Atlanta peronii Lesueur 1817

Introduction

Atlanta peronii is the type species in the genus Atlanta. It attains the largest size of all atlantids (to 10 mm shell diameter). The walls of the shell are unusually thick and strong. The keel is moderately tall and rounded in side profile, with a slightly truncated leading edge. The keel base is clear in young animals, but changes with age to a light brown and then a dark yellowish-brown. The keel inserts between the fourth and fifth whorls in shells larger than about 3 mm. The spire consists of 3-1/2 to 3-3/4 whorls. The whorl surfaces are smooth. The sutures between the first and second whorls are shallow, while those between subsequent whorls are incised. Full grown shells can consist of up to 6 whorls. The shell is transparent, but can become faintly yellow in large individuals. Eyes type b. Operculum type b. Radula type II, large with sexually-dimorphic marginal teeth. The species is cosmopolitan in tropical to subtropical waters. In Hawaiian waters, A. peronii is a strong nocturnal vertical migrator, ranging downward to a daytime maximum of 300 m and migrated into the upper 150 m at night.

Diagnosis

• Maximal shell diameter = 10 mm
• Shell with thick, strong walls, consisting of up to 6 whorls
  
• Shell transparent, but becomes a faint yellow in older animals
• Keel moderately tall and rounded, inserting between fourth and fifth whorls in shells larger than about 3 mm
  
• Keel base clear in young individuals, becoming brown to yellow-brown with age
• Spire consists of 3-1/2 to 3-3/4 whorls
• Sutures between the first and second whorls shallow; subsequent sutures incised
  
• Spire whorls smooth
• Eyes type b
• Operculum type b
• Radula type II; large, with sexually-dimorphic marinal teeth
  

Characteristics

1. Shell
1. Shell diameter to 10 mm
   
2. Shell with thick, strong walls; wall thickness = 22-25 µm (Richter, 1993)
   
3. Spire consists of 3-1/2 to 3-3/4 whorls; shell of full-grown individuals with up to 6 whorls
   
4. Surface of spire whorls smooth
5. Sutures between the first and second whorls are shallow; sutures between subsequent whorls are deeply incised (see the fourth SEM below)
   
   

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   Figure. Scanning electron micrographs of a 3.8 mm Atlanta peronii shell viewed from the right side. Shell at low magnification (left) and spire at high magnification (right). Scale bars = 0.5 mm (left) and 0.1 mm (right). © Roger R. Seapy
   
   

   

   Figure. Scanning electron micrographs of a 3.8 mm Atlanta peronii shell viewed from the right side tilted. Shell at low magnification (left) and spire at high magnification (right). Scale bars = 0.5 mm (left) and 0.1 mm (right) ©

6. Keel rounded in side view, with a slightly truncated leading edge (see title illustration)
   
7. Keel inserts between shell whorls beginning at about whorl 4-1/4; at a shell diameter of about 3 mm (see first SEM above)
   
8. Keel base clear in young individuals (see photograph below), but changes with age from a light brown to a dark yellowish-brown (see title illustration)
   
9. Shell transparent in small to moderate sized individuals (see photograph below), becoming faintly yellow in large specimens (Richter, 1993)
   
   

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   Figure. Photograph of a young Atlanta peronii. Shell diameter = 1.1 mm. ©

2. Eyes type b; with a transverse slit in the distal pigmented tissue
   
   

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   Figure. Right eye of Atlanta peronii, illustrating the clear transverse slit in the distal pigmented tissue that is diagnostic of the eye type. ©

3. Operculum type b (micro-oligogyre)
4. Radula type II
1. Radula large, with the number of tooth rows limited to about 60 (Richter, 1993)
2. Lateral teeth tall
   
3. Marginal teeth sexually dimorphic (Richter, 1993). Male teeth shaped as sharply-bent hooks, with the result that they are much shorter than the respective lateral teeth (see SEM on left below). In females (SEM on right below) the marginal teeth are longer and less sharply curved
   
   

