Shell equivalve, subcylindrical. Outline modioliform. Periostracum smooth and shining. Dorsal margin usually straight, ventral margin straight or slightly arcuate (distinctly arcuate in older specimens) with posterior end evenly rounded. Umbones nearly terminal. Smooth sculpture. Periostracum shining. Internal margin smooth.
shell brown, dark brown in older specimens; pale yellow zigzag lines in younger specimens. Internally the shell is usually purple above and white below the umbonal keel.
common size :
20-30 mm length. Height/length ratio=0.59, width/length=0.35, max. observed length 47 mm (Russo, 2001).
Remarkably similar to X. pulex (Lamarck, 1819), a south Australian species, from which it differs in the following characteristics: exteriorly above the umbonal keel the shell is brown (black in X. pulex), often paler below the keel; the umbones are terminal in X. pulex, nearly terminal in X. securis. Mature specimens of X. securis are significantly narrower than X. pulex.
BIOLOGY / ECOLOGY
Found exclusively in brackish waters, this species can tolerate very broad variations in salinity (1‰ to 31‰). X. pulex is strictly a marine species.
in upper reaches of estuaries where salinity is low for a considerable part of the year. In association with Mytilus galloprovincialis, attached with a byssus on hard substrata or oyster shells in numerous colonies. Specimens are found attached on algae, in masses of timber, stones or dead shells.