Rapid swimming and the median intersomitic ridge in calanoid copepods

The role of median intersomitic ridges during rapid swimming of calanoid copepods was examined with both morphological and theoretical techniques. The ridge between the maxillipeds and first pereiopods of 9 examined species was, with one exception (Metridia lucens Boeck, 1864), sufficiently large to allow roll stabilization when the copepods were moving at 100 body lengths per second. With much slower movement (10 body lengths per second), the major ridge in only two species (Euchaeta elongata Esterly, 1913 and Neocalanus cristatus (Kroyer, 1848)) was large enough to produce roll stabilization. Although all of the ridges have several functions, we postulate that during rapid swimming, the large ridge helps to maintain orientation and enables a more coherent response whether in escape or prey capture.