Larval Development of Pagurus longicarpus Say Reared in the Laboratory. III. Behavioral Responses to Salinity Discontinuities

Survival rate, although commonly used as an index of an organism's ability to meet a given environmental challenge ( Costlow, Bookhuout, and Monroe, 1960, 1962, 1966 ; Roberts,1971), is a rather crude estimate of the ability of an organism to adapt to its environment. Behavioral responses associated with the maintenance of an animal in its optimial environment are more suitable measures of adaptability (Shelford, 1915 ) . Orientative behavior reflects, at least in part, flue integration of an animal's various adaptive physiological changes in metabolism, reproduction, hormonal balance, etc., to challenges of the external milieu. Responses of various zoophankfers to salinity discontinuities were first studied by Harder ( 1952a, 1952b, 1954, 1957, 1968) . The ability to detect and avoid waters of reduced salinity has now been demonstrated experimentally for several meroplankters as well as holoplankters. A response to salinity discontinuities has been demonstrated in Mercenaria mercenania trochophores (Turner and George, 1955 ) , Phyllodoce trochopluores ( Lyster, 1965) , some copepods and the zoeal instars of Pisidia longicornis (Lance, 1962) , and Zoea I of Homarus amenicanus ( Scarratt and Raine, 1967). In the present study, the response of Pagurus longicarpus zoeal instars to salinity discontinuities of different magmuitudes was examined. The avoidance behavior was described and quantified. The interaction effect of developmental age and magnitude of the discontinuity on the response was examined. Further an attempt was made to determine if the receptor(s) is localized on certain structures.