A revised dichotomous key for the mangrove crab genus Scylla De Haan, 1833 (Brachyura, Portunidae)

Abstract

Abstract Species identification in mangrove crabs of the genus Scylla De Haan, 1833 is problematic, due to the high intraspecific variation and the continuity of diagnostic features. To determine the hierarchy of characters that are most useful in differentiating Scylla species from the Philippines, 1,185 mangrove crabs were collected from seven sites across the Philippines. Morphological species identification was contrasted with morphometric and molecular techniques. The hierarchy of characters in the existing dichotomous key resulted in the misidentification of 245 samples. Errors stem from asymmetry in the features of the chelipeds and the continuity in the features of the frontal lobe spines. Considering mainly morphological characters, the most reliable traits were the geometric patterns in the swimming and walking legs. Nonetheless, 27 individuals with morphological ambiguities exhibited unique patterns in digested nuclear ITS-1 fragments, unreported in any known species of Scylla. A revised hierarchy of characters resulted in a 97.8% match with molecular species identification.