Temperature, salinity, and prey effects on polyp versus medusa bud production by the invasive hydrozoan Moerisia lyonsi

Abstract

Hydrozoan species are renowned for flexible asexual reproduction, which may predispose them to be successful invaders. Polyps of the invasive hydrozoan Moerisia lyonsi (Boulenger, 1908) have very high rates of asexual production of both polyp and medusa buds. In order to determine how environmental factors affect asexual reproduction in M. lyonsi, the quantitative relationships between polyp bud and medusa bud production were studied in a 31-day laboratory experiment during August 2001. The combined effects of prey (4, 8, 12, 16 Acartia tonsa copepods polyp−1 day−1), temperature (20°C, 29°C), and salinity (5, 15, 25) were tested on the development times for polyp buds (DTp) and medusa buds (DTm), the total asexual reproduction rate (ARR, no. buds polyp–1 day–1), and the ratio of medusa bud to total bud production (Rm). Greater food consumption significantly and directly enhanced ARR and Rm and shortened DTp and DTm. A lower temperature (20°C) and higher salinity (25) reduced food consumption, lengthened development times, and decreased ARR and Rm, with opposite effects for the higher temperature (29°C) and lowest salinity (5). The patterns of variation of these reproductive parameters are more complex. DTm was most sensitive and was significantly and directly affected by all three measured factors. In addition to food consumption, direct effects were seen by temperature on DTp and by salinity on Rm. ARR was directly affected only by food consumption. Overall, DTp, DTm, and Rm were more sensitive to environmental differences than was ARR. More favorable conditions enhanced medusa bud production. The adaptive reproductive processes and their significance for the maintenance and dispersal of M. lyonsi are discussed.