SETTLEMENT AND POST-LARVAE BEHAVIOR OF MYTILUS-GALLOPROVINCIALIS - FIELD AND LABORATORY EXPERIMENTS

Field sampling carried out in the Ria de Vigo (NW Spain) from 1991 to 1993 showed that Mytilus galloprovincialis settle directly from the pl ankton onto substrates exposed to various environmental conditions: ex posed rocky shore; protected rocky shore; exposed, raft mussel-culture area; and protected, raft mussel-culture area. For direct settlement, competent larvae may use a wide variety of substrates: filamentous ny lon ropes; the byssus and intricately arranged material in the bottom of mussel beds; filamentous, thallus and membranous algae; and rugosit ies on adult mussel shells. The peak of settlement occurred from sprin g to early autumn and differences in the settlement abundance among lo calities were influenced by currents. After this peak, the settlement of larger post-larvae continued, associated with the increase in storm s during autumn that detach them from their original substrates. This dispersion phase allows for the possibility of colonising, or recruiti ng on other areas, even during the post-spawning season when the prese nce of small post-larvae is at its minimum. Laboratory experiments car ried out with post-larval stages from 0.250 to 2.000 mm showed that un der static water conditions they crawl and form clumps, but do not sea rch for a specific substrate. If they are not disturbed, they may rema in in their original place of settlement. Conversely, under moving wat er conditions they attach to a wide variety of substrates, particularl y to byssal filaments and thalli of red algae Ceramium rubrum. The con tact and attachment to substrates is carried out with a long mucous th read that also aids in forming clumps. The use of this mucus to settle results in a 'preference' for natural filamentous substrates but also in settlement on rugous hard surfaces. An alternative hypothesis to t he primary and secondary settlement pattern previously described in th e literature for Mytilus edulis is suggested.