Date of Award

Fall 1984

Project Type


Program or Major


Degree Name

Doctor of Philosophy


This study presents an analysis of physical and biological processes regulating the structure of rocky subtidal communities off coastal New Hampshire and southern Maine, USA, with emphasis on the ecology of the horse mussel Modiolus modiolus (L.). Quantitative sampling and multivariate analysis revealed 3 communities: (1) Modiolus community; (2) 30 m community of species with highest densities outside mussel beds; (3) 8-18 m community of species with maximal densities outside mussel beds.

Controlled field experiments, a natural grazing experiment, and long-term photographic monitoring (5 yrs) enabled patterns of Modiolus community structure to be related to the processes maintaining them. Bivalve, ophiuroid, and echinoid prey outside, but not inside, Modiolus beds were consumed by fish, crab, and lobster predators demonstrating that Modiolus beds provide a refuge from predation. The sea urchin Strongylocentrotus droebachiensis was the most significant agent of biological disturbance; urchin aggregations overgrazed the 8 m benthos causing radical changes in species composition, dominance, species richness, and diversity outside mussel beds. Infaunal assemblages within Modiolus beds changed the least because mussel bed structure damped the impact of overgrazing.

At exposed offshore sites, dense populations of Modiolus modiolus occurred at intermediate depths (11-18 m) but not at shallow depths (4-8 m) dominated by kelp (Laminaria digitata, Laminaria saccharina). Experiments indicated that chronic storm-generated disturbance caused massive dislodgement of mussels competitively overgrown by kelp, and could account for the scarcity of Modiolus at shallow depths. The hypothesis that by grazing kelp off mussels urchins decrease the risk of mussel dislodgement was tested by an urchin removal experiment. The removal of urchins from mussel beds led to exponential kelp recruitment, resulting in a 30-fold increase of mussel mortality (via kelp-induced dislodgement), compared to control beds with urchins. The Modiolus-Strongylocentrotus interaction is mutualistic, as mussels provide a refuge from predation for urchins. Thus, coexistence of kelp and mussel competitors is facilitated by mutualism. Patch recolonization experiments showed that kelps dominated all algal turf patches and 47% of mussel patches within 7 months. In contrast, Modiolus did not recover from simulated dislodgement disturbance, suggesting that the ability of kelps to bounce back from disturbance enhances their competitive superiority.