Copepod reproduction is unaffected by diatom aldehydes or lipid composition

We investigated whether reduced reproductive success of copepods fed with diatoms was related to nutritional imbalances with regard to essential lipids or to the production of inhibitory aldehydes. In 10-d laboratory experiments, feeding, egg production, egg hatching success, and fecal pellet production of Temora longicornis were measured for six different diatom species as well as for a nondiatom control diet (Rhodomonas sp.). The experiments were accompanied by determinations of fatty acids, sterols, and polyunsaturated aldehydes (PUA) in the food. Although diatoms were generally ingested at high rates, they yielded a variable egg production response in copepods, ranging from high egg production in four species (two strains of Thalassiosira rotula, Chaetoceros affinis, and Thalassiosira weissflogii) to low egg production in two species (Leptocylindricus danicus and Skeletonema costatum). Egg hatching rates decreased after 4 d in all diatom treatments, irrespective of the egg production rate and without any relationship to diatom aldehyde production. Similarly, no evidence was found that diatoms are per se nutritionally inferior to nondiatom food. The lack of a distinct mechanism for the observed inhibitory activity of diatoms suggests that the cause(s) might be more complex. We suggest, as one possible explanation, that hatching-specific nutritional deficiencies might be induced by incomplete digestion following from the low gut passage time of diatoms, as indicated by a strong correlation between egg viability and fecal pellet production. In the classic paradigm of pelagic productivity, diatom blooms in midand high latitudes, and upwelling ecosystems form the base for a short and efficient food chain via zooplankton to large exploitable fish stocks (Cushing 1989; Legendre 1990). This tight coupling of production has been fundamentally questioned by observations of an impaired recruitment success of copepods feeding on high concentrations of diatoms. In several experimental studies, maternal diatom diets reduced the viability of eggs, caused embryonic malformations, and increased larval mortality (Ianora et al. 2003; Paffenhöfer et al. 2005). A number of long-chain (poly)unsaturated aldehydes (PUA) of apoptotic activity have been identified and proposed as blocking agents in copepod embryogenesis (Miralto et al. 1999; Pohnert et al. 2002). They originate from the cleavage of fatty acid precursors by enzymes activated following cell breakage, and they inhibit cell proliferation, cell division, and phagocytosis in various animal cell types (Romano et al. 2003; Adolph et al. 2004). Diatom toxicity rather than diatom food quality or temperature-limited copepod growth, therefore, may restrain copepod cohort size and cause the frequently observed delay in zooplankton spring development (Miralto et al. 2003; Ianora et al. 2004). Although laboratory experiments have unequivocally shown that high concentrations of diatoms frequently may cause a reduced fecundity or egg viability in copepods, the ecological significance and underlying mechanism for the diatom effect are still controversial. Some studies suggest an impaired reproduction of copepods associated with various diatom blooms in the field (Miralto et al. 1999; Poulet et al. 2006; Vargas et al. 2006). Despite variable copepod egg production and egg hatching, however, no general negative relationship to diatom biomass has yet been established in various ecosystems (Laabir et al. 1998; Irigoien et al. 2002). Similarly, diatom toxicity as the cause for low egg viability is not without controversy. As an alternative explanation for impaired hatching success, laboratory and field studies have suggested the control of copepod recruitment by essential amino acids, polyunsaturated fatty acids (PUFA), or sterols, leading to the establishment of the ‘‘nutritional dietary deficiency hypothesis’’ (Jónasdóttir et al. 1995; Pond et al. 1996; Hassett 2004). However, the role of lipids or other food constituents in decoupling egg production and egg hatching has still not been clearly demonstrated and also remains controversial (Lacoste et al. 2001; Pohnert et al. 2002; Jónasdóttir et al. 2005). 1 Corresponding author ([email protected]). Present address: Department of Marine Ecology and Aquaculture, Danish Institute for Fisheries Research, Kavalergården 6, DK-2920 Charlottenlund, Denmark.