COMBINED EFFECTS OF SALINITY, TEMPERATURE AND FOOD ON EARLY DEVELOPMENT OF THE POLYCHAETE HYDROIDS ELEGANS

Effects of salinity, temperature and food concentration on early devel opment of the polychaete Hydroides elegans (Haswell) were examined in 4 laboratory experiments. Three 2-factor experiments tested the effect s of salinity (15 to 35 parts per thousand) and temperature (15 to 30 degrees C) on the survival and duration of development of H. elegans f rom newly-released oocyte to 2-cell, 2-cell to blastula, and blastula to trochophore stages respectively. A fourth 3-factor experiment teste d the effects of salinity (15 to 35 parts per thousand), temperature ( 15 to 30 degrees C), and concentration of the single-cell alga Isochry sis galbana (0 to 10(6) cells ml(-1)) on survival, settlement, and dur ation of development from trochophore to newly-settled juvenile. Withi n the experimental range, temperature had no effect on survivorship, b ut low temperature led to longer duration of development. Low salinity reduced survivorship and settlement, and lengthened the duration of d evelopment. Low food concentration reduced survivorship and settlement , and lengthened the duration of development from trochophore to newly -settled juvenile. At concentrations less than or equal to 10(3) cells ml(-1), >35% larvae survived through the 10 d experiment but lost the ir ability to become competent. Percentages of trochophores reaching s ettlement were similar at 10(4), 10(5), and 10(6) cells ml(-1). Durati on of development was shortest at concentrations of 10(5) cells ml(-1) , while trochophores at 10(4) and 10(6) cells ml(-1) had similar but l onger durations of development. Our data suggest that in Hong Kong wat ers, the decrease in salinity during the summer seems to override the benefits of high temperature and to be responsible for the decline in H, elegans settlement. The increase in phytoplankton concentration fro m early spring to early summer may contribute to the formation of sett lement peaks. Temperature, however, does not seem to be a limiting fac tor for early development and settlement of H, elegans.