Ontogeny of sinking velocity, body density, and phototactic behaviour in larvae of the coconut crab Birgus latro: Implications for larval dispersal and recruitment in the sea

As a basis for understanding the larval dispersal and recruitment processes of the coconut crab, Birgus latro, in the sea, we examined the ontogenetic changes in the passive sinking velocity, body density, and phototactic behaviour to different light intensities and wavelengths of the zoeal and megalopal stages of laboratory-reared larvae. Six levels of light intensity, from 310μmolm−2s−1 to 0.0031μmolm−2s−1 in 10% decrements, and 14 light wavelengths in the range of 400–660nm in 20nm increments were tested. The controls were tested in the dark. The coconut crab larvae showed negative buoyancy. The mean sinking velocity was 7.5mms−1 in the first zoeal stage, which increased significantly to ~10mms−1 after the third zoeal stage. The mean body density was 1.085–1.089gcm−3 in the first to third zoeal stages; it then decreased significantly to 1.072–1.075gcm−3 in the fourth zoeal and megalopal stages. This ontogenetic pattern in body density differs from those of other decapod crustacean species and may be related to the emigration behaviour of the megalopae from the sea to the land. The coconut crab larvae were generally photopositive, with the strength of their positive phototactic behaviour decreasing at lower light intensities and with age. Higher incidences of positive phototactic behaviour were recorded at spectra of 400–660nm, 400–620nm and 400–600nm for the first, second, and third zoeal stages, respectively. The spectral sensitivity of the fourth zoeal stage showed peaks at around 400–420nm and 500–580nm, suggesting that the larval distribution in the sea changes at the fourth zoeal stage. The overall phototactic behaviour of the megalopae was weak, and the behavioural responses varied at light wavelengths above 520nm. Thus, their phototactic behaviour may have little effect on their migration to the coast.