Atlanta frontieri Richter 1993

Introduction

Atlanta frontieri is a moderately large species (shell diameter to 5.5 mm) that is immediately distinguished by an elevated spiral ridge located at the outer edge of the second to fourth spire whorls. The spire shape is unusual; for the third and fourth whorls the spire shape is very low rounded, while the first two whorls form a raised cap. The keel is tall and rounded, inserting between the penultimate and final whorls in shells larger than about 2 mm. Eyes type b. Operculum type b. Radula type II, with the number of tooth rows (44-50) the lowest in the genus Atlanta and moderate sexual dimorphism in the marginal teeth. Recorded from the Indian Ocean and western North Pacific Ocean; vertical distribution unknown.

Diagnosis

• Shell diameter to 5.5 mm
• Shell spire of 4-1/4 to 4-1/2 whorls
• Spire shape low rounded (whorls 3 and 4), with inner whorls (1 and 2) forming a raised cap
  
• Elevated spiral ridge located on outer edge of the second to most of the fourth spire whorls
• Keel tall and rounded; inserting between penultimate and final shell whorls in shells larger than about 2 mm
• Eyes type b
• Operculum type b
• Radula type II, with low number of tooth rows (44-50) and moderate sexual dimorphism in the marginal teeth
  

Characteristics

1. Shell
   1. Shell moderately large (to 5.5 mm diameter)
   2. Shell transparent
   3. Spire consists of 4-1/4 to 4-1/2 whorls
      
   4. Thin spiral ridge along the outer margin of the spire whorls beginning on the second and ending at about 3-3/4 whorls (see second SEM below). Height of spiral ridge to about 20 µm on the second and third whorls  (Richter, 1993)
   5. Spire shape low with first and second whorls elevated, forming a low cone (see third and fourth SEM images below)
      
      

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      Figure. Scanning electron micrographs of the shell of Atlanta frontieri, viewed from the right side (left) and the spire (right). Scale bars = 0.5 mm (left) and 0.1 mm (right). © Roger R. Seapy
      

      

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      Figure. Scanning electron micrographs of the shell of Atlanta frontieri in tilted views; right side of shell () and spire (). Scale bars = 0.5 mm (left) and 0.1 mm (right). ©

   6. Larval shell with the elevated spiral ridges on outer sides of second to fourth whorls (see photographs and sketch below)
      
      

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      Figure. Larval shell of Atlanta frontieri in apertural (left) and apical (right) views. Photographs from Richter (1993, fig. 7).  Shell (left) diameter = 230 um.  Scale bar (= 100 µm) added to apical view (right). © 1993 G. Richter
      

      

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      Figure. Sketch of Atlanta frontieri larval shell in apertural view (from Tokioka, 1955, Fig. 4X'). Shell diameter = 460 µm. © 1955 T. Tokioka

   7. Keel moderately tall and rounded, inserting between the fourth and fifth whorls in animals larger than about 2 mm (see title illustration)
      
2. Eyes type b
3. Operculum type b
4. Radula type II
   1. Number of tooth rows limited to 44-50; lowest in the genus Atlanta (Richter, 1993)
   2. The lateral teeth are higher and thicker laterally than in the other two species (A. rosea and A. fragilis) in the A. peroni species group studied by Richter (1993)
      
   3. Moderate sexual dimorphism in the size and shape of the marginal teeth (Richter, 1993, fig. 21; see SEMs of male and female radulae below). The marginal teeth are somewhat longer and the tooth shafts more curved in females than in males
      
      

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      Figure. Portions of the male (left) and female (female) radulae in Atlanta frontieri. Note that in each tooth row, a lateral tooth is located on eachr side of the central tooth. Scale bars = 25 µm. Original photograph from Richter (1993, fig. 21) modified by addition of scale bars. © 1993 G. Richter

Comments

Tokioka (1955) was the first author to illustrate Atlanta frontieri in an apertural view of the larval shell (see drawing above); erroneously identifying the shell as belonging to A. inflata. In 1966 Frontier briefly described and illustrated the same species as A. peroni. In his monograph on the Heteropoda of the Indian Ocean, Richter (1974) followed Frontier's (1966) lead and incorrectly characterized the species at hand as A. peroni. Nineteen years later, Richter (1993) corrected his misidentification from 1974 and described A. frontieri as a new species. In this paper he also reported that A. frontieri shared a series of characters (notably the same eye, opercular and radular types) with A. peroni and the other two members (A. rosea and A. fragilis) of the A. peroni species group.

Atlanta frontieri has been recorded from the western North Pacific by Tokioka (1955; see above), although the species has not been reported from Hawaiian waters (Seapy, 1990) or off the U. S. west coast (Seapy, pers. obs.). Also, Newman (1990) did not record it  from eastern Australian or New Guinean waters. The majority of records of the species are from the northern Indian Ocean, where Richter (1974) recorded it as the sixth most abundant atlantid in his samples. Seapy (et al., 2003) also reported it from the southeastern Indian Ocean. Thus, A. frontieri must be regarded as an Indo-Pacific species. The vertical distribution of A. frontieri has not been investigated.

References

Frontier, S. 1966. Notes morphologiques sur les Atlanta recoltees dans le plancton de Nosy Be (Madagascar). Cahiers O.R.S.T.O.M. Oceanographie 4(2): 131-139.

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

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

Seapy, R., C. M. Lalli and F. E. Wells. 2003. Heteropoda from Western Australia Waters, pp. 513-546. In: F. E. Wells, D. I. Walker and D. S. Jones (eds), The marine flora and fauna of Dampier, Western Australia. Perth: Western Australian Museum.

Tokioka, T. 1955. On some plankton animals collected by the Syunkotu-Maru in May-June 1954. II. Shells of Atlantidae (Heteropoda). Publications of the Seto Marine Biological Laboratory 4: 227-235.