Influence of natural particle diets on egg laying and hatching success of Acartia tonsa in East Lagoon, Galveston, Texas

Abstract

Factors affecting egg production and hatching success of the copepod Acartia tonsa Dana were determined by measuring environmental variables and daily egg production of field collected females in the laboratory. During five periods between April and November of 1981, three day experiments were run at ambient particle concentrations and two dilutions of (1) natural particles collected over the water column, and (2) some modification of (1). Other A. tonsa were fed Thalassiosira weissflogii to isolate the effects of temperature and salinity on egg production. At 15(DEGREES)C, females produced (TURN) 30% of their body carbon (or nitrogen) as eggs per day, whereas at 28(DEGREES)C, they produced at least their own biomass as eggs per day ((TURN) 120-140%). Specific egg production rates were more strongly correlated with temperature for experiments with the unialgal diet than those with natural particle diets at ambient concentrations. Salinity significantly affected egg production, but not as greatly as temperature. Egg viability was highest in the spring, but was not related to diet or the percentage of females with spermatophores. Although specific egg production rates per unit carbon were consistently higher than those per unit nitrogen, their correlations with temperature and chlorophyll were always higher per unit nitrogen than carbon. Daily egg production was more highly correlated with food concentration measured the same day than that measured the previous day. Therefore, specific nitrogen rates and food variables measured the same day were tested in stepwise multiple regressions. For the entire data set, a higher coefficient of determination was found for a multiplicative model of the independent variables than for an additive model. Final variables admitted into the former were temperature and chlorophyll. Considering ambient concentrations only, final variables admitted into a multiplicative model were temperature, salinity, ash free dry weight, percent phaeopigments, and C:N ratio. Partial correlation coefficients between specific egg production and admitted variables were all significant. Specific egg production rates predicted from the observed range of food and environmental variables suggested that egg production as well as mortality in the field may cause changes in population abundances.