PERSISTENCE OF SUBTLE DEPARTURES FROM SYMMETRY OVER MULTIPLE MOLTS ININDIVIDUAL BRACHYURAN CRABS - RELEVANCE TO DEVELOPMENTAL STABILITY

Fluctuating asymmetry (FA) is generally viewed as a population-level c haracter. It is described by some measure of the variance of the diffe rence between the right and left sides for a collection of individuals . Very little is known of the developmental origins of FA, despite the fact that FA is widely used to estimate developmental stability. We p resent a novel technique for examining the growth trajectory of the as ymmetries that give rise to FA, and we explore two sample data sets fo r the brachyuran crab Hemigrapsus nudus. We have traced the fate of th ese small, random deviations from perfect symmetry through three succe ssive molts of individual crabs. Invertebrates that molt, and hence gr ow in discrete steps, provide an easily preserved record of their grow th. Model II regressions of measurements from one molt versus the prev ious molt can help describe the stability of subtle departures from sy mmetry over time. Although any number of different patterns may occur, we identify four general cases: a) asymmetries vary at random through out growth (random determination), b) asymmetries remain unchanged in sign and magnitude (constant asymmetries), c) asymmetries increase in proportion to character size and hence increase with growth (size-depe ndent asymmetries), and d) asymmetries persist, but are reduced in mag nitude (damped asmymmetries). Data from ten Hemigrapsus nudus, measure d for between 21 and 28 metrical, limb-segment characters over three s uccessive molts, yielded associations most similar to our pattern 'b', although some subtle departures in the direction of pattern 'c' were also observed. Persistent asymmetries accounted for 26% and 20% of the variance among asymmetries between molts 1 and 2, and molts 2 and 3 r espectively. Thus, in spite of large and rapid increments in the exter nal size of the crab, these subtle asymmetries tended to persist in bo th direction and magnitude, from molt to molt. This result suggests ei ther i) that individual crabs have a genetic predisposition towards as ymmetry in a particular direction but contribute to a continuous and n ormal distribution of R-L differences at the population level, or ii) that these subtle asymmetries arose at some earlier ontogenetic stage and were preserved through growth. Either interpretation has important ramifications for the study of FA. The first suggests that under some circumstances FA may not provide a valid measure of developmental ins tability, because subtle departures from symmetry in an individual may have a genetic basis. The second implies that subtle departures from bilateral symmetry are not 'corrected' as an individual grows.