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Containing group: Atlantidae

Introduction

Along with Atlanta peronii, Oxygyrus inflatus attains the largest shell size (diameter to 10 mm) in the family Atlantidae. The larval shell is calcareous and displays a distinctive pattern of evenly-spaced, zigzag-shaped spiral ridges oriented in the axis of shell growth and covering the shell surface. Unlike all other atlantids, the adult shell (teleoconch) is composed of a cartilaginous material, conchiolin. With growth the teleoconch overgrows the larval shell (protoconch), eventually surrounding it. An exposed shell spire, as seen in all other atlantids, is lacking and the spire is termed involute. The keel is glass-like, composed of conchiolin, and has a truncate anterior edge. The eyes are type c; characterized by a solid black, cup-shaped base. The operculum is unique among atlantids, with a broadly triangular shape (approaching trapezoidal) and lacking an apical gyre. The radula is type I, very large, with an elongate, triangular shape. The central (rachidian) tooth bears three cusps, in contrast with the single median cusp seen in all other atlantids. The geographical distribution is cosmopolitan in tropical to subtropical waters.

Diagnosis

• Larval shell (protoconch) calcareous, with evenly-spaced, zigzag-shaped ridges that are oriented in the axis of shell growth and cover the shell surface
  
• Adult shell (teleoconch) and keel cartilaginous, composed of conchiolin
• Shell spire involute
• Eyes type c, with a solid black, cup-shaped base
• Operculum broadly triangular, nearly trapezoidal in shape, and lacks a gyre region
  
• Radula type I; large with an elongate, triangular shape
  
• Central (rachidian) tooth with three cusps
  

Characteristics

1. Shell
   1. Larval shell calcareous with evenly-spaced, zigzag-shaped spiral ridges oriented in the axis of shell growth and covering the shell surface (see scanning electron micrographs below)
      
   2. Adult shell and keel cartilaginous, composed of conchiolin.  The adult shell overgrows the larval shell and eventually covers it except for the central portion, represented by the involute spire (see electron micrographs below)
      
      

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      Figure. Scanning electron micrographs of the shell of a young Oxygyrus inflatus (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. © Roger R. Seapy
      
      

      

      Figure. Scanning electron micrographs of the shell of the Oxygyrus inflatus shown above, viewed from the 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 resulted in a slight elevation of the base of the teleoconch from the underlying protoconch. ©

2. Eyes type c, which is distinguished by a solid black, cup-shaped base that holds the spherical lens. This eye type is found in only two other species of heteropods; Atlanta helicinoidea and the Cardiapoda richardi
   
   

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

3. Operculum broadly triangular, approaching trapezoidal, and lacks an apical gyre; unique among the atlantids. Also unlike other atlantids, only one-half of the aperture is covered by the operculum when the animal retracts into its shell  (Richter and Seapy, 1999)
   
   

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   Figure. Ventral views of opercula from Oxygyrus inflatus. Left: sketch of operculum from Spoel (1976, Fig. 133), © 1976 S. van der Spoel. Right: scanning electron micrograph of an operculum attached to the opercular lobe of the foot. ©

4. Radula
   1. Radula considerably larger than in other atlantids, with an elongate triangular shape and a broad base
      
      

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      Figure. Radula of Oxygyrus inflatus; whole radula (left) and lower half of radula enlarged (right). Modified from Richter (1961, figs. 27a,b) by addition of scale bars. © 1961 G. Richter

   2. Central tooth tricuspid, with the three cusps of equal length in juveniles, but with the median cusp substantially longer in adults (see sketch below)
      
      

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      Figure. Sketch of a central (rachidian) tooth from the radula of an adult Oxygyrus inflatus. From Richter (1961, fig. 2c). © 1961 G. Richter

   3. Lateral teeth (innermost tooth in the two drawings below) with a strong process on the inner side that bears an accessory cusp; the process and cusp become larger in adults (see second drawing below). The adjacent pair of marginal teeth are monocuspid and become more strongly hooked distally in adults
      
