Decoupling of copepod grazing rates, fecundity and egg-hatching success on mixed and alternating diatom and dinoflagellate diets

Abstract

Experiments were conducted over 10 to 20 d periods to study the grazing and repro- ductive success of the copepod Temora stylifera fed on unialgal cultures of the diatom Thalassiosira rotula (THA) or the dinoflagellate Prorocentrum minimum (PRO), as well as mixtures of THA and PRO (MIX experiments) and alternating diets of THA and PRO switched daily (SWITCH experi- ments). Adult females ate both THA and PRO, and while rates of feeding on the 2 diets were similar in terms of carbon ingestion, egg production was generally higher on the diatom diet. In contrast, copepod egg-hatching success was low on the diatom diet, declining rapidly after 2 d from >80 to 0% by Day 17. The diminution in hatching success was slower when females were fed MIX or SWITCH diets, but nonetheless diminished to 0 and < 25% by the end of the experiment, depending on the incubation method. Only in the case of the PRO diet was egg viability high and stable with time (87 to 96%), regardless of whether female and male couples were incubated as individual couples in crystallizing dishes or as triplicate couples in rotating bottles. However, in most other cases, the incu- bation method (crystallizing dishes vs rotating bottles) had very strong effects on egg and fecal pel- let production, and hatching success. Higher egg production rates were generally obtained when females were incubated in crystallizing dishes, whatever the diet, although fecal pellet production rates were significantly higher in the rotating bottle experiments in most cases. Egg-hatching success was also strongly affected by incubation method, with generally higher hatching rates in the rotating bottles. This was probably due to the fragility of non-viable eggs, which were more easily destroyed by mechanical disturbance in rotating bottle experiments. The results support the recent discovery that reproductive failure in copepods can be due to deleterious antimitotic compounds present in some diatoms that arrest normal embryonic division. Reduction in egg viability was not only visible when females were fed unialgal diatom diets, but also when they were fed mixed diets. However, on mixed diets there was a 'dilution effect' in that hatching was reduced by approximately half, and this took about twice as long to occur. The evolutionary advantages for diatoms in producing antimitotic compounds are discussed, as well as questions of why copepods feed on diatoms with impunity, even though some diatoms are detrimental to copepod reproductive success.