Atlanta gibbosa
Roger R. SeapyIntroduction
Atlanta gibbosa is of moderate size (to 4 mm). The shell is completely transparent and colorless, and the walls are thin. The shell surface is smooth, lacking any sculpture. The keel is tall with a truncate leading edge. The keel penetrates between the penultimate and last whorls in shells larger than about 2 mm. The shell spire consists of about 6 whorls and reaches a total of 7-1/2 whorls in full-grown specimens (highest among all Atlanta species). The spire shape is unique among the species with inclined spires; broadly conical with a wide base, a narrow, rounded apex, and an apical angle of about 85°. The spire sutures are moderately deep. The left side of the shell has a conspicuously wide umbilicus. Eyes type b; lens large in adults. Operculum type b, and very thin. Radula type II, with a growth angle of about 15°. Radula with strong sexual dimorphism in width and tooth morphology. The radula is about 25% wider in females than in males. The marginal teeth are much shorter in males than in females, and their shape is very different in males (as sharply-curved, short hooks) than in females (elongate, with more weakly-curved hooks). Also, in the male radula the short pair of marginal teeth leave a conspicuous gap between their hooked ends and those of the lateral teeth. The species is most probably limited to the Indian and Pacific Oceans, and is found in tropical to subtropical waters.Diagnosis
- Maximal shell diameter = 4 mm
- Shell transparent, colorless
- Shell surface smooth, without sculpture
- Keel tall, with truncate leading edge
- Keel penetrates between last two whorls in shells larger than 2 mm
- Spire with about 6 whorls; more than any other Atlanta species
- Spire shape broadly conical, with an apical angle of 90°
- Sutures moderately deep, clearly separating spire whorls
- Left side of shell with a very wide umbilicus
- Eyes type b
- Operculum type b
- Radula type II
- Growth angle of radula about 15°
- Radulae sexually dimorphic; width about 25% wider in females than in males
- Marginal teeth sexually dimorphic; short with sharply-curved hooks in males and elongate with gradually-curved hooks in females
Characteristics
- Shell
- Shell of moderate size (to 4 mm diameter), with up to 7-1/2 whorls (highest among all species of Atlanta)
- Shell transparent and colorless
- Shell fragile, with thin walls
- Keel tall, with a truncate leading edge (see title illustration)
- Keel penetrates between penultimate and last shell whorls in shells larger than about 2 mm diameter
Click on an image to view larger version & data in a new windowClick on an image to view larger version & data in a new windowFigure. Shell of Atlanta gibbosa, with views of the right side (left) and spire (right). Images from Richter (1972, fig. 1) and Richter (1974, fig. 13, right), modified by addition of scale bars (=1.0 mm and 0.5 mm, respectively). © 1972 and 1974 G. Richter
- Spire only moderately tilted relative to shell plane; lowest among the species of Atlanta with inclined spires
- Spire of about 6 whorls; highest among the species of Atlanta with inclined spires
- Spire broadly conical in side view, with a broad base and a narrow, somewhat rounded apex, and with an apical angle of about 85° (Richter, 1990)
- Spire sutures moderately deep and distinctly separate adjacent whorls (see above image)
- Internal walls of spire intact; not decalcified as in A. inclinata and A. tokiokai
- Left side of shell with unusually wide umbilicus
Click on an image to view larger version & data in a new windowFigure. Shell of Atlanta gibbosa; views of the left side (left) and left side, umbilical region (right). Images from Richter (1972, fig. 1, lower) and Richter (1990, fig. 18), respectively, modified by addition of scale bars (= 1 mm and 100 µm, respectively). © 1972 and 1990 G. Richter
- Shell of moderate size (to 4 mm diameter), with up to 7-1/2 whorls (highest among all species of Atlanta)
- Eyes type b
- Lens conspicuously large in adults, suggesting that the species prefers deeper daytime depths (Richter, 1974)
- Lens conspicuously large in adults, suggesting that the species prefers deeper daytime depths (Richter, 1974)
- Operculum type b
- Operculum very thin
- Radula type II
- Radula with a growth angle of about 15°
- Sexual dimorphism in radula width; about 25% greater in females than in males
- Sexual dimorphism in marginal teeth shape and length (see pair of images above). In males the marginal teeth are short, forming sharply-curved hooks, while in females the teeth are elongate and more gradually-curved
- Rachidian teeth with a broad, low basal plate and a long, slender cusp.
- Rachidian tooth width comparable with that in A. meteori, but about 40% wider than in A. inclinata and about 75% wider than in A. tokiokai
Comments
Atlanta gibbosa and A. meteori comprise the Atlanta gibbosa species group. The two species share a series of characters (summarized in Richter and Seapy, 1999): (1) shell is transparent, colorless, and fragile with thin walls, (2) keel is tall, with a truncate leading edge, (3) keel penetrates between the penultimate and last whorls in shells larger than about 2 mm, (4) eye is type b, (5) lens of the eye in adults is conspicuously large, suggesting that the species prefer greater daytime depths (Richter, 1974), (6) operculum is type b, (7) radula is type II, with a moderate growth angle of 12-15°, and (8) lateral teeth are relatively straight; not noticeably curved as in most species, and (9) rachidian teeth are wide, with a broad base; considerably wider than in the Atlanta inclinata species group.
Atlanta gibbosa has been recorded from the Indian and Pacific Oceans, although it has not been reported from the Atlantic Ocean with the exception of a single record by Souleyet (1852) that cannot be verified from voucher material (discussed by Richter, 1990).
References
Richter, G. 1972. Zur Kenntnis der Gattung Atlanta (Heteropoda: Atlantidae). Archiv fur Molluskenkunde 102: 85-91.
Richter, G. 1974. Die Heteropoden der "Meteor" Expedition in den Indischen Ozean 1964/65. "Meteor" Forschungs-Ergebnisse 17: 55-78.
Richter, G. 1990. Zur Kenntnis der Gattung Atlanta (IV). Die Atlanta inclinata-Gruppe (Prosobranchia: Heteropoda). Archiv fur Molluskenkunde 119: 239-275.
Richter, G. and R. R. Seapy. 1999. Heteropoda, pp. 621-647. In: D. Boltovskoy (ed.), South Atlantic Zooplankton. Leiden: Backhuys Publ.
Souleyet, L. F. A. 1852. In: J. F. T. Eydoux and L. F. A. Souleyet. Voyage autour du monde execute pendant les annees 1836 et 1837 sur la corvette 'La Bonite'commandee par M. Vaillant, capitaine de vaisseau, publie par ordre du Gouvernement sous les auspices du Departement de la marine. Zoologie. 2: 1-664. A. Bertrand, Paris.
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California State University, Fullerton, California, USA
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- First online 17 July 2010
- Content changed 17 July 2010
Citing this page:
Seapy, Roger R. 2010. Atlanta gibbosa http://tolweb.org/Atlanta_gibbosa/28761/2010.07.17 in The Tree of Life Web Project, http://tolweb.org/
. Version 17 July 2010 (under construction).