Atlanta californiensis Seapy and Richter, 1993

Introduction

Atlanta californiensis Seapy and Richter, 1993 is one of the three most recently described species; the other two, A. fragilis and A. frontieri, were described by Richter in the same year. It has the most restrictive geographical distribution of the atlantids, limited to the Transition Zone faunal province of the North Pacific Ocean (which includes the California Current off the west coast of North America and extends westward in a narrow band to eastern Asia). Its shell is most similar in external appearance to those of A. gaudichaudi and A. peroni, but morphological features of its body and spire interior ally it most closely with A. inflata and the A. inflata species group.

Diagnosis

An Atlanta with:

• Restricted geographically to Transition Zone faunal province of North Pacific Ocean.
• Shell flattened, up to 3.5 mm diameter, with a brown keel base; small, low and globular spire with 3-1/4 whorls.

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, with 3-1/4 whorls, a smooth surface, 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 = 1.0 mm. Right - hign magnification of shell spire. Photographs copyright ? 2005 R. Seapy

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

6. Umbilicus wide, but narrows rapidly with penultimate whorl.
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Figure Shell of Atlanta californiensis, left side, southern California waters. Left - low magnification, scale = 1.0 mm. Right - high magnification of umbilical region. Scale bar = 0.1 mm. Photographs copyright ? 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. Photograph copyright © 2005 G. Richter.

2. Eye
1. Eye type a (see Atlanta page), 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); thin, transparent and oval in shape.
2. Spiral gyre portion lacks spines.
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Figure Type c (monogyre) operculum of Atlanta californiensis, southern California waters, scale = 0.5 mm. Photograph copyright ? 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 two photographs - low magnification views of male (with 99 tooth rows) and immature female (with 56 tooth rows), respectively; scale = 0.1 mm. Right two photographs - high magnification views of sections of male and female radulae, respectively. Arrows indicate row 60 on each radula, scale = .05 mm. Photographs copyright ? 2005 G. Richter.

Comments

The above information is from Seapy and Richter (1993).

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.

Distribution

Vertical distribution

Daytime opening-closing BONGO plankton net samples collected by R. Seapy in San Pedro Basin, off southern California (Seapy, unpubl.) suggest that Atlanta californiensis is found between the surface and about 150 m (the range of the epipelagic zone in these waters). Nighttime samples have not been collected, so it is not known if this species undergoes nocturnal vertical migration. Such a migration probably does occur, however, since other species of atlantids whose ranges extend through the epipelagic zone in Hawaiian waters showed nocturnal vertical migration behavior (Seapy, 1990).

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 and (shaded blue) and records (red dots) of Atlanta californiensis. The 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. 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. Modified from Seapy and Richter (1993).

References

McGowan, J. A. 1967. Distributional atlas of pelagic molluscs in the California Current region. Calif. Coop. Ocean. Fish. Invest., Atlas 6, 218 pp.

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

Seapy, R. R. 1990. Patterns of vertical distribution in epipelagic heteropod molluscs off Hawaii. Mar. Ecol. Progr. Ser. 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.