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 (<1 cm 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. Viewed from the right side, the direction of shell coiling is dextral  ("right-handed")
2. Shell laterally compressed, with the spire projecting to varied degrees from the right side, except in Oxygyrus keraudreni, in which the spire is turned inward (involute)
3. A flattened keel of varied height extends outward from 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, with shell tilted to illustrate aperture and low conical spire. Shell diameter = 2.1 mm. © 2005

2. Tentacles large and of equal size, extending anteriorly from beneath the eyes
   
   

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   Figure. Head, proboscis and tentacles in Atlanta peroni. Tentacles project forward from beneath the eyes on either side of the proboscis. © 2005

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. © 2005

4. Eye morphology of three types (termed a, b and c; distinguished by Richter, 1961). The eye types differ in the pigmented region between the lens and the retina. Type a and b eyes have a dorsal portion of the pigmented wall that is transparent, forming a window. In type c eyes, however, a dorsal window is lacking (see example of Atlanta helicinoides below). Type a and b eyes differ in the absence (in the former) or presence (in the latter) of a narrow transverse slit between proximal and distal pigmentation (see example of type b eye in A. peroni).
   

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   Figure. The three types of eye morphology (types a, b and c) in the Family Atlantidae, with the eyes in dorsal view. Drawing modified from Richter and Seapy (1999, Fig. 2) © 2005

   
   

   Figure. Type c eye of Atlanta helicinoides. © 2005

5. Operculum
1. The operculum is cartilaginous and flexible, and is attached to the opercular lobe of the foot (see title illustration and photograph of Oxygyrus keraudreni)
2. Operculum serves to close off the aperture after the animal retracts into its shell (see photograph of Atlanta helicnoides)
3. Operculum is present in all larvae, and growth is spiral resulting in a gyre. After larval metamorphosis, however, opercular growth is in a straight line. Thus, in the adult operculum the gyre is apical in location (see sketches below)
4. Three types of opercula (termed a, b, and c; distinguished by Richter, 1961) are found in two of the three atlantid genera (Protatlanta and Atlanta). The third genus, Oxygyrus, is markedly different, having an operculum that is broadly triangular and lacks an apical gyre (see the Oxygyrus keraudreni page). In the three types, the larval (or gyre) region of the operculum (shaded blue in the drawings below) is apical in location. The type c (or monogyre) operculum differs from the type a and b opercula (oligogyre) in the number of turns of the gyre ("single" in the monogyre and "a few" in the oligogyre opercula). The two types of oligogyre opercula are further distinguished by the relative size of the gyre; hence, the prefixes macro- and micro-.
   
   

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   Figure. The three types of opercular morphologies (a, b, c) in atlantids (excepting Oxygyrus) . The apical larval portion of the operculum is shaded blue. Modified from Richter and Seapy (1999; Fig. 3).© 2005

5. Opercular gyre lacking ornamentation with three exceptions (A. plana, A. echinogyra, and A. turriculata)
   
   
6. Radula
1. Radular morphology of two distinctive types (I and II; Richter, 1961). Differences between the two types, found in all heteropods, are explained on the Pterotracheoidea page.
   
   

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

2. Rachidian, or central, tooth with one (Atlanta and Protatlanta; see example in Atlanta californiensis above) or three (Oxygyrus) short cusps.

Comments

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

Genus	Shell and keel compositon	Shell spire	Spiral portion of operculum
Atlanta	Calcareous shell and keel	Present (evolute)	Present
Protatlanta	Calcareous shell and conchiolin* keel	Present (evolute)	Present
Oxygyrus	Conchiolin shell and keel	Absent (involute)	Absent

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

Because of the presence of a shell, 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). Photograph 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).