The smaller your mouth, the longer your snout: predicting the snout length of Syngnathus acus, Centriscus scutatus and other pipette feeders.

Like most ray-finned fishes (Actinopterygii), pipefishes (Syngnathoidei) feed by suction. Most pipefishes reach their prey by a rapid dorso-rotation of the head. In the present study, we analysed the feeding kinematics of the razor fish, Centriscus scutatus, and of the greater pipefish, Syngnathus acus in detail. We found capture times of as little as 4-6ms for C. scutatus and 6-8ms for S. acus. We then hypothesized that the long snout of pipefishes is optimal for such fast feeding. To test this, we implemented in a mathematical model the following considerations. To reach the prey as fast as possible, a low moment of inertia increases the head's angular speed, whereas a long snout decreases the angle over which the head must be turned. The model accurately predicted the snout lengths of a number of pipefishes. We found that the optimal snout length, with which a prey will be reached fastest, is inversely related to its cross-section. In spite of the small cross-section, the development of a long snout can be an evolutionary advantage because this reduces the time to approach the prey.