The Megalopa Stage in Grapsidae Crabs (Decapoda, Brachyura), a First Complete Overview Based on New Data from Plankton-Collected Megalopae Identified by DNA Barcodes

The family Grapsidae is composed of 39 species belonging to seven genera. Currently, larval data are known for 24 out of the 39 species, but the megalopa stage remains unknown for the genera Goniopsis, Grapsus, Leptograpsus, and Planes, although megalopae collected in the plankton have been tentatively attributed to Grapsus and Planes. Thanks to the MALASPINA and MAF research projects, a significant number of megalopae were collected from open-ocean plankton worldwide, and, using DNA barcoding (16S and/or COI genes), a high percentage of them were identified. At the molecular level, Grapsidae have been widely studied, so the availability of barcode sequences in public databases has allowed us to identify the megalopa of Goniopsis pulchra, Grapsus grapsus, Leptograpsus aff. variegatus, Pachygrapsus socius, P. transversus, and Planes minutus. In the present work, these megalopae are described in detail and compared with those previously known. Consequently, for the first time, the morphology of the megalopa stage can be compared across all grapsid genera, and a set of characteristics is defined to identify the grapsid megalopa from the rest of the brachyuran megalopae.