J. Exp. Mar. Biol. Ecol., 1991, Vol. 145, pp. 33-48 Elsevier 33 JEMBE 01523 Mussel facilitation of barnacle survival in a sheltered bay habitat Erich G. Stephens and Mark D. Bertness GraduaL:', P~r, ram in Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island, USA (Received 4 April 1990; revision received 30 August 1990; accepted 24 September 1990) Abstract: In New England, the blue mussel Mytilus edulis (Linnaeus) often lives infaunally on cabbie beaches in sheltered bays. We examined the lifestyle ofinfaunal mussels in sheltered bays and their relationship with the acorn barnacle Semibalanus balanoides (Linnaeus) living on cobbles. Results of transplant experiments with M. edulis suggest that heat stress associated with hard substrata restricts high intertidal mussels to infaunal habitats in sheltered bays. At high tidal heights, infaunal mussels are buffered from thermal stress and have higher survivorship and growth rates than epifaunal mussels on hard surfaces. Infaunal mussels bind cobbles together with byssal threads and reduce disturbance mortality to barnacles living on cobbles. Disturbance, however, was not a major source (< 10%) of barnacle mortality in the population studied. In contrast, infaunal mussel binding of cobbles and sediment was found to buffer cobbles from temperature extremes, insulating barnacles on cobbles from thermal stresses and substantially decreasing barnacle mortality on cobbles. Our work shows that the relationship between M. edulis and S. balanoides varies across habitats. Under benign thermal conditions at low intertidal heights and on wave-splashed shores, mussels outcompete barnacles for space on hard substrata. In contrast, at high tidal heights on thermally stressful cobble beaches, infauaal mussels may buffer-barnacles from thermal stress and increase barnacle survivorship. Our results support a growing body of literature which suggests that intra- and interspecific facilitation mechanisms may commonly be important in physically stressful environmen|s. Key words: Barnacle-mussel interaction; Mytilus edulis; Semibalanus balanoides INTRODUCTION Experimental studies of marine intertidal communities have contributed dispro- portionately to understanding the generation of pattern in natural communities. Physical gradients in intertidal habitats are compact, conspicuous, and quantifiable, and many intertidal organisms are sessile or slow-moving and amenable to experimental manipu- lation. Consequently, physical and biotic factors that influence distribution patterns have been frequently examined in intertidal communities (Connell, 1961a,b; Paine, 1966; and many others). Nonetheless, while c-~mpetitive (see Buss, 1986, for review) Correspondence address: E.G. Stephens, Graduate Group in Population Biology, University of California at Davis, Davis, CA 95616, USA. Reprint-request address: M. D. Bertness, Graduate Program in Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA. 0022-0981/91/$03.50 © 1991 Elsevier Science Publishers B.V. (Biomedical Division)