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   Figure. Sections of radulae from male (left) and female (right) Atlanta peronii. Shape and length of marginal teeth differ between the two sexes. Image modified from Richter (1993, Fig. 19). © 1993 G. Richter

Comments

An Atlanta peronii species group was recognized by van der Spoel (1976) that included three species; A. peronii, A. gaudichaudi and A. pacifica. Although nobody has formally placed A. pacifica in synonymy with A. gaudichaudi, Spoel characterized it as either very close to if not synonymous with A. gaudichaudi. Richter (1993) revised the species composition of the A. peronii species group to include four species (A. peronii, A. rosea, A. fragilis, and A. frontieri). Among these species A. rosea was described by Souleyet in 1852 but had been synonymized with A. peronii by Tesch (1908, 1949). However, Richter showed that A. rosea was valid and resurrected the species. He also described Atlanta fragilis and A. frontieri as new species. And, lastly, he separated A. gaudichaudi from the A. peronii species group to form its own group that also included A. plana and A. echinogyra. The two species groups are primarily distinguished by radular differences (type II radula with bicuspid lateral teeth in the former group; type I radula with monocuspid lateral teeth in the latter).

In comparisons with the numbers of other heteropod and/or atlantid species collected in several studies, Atlanta peronii was moderately abundant to abundant. From plankton and dredge samples collected in the western Indian Ocean off Australia (Seapy, et al., 2003), A. peronii ranked third (9.4% of the total) among nineteen heteropod species. Of the twenty-two species of heteropods collected during the Meteor Expedition in the northern Indian Ocean (Richter, 1974) Atlanta peroni was fifth most abundant (8.2% of the total). His identification and characterization of the species, however, was incorrect, which he resolved in 1993 with the description of Atlanta frontieri. Three studies from Hawaiian waters of the North Pacific (Seapy, 1990a,b and 2008) reported the abundance of A. peronii. In the first paper data from five sampling periods were combined and it ranked fifth (11.5% of the total) among thirteen species of atlantids. The second paper was a study of diel vertical distributions of heteropods off leeward Oahu, and A. peronii ranked fourth (28.5% of the total) among eleven species of atlantids and sixth (8.3%) among heteropod species. In the last paper, a study of diel vertical distributions at three stations located 1, 5, and 15 nmi off leeward Oahu, A. peronii was fifth most abundant among twelve heteropod species (2.7% of the combined mean nighttime densities) in a fall sampling period and fourth of seventeen species (7.6%) in a spring period.

Diel vertical distribution patterns of A. peronii off the leeward coast of Oahu, Hawaii was studied by Seapy (1990b, 2008). In the former study, the species ranged downward to a daytime depth of 200-300 m (the maximal depth interval sampled), and migrated into the upper 150 m of the water column at night. In the latter study, vertical distribution patterns were studied at three stations located at 1, 5 and 15 nmi offshore. The species was recorded to a daytime depth of 160-200 m at the 5 and 15 nmi stations, while at night a slight upward migration was suggested since the deepest depth interval containing specimens was 120-160 m at both stations. Also, the abundance of A. peronii decreased shoreward by 50% from the 15 nmi to the 1 nmi station (comparison based on mean nighttime densities at each station).

References

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

Richter, G. 1993. Zur Kenntnis der Gattung Atlanta (V). Die Atlanta peroni-Gruppe und Atlanta gaudichaudi (Prosobranchia: Heteropoda). Archiv für Molluskenkunde 122: 189-205.

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 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.

Souleyet, L. F. A. 1852. Heteropodes, pp. 289-392. In: J. F. T. Eydoux and L. F. A. Souleyet, Voyage autour du monde exécuté pendant les années 1836 et 1837 sur la corvette 'La Bonite', commandée par M. Vaillant, capitaine de vaisseau, publié par ordre du Gouvernement sous les auspices du Département de la marine. Zoologie, 2. Atlas. A. Bertrand (ed), Société de Géographie, Paris.

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

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.

Tesch, J. J. 1949. Heteropoda. Dana Report 34, 53 pp., 5 plates.