Atlanta meteori Richter 1972

Introduction

Atlanta meteori attains a maximal shell diameter of 4 mm. The shell is transparent and colorless, and the walls are thin and fragile. The surface of the shell is smooth, without any sculpture. The keel is tall, with a truncate leading edge. The keel inserts between the penultimate and last shell whorls in shells larger than about 2 mm. The shell spire is comprised of about 5-1/2 to 5-3/4 whorls, and is strongly tilted relative to the shell plane. The spire is conical, with a rounded apex and an apical angle of 60-70°. The spire sutures are very shallow, with the result that the whorls are difficult to distinguish when the shell is viewed from the side. The umbilicus is very small and narrow. Eyes type b; lens conspicuously large in adults. Operculum type b; very thin and fragile. Radula type II and narrow, with a growth angle of 12-14°. Radulae not sexually dimorphic. Lateral teeth with accessory cusps. Marginal teeth with long, straight shafts and sharply bent hooks. Rachidian teeth broad and with a long median cusp. Atlanta meteori was previously considered to be an Indo-Pacific species in tropical to subtropical waters, but with its identification off northwestern Africa, it must now be regarded as cosmopolitan.

Diagnosis

• Shell diameter to 4 mm
• Shell transparent and colorless
• Surface of shell smooth, lacking sculpture
• Keel tall with truncate leading edge
• Keel penetrates between penultimate and last whorls in shells larger than about 2 mm
• Shell spire strongly tilted relative to shell plane
• Spire consists of 5-1/2 to 5-3/4 whorls
• Spire shape conical, with a rounded apex and an apical angle of 60-70°
• Spire sutures very shallow and difficult to resolve when shell is viewed from the side
  
• Umbilicus very small and narrow
  
• Eyes type b
• Operculum type b
• Radula type II
• Radula narrow, with a growth angle of 12-14°
• Lateral teeth with accessory cusps
• Marginal teeth with long, straight shafts and sharply bent hooks
• Rachidian teeth broad, with long median cusp
  

Characteristics

1. Shell
   1. Shell of moderate size (to 4 mm diameter)
   2. Shell transparent and colorless
   3. Shell fragile, with thin walls
   4. Keel tall, with a truncate leading edge that begins nearly 1/4 whorl from  the shell aperture (see title illustration)
      
   5. Keel penetrates between penultimate and last whorls in shells larger than about 2 mm (see first SEM below)
      
   6. Spire strongly tilted relative to shell plane (see third SEM below)
      
      

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      Figure. Shell of Atlanta meteori; views of the right side (left) and spire (right). Scale bars = 0.5 mm and 0.1 mm, respectively. © Roger R. Seapy
      

      

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      Figure. Shell of a 3.7 mm Atlanta meteori; views of the right side, tilted (left) and the spire, tilted (right). Scale bars = 0.5 mm and 0.1 mm, respectively. ©

   7. Spire of about 5-1/2 to 5-3/4 whorls
   8. Spire conical in side view, with an apical angle of 60-70°. Richter (1990) characterized the apical angle as 70°, and this value is in agreement with the apical angles of the adult shells shown above and in the spire photograph below. However, Richter's photographs of larval shells (see fourth SEM above and transmitted light photograph below) have apical angles of about 60°. Without pursuing an explanation for this discrepancy, the apical angle is given here as 60-70°
      

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      Figure. Shell spire in Atlanta meteori; photograph taken with shell positioned about 45° above the shell plane. Image cropped from the original in de Vera, et al. (2006, fig. 4b). © 2006 A. de Vera
      

      

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      Figure. Larval shell of Atlanta meteori, in apertural view. Image from G. Richter (1990, fig. 12), modified by addition of scale bar (=0.5 mm). © 1990 G. Richter
      

   9. 

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      Spire sutures very shallow, with the result that when the spire is viewed from the side, the sutures are difficult to distinguish (see SEMs of adult and larval shells above)
   10. Internal walls of spire intact (see transmitted light photograph below); not decalcified as in A. inclinata and A. tokiokai
       
       

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       Figure. Larval shell of Atlanta meteori, in side view using transmitted light. Photograph from Richter (1990, fig. 24), modified by addition of scale bar (=0.5 mm). © 1990 G. Richter

   11. Left side of shell with a very small and narrow umbilicus
       
       

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       Figure. Shell of Atlanta meteori; views of left side of shell (left) and umbilical region (right). Scanning electron micrographs from G. Richter (1990, figs. 8 and 20), modified by 180° rotation of second photograph and addition of scale bars (= 1.0 mm and 250 µm, respectively). © 1990 G. Richter

2. Eyes type b
   1. Lens conspicuously large in adults, suggesting that the species prefers deeper daytime depths (Richter, 1974)
3. Operculum type b
   1. Operculum thin and fragile
4. Radula type II
   1. Radula with a growth angle of 12-14°; slightly narrower than in A. gibbosa
   2. Radulae of males and females show no sexual dimorphism
   3. Lateral teeth with accessory cusp (see label on SEM below)
      

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      Figure. Section of the radula from Atlanta meteori. Image from Richter (1990, fig. 33), modified by addition of scale bar (= 100 um). © 1990 G. Richter

   4. Rachidian teeth with a tall median cusp; in contrast with the short median cusp in A. gibbosa
      
      

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      Figure. Rachidian tooth from adult portion of radula in Atlanta meteori. Image from Richter (1990, fig. 41), modified by addition of scale bar (= 10 µm). © 1990 G. Richter

References

de Vera, A., R. R. Seapy, and F. Hernandez. 2006. Heteropod molluscs from waters around the Selvagens Islands (Gastropoda: Carinarioidea). Vieraea 34:33-43.

Richter, G. 1972. Zur Kenntnis der Gattung Atlanta (Heteropoda: Atlantidae). Archiv für Molluskenkunde 102(1/3): 85-91.

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. 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.

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

Seapy, R. R. 2008. Offshore-inshore and vertical distributional patterns of heteropod mollusks off leeward Oahu, Hawaii. Marine Biology 154: 985-995.