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Morula marginalba (Blainville, 1832)

Description: Shell solid, with five spiral rows of nodules on body whorl, with a variable number of scaly ribs between. (The shell is usually eroded, so this fine sculpture is normally not present). Outer lip thick, curved, with four teeth internally, more or less the same size. Columella thickly calloused, smooth. External colour grey with black nodules; deep interior of aperture purple-grey, teeth white; outer lip cream or cream alternating with black or brown.

Size: Up to 35 mm in length, but usually less than 20 mm.

Distribution: Northern Australia, from north-western Australia to Twofold Bay, NSW. (See comparison below).

Habitat: On rocks on all intertidal shores. Abundant.

Comparison: There has been debate as to whether this species is distinct from the widespread tropical Indo-West Pacific species Morula granulata (Duclos, 1832). However, the most recent work by Tan (1995) makes a convincing case for it being a distinct species, and presents the following comparison of characters.

Character

M. granulata

M. marginalba
Outline of outer lip Parallel to shell axis about 2/3 way towards the anterior Curved
Pigmentation on inside of outer lip Adults (thickened outer lip): continuous dark brown or black margin

Growing shells: black throughout

 Adults (thickened outer lip): continuous cream margin, or cream alternating with brown/black; cream between tubercule rows, black at tubercule rows

Growing shells : cream margin when growing edge is between tubercules, alternating cream and black when growing edge is at tubercules
Apetural teeth inside outer lip Strongly dimorphic, two larger and two smaller, white More or less equal sized, white
Columella Stained dark brown/black at parietal region Stained light brown or overlaid with white inductra
Animal: sides of foot Dark green and subcutaneous aggregate white grains Dark green, light yellow-green and dense subcutaneous white pigment grains
Penis Flagellate, no penis flap Flagellate, with distinct penis flap

Given this interpretation of the two species, the distributions as given by Tan (1995) and elaborated by the Australian Museum collection is:

Morula granulata: Entire Indo-West Pacific Region, from eastern Africa to the eastern Pacific, including northern Australia from central Western Australia to Queensland. In Queensland, it occurs on the outlying area of the Great Barrier Reef.

Morula marginalba: Northern Australia, from north-western Australia to Twofold Bay, NSW, on the coast and continental islands, and other islands of the central Indo-West Pacific.

Figs 1a, 2a: The Basin, Pittwater, NSW (DLB2432)

Figs 1b, 2b: Cremorne Point, Sydney Harbour, NSW (DLB4956)

Copyright Des Beechey 2000