Atlanta inclinata Gray 1850

Introduction

Diagnosis

• Shell diameter to 6-7 mm
• Adult shell clear to light yellow
• Keel moderately elevated with a rounded to slightly truncate leading edge
  
• Spire tilted relative to shell plane and consists of about 5 whorls
• Spire shape globose or beehive shaped
• Internal spire walls decalcified
• Inner surfaces of spire whorls with radially-arranged lines
• Outer surfaces of spire whorls with small, low punctae that are scattered or form irregular spiral lines
• Innermost whorl adjacent to umbilicus on left side of shell with a low ridge from which the keel develops
• Eyes type b
• Operculum type c
  
• Radula type II
• Number of tooth rows comprising radula limited to about 60
• Radula large, with a growth angle of about 16°
• Lateral teeth on radula monocuspid
  

Characteristics

1. Shell
   1. Maximal shell diameter large (to 6-7 mm)
   2. Shell clear to light yellow in color
   3. Spire inclined relative to the shell plane
      
      

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      Figure. Scanning electron micrographs of a 3.6 mm Atlanta inclinata; views of the right side (left) and the spire (right). Scale bars = 1.0 mm (left) and 0.1 mm (right). Images modified by addition of scale bars from Richter (1990, figs. 3 and 15). © 1990 G. Richter

   4. Spire consists of about 5 whorls (see image below of juvenile shell with a total number of whorls of about 5-1/3)
      
      

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      Figure. Scanning electron micrograph of Atlanta inclinata juvenile shell. Image modified by addition of scale bar from Richter (1990, fig. 26). Scale bar = 0.5 mm. © 1990 G. Richter

   5. Spire shape globose (or beehive shaped), with shallow but distinct sutures
      
      

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      Figure. Scanning electron micrograph of the larval shell of Atlanta inclinata in side view, illustrating the shallow but distinct spire sutures. Image modified by addition of scale bar from Richter (1990, fig. 10). Scale bar = 0.5 mm. © 1990 G. Richter

   6. Spire surface with small, low punctae (or tubercles) that are usually scattered but can form irregular spiral lines
      
      

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      Figure. Scanning electron micrograph of Atlanta inclinata shell spire. Image from Leslie Newman (pers. comm.). Scale bar = 100 µm. © 1990 L. J. Newman

   7. Inner walls of spire whorls with radially-arranged lines, which can only be seen using transmitted light (click on image below to resolve the radial lines, seen most clearly in the upper portion of the outermost whorl)
      
      

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      Figure. Shell spire in Atlanta inclinata, viewed using transmitted light to illustrate radial lines on inner walls of the spire. Image from Richter (1990, fig. 30). © 1990 G. Richter

   8. Internal walls of spire whorls decalcified (only seen using transmitted light)
      
      

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      Figure. Larval shell of Atlanta inclinata in side view photographed using transmitted light. Image modified by addition of scale bar from Richter (1990, fig. 22). Scale bar = 0.5 mm. © 1990 G. Richter

   9. Umbilical region on left side of shell with a low and nearly indistinct ridge on the rounded whorl adjacent to the umbilicus (see second image below). The ridge marks the line where, on the following whorl, the keel develops
      
      

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

      

      Figure. Scanning electron micrographs of the left side of the shell in Atlanta inclinata; entire shell (left) and umbilical region (right). Images modified by addition of scale bars from Richter (1990, figs. 4 and 16). Scale bars = 1.0 mm (left) and 0.1 mm (right). © 1990 G. Richter

2. Eyes type b
   1. xx
3. Operculum type c
   1. xx
4. Radula type II
   1. Radula large and with a growth angle of about 16°
      
      

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

      
      

      Figure. Radula of Atlanta inclinata in dorsal view. Image modified by addition of scale bar from Richter (1990, fig. 37). Scale bar = 250 µm. © 1990 G. Richter

   2. Number of tooth rows limited to about 60
   3. Lateral teeth monocuspid
      
      

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

      
      

      Figure. Section of radula consisting of four tooth rows in Atlanta inclinata. Note that the lateral teeth are located medial to the two pair of marginal teeth in each tooth row. Image modified by addition of scale bar from Richter (1990, fig. 32). Scale bar = 100 µm. © 1990 G. Richter
      

   Comments

   Historically, the species of Atlanta with inclined spires (i.e., tilted strongly relative to the shell plane) have been treated together in a single A. inclinata species group (van der Spoel, 1976). In his 1990 paper, however, Richter described and characterized the four species of Atlanta with inclined spires, and concluded that they formed two distinctively different species groups. He placed two of the species, A. inclinata and A. tokiokai, in the A. inclinata species group and the other two, A. gibbosa and A. meteori, in an A. gibbosa species group. Richter remarked on the striking similarities of the former species pair and regarded them as closely related, or sister species . Features shared by A. inclinata and A. tokiokai (summarized in Richter and Seapy, 1999) include: (1) spire large and globose, (2) internal spire whorls decalcified, (3) internal wall of spire whorls with radially-arranged lines, (4) external surface of spire whorls with sculpture of small punctae that continue on to the first teleoconch whorl, (5) eyes type b, and (6) operculum type c. The main differences between the two species are: (1) the punctae on the spire are smaller and scattered or form irregular spiral lines in A. inclinata, while in A. tokiokai the punctae are arranged in distinct spiral lines, (2) the spire sutures are shallow, but clearly separate the spire whorls in the former species, while they are very shallow and difficult to distinguish in the latter species, (3) a low, nearly indistinct ridge is present on the whorl adjacent to the umbilicus on the left side of the shell in the former species, while this ridge is strongly developed in the latter species, (4) the area between the aforementioned ridge and the umbilicus is rounded in the former species but is flattened in the latter species, (4) the radula is large, with a growth angle of about 16°, and the lateral teeth are moncuspid in the former species, while the radula is small and ribbon-like, with a growth angle of about 9°, and the lateral teeth have an accessory cusp in the latter species.
   
   

Other Names for Atlanta inclinata Gray 1850

• Atlanta inclinata Souleyet 1852

References

Richter, G. 1990. Zur Kenntnis der Gattung Atlanta (IV). Die Atlanta inclinata-Gruppe (Prosobranchia: Heteropoda). Archiv fur Molluskenkunde 119: 239-275.

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

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