Atlantidae Rang, 1829

Introduction

Atlantids are the most species-rich family of heteropods, containing over 60% of all species. They are small (< 1 cm), and their bodies can be retracted entirely into their shells and closed off by a chitinous operculum on the opercular lobe of the foot. The head has a pair of large tentacles anterior to the eyes. A large muscular sucker, located on the posteroventral margin of the swimming fin, is used to hold prey fast while feeding. Atlantids are found primarily between the surface and 200 m in tropical to subtropical waters. Many species undergo vertical migration from daytime depths into shallower waters at night.

Brief Diagnosis

A heteropod with:

• Microscopic size (to about 10 mm) 
• Body retractable into shell.

Characteristics

1. Shell
1. Shell with dextral (right-hand) coiling and laterally flattened.
2. Body retractable into shell; aperture closed by a chitinous operculum.
   


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Figure. Left: Side view of Atlanta helicinoides, viewed from the right side; body retracted into shell. Right: Ventral, or apertural, view of Atlanta plana; shell diameter 2.1 mm. © R. Seapy

2. Foot
1. Sucker large and muscular to hold prey after capture.
   
2. Opercular lobe and operculum present.
   

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   Figure. Right side view of Oxygyrus keraudreni. Operculum, located beneath opercular lobe in photograph, not visible. © R. Seapy.

3. Head
1. Eyes well-developed, with a large spherical lens and a basal ribbon-like retina.
2. Wall of eye partially to completely pigmented
   
3. Tentacles large and may extend beyond contracted proboscis.
   

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   Figure. Right side view of head, proboscis and tentacles of Atlanta peroni. © R. Seapy.

4. Radula
1. Rachidian tooth (central tooth in each tooth row) with one (Atlanta and Protatlanta) or three (Oxygyrus) short cusps.
   

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   Figure. Radula of Atlanta californiensis. Scanning electron micrograph. © 1993 G. Richter.

Comments

The family Atlantidae includes three genera, two of which (Protatlanta and Oxygyrus) are monotypic. The genera can be distinguished by the following characters:

* Conchiolin, especially in fresh specimens, can be recognized by its extreme transparency.

Because of the presence of a shell into which their bodies can be retracted, sinking should be a major problem in the atlantids. As noted elsewhere, the carinariids and pterotracheids have enlarged, elongated bodies containing gelatinous tissues in which heavier sulfate ions are replaced by lighter chloride ions to achieve neutral buoyancy. Laboratory observations of Oxygyrus keraudreni by Land (1982) have shown that animals will alternately swim upwards for several seconds and then sink back down with their bodies extended from their shells for about 10 seconds.  In-situ field observations of atlantids at night by Gilmer (in Lalli and Gilmer, 1989) indicated very different behavior than was exhibited during the day. Individuals were motionless, attached to long strands of mucus that appear to originate from the foot.

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Figure. In-situ photograph of an unidentified atlantid attached to mucus strands (= MS), that extend about 45 mm above the animal. Note that the mucus appears to come from the foot (= F). Modified from Lalli and Gilmer (1989, fig. 11). © 1989 Ronald Gilmer

References

Lalli, C. M. and R. W. Gilmer. 1989. Pelagic snails. The biology of holoplanktonic gastropod snails. Stanford: Stanford University Press. 259 pp.

Land, M. F. 1982. Scanning eye movements in a heteropod mollusc. Journal of Experimental Biology 96: 427-430.

Richter, G. 1961. Die Radula der Atlantiden (Heteropoda, Prosobranchia) und ihre Bedeutung fur die Systematik und Evolution der Familie. Zeitschrift fur Morpholologie und Okologie der Tiere 50: 163-238.

Richter, G. and R. R. Seapy. 1999. Heteropoda, pp. 621-647. In: D. Boltovskoy (ed.), South Atlantic Zooplankton. Leiden: Backhuys Publishers.

Seapy, R. R. 1990. The pelagic family Atlantidae (Gastropoda: Heteropoda) from Hawaiian waters: a taxonomic survey. Malacologia 32: 107-130.

Spoel, S. van der. 1976. Pseudothecosomata, Gymnosomata and Heteropoda (Gastropoda). Utrecht: Bohn, Scheltema and Holkema. 484 pp.

Spoel, S. van der, L. Newman and K. W. Estep. 1997. Pelagic molluscs of the world. World Biodiversity Data Base CD-ROM Series. Amsterdam: Expert Center for Taxonomic Identification (ETI).