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
• Outer surface of spire whorls with small, low punctae that are scattered or form irregular spiral lines
• Inner surface of spire with radially-arranged lines
• Innermost whorl adjacent to umbilicus rounded, with wide and low ridge
  
• Eyes type b
• Operculum type c
  
• Radula type II
• Radula large, with a growth angle of about 16°
• Number of tooth rows in radula limited to about 60
• Lateral teeth monocuspid
• Rachidian teeth about 20% wider than in A. tokiokai
  

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

   4. Spire consists of about 5 whorls (see SEM below of juvenile shell)
      
      

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      Figure. Juvenile shell of Atlanta inclinata, viewed from the right side. Image from Richter (1990, fig. 26), modified by addition of scale bar (= 0.5 mm). © 1990 G. Richter

   5. Spire shape globose (beehive shaped), with shallow but distinct sutures evident when spire is viewed from the side (see larval shell below)
      
      

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      Figure. Larval shell of Atlanta inclinata in side view. Image modified from Richter (1990, fig. 10) by addition of scale bar (= 0.5 mm). © 1990 G. Richter

   6. Spire surface with small, low tubercles (or punctae) that are usually scattered, but can form irregular spiral lines. Tubercles occasionally continue on to first teleoconch whorl
      
      

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

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

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      Figure. Shell spire of Atlanta inclinata, viewed using transmitted light. Image from Richter (1990, fig. 30). © 1990 G. Richter

   8. Internal walls of spire whorls decalcified (see transmitted light photograph below)
      
      

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      Figure. Transmitted light photograph of an Atlanta inclinata larval shell in apertural view. Image from Richter (1990, fig. 22), modified by addition of scale bar (= 0.5 mm). © 1990 G. Richter

   9. A very low, wide spiral ridge is present on the rounded whorl adjacent to the umbilicus (see second SEM below)
      

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      Figure. Shell of Atlanta inclinata, viewed from the left side (left) and enlarged to show umbilical region (right). Images from Richter (1990, figs. 4 and 16), modified by addition of scale bars (= 1.0 mm and 100µm, respectively). © 1990 G. Richter

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

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      Figure. Radula of Atlanta inclinata in dorsal view. Image from Richter (1990, fig. 37), modified by addition of scale bar (= 100 µm). © 1990 G. Richter

   2. Number of tooth rows limited to about 60
   3. Lateral teeth monocuspid (see SEM below)
      
      

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      Figure. Section of radula with about four tooth rows in Atlanta inclinata. Image from Richter (1990, fig. 32), modified by addition of labels for rachidian and lateral teeth and scale bar (= 100 µm). © 1990 G. Richter 

   4. Rachidian teeth in the adult portion of the radula (see SEM above) about 20% wider than in A. tokiokai, but about 30% narrower than in A. gibbosa and A. meteori (see respective species pages)
      
      

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      Figure. Rachidian tooth from adult portion of radula in Atlanta inclinata. Image from Richter (1990, fig. 39), modified by addition of scale bar (= 10 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 characterized four species of Atlanta with inclined spires, and concluded that they belonged to 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. The most comprehensive reference to the species of Atlanta with inclined spires is the paper by Richter (1990); the paper is noteworthy for the organization of the plates enabling direct comparisons between the four species. The species pages on atlantids with inclined spires, beginning with this one, have drawn heavily from this paper.
   
   The two species in the Atlanta inclinata species group are very similar and Richter (1990) argued that they must be closely related. Features shared by A. inclinata and A. tokiokai (summarized in Richter and Seapy, 1999, p. 639) 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 whorl of the adult shell, (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 larger and are arranged in distinct spiral lines, (2) when the spire is viewed from the side, the sutures are shallow but clearly separate the spire whorls in the former species, while they are shallower and difficult to distinguish in the latter one, (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 one, (4) the area between the aforementioned ridge and the umbilicus is rounded in the former species but is flattened in the latter one, (5) the radula is large, with a growth angle of about 16° in the former species, and is small and ribbon-like, with a growth angle of about 9° in the latter one, (6) the lateral teeth are moncuspid in the former species, and have an accessory cusp in the latter one, and (7) the rachidian teeth in the adult portion of the radula are about 20% wider in the former than 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 für Molluskenkunde 119: 239-275.

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

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