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The biology and functional morphology of Arca noae (Bivalvia: Arcidae) from the Adriatic Sea, Croatia, with a discussion on the evolution of the bivalve mantle margin

In the Croatian Adriatic, Arca noae occurs from the low intertidal to -60 m, can live for >15 years and is either solitary or forms byssally-attached clumps with Modiolus barbatus. The shell is anteriorly foreshortened and posteriorly elongate. The major inhalant flow is from the posterior although a remnant anterior stream is retained. There are no anterior but huge posterior byssal retractor muscles and both anterior and posterior pedal retractors. The ctenidia are of Type B (1a) and the ctenidial-labial palp junction of Category 3. The ctenidia collect, filter and undertake the primary sorting of potential food in the inhalant water. The labial palps are small with simple resorting tracks on the ridges of their inner surfaces. The ciliary currents of the mantle cavity appear largely concerned with the rejection of particulate material. The mantle margin comprises an outer and an (either) inner or middle fold. The outer fold is divided into outer and inner components that secrete the shell and are photosensory, respectively. The latter bears a large number of pallial eyes, especially posteriorly. The inner/middle fold of A. noae, possibly representative of simpler conditions, may have differentiated into distinct folds in more recent representatives of the Bivalvia.

Arca noae; Arcidae; Adriatic Sea; distribution; anatomy; mantle margin; evolution

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