Does resource availability influence the vital rates of the tropical copepod Apocyclops royi (Lindberg, 1940) under changing salinities?

The physiology of invertebrates inhabiting many coastal ecosystems is challenged by strong temporal fluctuations in salinity. We investigated how food availability influences vital rates in the tropical cyclopoid copepod Apocyclops royi subjected to different salinities (5–32 PSU). We hypothesized that (i) mortality decreases and egg production rate increases with food availability; (ii) under suboptimal salinity, mortality increases and the egg production rate is reduced and (iii) the threshold concentration for egg production (the lowest food concentration where egg production is initiated) shifts to higher food concentrations when challenged by salinity. Surprisingly, A. royi survived, ingested food and produced eggs at all tested salinities. Mortality rate was, however, dependent on salinity level, but not on food availability. Mortality increased (~12% h−1) during short-term (1 h) salinity acclimatization to 5 PSU and during the following 24-h incubations (~5% d−1) compared with higher salinities. Feeding and egg production rates increased with food availability up to an optimum at all salinity levels, with no effect of salinity on the lowest food concentration initiating egg production. This reveals a high-salinity tolerance by A. royi and may partly explain why this particular copepod is so successful compared with its congeners in occupying extreme habitats.