Growth of the mussel Mytilus edulis L. in the Wadden Sea affected by tidalemergence and barnacle epibionts

On the tidal Rats in the Wadden Sea mussels Mytilus edulis form extensive e pibenthic beds. Near the island of Sylt (German Eight, North Sea), these ex tend from mid intertidal to shallow subtidal zones and are often heavily ov ergrown by barnacles (Semibalanus balanoides and Balanus crenatus). The eff ects of tidal emergence and barnacle epibionts on growth in M. edulis were investigated by field experiments and surveys. The results showed that muss el cohort lengths increased from the mid intertidal (25-35 mm shell length) over low intertidal (30-45 mm) to the subtidal zone ( 15-60 mm). In the lo w intertidal zone M. edulis were heavily overgrown by barnacles (2.2 +/- 0. 4 g dry weight mussel(-1)). while balanid epigrowth was significantly lower subtidally (0.9 +/- 0.4 g mussel(-1)) as well as in the mid intertidal zon e (0.7 +/- 0.3 g mussel(-1)). Cross-transplantation experiments of clean mu ssels (25-35 mm length) showed that from July to September growth was about 3 mm in the intertidal zone and significantly increased to about 10 mm sub tidally. Mussel origin had no influence upon growth. Further field experime nts showed significantly lower growth in mussels with barnacles than withou t. Living and experimentally sealed barnacles did not differ in their effec t, indicating that food competition was not responsible. This investigation suggests that fluctuations in the extent of barnacle epigrowth may modify growth patterns in M. edulis which are otherwise governed by tidal exposure . (C) 2001 Elsevier Science B.V. All rights reserved.