Atlanta turriculata d'Orbigny 1836

Introduction

Atlanta turriculata is a small species, with a maximal shell diameter of about 1.8 mm. The keel is tall and transparent, inserting between the last and penultimate whorls in animals larger than 1.3 mm. The spire (protoconch) consists of 4-1/4 whorls, with the first three whorls strongly elevated laterally and forming a "turret". A prominent spiral ridge extends outward from the middle of the spire whorls, beginning on the second whorl. Shell transparent; spire tissues are light brown to reddish brown. Eyes type a, operculum type a, and radula type I. Geographic distribution limted to the Pacific and Indian Oceans in tropical to subtropical waters. Vertical range in Hawaiian waters in the upper half of the epipelagic zone (0-200 m) during the day, with migration into the upper 100 m at night.

Diagnosis

• Maximal shell diameter ca. 1.8 mm
• Keel tall and colorless, inserting between the last and penultimate whorls in shells >1.3 mm diameter
• Spire elevated, with the first three whorls forming a narrow "turret"
• Prominent spiral ridge extends outward from the middle of each spire whorl
• Color of spire light brown to reddish brown
  
• Eyes type a, operculum type a, and radula type I
  

Characteristics

1. Shell
1. Small, with shell diameter <2 mm (maximal size 1.7 mm in Hawaiian waters and 1.8 mm in northern and northwestern Indian Ocean; Seapy, 1990a and Richter, 1974, respectively)
2. Keel tall and rounded (not truncate); keel starts slightly behind the shell aperture and increases gradually in height to a maximum dorsally (see title illustration and first image below). Between the keel origin and aperture, the two valves are separated forming a narrow slit
   
3. Keel inserts between the last and penultimate shell whorls in shells with diameters (excluding the keel) larger than 1.3 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

   Figure. Low (left) and high (right) magnification scanning electron micrographs of the shell and shell spire, respectively, in Atlanta turriculata, viewed from the right side. Scale bars = 0.5 and 0.1 mm, respectively. ©
4. Spire consists of 4-1/4 whorls, with the first three whorls extending outward from the right side of the shell and forming a "turret" (hence the specific epithet, turriculata). Elevated ridges and punctae on the second and third whorls (similar to that seen in A. fusca, but less ornate). Prominent spiral ridge beginning on the second whorl and centered on each whorl (see particularly the high magnification views of the spire above and below)
   
   

   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. Low (left) and high (right) magnification scanning electron micrographs of the shell and shell spire in Atlanta turriculata, respectively, viewed at a 60° tilt. Scale bars = 0.5 and 0.1 mm, respectively.© Roger R. Seapy
   

   

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

   
   

   Figure. Dorsal view of juvenile Atlanta turriculata, with the turret-shaped spire projecting outward at right angles to the shell plane. Note the prominent outwardly-directed spiral ridge on the fourth whorl. Shell diameter ca. 0.7 mm. ©

5. Shell transparent; spire tissues light brown to reddish-brown (see title illustrations and photograph above)
   
2. Eyes type a; lacking a transverse slit in the distal pigmented tissue
3. Operculum type c (monogyre); unique to this species are two rows of short spines that spiral outward from the gyre center (illustrated also by drawings in Tokioka, 1961)
   
   

   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. Operculum of Atlanta turriculata. Left: low magnification view of operculum. Right: high magnification view of gyre region, with two spiral rows of short spines. Scale bars = 0.1 mm. ©
   

4. Radula type I, small and ribbon shaped. Tooth morphology same as in Atlanta fusca

Comments

Atlanta turriculata is an Indo-Pacific species. It has previously been reported from the Atlantic Ocean, but these records probably represent aberrant forms (with tall spires) of the closely-related species, A. fusca (discussed by Richter and Seapy, 1999). A. turriculata  occurs in tropical to subtropical waters. In Hawaiian waters it was recorded between the surface and 200 m (the upper half of the 400-m deep epipelagic zone) during the day and to a maximal depth of 90 m at night, with most individuals in the upper 45 m (Seapy, 1990b). These data support an hypothesis of nocturnal vertical migration. Based on nighttime abundance data from three stations at increasing distances off leeward Oahu, A. turriculata decreases in population size from offshore (15 nmi) to inshore (1 nmi) waters (Seapy, 2008).

References

Tokioka, T. The structure of the operculum of the species of Atlantidae (Gastropoda: Heteropoda) as a taxonomic criterion, with records of some pelagic mollusks in the North Pacific. Publications of Seto Marine Biological Laboratory 9: 267-332.

Richter, G. 1961. Die Radula der Atlantiden (Heteropoda, Prosobranchia) und ihre Bedeutung f?r die Systematik und Evolution der Famiie. Morphol. ?kol. Tiere 50: 163-238.

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

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

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.