Atlantidae Rang, 1829

Introduction

Atlantids are the most species-rich family of heteropods, containing over 60% of all species. They are microscopic (< 1 cm shell diameter), and bear a coiled shell into which they can retract their bodies. After retraction into the shell, the shell aperture is closed off by a chitinous operculum attached to 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 family of heteropods with:

• Microscopic size (less than 10 mm shell diameter) 
• Body retractable into coiled shell; shell aperture closed off by chitinous operculum
  
• Swimming fin with large, muscular sucker that is used to hold prey while feeding
  

Characteristics

1. Shell
1. Shell with dextral or "right-hand" coiling (viewed from right side of shell)
   
2. Shell laterally compressed with a flat keel of varied height extending outward from the shell around most to all of the shell's circumference
   


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Figure. Left: Atlanta helicinoides viewed from right side, with body retracted into shell. Right: Scanning electron micrograph of Atlanta plana, shell diameter = 2.1 mm. Shell tilted to illustrate aperture and low conical spire. © R. Seapy

2. Operculum
1. Operculum closes off shell aperture after animal retracts into the shell (see figure above)
2. Operculum attached to opercular lobe of the foot (see title illustration and figure below)


3. Foot
   
1. Anterior portion of foot forms a muscular swimming fin, with a large sucker on the ventro-lateral margin that is used to hold prey during feeding
   
2. Opercular lobe develops from the posterior part of the foot and bears a chitinous operculum
   

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   Figure. Oxygyrus keraudreni viewed from the right side. Operculum, like the shell and keel, is transparent and not visible in photograph. © R. Seapy.

4. Head
1. Eyes well-developed, consisting of a large spherical lens that rests in a partially to entirely pigmented base
2. Tentacles large and of equal size, extending anteriorly from beneath the eyes
   
   

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   Figure. Head in Atlanta peroni. Tentacles extend anteriorly on either side of the proboscis dorsum. © R. Seapy.

5. 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).