First zoeal stage of the Indian Ocean hydrothermal vent crab, Austinograea rodriguezensis Tsuchida & Hashimoto, 2002 (Decapoda: Brachyura: Bythograeidae).

First stage zoeas of the vent crab Austinograea rodriguezensis Tsuchida & Hashimoto, 2002 (Decapoda, Brachyura, Bythograeidae) were collected by a remotely operated vehicle from the Kairei Hydrothermal Vent, Central Indian Ridge, in January 2016 during the INDEX2016 cruise of the French registered RV Pourquoi Pas? The zoeal appendages are illustrated using confocal laser scanning microscopy and described. A comparison between first stage zoeal morphology of Gandalfus yunohana (Takeda, Hashimoto & Ohta, 2000) and A. rodriguezensis is made and, although remarkably similar, they can be distinguished by dorsal pleonal spinulation. Tdhe number of possible zoeal stages of A. rodriguezensis is discussed and considered to be four. Several deep-water brachyuran zoea 1 have been described with distinct elongated dorsal carapace and pleonal spinulation, including A. rodriguezensis, G. yunohana, Calocarcinus africanus Calman, 1909, Dorhynchus thomsoni Thomson, 1873. These are compared with the zoeas of three species of Geryoninae Colosi, 1924 for which the larval phases are described. These geryonine crabs are also deep-water species, but their zoeas are only described with rostral, dorsal and lateral carapace spines and are without the elongated dorsal spinulation of the carapace and pleomeres. This is attributed to the fact these geryonines have an extensive distribution range compared to those of A. rodriguezensis, G. yunohana, C. africanus and D. thomsoni, which are restricted to discrete habitats. The elongated dorsal spinulation of the carapace and pleomeres of these latter four species is attributed to the fact that the length of these spines increases the effective size of these without extra cost of growth and help the dispersal of the zoeal stages. Videos of A. rodriguezensis female behaviour with respect to aeration of eggs and the subsequent hatching near vent plumes are provided as supplementary information.