Atlanta californiensis Seapy and Richter 1993

Introduction

Atlanta californiensis is a small species with a transparent, flattened shell (diameter to 3.5 mm), with a orange to red-brown keel base. The shell spire is low and globular in shape, consisting of 3-1/4 whorls and clear to violet and purple sutures. Internal walls of spire partially decalcified, which allies A. californiensis most closely with A. inflata. Morphology of the eye is type a, and that of the operculum type c.  The geographical distribution of the species is unusual among atlantids; it is limited to the Transition Zone faunal province of the North Pacific Ocean.

Diagnosis

A species of Atlanta, belonging to the Atlanta inflata species group, with the following characteristics:

• A small species (to 3.5 mm shell diameter), with a laterally-flattened, transparent shell
  
• Shell spire low and globular, consisting of 3-1/4 whorls, with partially decalcified internal walls, lacking surface sculpture, and having sutures that range from transparent to purple
  
• Keel with a brown base
  
• Geographic distribution restricted to the Transition Zone faunal province of North Pacific Ocean

Characteristics

1. Shell
1. Shell moderately small (to 3.5 mm, with 4-2/3 whorls)
2. Shell flattened and transparent, except keel base that is orange to red-brown
3. Keel penetrates between last and penultimate whorls in shells larger than about 2 mm diameter
4. Shell spire low and globular in shape, consisting of 3-1/4 whorls, a smooth surface (lacking sculpture), and sutures ranging from clear to light violet to purple
5. Spire coloration variable, either uniformly clear to light yellow, brown or violet, or mottled light to dark yellow-brown to brown

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Figure. Shell of Atlanta californiensis, right side, southern California waters. Left: low magnification, scale bar = 1.0 mm. Right: high magnification of shell spire. © 2005 R. Seapy



Figure. Shell of Atlanta californiensis, tilted view of right side, southern California waters. Left: low magnification, scale bar = 1.0 mm. Right - high magnification of shell spire, scale bar = 0.1 mm. © 2005 R. Seapy

6. Umbilicus wide, but narrows rapidly with penultimate whorl


Figure. Shell of Atlanta californiensis, left side, southern California waters. Left: low magnification, scale bar = 1.0 mm. Right: high magnification of umbilical region. Scale bar = 0.1 mm. © 2005 R. Seapy

7. Internal walls of shell spire partially decalcified and replaced with thin, chitinous membrane

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Figure. Shell of Atlanta californiensis, immature specimen, frontal view, southern California waters. Transmitted light photograph. © 2005 G. Richter.

2. Eyes
1. Eye type a, with a clear, spherical lens and a black pigmented base that is interrupted dorsally by a triangular-shaped, transparent window
3. Operculum
1. Operculum monogyre (type c; see below); thin, transparent and oval in shape
2. Spiral gyre portion lacks spines


Figure. Type c (monogyre) operculum of Atlanta californiensis, southern California waters, scale bar = 0.5 mm. © 2005 R. Seapy

4. Radula
1. Radula large relative to size of the animal
2. Radula elongate and narrowly triangular, with distinct sexual dimorphism (male radula broader, with a wider growth angle and greater number of tooth rows than in females)
   
   
   

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   Figure. Radulae of Atlanta californiensis, southern California waters. Left: low magnification views of male (with 99 tooth rows) and immature female (with 56 tooth rows), scale bar = 0.1 mm. Right: high magnification views of sections of male and female radulae, respectively. Arrows indicate row 60 on each radula, scale bar = .05 mm. © 2005 G. Richter.

Comments:

Morphological features of the body (with type a eye and type c operculum) and shell interior (i.e., with partial decalcification of the inner walls of the shell spire and replacement by a thin, flexible chitinous membrane) ally Atlanta californiensis most closely with A. inflata. The external shell morphologies of the two species, however, are quite different. The shell of A. californiensis is laterally flattened, with a spire consisting of 3-1/4 whorls and having a smooth surface (similar to the shells of A. gaudichaudi and A. peroni), while that of A. inflata is laterally "inflated" (about 40% of the shell diameter) and has a spire of 4-1/2 whorls, usually bearing evenly-spaced, raised spiral ridges. Also, the radulae of the two species differ; they are broader and consist of fewer tooth rows in A. californiensis than in A. inflata (see Seapy and Richter, 1993).

Distribution

The vertical distribution of Atlanta californiensis during daytime hours was studied based on specimens collected  using opening-closing BONGO plankton net samples in waters overlying San Pedro Basin, southern California (Seapy and Richter, 1993). The daytime vertical range extends from the surface to about 150 m (the depth range of the epipelagic zone in nearshore basins off southern California). Comparable discreet depth interval samples were not collected at night, so it is not known if this species undergoes nocturnal vertical migration. Such a migration probably does occur, however, since a number of other species of atlantids whose ranges extend throughout the 400-m deep epipelagic zone in Hawaiian waters were shown to be vertical migrators (Seapy, 1990).

Seasonal distribution

Replicated, monthly oblique samples through the epipelagic zone in San Pedro Basin (Cummings and Seapy, 2003) showed that A. californiensis occurs in low abundance (less than 25 individuals per 1,000 m³) throughout the year except in the summer (to 88 per 1,000 m³). The seasonal maxima are correlated with the period of the year when California Current flow is strongest and shoreward transport of animals from offshore waters would be predicted.

Geographic distribution

Among the Heteropoda, two species (Atlanta californiensis and Carinaria japonica) are members of the Transition Zone faunal province of the North Pacific Ocean (Seapy and Richter, 1993; Seapy, 1974, respectively). This faunal province extends in a narrowing band from the western coast of North America (between British Columbia and southern Baja California) to the coast of Japan. It is bounded to the north by the Subarctic Pacific faunal province and to the south (at the Subarctic Boundary) by the Central North Pacific faunal province.

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Figure. Location of the Transition Zone faunal province (shaded violet) and records (red dots) of Atlanta californiensis. The black dots are station records for Atlanta sp.of McGowan (1967), many of which, particularly those north of about 34° latitude, are probably A. californiensis. The Transition Zone province is separated from the Central North Pacific province by the Subarctic Boundary and its southeasterly extension to Baja California; it merges more broadly with the Subarctic Pacific province to the north. Drawing modified from Seapy and Richter (1993).

References

Cummings, F. A. and R. R. Seapy. 2003. Seasonal abundances of euthecosomatous pteropods and heteropods from waters overlying the San Pedro Basin, California. Veliger 46: 305-313.

McGowan, J. A. 1967. Distributional atlas of pelagic molluscs in the California Current region. California Cooperative Oceanic Fisheries Investigations, Atlas 6, 218 pp.

Seapy, R. R. 1974. Distribution and abundance of the epipelagic mollusk Carinaria japonica in waters off Southern California. Marine Biology 24: 243-250.

Seapy, R. R. 1990. Patterns of vertical distribution in epipelagic heteropod molluscs off Hawaii. Marine Ecology Progress Series 60: 235-246.

Seapy, R. R. and G. Richter. 1993. Atlanta californiensis, a new species of atlantid heteropod (Mollusca: Gastropoda) from the California Current. Veliger 36: 389-398.