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Anachis atkinsoni (Tenison-Woods, 1875)

Description: Shell variable in shape, with height to width ratio in the range 2.3-3.0; transparent, whorls slightly convex, aperture about 1/3 of shell length. Axial sculpture completely absent, or present as weak or strong axial ribs. Spiral sculpture restricted to a few cords on base, and sometimes also faint threads on body whorl. Outer lip sinuous in profile with a sinus posteriorly, not thickened externally, thickened but smooth internally. Columella smooth, callus sharp-edged. Siphonal canal slightly upturned, notch moderately deep. Colour variable; transparent when fresh, aging to opaque white or fawn; usually with a spiral row of white spots below suture and another at middle of body whorl, these coinciding with axial ribs when present; sometimes axial zigzag brown lines; usually brown blotch inside outer lip posteriorly.

Size: Adults 2.5-4 mm in length.

Distribution: Endemic to Australia; northern Queensland around southern Australia to at least Cape Leeuwin, south-western WA.

Habitat: Intertidal on seagrass and algae, and down to about 50 m, in estuaries and on the open coast. Very common, particularly as empty beach shells.

Comparison: See A. fulgida

Synonymy: Columbella speciosa Angas, 1877 is a synonym based on a specimen with zigzag axial tan lines from Port Jackson, NSW.

Remarks: This species is undoubtedly one of the most variable of Australian molluscs. Adult size varies from 2.5-4 mm. Shell shape may be tall and slender (Fig. 1) or relatively short and stout (Fig. 2). Shells may be smooth, or possess strong axial ribs on the spire, on the last whorl, or on both. The colour pattern usually includes a spiral row of white spots, but axial brown lines may be present in a variety of configurations (Fig. 3). The distribution limits are not clear, due to the extreme variability of the species. Axially ribbed specimens occur throughout the range, but very strongly ribbed shells that appear to be an extreme form of this species occur in Port Curtis, Gladstone, and further north in Torres Strait.

Fig. 1  Off Chinamans Beach, Middle Harbour, NSW (C.334813)

Fig. 2  Middle Harbour, NSW (C.335157)

Fig. 3  a. Sow and Pigs Reef, Port Jackson, NSW (C.334736) b. Fishermans Bay, Port Hacking, NSW (C.335112) c. Georges Bay, St. Helens, Tasmania (C.unreg) d,e. Shelly Beach, Bermagui, NSW (C.335054)