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 sphe re. Some species, such as Pleuromamma xipluas and Labidocera madurae, respo nd with very large spikes (1-4 mV), whereas maximum spike heights are an or der of magnitude smaller in others, such as Undinula vulgaris and Neocalanu s gracilis. A comparative study of the escape responses showed that all spe cies reacted within 10 ms of the initiation of a hydrodynamic stimulus. How ever, U. vulgaris and N. gracilis had significantly shorter reaction times (minimum reaction times: 1.5 ms and 1.6 Ins) 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 ext ensive myelination of sensory and motor axons in the two species with the s horter reaction times. Axons of the other two species resembled typical cru stacean 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 su perfamilies.