Meeting Abstract
In fish fin evolution there is a trend towards a higher range of motion and flexibility, but little attention has been given to the spiny and less flexible fins of Acanthomorpha. In some Centrarchidae fishes, the spiny portion of the dorsal fin reaches similar sizes to the soft dorsal fins and is thought to have a role in predatory defense and/or in locomotion. We sampled green, redear and bluegill sunfish from Illinois tributaries and analyzed dorsal fin morphometrics. We also collected kinematics data during steady swimming, hovering, routine maneuvers and fast starts, as well as in response to hydrodynamic stimuli delivered to the dorsal fin. Geometric morphometrics separated the three species into two groups but was not effective in separating the green sunfish from the bluegill. The main variables driving the differences were relative position along the cranio-caudal axis and height of the spiny and soft dorsal fins. In terms of ontogenetic changes, we found that the soft dorsal fin grows faster in height than the spiny dorsal fin. This suggests that the fins are under selective pressure, although not necessarily as a predatory deterrent. There were no significant differences in escape responses between green sunfish and bluegill sunfish of similar sizes, but the spiny dorsal fin was deployed by both species. Bluegills are able to detect hydrodynamic stimulus delivered to the fin and respond by temporarily collapsing and then re-erecting the spiny dorsal fin while moving the soft dorsal fin laterally. These results seem to indicate that the dorsal fin is capable of detecting flow and provide stability in different locomotor behaviors. Stability demands are likely to have played a role in the evolution of spiny fins in Acantomorpha.
