An ecophysiological framework for the morphological evolution of bluegill sunfish

Body shape affects the capacity and efficiency of swimming in fishes, and places constraints on foraging and reproductive performance. Hence, fitness components, such as aerobic swimming capacity and efficiency, can be determined from analysis of swimming energetics using active respirometry. In particular, body shape adaptations, such as streamlining, aim at reducing hydrodynamic drag (resistance), thereby increasing swimming efficiency in the presence of water flow, which is a principal contributor to resistance for fish inhabiting rivers. For two populations of bluegill sunfish, one from the Brazos River and the other from Moelman?s Slough (a Brazos River oxbow lake), the metabolic transport rate (MTR) was determined to evaluate differences in swimming efficiency. The standard cost of swimming (SCOS) was also determined to evaluate differences in swimming capacity, which represents the overall capacity of the skeletal muscles to generate mechanical power to overcome hydrodynamic resistance. The MTR and the SCOS describe holistic swimming performance, where the MTR specifies the hydrodynamic response due to swimming, and the SCOS specifies the physiologic response due to swimming. The differences in swimming performance are mainly attributed to factors affecting hydrodynamic resistance and could be predicted by morphology; because body shape, like water flow, is also a principal contributor to resistance. Multivariate body shape, from generalized Procrustes analysis, was used to assess the influence of multiple shape traits on swimming costs. This measure of shape related to swimming performance using partial least-squares analysis showed the two bluegill populations to be significantly different. The results were as follows: the shallow-bodied condition in bluegills was highly correlated with efficient swimming and low swimming capacities; whereas, deep-bodied bluegills were highly correlated with inefficient swimming and high swimming capacities. This is an empirical case of divergent natural selection. For convergence, however, the position of the caudal peduncle is consistent with optimal swimming speed (Um), which depends on standard metabolic rate (SMR), or metabolic maintenance. Bluegills with erect caudal peduncles have a high range of swimming speeds without suffering much cost of swimming ability compared to bluegills with prone caudal peduncles. The adaptive physiological response to high Um is due to a low SCOS because swimming efficiency is low and metabolic maintenance is high. In other words, bluegills that are inefficient swimmers and require a high energy intake cannot survive unless they gain the ability to increase their foraging capacity by thrust or metabolic power reduction. This is perhaps one of the most remarkable adaptive physiological responses due to the joint effects of shape and SMR.