Reference : Biol. Bull., 140 : 489-501. (June, 1971) LARVAL DEVELOPMENT OF PAGURUS LONGICARPUS SAY REARED IN THE LABORATORY. III. BEHAVIORAL RESPONSES TO SALINITY DISCONTINUITIES 1 MORRIS H. ROBERTS, JR. 2 J'iri/initi Institute of Marine Science, Gloucester Point. I'lnjinia 23062 Survival rate, although commonly used as an index of an organism's ability to meet a given environmental challenge (Costlow, Bookhout, 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 mainte-nance of an animal in its optimal environment are more suitable measures of adaptability (Shelford, 1915). Orientative behavior reflects, at least in part, the integration of an animal's various adaptive physiological changes in metabolism, reproduction, hormonal balance, etc., to challenges of the external milieu. Responses of various zooplankters 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 Mcrccnarla mercenaries trochophores (Turner and George, 1955), Phyllodocc trochophores (Lyster, 1965), some copepods and the zoeal instars of Pisidla longicornis (Lance, 1962), and Zoea I of Homarns anicricanus (Scarratt and Raine, 1967). In the present study, the response of Pagitnts longicarpus zoeal instars to salinity discontinuities of different magnitudes was examined. The avoidance be-havior 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. MATERIALS AND METHODS The apparatus used to study responses to salinity discontinuities consisted of two plastic cylinders of 7.9 cm diameter, 46 cm height, marked off into ten 4 cm segments and filled to a depth of 40 cm. The volume of each segment was about 198 ml. A glass tube was cemented at the bottom of each cylinder througb which water could be introduced from a separatory funnel. Two centimeters above the bottom of the cylinder, a canula was inserted through a small neoprene stopper, the tip of the canula reaching to the central axis of the cylinder. Externally the canula was fitted witb a short plastic tube and pincb clamp to which a large-bore syringe could be attached ( Fig. 1 ) . To establish a discontinuity, about one liter of water of reduced salinity was first poured directly into one column. High salinity water was then slowly 1 Contribution Number 381 from the Virginia Institute of Marine Science, Gloucester Point, Virginia 23062. 2 Present address : Department of Biology, Providence College, Providence, Rhode Island 02918. This paper is part of a dissertation submitted to the School of Marine Science of the College of William and Mary in partial fulfillment of the requirements for the Doctor of Philosophy Degree. 489
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