The Swimming of Nymphon Gracile (pycnogonida)

Most arthropods move by means of metachronal waves of activity passing over the paired appendages, and the resulting gaits have been studied extensively, with regard both to their locomotory effectiveness (Manton, 1953) and to co-ordination (Beth6, 1930; von Hoist, 1935). Most attention has been focused on terrestrial or benthic arthropods and the mechanism of co-ordination of insect walking especially has aroused considerable interest (see reviews by Hughes, 1965; Wendler, 1966; Wilson, 1966). Here, as in all walking animals, the ambulatory appendages perform the dual function of support and propulsion, and gravity is of considerable importance in determining the gait employed. During swimming, however, it is frequently more important that the movements of the locomotory appendages be co-ordinated so as to ensure some degree of hydrodynamic stability if efficient progression is to be achieved. This would appear to be especially so in the pycnogonid Nymphon gracUe where the four pairs of legs are arranged in almost radial symmetry about the body. During swimming these beat dorso-ventrally, and the animal moves through the water dorsal side first (Knight-Jones & Macfadyen, 1959). The forces generated by the leg during the swimming beat have been discussed earlier (Morgan, 1971). In this paper the swimming gait of Nymphon is described and some aspects of coordination between the swimming legs are considered. Investigations into the gait patterns of invertebrates generally involve the amputation of one or more of the locomotory appendages. In insects this results in a marked change in phase between the remaining legs (Hughes, 1957; Wendler, 1966; Wilson, 1966; Delcomyn, 1971) and Wilson (1967) found that removal or immobilization of the third leg pair likewise effected a phase shift in the walking legs of tarantula spiders. Comparable results have been reported for opilionids (Bethe", 1930) and araneid spiders (Jacobi-Kleeman, 1953). The effect of similar amputations on the pattern of swimming leg movement in Nymphon are described here together with the results of experiments involving forced cyclical leg movements.