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Pisinna olivacea (Frauenfeld, 1867)

Description: Shell minute, solid, variable in shape, usually eroded, spire convex in outline. Protoconch of 1½ whorls, sculptured with spiral rows of minute pits, with distinct junction to teleoconch. Teleoconch up to about 3 weakly to moderately rounded whorls; first teleoconch whorl smooth, latter whorls with oblique axial ribs, absent on last third of last whorl. Spiral groove below suture, forming nodules between groove and suture. Aperture circular to D-shaped, sharp edged; outer lip without varix. Colour red to orange-brown, often with sutural nodule band white.

Size: Up to 2.3 mm in length.

Distribution: Endemic to Australia: Noosa Heads, Queensland, southwards to eastern Victoria and Tasmania.

Habitat: "Lives in the middle to lower littoral and shallow subtidal on short algae and beneath stones etc." (Ponder & Yoo, 1976). Common in beach washup.

Comparison: This tabulation compares all the species of Pisinna that occur in NSW. Some of the species are quite variable, particularly in the presence/absence and strength of axial ribs, so quite a few specimens are needed for a confident identification.

Continental shelf species - Not found more shallow than about 20 m depth, and not in beach washup:

P. laseroni Mid-sized for genus; blunt, smooth shell with very strong varix.

P. nitida Largest species in the genus; smooth, straight sided.

P. paucirugosa Large for genus; like P. nitida but wider and with weak axials

P. tasmanica Mid-sized for genus, smooth or ribbed.

P. varicifera relata Large for genus; fine axials, sutures often deeply grooved.

Intertidal and shallow water species

 1. With axial ribs, even if faint; in order of decreasing abundance in NSW.

P. olivacea Axials absent on at least first whorl. Spiral groove below suture making row of nodules.

P. albizona Either smooth or axially ribbed. White band below suture.

P. frauenfeldi Large for genus (3.7 mm), axials on all whorls, usually white spiral band on middle of last whorl.

P. salebrosa The widest species in the genus; strong oblique ribs with a depression around middle of whorls.

P. kershawi Close-set axials on all whorls, sometimes weak on first whorl. Like P. olivacea but no spiral groove below suture.

P. castella Three spiral rows of strong nodules.

P. oblata Very fine oblique axials on last whorl.

P. tumida Similar to P. olivacea but stronger axials and colour bands. Very rare in NSW.

2. Without axial ribs

P. approxima Small, translucent reddish-brown.

P. vincula Usually with 1-2 spiral cords and angled periphery on last whorl, but sometimes smooth and then very similar to P. approxima.

Synonymy: Rissoa diemenensis Petterd, 1884 is a synonym.

Remarks: The shape and sculpture of this species is variable. The number and strength of axial ribs is variable, as is their commencement point on the spire. The spiral groove below the suture varies in strength, as does the spiral row of nodules it defines.

Fig. 1: 4968-1 Little Manly Beach, Sydney, NSW (C.353046)

Fig. 2: 4970-1 Both specimens Ulladulla, NSW (C.302891)