieraea. 34: 45–54.
↑Richter G. & Seapy R. R. (1999). Heteropoda. In: D. Boltovskoy (ed.). South Atlantic zooplankton, 1. Backhuys, Leiden; p. 621-647.
Richter G. & Seapy R.R. 1999. Heteropoda, pp.621–647. In: D. Boltovskoy (ed.), South Atlantic Zooplankton. Leiden: Backhuys Publ.
Gofas, S.; Le Renard, J.; Bouchet, P. (2001). Mollusca, in: Costello, M.J. et al. (Ed.) (2001). European register of marine species: a check-list of the marine species in Europe and a bibliography of guides to their identification. Collection Patrimoines Naturels, 50: pp.180–213
Rolán E., 2005. Malacological Fauna From The Cape Verde Archipelago. Part 1, Polyplacophora and Gastropoda.
Rosenberg, G., F. Moretzsohn, and E. F. García. 2009. Gastropoda (Mollusca) of the Gulf of Mexico, Pp.579–699 in Felder, D.L. and D.K. Camp (eds.), Gulf of Mexico–Origins, Waters, and Biota. Biodiversity. Texas A&M Press, College Station, Texas.
Spencer, H.; Marshall. B. (2009). All Mollusca except Opisthobranchia. In: Gordon, D. (Ed.) (2009). New Zealand Inventory of Biodiversity. Volume One: Kingdom Animalia. 584 pp
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