The need for speed. I. Fast reactions and myelinated axons in copepods.

A rapid and powerful escape response decreases predation risk in planktonic copepods. Calanoid copepods are sensitive to small and brief hydrodynamic disturbances: they respond with multiple nerve impulses to a vibrating sphere. Some species, such as Pleuromamma xiphias and Labidocera madurae, respond with very large spikes (1-4 mV), whereas maximum spike heights are an order of magnitude smaller in others, such as Undinula vulgaris and Neocalanus gracilis. A comparative study of the escape responses showed that all species reacted within 10 ms of the initiation of a hydrodynamic stimulus. However, U. vulgaris and N. gracilis had significantly shorter reaction times (minimum reaction times: 1.5 ms and 1.6 ms) than the other two, P. xiphias (6.6 ms) and L. madurae (3.1 ms). Examination of the first antenna and the central nervous system using transmission electron microscopy revealed extensive myelination of sensory and motor axons in the two species with the shorter reaction times. Axons of the other two species resembled typical crustacean unmyelinated fibers. A survey of 20 calanoids revealed that none of the species in two of the more ancient superfamilies possessed myelin, but myelination was present in the species from three more recently-evolved superfamilies.