Reference: Biol. Bull. 173: 461-473. (December, 1987) QUANTITATIVE GENETICS OF JUVENILE GROWTH AND SHAPE IN THE MUD CRAB EURYPANOPEUS DEPRESSUS [ THOMAS J. HILBISH AND F. JOHN VERNBERG Department of Biology and Belle W. Baruch Institute, University of South Carolina, Columbia, South Carolina 29208 ABSTRACT Rates of growth and development were measured for the first six molts following the crab 1 stage in the mud crab Eurypanopeus depressus. The genetic contribution to variation in growth rate, development rate, and shape was determined for each molt interval. Genetic variation in growth rate, measured as increases in both width and length, was evident at most molt intervals. There were also significant genetic effects upon the intermolt interval. Growth rates for each molt interval, calculated on a daily basis to remove the interaction between growth rate and development rate also showed genetic variation. There was no evidence that genetic variation in these parameters changed during early juvenile development; there were substantial levels of genetic variation in growth rate at most ontological stages. Despite high levels of genetic variation for growth rate in dimensions of the carapace, there was no evidence of genetic variation in shape. This analysis does not provide a quantitative estimate of the levels of genetic variance for these traits but does indicate that the magnitude of this source of variance must be very significant. INTRODUCTION Variance in many traits may have a large genetic component. High heritabilities have been demonstrated for many traits, including morphology (van Noordwijk et al, 1980; Boag, 1983), behavior (Arnold, 1981a, b; Via, 1984a, b), physiology (Curt-singer and Laurie-Ahlberg, 1981), and other traits that are ecologically important and have a strong influence upon fitness. Current studies focus on understanding the role of development in genetic variation for these traits. It is important to determine whether quantitative genetic variation for a trait is stable throughout the develop-ment of an organism. If the heritability of a trait changes during ontogeny, then natu-ral selection can only influence the trait during intervals of high heritability. Con-versely, if natural selection only occurs during certain periods of development, then the trait will be more free to vary during other portions of ontogeny. The mud crab Eurypanopeus depressus (Smith) has several features which will make it suitable for a quantitative analysis of growth and development. Like other arthropods, changes in size and shape in Eurypanopeus is restricted to a short interval following a molt while the new exoskeleton is still flexible. In addition, molting of the exoskeleton uniquely defines developmental events. In many nonarthropod species the accurate assessment of developmental progress or developmental staging is either difficult or is a largely arbitrary process. Accurate developmental assessment is essen-Received 22 June 1987; accepted 26 August 1987. 1 Contribution number 688 from the Belle W. Baruch Institute for Marine Biology and Coastal Re-search. 461
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