ORIGINAL PAPERP. H. Lenz á D. K. Hartline á A. D. DavisThe need for speed. I. Fast reactions and myelinated axons in copepodsAccepted: 8 January 2000Abstract A rapid and powerful esca pe response de-creases predation risk in planktonic copepods. Calanoidcopepods are sensitive to small and brief hydrodynamicdisturbances: they respond with multiple nerve impulsesto a vibrating sphere. Some spec ies, such as Pleuro-mamma xiphias and Labidocera madurae, respond withvery large spikes (1±4 mV), whereas maximu m spikeheights are an order of magnitude smaller in others, suchas Undinula vulgaris and Neocalanus gracilis. A com-parative study of the escape responses showed that allspecies reacted within 10 ms of the initiation of a hy-drodynamic stimulus. However, U. vulgaris and N. grac-ilis had signi®cantly shorter reaction times (minimumreaction times: 1.5 ms and 1.6 ms) than the other two,P. xiphias (6.6 ms) and L. madurae (3.1 ms). Examina-tion of the ®rst antenna and the central nervous systemusing transmission electron microscopy revealed exten-sive myelination of sensory and motor axons in the twospecies with the shorter reaction times. Axons of theother two species resembled typical crustacean unmyeli-nated ®bers. A survey of 20 calanoids revealed that noneof the species in two of the more ancient superfamiliespossessed myelin, but myelination was present in thespecies from three more recently-evolved superfamilies.Key words Crustacean á Escape behavior áMechanosensitivity á Phylogeny á MyelinationIntroductionTimeliness in an escape response is key to avoidingpredation for many organisms. Many zoopla nkton,inhabiting pelagic environments with little cover, dependon an eective escape reaction for survival. Among theseare the calanoid and cyclopoid copepods. These smallcrustaceans (<3 mm) usually dominate the marinezooplankton and form a key component of oceanic foodwebs. During an escape ``jump,'' a copepod propels itselfforward at 200±500 body lengths per second through thecoordinated power strokes of its four or ®ve pairs ofswimming legs, the pereiopods (Storch 1929; Strickler1975; Fields 1996). This escape response may lowerpredation risk by as much as 50% compared to lessevasive prey (Drenner et al. 1978; Browman et al. 1983;Trager et al. 199 4). The rate of work output per gram ofmuscle during such an escape jump is among the highestin the animal kingdom (Svetlichnyy 1987; Lenz andHartline 1999).Although all pelagic copepods exhibit a strong escaperesponse, species-speci®c variation exists. This may leadto dierences in susceptibility to certain predators,which may aect the structure of planktonic communi-ties (Brooks and Dodson 1965). Relative eectiveness ofthe escape response may contribute to observed cope poddistribution and abundance patterns (e.g., Kimmerer1991; Hays et al. 1997). A critical component of theescape behavior is the time it takes an animal to initiatea response . We have recently measured the reaction timeof a calanoid (Undinula vulgaris) and have found it to beexceptionally short (<3 ms; Lenz and Hartline 1999).We also discovered that many, but not all, calanoidcopepods have myelin-like sheaths enveloping many oftheir axons (Davis et al. 1999). The calanoids possessingmyelin sheaths may be presumed to have increased nerveimpulse conduction velocities by a factor of ten or more(Ritchie 1984). Although increased conduction velocitiesdue to myelination have been well documented in bothvertebrates and invertebrates, no studies have linked thisphysiological advantage to improved behavioral per-formance. In the present study, we quantify the escaperesponses in four species of calanoids. We correlate thereaction time with the presence or absence of myelin-likesheaths. We demonstrate that the two species with theJ Comp Physiol A (2000) 186: 337±345 Ó Springer-Verlag 2000P. H. Lenz (&) á D. K. Hartline á A. D. DavisBeÂkeÂsy Laboratory of Neurobiology,Paci®c Biomedical Research Center,University of Hawaii at Manoa,1993 East-West Rd. Honolulu, HI 96822, USAe-mail: [email protected]: +1-808-956-6984shortest reaction times are also the two with myelin,while the slower species lack it. It appears that by uti-lizing myelin, copepods in certain groups have increasedtheir escape eectiveness by improving their respon setime. A survey of 20 species showed that myelin is pre-sent in calanoids belonging to more recently evolvedsuperfamilies. The presence/absence of myelin leads topredictions of the physiol ogical and behavioral patternsin species not yet studied by these methods.Materials and methodsCollectionOur work focused on four calanoid species from three superfamiliesof the suborder Calanoida: Pleuromamma xiphias (superfamilyAugaptiloidea: family Metridinidae), Labidocera madurae (Cent-ropagoidea: Pontellidae), U. vulgaris (Megacalanoidea: Calanidae),Neocalanus gracilis (Megacalanoidea: Calanidae). L. madurae andU. vulgaris are common sub-tropical surface-dwelling species.These two species overlap in their distribution, although L. maduraeis found primarily near shore, and U. vulgaris is more widely dis-tributed. P. xiphias is a widespread mesopelagic species (daytimeoccupation of 0.1±1 km depths), which is characterized by exten-sive diel vertical migration (Haury 1988; Mauchline 1998). N.gracilis is a subtropical oceanic species, which inhabits the epipel-agic (surface to 0.1 km depth) regions (Ambler and Miller 1987;Mauchline 1998). Subsurface tows with a plankton net were used tocollect U. vulgaris and L. madurae from inside Kaneohe Bay, Oahu,Hawaii. Additional species collected from Kaneohe Bay includedL. pavo, Bestiolina similis and Acartia fossae. N. gracilis, as well asCandacia aethiopica, Centropages sp., and Euchaeta rimana werecollected ca. 2 km oshore from Kaneohe Bay with sub-surface nettows. Pleuromamma xiphias, as well as N. gracilis and E. rimanawere collected 4 km oshore from Kailua-Kona, Island of Hawaiiwith oblique plankton tows between 100 m and the surface atnight. Additional P. xiphias, Gaussia princeps and Euchirella sp.were collected at the Natural Energy Laboratory of Hawaii, Kea-hole Pt., Hawaii, using a net attached to the end of a pipe pumpingwater from 586 m depth. Calanoids from San Juan Strait, PugetSound, Washington were collected with oblique plankton towsfrom ca. 120 m (Calanus paci®cus, C. marshallae, Eucalanus bungii,Pareuchaeta elongata, Pseudocalanus moultoni).