Shifts in morphological and mechanical traits compensate for performance costs of reproduction in a wave-swept seaweed

1. In addition to metabolic costs associated with reproduction, morphological and mechanical changes accompanying reproductive effort can affect an organism's performance. Reproductive effort may have unavoidable costs in plants and plant-like taxa (i.e. extra mass or drag); however, the extent to which an organism can ameliorate these consequences by additional modifications of shape or tissue properties remains unknown in seaweed and plant taxa. 2. We investigated mechanical and morphological changes associated with reproduction and how these shifts influence functional performance in the winged kelp, Alaria marginata. 3. Compared to vegetative blades, reproductive blades were similar in width but were longer and had greater surface area. Reproductive sporophylls were also thicker and less ruffled. Tissue extensibility and breaking stress were not different in reproductive vs. vegetative blades. However, reproductive tissue exhibited greater tensile stiffness, flexural stiffness and force to break. 4. Reproductive blades experienced greater drag (despite decreased flapping) than did vegetative blades, but did not experience greater size-specific drag. Tissues cut into experimental blades of the same size and shape experienced greater drag when cut from reproductive tissue suggesting that the change in shape associated with the onset of reproduction ameliorates the cost of increased tissue stiffness. Nonetheless, increased blade breaking force in reproductive individuals resulted in increased blade safety factors (breaking force/drag experienced) in reproductive compared to non-reproductive sporophylls. 5. Synthesis: In A. marginata, decreased flexibility and increased surface area are mechanical costs associated with reproduction. Decreased blade ruffliness and increased strength associated with the onset of reproduction in A. marginata ameliorate the concomitant mechanical costs of decreased flexibility and increased size. Shifts in mechanical and morphological traits among plants and plant-like taxa may allow them to increase reproductive output without decreasing functional performance.