Larval release rhythms of four species (family Grapsidae and Ocypodidae) of intertidal crabs on a coastal flat in western central Taiwan

Abstract

Larval release rhythms of four crab species, Parasesarma tripectinis (Shen, 1940), Perisesarma bidens (De Haan, 1835), Hemigrapsus penicillatus (De Haan, 1835) and Baruna sinensis Tan and Huang, 1995, were studied in two different intertidal zones on a coastal flat. Of these, P. bidens is the only species that migrates between high and lower intertidal zones. In general, larval release patterns of these intertidal crabs are not too different from those of intertidal crabs living in other geographical regions, which often release larvae around new or full moons and nocturnal high tides. However, differences were apparent when compared with previous studies in that: (1) peak larval release periods of P. bidens occurred during submaximum tidal amplitudes, (2) a good portion of B. sinensis released their larvae during neap tides, (3) all four species released their larvae during both daytime and nighttime high tides although the percentage varied among these species, (4) three species released their larvae within 1 h after the time of high slack tide, but larval release of B. sinensis peaked between 1 and 2h after the time of maximum tidal height. Larval release pattern of Perisesarma bidens suggests that locomotory behaviour of high intertidal crabs may influence their larval release rhythms. Antipredatory adaptation may explain larval release rhythms of the four species examined in the present study. Larvae of Parasesarma tripectinis, P. bidens and Hemigrapsus penicillatus are more vulnerable to planktivorous fish predation and tend to release their larvae at nocturnal high tides during maximum tidal amplitudes. On the other hand, larvae of Baruna sinensis may be better adapted against planktivorous fish predation by having yellow-green colour. The timing of larval release of this species occurred throughout the lunar cycle and often at daytime 1-2 h after the time of high slack tide.