Atlanta

Introduction

Atlanta is the type genus of the Atlantidae, containing 20 of the 22 species in the family (the other two genera, Protatlanta and Oxygyrus, are monospecific). All species but one (A. californiensis from the temperate North Pacific) are found in tropical to subtropical waters. Most (12) are cosmopolitan in geographic distribution, although three are Indo-Pacific, two are found only in the Indian Ocean, two only in the Atlantic Ocean, and one in the Pacific Ocean.

Brief Diagnosis

An atlantid with:

• Shell and keel calcareous.
• Larval shell becomes spire of adult shell.

Characteristics

1. Eye morphology. Three eye types (termed a, b and c; after Richter, 1961) are present that differ primarily in patterns of pigmentation. Types a and b have a large part of the eye wall that lacks pigment forming a window. These two eye types differ in the absence (type a) or presence (type b) of a narrow transverse slit (extension of the distal part of the window) that almost completely separates distal from proximal pigmentation. Type c lacks a window.
   

   Click on an image to view larger version & data in a new window

   

   Figure. The three types of eye morphologies (a, b and c) in Atlanta. Drawing modified from Richter and Seapy (1999).

   

   Figure. Two (Types b and c) of the three eye types in Atlanta. Left - Type b eye of Atlanta peroni, Hawaiian waters. Right - Type c eye of Atlanta helicinoides, Hawaiian waters . A representative photograph of an animal with a Type a eye is not available. Photographs copyright © 2005 R. Seapy.

2. Operculum morphology. Three types of opercula (termed Types a, b, c; after Richter, 1961) are found in Atlanta. The third type (c) differs from the first two (a and b) in the number of turns of the spiral (larval) gyre (shaded blue in drawings below); "single" in the mono-gyre and "a few" in the oligo-gyre opercula. The two types of oligogyre opercula differ in the relative size of the gyre region; hence, the prefixes macro- and micro-.
   

   Click on an image to view larger version & data in a new window

   

   Figure. The three types of opercular morphologies (a, b, c) in Atlanta. The gyre (larval) portion of the operculum is shaded blue. Drawing modified from Richter and Seapy (1999).

3. Spire whorl number. The shell of Atlanta consists of three sequentially-deposited portions; a dome-shaped embryonic shell (protoconch I; shaded yellow below), a larval shell (protoconch II; shaded blue below) and the adult shell (teleoconch). In postmetamorphic animals, the number of whorls comprising the teleoconch increases with growth, while the number of whorls in the protoconch (forming the spire of the adult shell) is constant. The location on the shell where the protoconch ends and the teleoconch begins is often difficult to determine, but its approximate location is necessary in determining the number of whorls comprising the shell spire; an important species characteristic. The number of spire whorls ranges from two and one-half to six.
   

   Click on an image to view larger version & data in a new window

   

   Figure. Left: Larval shell of Atlanta plana, Hawaiian waters. Scale bar = 0.1 mm. The embryonic shell (protoconch I) is indicated in yellow. The remainder of the larval shell (protoconch II) is shaded blue. Right: Adult shell of Atlanta plana, Hawaiian waters. Scale bar = 0.5 mm. The embryonic and larval shells are colored blue to distinguish them from the remainder of the shell. Scanning electron micrographs copyright © 2005 R. Seapy.

4. Spire shape and ornamentation.  Spire shape is highly variable and can often serve as a distinctive species characterisitic. Examples include the tall, turreted spire of A. turriculata, the tilted (or "inclined") spire in the A. inclinata and A. gibbosa species groups, and the large, low rounded and elevated spire of A. inflata and A. helicinoides.  Similarly, spire ornamentation can be very distinctive, ranging from smooth (e.g., the A. lesueuri species group and three of the members of the A. peroni species group) to ornamented by low, elevated spiral ridges (e.g., A. inflata, A. helicinoides, and A. plana), and highly ornate (e.g., A. fusca, A. echinogyra and A. turriculata).

Nineteen of the 20 species of Atlanta are combined into seven species groups (after Richter and Seapy, 1999), each of which is characterized by similarities in overall shell and shell spire morphologies, and in some cases body coloration. Members of each group usually share the same opercular type and, in many cases, the same eye type. The table below summarizes this information. 

*One species, Atlanta peresi, is excluded from the table. It was described by Frontier (1966) from the western Indian Ocean. Although its adult shell morphology and opercular type were characterized by Frontier, its eye and radular morphologies are unknown. Subsequent studies of the atlantid heteropods from the Indian Ocean by Richter (1974) and Seapy et al. (2003) did not record A. peresi, and the species' validity remains in question. The interested reader is referred to Richter and Seapy (1999) for a further description and discussion.

References

Frontier, S. 1966. Notes morphologiques sur les Atlanta récoltées dans le plancton de Nosy Bé (Madagascar). Cah. ORSTOM, Sér. Océanogr. 4: 131-139.

Richter, G. 1961. Die Radula der Atlantiden (Heteropoda, Prosobranchia) und ihre Bedeutung für die Systematik und Evolution der Famiie. Morphol. Ökol. Tiere 50: 163-238.

Richter, G. 1974. Die Heteropoden der "Meteor"-Expedition in den Indischen Ozean, 1964/65. "Meteor" Forsch.-Ergebnisse 17(D): 55-78.

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

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

Seapy, R. R., C. M. Lalli and F. E. Wells. 2003. Heteropoda from western Australian waters, pp. 513-546. In: F. E. Wells, D. I. Walker and D. S. Jones (eds.), The marine flora and fauna of Dampier, Western Australia. Perth: Western Australia Museum.