Atlanta plana Richter 1972

Introduction

Atlanta plana is a moderately small species (maximal shell diameter = 4 mm). The shell is colorless except for the yellow-brown to red-brown keel base and violet spire sutures. The spire consists of 3-1/2 whorls and has a low, conical shape with rounded whorls and incised sutures. The spire bears two low spiral ridges on the second and third whorls. On the second half of the third whorl the spiral ridges are replaced by small punctae. The keel is moderately low with a rounded profile. Keel inserts between last two whorls in shells larger than about 3 mm. Eyes type a. Operculum type b, with a gyre that bears a spiral row of narrow and low, outwardly-directed spines. Radula type I, with unlimited numbers of tooth rows and no sexual dimorphism. Geographic distribution Indo-Pacific. Vertical distribution in Hawaiian waters extends to about 200 m during the day, with limited migration (mostly from 45-90 m) into the upper 45 m at night.

Diagnosis

• Shell small, with a maximal diameter of 4 mm
• Shell colorless except for a yellow-brown to red-brown keel base and violet spire sutures
• Spire low conical with rounded whorls and incised sutures
• Spire consists of 3-1/2 whorls.
  
• Two low spiral ridges present on the second and third whorls; ridges replaced with small punctae at whorl 3-1/2
  
• Keel moderately low, with rounded profile
• Keel inserts between last two whorls in shells larger than about 3 mm
• Eyes type a
• Operculum type b; gyre that bears a spiral row of flat, narrow, outwardly-directed spines
• Radula type I

Characteristics

1. Shell
   1. Shell moderately small; diameter to about 4 mm
   2. Spire small, consisting of 3-1/2 whorls
   3. Spire shape low conical, with rounded whorls and incised sutures
      
   4. Spire sutures violet (see title illustration)
      
   5. Two low spiral ridges present on the second and third spire whorls. In the last half of the third whorl, the ridges break up and are replaced by spiral  rows of small, low punctae.
   6. Keel somewhat low and rounded in profile, inserting between the penultimate and the last shell whorls in shells larger than about 3 whorls
      
   7. Keel base yellow-brown to red-brown
      
      

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      Figure. Shell of Atlanta plana viewed from the right side (left) and the spire (right). Scale bars = 0.5 mm (left) and 0.1 mm (right). © Roger R. Seapy
      
      

      
      

      Figure. Shell of Atlanta plana in tilted view from the right side (left) and the spire (right). Scale bars = 0.5 mm (left) and 0.1 mm (right). ©

   8. Larval shell with a low conical spire, composed of rounded whorls and incised sutures (see second SEM below). Two spiral ridges are present on the second whorl and extend onto the third whorl. A raised, thin spiral ridge is located on the outer edge of the shell whorls, begining at the same point as the spiral ridges on the second whorl and continuing to the shell aperture (see first SEM below)
      
      

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      Figure. Larval shell of Atlanta plana, viewed from the right side (left) and the right side at about a 60° tilt (right). Scale bars = 100 µm. ©

2. Eyes type a, without a transverse slit in the distal pigmented tissue
3. Operculum type b (micro-oligogyre)
   1. Gyre of operculum small, bearing a spiral row of up to 35-40 outwardly-directed, narrow, low spines (see second photograph below)
      
      

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      Figure. Operculum of Atlanta plana (left), with close up of gyre region (right), which bears a spiral row of flattened and outwardly-directed spines. Scale bars = 100 µm (left) and 50 µm (right). ©

4. Radula type I
   1. Number of tooth rows unlimited
   2. No sexual dimorphism in teeth or in radula size or shape
      
      

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      Figure. Radula of Atlanta plana; section of radula with about 5 rows (left) and close up of two central teeth (right). Scanning electron micrographs from Richter (1987, figs. 48 and 56), modified by addition of scale bars (= 100 µm and 25 µm, respectively). © 1987 G. Richter

Comments

The shells of Atlanta plana and A. gaudichaudi are very similar in appearance (discussed by Richter, 1974 and Seapy, 1990a). Both species are moderately small (maximal shell diameters of 3 and 4 mm, respectively) and have spires composed of about the same number of whorls (3-1/2). The spires in both species have a low conical shape with incised sutures. The shells differ most conspicuously in the presence (A. plana) or absence (A. gaudichaudi) of spiral sculpture on the second and third spire whorls. Their soft part morphologies also differ; the eye types are different (a and b, respectively), and the opercular gyres either possess (A. plana) or lack (A. gaudichaudi) spines.

Atlanta plana can be an abundant species in Hawaiian waters (Seapy, 1990a,b and 2008), with the result that it is well suited for quantitative ecological studies. Diel patterns of vertical distribution of heteropods were examined based on replicated, opening-closing net samples (Seapy, 1990b). Atlanta plana ranged downward to 200 m during the day, although it was only abundant in waters shallower than 90 m. At night there was an upward migration that was most pronounced from the 45-90 m depth interval into the upper 45 m of the water column. In the 2008 study, which was based on duplicated, opening-closing MOCNESS sample series, the species ranged downward maximally to  160-200 m at a station 5 nmi off the coast (comparable in location to the area sampled in the 1990b study). Similar to the earlier findings, densities were higher in the upper 40 m during the night than the day, although the differences were far less striking. Onshore-offshore comparisons of abundance patterns were markedly different between fall and spring sampling periods; maximal numbers were recorded at the 5 nmi station in the fall, while abundances in the spring decreased progressively shoreward by three-fold from a maximum at the 15 nmi station to a minimum at the 1 nmi station.

References

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

Richter, G. 1987. Zur Kenntnis der Gattung Atlanta (III), Atlanta inflata, A. helicinoides, A. echinogyra und A. plana (Prosobranchia: Heteropoda). Archiv für Molluskenkunde 117: 177-201.

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

Seapy, R. R. 1990b. 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.