Oxygyrus keraudreni Benson 1835

Introduction

Along with Atlanta peroni, Oxygyrus keraudreni attains the largest size (shell diameter to 10 mm) among the Atlantidae. Body coloration is light bluish-purple, with the color darkening with age. The larval shell is calcareous and displays a distinctive pattern of zigzag-shaped spiral ridges that are evenly spaced and cover the shell surface. The teleoconch is composed of conchiolin, a transparent cartilaginous material, and its surface lacks sculpture. With growth the teleoconch overgrows the protoconch and eventually surrounds it. A shell spire, as seen in all other atlantids, is lacking and the spire region is termed involute. The conchiolin keel is glass-like and has a truncate anterior edge that ends at the shell aperture.

The type c eye morphology is only seen in one other species of atlantid (A. helicinoides) and one carinariid (Cardiapoda richardi). The shape of the operculum is unique among atlantids, broadly triangular, approaching trapezoidal in shape, and lacking the apical spiral part present in all other atlantids. Also unique to the operculum is that when the animal retracts into its shell, only about one-half of the operculum covers the shell aperture. The radula is very large and is triangular in shape. The geographical distribution is cosmopolitan in tropical to subtropical waters.

Characteristics

1. Shell
   1. Larval shell calcareous with prominent, evenly-spaced zigzag-shaped spiral ridges covering the surface
      
   2. Adult shell and keel composed of transparent conchiolin, overgrowing the calcareous larval shell and eventually covering it except for the central portion that is represented by the involute spire
      
      

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      Figure. Scanning electron micrographs of the shell of a young adult (shell diameter = 2.2 mm), viewed from the right side. Left: low magnification; scale bar = 0.5 mm. Right: high magnification; scale bar = 0.1 mm. ©

      

      Figure. Scanning electron micrographs of the shell of the young adult Oxygyrus keraudreni shown above, viewed from ventro-lateral (apertural) side. Left: low magnification; scale bar = 0.5 mm. Right: high magnification; scale bar = 0.1 mm. Note that the critical point drying method used to prepare the specimen for examination under the scanning electron microscope resulted in a slight elevation of the base of the teleoconch from the underlying protoconchl. ©

2. Eyes type c; distinguished by a solid black base of hemispherical shape, in which the spherical lens is situated
   
   

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   Figure. Right eye in Oxygyrus keraudreni. Photo of an enlarged portion of the title illustration. ©

3. Operculum broadly triangular, approaching trapezoidal, and lacking an apical spiral portion (gyre)
   
   

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   Figure. Sketch of an operculum from Oxygyrus keraudreni. Modified from Spoel (1976, Fig. 133).© 1976 S. van der Spoel

4. Radula
   1. Very large in comparison with other atlantids; shape elongate triangular, with a broad base
      
      

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      Figure. Dorsal view of the radula from an adult Oxygyrus keraudreni. Scale bar = 0.1 mm. Modified from Richter (1961, Fig.27a). © 1961 G. Richter

   2. Central tooth tricuspid, with all three cusps pointed in juveniles but lateral lobes flattened in adults. Lateral teeth with a strong process on the inner side and a pointed accessory cusp. Marginal teeth monocuspid (Richter, 1961)
      
5. Larva
   1. Innermost tissues dominated by the brown digestive gland; remaining tissues posterior to the head bluish-purple
      

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      Figure. Oxygyrus keraudreni larva, viewed from the right side. © 2005

   2. Larval shell with zigzag spiral ridges on the surface (see enlarged view of photo above and high magnification shell scanning electron micrographs above)
   3. Embryonic shell low, dome-shaped and apical in early larvae (see left sketch below). With growth the outer whorl of the larval shell progressively overrides the embryonic shell (second sketch), and eventually completely overgrows it (third sketch), resulting in the involute spire seen in late larvae and adults
      
      

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      Figure. Sketches of the larval shells of Oxygyrus keraudreni at early (left) to late (right) stages of larval growth. Scale bar = 0.5 mm. Modified from Richter (1968, Figs. 15). © 1968 G. Richter

Comments

Oxygyrus keraudreni is a shallow-dwelling species that normally occurs in low abundances. In the Indian Ocean, Richter (1982) found that O. keraudreni was essentially limited to the upper 100 m of the water column, with 90% between 50 m and the surface. Similarly, off Hawaii Seapy (1990) recorded low numbers of individuals whose vertical range was limited to the upper 90 m of the water column, with most individuals ocurring at night in the upper 45 m. In another study from the same area off Hawaii, Seapy (2008) found that O. keraudreni was captured in nighttime tows but was either absent or nearly so from comparable daytime ones. The results from the latter two studies suggest the possibility of daytime net avoidance.

References

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. 1968. Heteropoden und Heteropodenlarven im Oberfl?chenplankton des Golfs von Neapel. Pubblicazioni della Stazione Zoologica di Napoli 36: 346-400.

Richter, G. 1982. Mageninhaltsuntersuchungen an Oxygyrus keraudreni (Lesueur) (Atlantidae, Heteropoda). Beispiel einer Nahrungskette im tropischen Pelagial. Senckenbergiana Maritima 14: 47-77.

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

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

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

Spoel, S. van der. 1976. Pseudothecosomata, Gymnosomata and Heteropoda (Gastropoda). Utrecht: Bohn, Scheltema & Holkema. 484 pp.