      

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      Figure. Drawings of juvenile (left) and adult (right) lateral and paired marginal teeth in Oxygyrus inflatus. From Richter (1961, fig. 12e). © 1961 G. Richter

5. Larva
   1. Larval tissues posterior to the head region light bluish-purple; digestive gland brown (see photograph below)
      
      

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

   2. Larval shell with evenly-spaced, zigzag-shaped spiral ridges that run parallel with the axis of coiling and cover the shell surface (see the scanning electron micrograph below)
      
      

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      Figure. Larval shell of Oxygyrus inflatus viewed from beneath. Scanning electron micrograph modified from Thiriot-Quiévreux (1973, fig. 6a) by addition of scale bar (= 100 µm). © 1973 C. Thiriot

   3. Embryonic portion (protoconch I) of the larval shell low and dome-shaped (see the scanning electron micrographs below). The embryonic shell is only seen in early larvae because it is subsequently overgrown by protoconch II. In the three sketches below, protoconch II can be seen to progressively override and eventually completely overgrow protoconch I. The result is that an exposed spire (as seen in all other atlantids) is lacking in adults,  (see first two scanning electron micrographs above); and the spire is termed involute
      
      

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      Figure. Scanning electron micrographs of fossil Oxygyrus inflatus early larval shells in apertural (left) and apical (right) views. Original photographs graciously provided by A. Janssen. Scale bars = 50 µm. © 2011 Arie Janssen

      

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      Figure. Sketches of the larval shells of Oxygyrus inflatus at early (left) to late (right) stages of larval growth. Scale bar = 500 µm. Modified from Richter (1968, Figs. 15). © 1968 G. Richter

Comments

Taxonomy: This species has been recognized as Oxygyrus keraudreni (Lesueur, 1817) for over a century by all but a few authors; despite the fact that two early workers (d'Orbigny, 1835, and Souleyet, 1852) had shown that what Lesueur had described as Atlanta Keraudrenii was not a species of Atlanta. Both authors, however, politely continued to use Lesueur's well-known name and authorship for reasons of 'nomenclatural stability'. Erroneously interpreting the name Atlanta Keraudrenii Lesueur, Rang (1827) was the first to describe and illustrate the species now known as Oxygyrus keraudreni.

The generic name Oxygyrus was introduced in 1835 by Benson. Following a lengthy description of the genus, and at the very end of the paper, Benson named the species Oxygyrus inflatus with a very brief description in Latin and English:

"It only remains to give the specific character of Oxygyrus. O. inflatus. Shell tumid, whorls transversely and closely striate; sutures deeply cut."

Interestingly, Benson referred to neither Lesueur (1817) nor Rang (1827) in his paper. Fifteen years after Benson's publication, Gray (1850, p. 101) merged Lesueur's specific epithet with Benson's genus to create Oxygyrus keraudreni, which he then attributed to Lesueur! Based on Benson's short description of O. inflatus, and overlooking the preceding 2-1/2 pages of Benson's generic description, Tesch (1906) was unable to interpret the species name, and two years later in his 1908 review of the taxonomy of the family Atlantidae, Tesch (p. 5) included "O. inflatus? Benson" in his list of synonyms for O. keraudreni, but with no explanation or further discussion. Since the latter paper there have only been two authors (Thiele, 1929 and Wenz, 1941) who have recognized O. inflatus as the type species of Oxygyrus.

The preceding discussion is based on an as-yet unpublished manuscript by Arie W. Janssen, graciously provided to R. Seapy. Janssen is of the opinion that the original description of Atlanta keraudreni by Lesueur (1817) refers to a juvenile Atlanta species, most probably A. peronii (Lesueur, 1817). As noted above, the specific epithet keraudreni has persisted until the present as Oxygyrus keraudreni (Lesueur, 1817), which has been used by nearly all authors over the past century. To avoid all possible future uncertainties, Janssen intends to designate a neotype for Lesueur's taxon, making it a synonym of Atlanta peronii. And, Oxygyrus inflatus Benson, 1835 must now be recognized as the valid species name.

