Dissodactylus xantusi and Dissodactylus lockingtoni (Decapoda: Brachyura: Pinnotheridae): larval development of two sympatric Pacific species

Abstract

Dissodactylus xantusi and D. lockingtoni are small pinnotherid crabs living with D. nitidus as ectosymbionts on the Pacific sand dollars Encope grandis, E. californica, E. micropora, and Mellita longifissa. Like D. nitidus, D. xantusi and D. lockingtoni develop through four zoeal stages before metamorphosing to the megalopa. This is in contrast to the Atlantic species with known larval development, which all have three zoeal stages. All larval stages are described in detail, including the distribution of chromatophores and the types of setae. Beginning with the first zoea, the mean durations of the stages were 4.0, 3.8, 3.3, and 4 days for D. xantusi, the first megalopa appearing 15 days after hatching. The mean durations of the first three zoeal stages of D. lockingtoni were 3.3, 4.0, and 4.0 days, the only moult to a fourth zoea taking place 12 days after hatching. Laboratory-reared megalopae of D. lockingtoni were not obtained, but wild megalopae of both species were collected from hosts. Compared with Atlantic species of Dissodactylus, whose larvae differ interspecifically, zoeae of D. xantusi, D. lockingtoni, and D. nitidus are morphometrically and meristically very similar. In the first zoeal stage the relative length of the dorsal spine of the carapace is the only distinction between D. lockingtoni and the other two species, the differences in carapace spination becoming a little more pronounced as development proceeds. Megalopae of the three species can be readily distinguished on the basis of size, shape, chromatophore pattern, and setation. Dissodactylus xantusi shares with D. nitidus an oversize seta on the fifth leg that is not found in any megalopa of other Dissodactylus species. Collectively, larvae of the three Pacific species resemble more closely those of other members of the "large-palped," adult-defined subgroup (D. crinitichelis, D. mellitae, D. nitidus, D. primitivus) than those of members of the "small-palped" subgroup (D. stebbingi, D. rugatus), which has recently been proposed to be included in a separate genus, Clypeasterophilus.