Effects of temperature and salinity on the life cycle ofOphryotrocha diadema(polychaeta, dorvilleidae)

Abstract

Ophryotrocha diadema is stenohaline. The complete life cycle has only been observed at salinities of 30, 35 and 40 ‰. At an optimum salinity, 35 ‰, reproduction occurs between 11 and 27 °c. Experiments were performed in 5 ‰ steps of salinity from 15 to 45 ‰ combined with temperatures of 15, 18,21 and 25 0c. At salinities of 15 and 20 ‰ all animals, both adults and larvae, died within two days. In the remaining 20 salinity/temperature combinations adults were kept for 3 weeks and larvae for a minimum of one month or until they were sexually mature. For adults, mortality, number and size of egg masses, and reproductive success were recorded. Corresponding parameters for larvae were mortality, growth rate and time to beginning of reproduction. Optimum conditions for survival were obtained at 35 ‰ salinity and 18 ac. The optimum for egg production (reproductive rate) was 35 ‰ and 25 °c. The maximum growth rate was observed at 30 and 35 ‰ salinity and 25°C. These two combinations also represented the shortest time of development to sexual maturity. The number of eggs per egg mass was significantly salinity dependent, with an optimum at 35 ‰, but was not affected by temperature, except at 40 ‰ salinity, or by salinity/temperature interaction. Both adults and larvae demonstrated good survival at 25 ‰ salinity, but fertilization and/or early development was blocked. This was also true at 45 ‰ where the rate of survival was much lower than at 25 ‰. At low temperatures, 15 and 18°C, survival was better at 40 than at 25 ‰ salinity. At 21 and 25 °C the relations are reversed. The growth rate figures indicate a similar change, which occurs between 15 and 18 ac. At 15 ac the larvae maintained at 40 ‰ salinity were superior, but at higher temperatures, the higher survival was observed for larvae reared at 25 ‰. The coefficient of population growth, r, was calculated from the exponential equation N_t = N₀ × e^rt for the four most favourable salinity/temperature combinations. The highest value was obtained at 25°C and 35 ‰ salinity.