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Containing group: Atlanta

Introduction

Atlanta oligogyra is a moderately small species (maximal shell diameter of 2.1 mm in the Pacific and 2.8 mm in the Atlantic and Indian Oceans). The shell is transparent, with relatively thick and strong walls. The spire is light violet, compact and low, with a smooth surface, and consisting of about 2-1/2 whorls. In tilted or oblique views, the shape of the spire is low and rounded. The sutures between the first and second spire whorls are shallow, giving the spire a dome-like appearance. The keel is moderately elevated, with a rounded leading edge and a clear to brown base. Eyes type a. Operculum type b. Radula type I, small and ribbon-like. Geographic distribution is cosmopolitan in tropical to subtropical waters. In Hawaiian waters A. oligoyra is limited to the upper 150 m of the water column, with most individuals in the upper 100 m and some evidence for nocturnal vertical migration from 50-100 m during the day into the upper 50 m at night.

Diagnosis

• Maximal shell diameter = 2.1 mm (Pacific Ocean) and 2.8 mm (Atlantic and Indian Oceans)
  
• Shell transparent with relatively thick, strong walls
• Spire small and low, with a smooth surface and a rounded profile
  
• Spire consists of about 2-1/2 whorls
• Spire sutures between first and second whorls shallow, giving spire a dome-like appearance
  
• Keel moderately tall, with rounded leading edge and a clear to brown base
  
• Eyes type a
  
• Operculum type b
• Radula type I, small and ribbon-like
  

Characteristics

1. Shell
   1. Maximal shell size mostly less than 3 mm and greater in the Atlantic (2.8 mm; Richter, 1986) and Indian (3.1 mm; Richter, 1974) Oceans than in the Pacific Ocean off eastern Australia (2.1 mm; Newman, 1990) and Hawaii [2.0 mm; Seapy, 1990a,b)
   2. Shell with relatively thick and strong walls (Richter, 1986)
      
   3. Keel moderately tall, with a rounded leading edge
      
   4. Spire consists of about 2-1/2 whorls
   5. Spire shape low, rounded (see fourth SEM image below)
      
   6. Spire sutures between first and second whorls shallow, thus giving the spire a smooth, dome-like appearance when viewed from the side (see fourth SEM image below); sutures between second and third and between third and fourth whorls deep, with the result that whorls subsequent to the second one are easily distinguished from each other (see third and fourth SEM images below)
      
      

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      Figure. Scanning electron micrographs of a 1.1 mm Atlanta oligoyra. Shell viewed from right side (upper left), and right side tilted (upper right); shell spire viewed from right side (lower left) and right side tilted (lower right). Scale bars = 0. 5 mm (upper and lower left) and 100 µm (upper and lower right). ©

2. Coloration
   1. Shell spire light violet in waters off Hawaii (see title illustration) and off eastern Australia (Newman, 1990). Richter (1974) described the spire color in Indian Ocean specimens as red-violet. In his 1986 paper, Richter noted that in live or recently preserved animals, the shell has a weak rose color
      
   2. Keel base brown in shells from the Indian and Atlantic Oceans (Richter, 1974, 1986), although in Hawaiian waters, the keel base in juvenile and smaller adults is clear (Seapy, 1990a)
      
3. Eyes type a
   1. In contrast with the proportionately large lens of A. lesueurii (Richter, 1986), the lens of A. oligogyra is comparable in size to those of other Atlanta species
4. Operculum type b
5. Radula type I
1. Radula small; long and narrow (Richter, 1986, fig. 6)
2. Lateral teeth monocuspid or with a very small accessory cusp (Richter, 1986, fig. 15)

Comments

Atlanta oligogyra is very similar in general appearance to A. lesueurii. Although Tesch (1908) recognized both species as valid, in his monograph on the Heteropoda of the Dana Expedition he made A. oligogyra a junior synonym of A. lesueurii. Like Tesch (1949), van der Spoel (1976) also rejected A. oligogyra as a valid species.  In his Meteor Expedition report on the Heteropoda of the Indian Ocean, however, Richter (1974) resurrected A. oligogyra as a valid species, describing clear differences between it and A. lesueurii. He subsequently (1986) published a paper presenting a direct comparison of the two species.

As described in Richter (1974, 1986; Seapy, 1990a; Richter and Seapy, 1999), A. oligogyra is distinguished from A. lesueurii by several characteristics. First, and most distinctively, the  eyes of A. oligogyra are type a (lacking a transverse slit in the distal pigmented tissue) and the lens is of average size, while the eyes of A. lesueurii are type b (possessing a transverse slit in the distal pigmented tissue) and the lens is large in relation to the basal portion of the eye. Second, although both species have small spires, that of A. oligoygra is low and dome-shaped with a shallow suture between the first and second whorls, while that of A. lesueurii is somewhat elevated with distinctive first and second whorls, caused by the deep suture between these two whorls. Third, the keel of A. oligogyra is moderately tall with a rounded leading edge, while that of A. lesueurii becomes very tall in adults with a distinctive truncate leading edge.

The diel vertical distribution of Atlanta oligogyra was studied in Hawaiian waters by Seapy (1990b). The species was recorded in low abundances between the surface and 150 m  (maximal density = 1.4 per 1,000 m³ in the 0-50 m depth interval at night). There was limited evidence of nocturnal vertical migration from maximal numbers in the 50-100 m depth interval during the day into the upper 50 m at night.

References

Newman, L. J. 1990. The heteropod fauna (Prosobranchia, Heteropoda) of Australian and Papua New Guinean waters: a taxonomic and distributional review. Ph.D. thesis, University of Queensland, Australia.

Richter, G. 1974. Die Heteropoden der "Meteor" Expedition in den Indischen Ozean 1964/65. "Meteor" Forschungs-Ergibnisse Ser. D, No. 17, pp. 55-78.

Richter, G. 1986. Zur Kenntnis der Gattung Atlanta (II). Atlanta lesueuri Souleyet und Atlanta oligogyra Tesch (Prosobranchia: Heteropoda). Archiv f?r Mollusken-kunde 117: 19-31.

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

Seapy, R. R. 1990a. The pelagic family Atlantidae (Gastropoda, Heteropoda) from Hawaiian waters: a faunistic survey. Malacologia 32(1): 107-130.

Seapy, R. R. 1990b. Patterns of vertical distribution in epipelagic heteropod molluscs off Hawaii. Marine Ecology Progress Series 60: 235-246.

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

Tesch, J. J. 1908. Systematic monograph of the Atlantidae (Heteropoda) with enumeration of the species in the Leyden Museum. Notes from the Leyden Museum 30: 1-30.

Tesch, J. J. 1949. Heteropoda. Dana Report 34: 1-35.