Abstract
Mytilaster minimusis widely distributed along the rocky intertidal of much of the Mediterranean's coastline and the Adriatic Sea. Populations are, however, threatened by the Lessepsian invaderBrachidontes pharaonisthat occupies the same habitat and is more tolerant of environmental extremes. This is the first study of the anatomy ofM. minimusin relation to its evolution and adaptations towards an intertidal life on the karsted limestone shores that characterize much of the Mediterranean. In most anatomical respectsM. minimusis a typical mytilid but is small (<16 mm shell length) and, post-juvenile, greatly deformed concomitant with its niche of colonizing pits in the karsted rocks. It is thus generally squatter, that is, dorso-ventrally flattened, laterally expanded and antero-posteriorly foreshortened in comparison withM. galloprovincialis. A pair of statocysts has been identified in the visceral mass. Most interest, however, resides in the fact the posterior byssal retractor muscles, like the shell, are foreshortened to comprise one paired block and the posterior pedal retractor muscles are situated beneath these not anterior to them as in other mytilids. These adaptations equipM. minimusfor a compressed, squat, life in the intertidal karst. In addition to competition from the introducedB. pharaonisin the Mediterranean,M. minimusis facing competitive exclusion from the nativeMytilus galloprovincialisthat, as a result of intensive and increasing mariculture, is coming to dominate Croatian shorelines. This study is, therefore, prescient in laying the foundations for future research on what is becoming a threatened native Mediterranean species.
Published Version
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