Complete

Containing group: Atlanta

Introduction

Atlanta helicinoidea is a small species, with a maximal shell diameter of 2 mm. The last whorl in adult shells is broadly oval to rounded. The keel is moderately tall and rounded in undamaged specimens, with a posteriorly-slanted corrugated texture. The spire is low conical, consisting of 4-1/2 to 4-3/4 whorls. The second through fourth whorls have spiral ridges that increase from two in the second whorl to four in the first half of the fifth whorl. Spire coloration is dimorphic; light purple or light yellowish tan, due to underlying tissue pigmentation. Eyes type c, with a black cup-shaped base (unique in the genus), operculum type c, and radula type I. The species is cosmopolitan in tropical to subtropical waters, and is limited to the upper 100 m of the water column in Hawaiian waters. The species shares many similarities with A. inflata, but is immediately distinguishable by its eye morphology.

Diagnosis

• Shell diameter <2.0 mm
• Keel moderately tall and rounded, with a posteriorly-directed corrugated texture
  
• Spire low conical, consisting of 4-1/2 to 4-3/4 whorls
• Spire whorls with prominent spiral ridge sculpture
  
• Spire coloration either light purple or light yellow to yellow-tan
• Eyes type c, with a black, cup-shaped base (unique in the genus Atlanta)
  
• Operculum type c, radula type I
  

Characteristics

1. Shell
   1. Small, with a maximal diameter of 2.0 mm
   2. Keel moderately tall and rounded in undamaged specimens (see especially the tilted view of the shell in first figure below)
      
   3. Keel corrugated, with corrugations angled posteriorly (see first figure below)
      
   4. Spire consists of about 4-1/2 to 4-3/4 whorls (see first two figures below)
      
   5. Spire whorls with rows of spiral ridges that are moderately to strongly expressed in Hawaiian waters. Minimally (see the first figure below), the number of ridges begin at the end of the second whorl with one or two ridges and end in the first half of the fifth whorl with five ridges in the fourth whorl. Maximally expressed (see the second figure below), two ridges appear at the beginning of the second whorl and increase to three ridges on the third whorl and up to five ridges on the fourth and first half of the fifth whorl.
      
      

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

      
      

      

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

      

      

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

      Figure. Scanning electron micrographs of the shell of a 2.0 mm Atlanta helicinoidea, viewed from the right side (upper left), the right side tilted (lower left), and the spire from the right side (upper right) and right side tilted (lower right). Scale bar in upper left SEM = 0.5 mm. ©

      

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

      
      

      

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

      

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

      
      

      

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

      Figure. Scanning electron micrographs of the shell of a 1.4 mm diameter Atlanta helicinoidea, viewed from the right side (upper left), the right side tilted (lower left), and the spire from the right side (upper right) and right side tilted (lower right). Scale bars = 0.5 mm (upper and lower left) and 0.1 mm (upper and lower righti. © Roger R. Seapy
      

2. Coloration
   1. Keel base coloration variable, ranging from clear to brown or yellowish brown.
      
   2. Spire either light purple or light yellow to yellow-tan, due to pigmentation of underlying tissues.
      
      

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

      

      Figure. Atlanta helicinoidea, viewed from right side, with the body fully extended from the shell (left), and with the spire region magnfied (right) to illustrate the light purple coloration due to the underlying tissues. © Roger R. Seapy
      

      

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

      
      
      Figure. Juvenile (left) and young adult (right) examples of Atlanta helicinoidea that illustrate the yellow to yellow-tan coloration of the spire. ©
      
3. Eyes type c, with a black, cup-shaped base (see title illustration). This eye morphology is unique in the genus Atlanta, although it is also seen in Oxygyrus keraudreni.
4. Operculum type c
   
5. Radula type I
   

Comments

The spiral ridge sculpture on the shell spire described above applies to shells from Hawaiian waters and the North Atlantic, as reported by Richter (1987). However, among his specimens from the Arabian Sea of the Indian Ocean, Richter also found shells with reduced spiral sculpture, having a single ridge beginning at whorl 1-1/2 and two additional ridges on the fourth whorl (see left image below).

Other Names for Atlanta helicinoidea Gray 1850

• Atlanta helicinoides Souleyet 1852

References

Richter, G. 1987. Zur Kenntnis der Gattung Atlanta (III): Atlanta inflata, A. helicinoides, A. echinogyra und A. plana (Prosobranchia: Heteropoda). Archiv fur Mollusken-Kunde 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.