Underwater compensation for exaggerated weaponry: The role of morphology and environment on crab locomotor performance

Abstract

Exaggerated morphologies may increase fitness, but they might be costly to bear; heavy weight, for instance, might hinder locomotion. Evidence supporting these costs are sparse because animals that move on land or swim have traits reducing those costs, called compensatory traits. Animals that walk underwater, however, are under different environmental pressures than land animals. Buoyancy, for instance, reduces the effective weight of any object, which could decrease the locomotion costs of carrying exagerrated traits. Hence, underwater species might maintain performance without compensation. To test this, we compared males of the freshwater anomuran Aegla longirostri that bear an exaggerated claw to females (the natural control). We first tested whether the exaggerated claw decreased male locomotor performance. Next, we tested if sexual dimorphism in performance is associated with differences in leg asymmetry, length, and muscle size. Lastly, we tested if large males have proportionally heavier legs than smaller males. Unexpectedly, females are faster than males while also having relatively longer legs than males. Therefore, females might walk faster because of the longer legs, which might be unrelated to the male exaggerated claw. Furthermore, larger males did not have proportionally heavier legs than smaller males, further suggesting no compensation. Hence, even though aeglid's claw weigh ~25% of their total body weight, we did not find evidence for burden or compensation on males. The environment might thus decrease the costs of exaggerated traits.

Moving in fast waters: the exaggerated claw gape of the New River crayfish ( Cambarus chasmodactlyus ) aids in locomotor performance

“…Small changes in eyestalk form could result in significant reductions in flight agility and manoeuvrability [7]. Thus, the environment strongly influences the size, shape and presence of exaggerated morphological structures [3,4,[8][9][10][11] .…”

“…Small deviations in shape or position of the rostra could significantly hinder billfish locomotor and hunting performance. Overall, the shape and design of exaggerated morphologies in aquatic animals are likely strongly influenced by the animal's ability to move throughout their environment [11].…”