Mytilaster minimus is widely distributed along the rocky intertidal of much of the Mediterranean's coastline and the Adriatic Sea. Populations are, however, threatened by the Lessepsian invader Brachidontes pharaonis that occupies the same habitat and is more tolerant of environmental extremes. This is the first study of the anatomy of M. minimus in relation to its evolution and adaptations towards an intertidal life on the karsted limestone shores that characterize much of the Mediterranean. In most anatomical respects M. minimus is 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 with M. 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 equip M. minimus for a compressed, squat, life in the intertidal karst. In addition to competition from the introduced B. pharaonis in the Mediterranean, M. minimus is facing competitive exclusion from the native Mytilus galloprovincialis that, 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.