Atlanta gaudichaudi Gray 1850

Introduction

Atlanta gaudichaudi is a small species (maximal shell diameter = 3 mm). Shell surface is colorless, although the keel base is yellow-brown or red-brown. The shell spire consists of 3-1/4 to 3-1/2 whorls. Surface of spire whorls lack sculpture except for a small spiral ridge on the outer edge of the whorls. Spire shape is low conical with a helicoid shape (due to rounded whorls and incised sutures). The keel is tall, with a truncate leading edge. The keel does not insert between the penultimate and last shell whorls. Eyes type b. Operculum type b. Radula type I. This species differs from the other two in the A. gaudichaudi species group in that the shell spire lacks raised sculpture and the gyre of the operculum lacks spirally-arranged hooks or spines. The species has a cosmopolitan distribution.

Diagnosis

• Shell small; maximal diameter = 3 mm
• Shell colorless and with a smooth surface that lacks raised sculpture, except for a small spiral ridge on the outer edge of the spire whorls
  
• Spire low conical, with a helicoid shape that is due to rounded whorls and incised sutures
  
• Spire consists of 3-1/4 to 3-1/2 whorls
  
• Keel tall, with a truncate leading edge and a yellow-brown or red-brown base
  
• Keel does not insert between last two shell whorls
• Eyes type b
• Operculum type b
  
• Radula type I
  

Characteristics

1. Shell
   1. Shell small; maximal diameter = 3 mm
   2. Shell colorless, except for a yellow-brown or reddish-brown keel base
   3. Shell surface smooth, lacking surface sculpture except for a fine spiral ridge on the outer edge of the spire whorls (see first SEM of larval shell below)
      
   4. Spire consists of 3-1/4 to 3-1/2 whorls
   5. Spire shape low conical with a helicoid shape, due to rounded whorls and incised sutures (see fifth SEM)
   6. Keel tall, with a truncate leading edge (see first SEM below)
      
   7. Keel does not insert between penultimate and last shell whorls
      
      

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      Figure. Shell of Atlanta gaudichaudi; right side (left) and spire (middle and right). Left and middle images modified from Richter (1974, fig. 8). Right image modified from Richter (1993, fig. 11). © 1974 and 1993 G. Richter
      

      

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      Figure. Shell of Atlanta gaudichaudi; right side in tilted (left) and apertural (right) views. Images modified from Richter (1974, fig. 8) and Richter (1993, fig. 12), respectively. Scale bars = 0.5 mm (left) and 100 µm (right). © 1974 and 1993 G. Richter

   8. Larval shell with a low conical, helicoid apex and a low, fine spiral ridge on outer edge of the whorls (see first SEM below). Internal walls of larval shell intact (see dark-field photograph below); not decalcified as in members of the A. inflata species group.
      
      

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      Figure. Larval and post-larval shells of Atlanta gaudichaudi. Scanning electron micrograph of larval shell (left); view of right side, tilted (from Newman, 1990b, fig. 3b). Dark field, transmitted light photograph of of post-larval shell (right) in apertural view (from Richter, 1993, Fig. 13). © 1990 L. J. Newman and 1993 G. Richter

2. Eyes type b, with a transverse slit in distal pigmented tissue
3. Operculum type b (micro-oligogyre), lacking the spirally-arranged spines or hooks seen in the gyres of the other two species in the species group, A. plana and A. echinogyra
   
4. Radula type I
1. Number of tooth rows unlimited
2. No sexual dimorphism in radulae or radular teeth
   
   

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   Figure. Sections of radulae from two large adult Atlanta gaudichaudi (shell sizes and sexes not known). Image modified by addition of scale bar from Richter (1993, Fig. 15). Scale bar = 50 µm. © 1993 G. Richter

3. Lateral teeth (immediately adjacent to central teeth in SEM below) monocuspid, lacking a secondary spine (Richter, 1993)
   
   

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   Figure. Medial region of Atlanta gaudichaudi radula. Modified from Newman (1990b, fig. 3D) by addition of scale bar. Scale bar = 10 µm. © 1990 L. J. Newman

   

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Comments

Atlanta gaudichaudi can be easily distinguished from the other two members of the A. gaudichaudi species group by the absence of any surface sculpture on the shell spire and the lack of spirally-arranged spines (A. plana) or hooks (A. echinogyra) on the gyre of the operculum.

This species is most often confused with smaller A. peronii, which also has a smooth shell surface (lacking sculpture), comparable number of spire whorls, and the same eye, opercular and radular types (see the species summary table on the genus Atlanta page). However, the size and shape of the spire were reported by Richter (1993) to enable one to distinguish the two species. Richter noted that the spire of A. gaudichaudi is tall with a low conical, helicoid shape (described above), while that of A. peronii is very low conical (see the first two image pairs below). However, comparisons of the spire diameters in the two species (see the two pairs of spire SEMs below) at three complete whorls indicates that the diameters are reasonably comparable

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Figure. Shell spires of Atlanta  gaudichaudi (left) and A. peronii (right) in apertural and tilted views, respectively. To make the sizes of the images approximately comparable, each was cropped (from the fifth SEM above and the fourth SEM on the A. peronii species page). Scale bars = 100 µm. © 1993 G. Richter (left) and Roger R. Seapy (right)

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

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

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

Figure. Shell spires of Atlanta gaudichaudi (left) and A. peronii (right). Shell diameter at three complete whorls is indicated by white bars; 244 µm (left) and 253 µm (right). The images were created, respectively, from the third SEM image above and the second SEM (rotated 90° clockwise) on the A. peronii species page. Scale bars = 100 µm. © 1993 G. Richter (left), and Roger R. Seapy (right)

The geographic distribution of Atlanta gaudichaudi is cosmopolitan in tropical to subtropical waters. However, in the Pacific Ocean its distribution is widespread but not uniform. It has been recorded from the western North Pacific (Kuroshio Current waters off Japan; Tokioka, 1955, Okutani, 1957), western South Pacific (Great Barrier Reef waters off eastern Australia and off New Guinea; Newman, 1990a,b), and eastern North Pacific (Gulf of California; Seapy and Skoglund, 2001 and O. Angulo, pers. comm.), but not from the central North Pacific (Hawaiian waters; Seapy, 1990). 

Other Names for Atlanta gaudichaudi Gray 1850

• Atlanta gaudichaudii Souleyet 1852

References

Newman, L. J. 1990a. Holoplanktonic molluscs (Gastropoda: Thecosomata and Heteropoda) from the waters of Australia and Papua New Guinea: their taxonomy, distribution and biology. Ph.D. thesis. University of Queensland, Brisbane Australia.

Newman, L. J. 1990b. The taxonomy, distribution and biology of Atlanta gaudichaudi Souleyet, 1852 (Gastropoda, Heteropoda) from the Great Barrier Reef, Australia. American Malacological Union Bulletin 8: 85-94.

Okutani, T. 1957. Holoplanktonic Gastropoda in the "Kuroshio" Area, south of Honshu, May 1955. Records of Oceanographic Works in Japan, Special Number (New Series): 134-142.

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

Richter, G. 1993. Zur Kenntnis der Gattung Atlanta (V). Die Atlanta peroni-Gruppe und Atlanta gaudichaudi (Prosobranchia: Heteropoda). Archiv für Molluskenkunde 122: 189-205.

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

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

Tokioka, T. 1955. Shells of Atlantidae (Heteropoda) collected by the Sôyô-Maru in the southern waters of Japan. Publications of the Seto Marine Biological Laboratory 4: 237-250, 2 plates.