Functional analysis of fiddler crab foraging: sex-specific mechanics and constraints in Uca pugnax (Smith)

Abstract

Intake rate (μg Chl a·g −1 sediment dry weight · s −1), scoop rate (feeding motions · s −1 of the feeding chelae) and extraction rate (μg Chl a removed · g −1 sediment dry wt · s −1) were characterized for male and female fiddler crabs Uca pugnax (Smith) feeding on patches of diatoms in the laboratory. Female intake rate remained constant on patches with Chl a concentrations ranging from 50 to 200 μg Chl a · g −1 3ediment dry wt, whereas male intake rate increased between 50 and 100 μg · g −1 Chl a, and plateaus at a food level of 200 μg Chl a · g −1. The ratio of female: male intake rate declined from a value of 2.3 at 50 μg Chl a · g −1, to ≈1.3 at 200 μg · g −1. These patterns were the result of sex-specific patterns in scoop and extraction rates vs. food concentration. Scoop rate increased with food concentration, but the extraction rate fell as individuals fed more quickly, and extraction became time limited. Females exhibited higher overall scoop rates than males, and maintained higher per chelae scoop rates except at very low food concentrations. Females also had higher extraction rates at equivalent feeding rates. In general, because of their more rapid feeding, females were always extraction-time limited, whereas males were scoop rate limited at low food levels, and extraction-time limited at high food levels. These results illustrate the danger of extrapolating intake rate from scoop (feeding) rate, as there was a nonlinear relationship between the two. In addition, the cost of the male master chelae in terms of decreased intake rate is not constant, but varies with food concentration. The overall decrease in male fitness due to this feeding cost is therefore dependent on the distribution of food abundance in the habitat.