The lower rocky intertidal zone of many moderately exposed and exposed shores in the Gulf of Maine, USA, is co-dominated by two species of red macroalgae, Chondrus crispus Stackhouse and Mastocarpus stellatus (Stack. in With.) Guiry. These species are anatomically, morphologically, ecologically and phylogenetically similar. We quantified: 1. (1) mechanical properties of the stipe; and 2. (2) flow forces on the stipe relative to thallus area and biomass of these species, to determine mechanical and morphological characteristics that could explain the greater winter dislodgement of C. crispus thalli in mixed stands. Although stipes of both species broke at the same mean force, C. crispus stipes were relatively thick, weak and extensible compared to the relatively thin, strong and stiff stipes of M. stellatus. Risk of breakage increased with size in both species because: 1. (1) their stipes weakened with increasing cross-sectional area; and 2. (2) cross-sectional area of the stipe failed to increase in linear proportion to frond area. Drag on C. crispus thalli of > 3 g fresh weight was greater than on M. stellatus of the same biomass, whereas drag on smaller fronds ( < 3 g fresh weight) of the two species was similar. Drag on large C. crispus thalli was greater mainly because they exhibit greater surface area for a given biomass than do large M. stellatus. Dislodgement by hydrodynamic forces has more severe ecological consequences for M. stellatus because regeneration from holdfasts is slower than in C. crispus. A reduced surface area:biomass ratio coupled with greater strength may lessen wave-induced disturbance and be important to maintaining a high abundance of M. stellatus in the low intertidal zone of wave-swept shores. © 1993.
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