Protatlanta souleyeti (Smith 1888)

Introduction

Protatlanta souleyeti is a small species (<2 mm shell diameter) that is distinguished from all other atlantids by its calcareous shell and glass-like, cartilaginous keel, composed of conchiolin. The truncated keel extends from the shell aperture to about one-half of the shell's circumference in adult animals. The shell spire consists of three whorls and is slightly elevated and rounded, with incised sutures. In all P. souleyeti from the world oceans the shell spire lacks surface sculpture, although in the Atlantic Ocean there is a second form that has a low spiral ridge centered on the second whorl and extends to the third whorl, and a lower and shorter secondary spiral ridge inside the first one. The eyes lack a transverse slit in the distal pigmented tissue (type a, Richter, 1961), and the operculm has a large apical gyre (macro-oligogyre; type a, Richter, 1961). Geograpic distribution is cosmopolitan at tropical to subtropical latitudes.

Characteristics

1. Shell
   1. Adult shell small (<2 mm), calcareous, and with a transparent cartilaginous keel of conchiolin (compare title illustration with first scanning electron micrograph below)  
      
   2. Keel tall and truncated, extending from the shell aperture to about one-half of the circumference of the shell
      
      

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      Figure. Scanning electron micrographs of the shell of Protatlanta souleyeti, viewed from the right side. Left: Low magnification; scale bar = 0.5 mm. Right: High magnification of the spire; scale bar = 0.1 mm. ©

      

      Figure. Scanning electron micrographs of the shell of Protatlanta souleyeti, tilted about 60°. Left: Low magnification view; scale bar = 0.5 mm. Right: High magnification view; scale bar = 0.1 mm.©

   3. Spire slightly elevated and rounded, with incised sutures, consisting of three whorls (see right photo above)
      
   4. Surface of spire whorls smooth (see high magnification views above), except for a variant with which it co-occurs in the Atlantic Ocean. In this form a low, spiral ridge is located in the middle of the whorl beginning on the second whorl and ends about three-fourths of a turn on the third whorl. A less conspicuous and shorter spiral ridge is located inside the primary ridge on the second whorl
      
      

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      Figure. Shell spire of second form of Protatlanta souleyeti present in the Atlantic Ocean, having low, spiral sculpture on second and third whorls. Modified from Richter and Seapy (1999, fig. 5a). © 1999 G. Richter

   5. Shell aperture large, ovoid in shape (see second photo above)
      
   6. Umbilicus large (Tesch, 1949)
      
2. Digestive gland, contained in the shell spire of adults, is brownish- to reddish-orange (see title illustration)
   
3. Eyes type a; cuboidal in dorsal view, and with distal pigmented tissue lacking a transverse slit (after Richter, 1961)
   
4. Operculum type a; macro-oligogyre, with a large gyre in the apical part of the operculum (after Richter, 1961)
   
5. Radula large and triangular in shape. Central rachidian tooth monocuspid with a large, low basal plate. Lateral teeth monocuspid with a low process on the inner side. Marginal teeth monocuspid (Richter, 1961; Richter and Seapy, 1999)
   
6. Larval shell with a smooth surface and a large ovoid aperture
   
   

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   Figure. Sketch of Protatlanta souleyeti larval shell in side or apertural view. Scale bar = 0.1 mm. Modified from Richter (1968, Fig. 6). © 1968 G. Richter

References

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

Richter, G. 1968. Heteropoden und Heteropodenlarven im Oberflächenplankton des Golfs von Neapel. Pubblicazioni della Stazione Zoologica di Napoli 36: 346-400.

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.

Tesch, J. J. 1949. Heteropoda. Dana Report, No. 34. 53 pp.