ONTOGENY OF SUBTLE SKELETAL ASYMMETRIES IN INDIVIDUAL LARVAE OF THE SAND DOLLAR DENDRASTER EXCENTRICUS.

Abstract

Fluctuating asymmetry (FA), defined as the distribution of small random deviations from bilateral symmetry in a sample of organisms, has received considerable attention in recent evolutionary and behavioral literature (Markow 1994). The degree of asymmetry exhibited by a single organism presumably results from small independent disruptions of normal development caused by random differences of internal and external environment on each side. FA is used in evolutionary studies as a measure of genetic quality, developmental robustness, and environmental tolerance. FA may reflect genetic quality because individual asymmetry is often inversely correlated with heterozygosity (Mulvey et al. 1994), and hybrids are often more asymmetrical than either of their parent species (Graham 1992). Behavioral studies of the good genes model of mate choice commonly assume individual asymmetry is inversely correlated with genetic quality. These studies have also extended the association between genotypic and phenotypic quality, and asymmetry by showing that females often prefer symmetrical males and that symmetrical males, may be more successful foragers (M0ller 1992, 1994; and reviewed in M0ller and Pomiankowski 1994). Asymmetry is also associated with environmental stress such as pollution and thermal stress (Graham et al. 1993). Paleontological studies have attempted to apply these results to fossil groups. Smith (1994) measured FA in trilobite species throughout the Cambrian and early Ordovician, in an attempt to determine if developmental robustness increased after the Cambrian explosion. Finally, Palmer et al. (1994) have looked to random asymmetries for clues to the origin of large genetically determined asymmetries like those seen in fiddler-crab claws and gastropod torsion. Although FA and other patterns of asymmetry are assumed to result from developmental processes, few studies have directly addressed changes in asymmetry during ontogeny (but see Chippindale and Palmer 1993; and for an indirect approach see Hallgrimson 1993). Such studies are of interest because associations between deviations from symmetry among traits and within traits through time may yield insights into both the causes of asymmetries and mechanisms controlling the development of bilaterally symmetrical characters. The way asymmetries vary during ontogeny can be used to determine the relative contributions of intrinsic and ex-