Atlanta fusca Souleyet 1852

Introduction

Atlanta fusca is a small species (shell diameter to 2.0 mm) that is darkly pigmented (brown to reddish-and golden-brown). The keel is tall and colorless, and inserts between the last and penultimate shell whorls in animals larger than 1.5 mm. The spire is tall and conical, consisting of 4 whorls and possessing a complex pattern of spiral sculpture that ends on the protoconch and is replaced by spiral rows of small punctae on the teleoconch. Coloration ranges from brown to amber and reddish-brown. Eyes type a, operculum type a, and radula type I. Geographic distribution is cosmopolitan in tropical to subtropical waters.

Diagnosis

• Maximal shell diameter = 2.0 mm
• Keel tall and colorless, and inserts between last and penultimate whorls in shells > 1.5 mm diameter
  
• Spire tall and conical, with ornate, complex sculpture
• Coloration brown to amber and reddish-brown
  
• Eyes type a; operculum type a; radula type I
  

Characteristics

1. Shell
   1. Small, with maximal shell diameter = 2.0 mm
   2. Keel tall and rounded
   3. Keel inserts between the last and penultimate shell whorls in shells larger than 1.5 mm
      
      

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      Figure. Scanning electron micrographs of Atlanta fusca shell. Left: right side, perpendicular to shell plane. Right: right side, tilted 60°. Scale bars = 0.5 mm. ©

   4. Spire (protoconch) of four whorls, with ornate and complex surface sculpture and a prominent spiral ridge on the outer margin of the whorls that progresses in height and begins on the second whorl (see enlarged views of the two images below)
      

   5. Spire sculpture replaced on teleoconch by spiral rows of small punctae (see enlarged view of second image below)
      

      

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      Figure. High magnification scanning electron micrographs of shell spire in Atlanta fusca. Left: view at right angle to shell plane. Right: view at 60º tilt. Scale bars = 0.1 mm. ©

   6. Shell and spire coloration light to dark brown, amber and reddish-brown (see title illustrations and below)
      
      

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      Figure. Adult specimen of Atlanta fusca with dark coloration. ©

2. Eyes type a; lacking a transverse slit in the distal pigmented tissue
3. Operculum type a (macro-oligogyre)
   
4. Radula type I, small and ribbon shaped. Like Atlanta turriculata, each tooth row includes (1) a monocuspid central tooth, with a postero-lateral process on each side of the tooth, (2) biscuspid lateral teeth, each with a strong rectangular process on the inner side, and (3) a monocuspid marginal tooth
   
5. Larva
   1. The two lobes of the velum in early larvae give rise to the six-lobed velum in late larvae
      
      

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      Figure. Early (left) and late (right) stage larvae of Atlanta fusca. Drawings combined from Franc (1948, Figs. 10, 11). © 1948 A. Franc

      

      Figure. Late veliger larva of Atlanta fusca with six, ciliated velar lobes. Scale bar = 0.5 mm. Modified from Richter (1968, Fig. 11). © 1968 G. Richter

   2. Surface sculpture of larval shell (following the apical embryonic portion) of multiple, parallel spiral ridges
      
      

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

      

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      Figure. Larval shell of Atlanta  fusca. Left: Scanning electron micrograph of shell viewed in the shell plane. Scale bar = 50 µm. Modified from Thiriot-Quiévreux (1973, Fig. 6b). © 1973 C. Thiriot  Right: Sketches of three stages in the development of the larval shell. Scale bar = 250 µm. Modified from Richter (1968, Fig. 10). © 1968 G. Richter

Comments:

Atlanta fusca is a cosmopolitan species, found at tropical to subtropical latitudes. It is limited to the upper 100 m of the water column in Hawaiian waters (Seapy, 1990b). Comparison of day and night vertical distribution of abundances suggested that a portion of the population from 50-100 m migrated into the upper 50 m at night

References

Franc, A. 1948. Veligeres et mollusques gasteropodes des Baies d'Alger et de Banyuls. Journal de Conchyliologie (Paris) 88:13-35, plates 1 and 2.

Richter, G. 1968. Heteropoden und Heteropodenlarven im Oberfl?chenplankton 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. 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.

Thiriot-Quievreux, C. 1973. Heteropoda. Oceanography and Marine Biology Annual Review, H. Barnes (ed.) 11: 237-261.