EFFECTS OF TEMPERATURE AND SALINITY ON THE LARVAL DEVELOPMENT OF THE CHINESE MITTEN CRAB ERIOCHEIR-SINENSIS (DECAPODA, GRAPSIDAE)

Larvae of the Chinese mitten crab Eriocheir sinensis H. Milne-Edwards were reared in the laboratory under 25 different combinations of constant temperature (6 to 18-degrees-C) and salinity (10 to 32 parts per thousand S). Different schedules of changing salinity were tested only in the final larval stage, the megalopa. Pelagic larval development comprises a total of 5 zoeal stages and 1 megalopa. An additional (stage VI) zoea and in one case an additional megalopa (transitional to the juvenile) were occasionally observed under unfavourable conditions with low salinity (less-than-or-equal-to 15 parts per thousand S). Successful development from hatching to metamorphosis occurred only at temperatures greater-than-or-equal-to 12-degrees-C. With increasing temperature, both overall survival and range of salinity tolerance increased, whereas development duration decreased exponentially. Regression equations describing this relationship are given for different larval stages and salinities. Development time increased at unfavourably low or high salinities (i.e. where mortality also increased). Zoea I was very euryhaline, with an optimum in slightly brackish water (25 parts per thousand S). During subsequent zoeal development, the larvae became increasingly stenohaline, and their optimum shifted to seawater. The megalopa, in contrast, was euryhaline again and developed fastest in lower salinities (15 to 25 parts per thousand S). When larvae were reared in seawater until they reached the megalopa stage and then transferred to lower salinities, development to the first juvenile instar was also possible at less-than-or-equal-to 5 parts per thousand S, exceptionally even in freshwater. Thus, metamorphosis of E. sinensis is, after a gradual adaptation during megalopa development, possible in principle at any salinity. An optimum, however, was found in the range from 15 to 25 parts per thousand S. These patterns of ontogenetic change in salinity tolerance are interpreted as an adaptation to patterns of larval dispersal presumably occurring in the field: (1) hatching in brackish water of outer estuaries; (2) offshore (near-surface) transport during zoeal development, with late stages living predominantly under marine conditions; (3) onshore (near-bottom) transport of the megalopa; (4) settlement in any part of an estuary, probably most frequently at ca 15 to 25 parts per thousand S.