ESTIMATES OF THE COMPRESSIBILITY OF SOME MARINE CRUSTACEANS

Direct mcasurcment of the volume changes of planktonic euphausids and intertidal isopods, due to prcssurc changes of a few atmospheres, indicated specific compressibilities of the animal tissues from 15 to 40% lower than the compressibility of seawater. Similar measurements on an intertidal amphipod provide no support for the hypothesis that the sensory system by which these animals perceive small pressure changes is based on an ultrathin layer of gas. INTRODUCTION pretation of the reactions of a small interSpecific compressibility-the volume tidal amphipod, Synchelidium sp., to small change of a substance per unit pressure per changes in pressure ( Enright 1961, 1962). unit volume-has been measured on a variA micropiezometer was, therefore, designed ety of inorganic and simple organic fluids. and used to measure the compressibility of Although this property of an organism is small volumes of this and two other small relevant to several problems in marine crustaceans over pressure changes of only a biology, including buoyancy, sound reflecfew atmospheres. tion, and pressure sensitivity, there are apThis work was performed during the tenparently no published data on the comurc of a National Science Foundation prepressibility of whole organisms. Furtherdoctoral fellowship, and supported in part more, the available estimates for isolated by grant G-7141 of the National Science animal tissues are not in good agreement. Foundation to Dr. E. W. Fager. The adHenderson and Brink ( 1908)) in a single vice of Dr. Fagcr, with both the research experiment with vertebrate muscle, arrived and the manuscript, is gratefully acknowlat an estimate of 36 x lOA per bar, which edged. Mr. A. 0. Flechsig and Mr. Leo is about 15% less than their estimate for disMiller assisted with the equipment design. tilled water; and Aggazzotti and Ligabue Dr. R. S. Arthur, Dr. R. Wever, and Dr. T. (1943), also working with vertebrate muscle, F 1 o som gave helpful suggestions for the obtained a value more than 50% greater in tcrprctation of the results. than for water: 78 x 10-O per bar. METHODS Compressibility has usually been measured on large volumes of the substanceThe piczometer was analogous, in princiliters of a fluid, for example-by techniques plc, to a mercury thermometer ( Fig. 1). A volume of fluid, or fluid plus animals, was which can give great precision, but are not placed in a glass chamber, in the mouth of readily applicable to the small organisms which was fixed a calibrated capillary tube that comprise the bulk of the marine fauna. containing a small amount of marker fluid. Precision can also be increased by employThis chamber was placed in a transparent ing pressure changes of hundreds of atmopressure cylinder. When pressure is apspheres, but such changes are beyond the plied to this system, the displacement of the tolerance of most living organisms (Regnard marker in the capillary represents the delSS5), and require indirect methods of crease in volume of the fluid contained in observation. A measure of compressibility the measuring chamber, less the decrease in was needed in connection with the intervolume of the chamber itself. The chamber was made from a standard-taper Pyrex l Present address: Dcpartmcnt of Zoology, Unitubing joint; the calibrated tubing was a versity of California at Los Angeles. lo-cm length of a small-bore thermometer