Copepod swimming behavior, respiration, and expression of stress-related genes in response to high stocking densities

- We challenge the perception of copepods being stressed at high-density conditions with swimming-behavior, respiration and gene-expression as physiological end-points.
- Adult individuals of the calanoid copepod, Acartia tonsa, do not exhibit sign on any stress response to densities up to 10,000 individuals per liter.
- Limitations in copepod densities must be because of other parameters than density itself, for instance indirect regulation of female fecundity.

Using copepod nauplii as live feed in aquaculture hatcheries could solve high mortality rates of first-feeding fish larvae due to malnutrition. However, implementing the use of copepod nauplii on an intensive production scale requires a stable production at preferably high densities, which is problematic for calanoid copepod species like Acartia tonsa. In the present study, we evaluated the response of copepods experiencing stress under high-density conditions by assessing the acute stress level of A. tonsa. Control density was at 100 ind. L−1 while the treatments were increased stepwise up to 10,000 ind. L−1. Three biological/physiological end-points were studied: swimming behavior, respiration rate and expression level of stress-related genes.None of the elevated densities caused any significant change in swimming behavior, respiration rate or gene expression level. This study suggests that adults of A. tonsa do not exhibit any measurable acute stress response when exposed to high culture densities for 12 h.