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

Experiments were conducted over 10 to 20 d periods to study the grazing and reproductive success of the copepod Temora sWera 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 al ternating diets of THA and PRO switched daily (SWITCH experiments). Adult f emales 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 th e diatom diet, declining rapidly after 2 d from > 80 to 0% by Day 17. The d iminution in hatching success was slower when females were fed MIX or SWITC H diets, but nonetheless diminished to 0 and < 25% by the end of the experi ment, 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 whet her female and male couples were incubated as individual couples in crystal lizing dishes or as triplicate couples in rotating bottles. However, in mos t other cases, the incubation method (crystallizing dishes vs rotating bott les) had very strong effects on egg and fecal pellet production, and hatchi ng success. Higher egg production rates were generally obtained when female s 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 w ere more easily destroyed by mechanical disturbance in rotating bottle expe riments. The results support the recent discovery that reproductive failure in copepods can be due to deleterious antimitotic compounds present in som e 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 'dilut ion effect' in that hatching was reduced by approximately half, and this to ok 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 detri mental to copepod reproductive success.