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

Other Names for Oxygyrus inflatus Benson 1835

• Oxygyrus keraudreni Lesueur, 1817

References

Benson, W. H. 1835. Account of Oxygyrus; a new genus of pelagian shells allied to the genus Atlanta of Lesueur, with a note on some other pelagian shells lately taken on board the ship Malcolm. Journal of the Asiatic Society of Bengal 4: 173-176.

Gray, J. E. 1850. Catalogue of the Mollusca in the collection of the British Museum, 2. Pteropoda. British Museum/E. Newman, London. 45 pp.

Janssen, A. W. In Prep. Late Quaternary to Recent holoplanktonic Mollusca (Gastropoda) from the eastern Mediterranean: systematics, morphology and palaeogeographical implications (provisional title, intended to be published eventually in Bollettino Malacologico).

Lesueur, [C. A.] 1817. M?moire sur deux nouveaux genres de mollusques, Atlante et Atlas. Journal de Physique, de Chimie, et d'Histoire naturelle 85: 390-393, 2 plates.

Orbigny, A. d'. 1834-1847. Voyage dans l'Am?rique m?ridionale (le Br?sil, la r?publique orientale de l'Uruguay, la r?publique Argentine, la Patagonie, la r?publique du Chili, la r?publique de Bolivia, la r?publique du P?rou), ex?cut? pendant les ann?es 1826, 1827, 1828, 1829, 1830, 1831, 1832 et 1833, 5. Bertrand and Strasbourg, Levrault, Paris. pp. 49-72, 129-176, pls. 3-8, 17-23, 25, 55 (1835).

Rang, [P. C. A. L]. 1827. Observations sur le genre Atlante. M?moires de la Soci?t? d'Histoire naturelle de Paris 3: 372-381, pl. 9, figs. 4-6 and 8.

Richter, G. 1961. Die Radula der Atlantiden (Heteropoda, Prosobranchia) und ihre Bedeutung f?r die Systematik und Evolution der Familie. Zeitschrift f?r Morphologie und ?kologie der 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. Backhuys Publishers, Leiden.

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

Seapy, R. R. 1990b. Patterns of vertical distribution in epipelagic heteropod molluscs off Hawaii. Marine Ecology Progress Series 60: 235-248.

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

Souleyet, [L. F. A.] 1852. Heteropodes, pp. 289-392. In: [J. F. T. Eydoux] and [L. F. A.] Souleyet, Voyage autour du monde ex?cut? pendant les ann?es 1836 et 1837 sur la corvette 'La Bonite', command?e par M. Vaillant, capitaine de vaisseau, publi? par ordre du Gouvernement sous les auspices du D?partement de la marine. Zoologie, 2. Atlas. A. Bertrand (ed), Soci?t? de G?ographie, Paris.

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

Tesch, J. J. 1906. Die Heteropoden der Siboga-Expedition. Monograph 51, 112 pp, 14 plates. E. J. Brill, Leiden.

Tesch, J. J. 1908. Systematic monograph of the Atlantidae (Heteropoda) with enumeration of the species in the Leyden Museum. Notes from the Leyden Museum 30: 1-30, 5 plates.

Thiele, [K. H.] J. 1929. Handbuch der systematischen Weichtierkunde, Fischer, Jena 1(1): 1-376.

Thiriot-Quievreux, C. 1973. Heteropoda. Oceanography and Marine Biology Annual Review 11: 237-261.

Wenz, W. 1941. Handbuch der Pal?ozoologie, 6. Gastropoda, 1. Allgemeiner Teil und Prosobranchia, 5. Borntr?ger, Berlin. pp. 961-1200.