Comparison of Patterns of Asymmetry in two Portunus pelagicus Populations from Manila Bay and Tayabas Bay

Abstract

Specimens of the blue swimmer crab Portunus pelagicus, were collected from Tayabas Bay and from Manila Bay to analyze patterns of asymmetry in their leg appendages. Selected traits of interest included three measurable characters in each cheliped and two characters in each fifth pereiopod. Asymmetry analysis revealed that bilateral measurements of the cheliped traits of all groups exhibited directional asymmetry (DA), and antisymmetry, in the case of male adult crabs from Tayabas Bay. Variations for the bilateral measures for pereiopod traits, exhibited fluctuating asymmetry (FA), which is more pronounced among juvenile crabs in Tayabas Bay and among adults in Manila Bay. A combination of site and age group effects appears to be major influences on the variation of a transformed FA index for the pereiopod traits. Since the Manila Bay site was found to have very low dissolved oxygen levels compared to the Tayabas Bay site, much of the FA detected for the crabs in Manila site could be attributed to hypoxic stress which can disturb developmental stability. Although the Tayabas Bay crabs show less ability to buffer stress effects in early life stages, such ability becomes more developed as they mature. The patterns of asymmetry among the crabs observed in the present study suggest that individuals have varying leg morphologies as their means of coping with long-term or frequently-occurring stress factors in their habitats.