Temperature-dependent egg production and egg hatching rates of small egg-carrying and broadcast-spawning copepods Oithona similis, Microsetella norvegica and Microcalanus pusillus

Abstract

Reproductive rates of copepods are temperature-dependent, but poorly known for small copepods at low temperatures, hindering the predictions of population dynamics and secondary production in high-latitude ecosystems. We investigated egg hatching rates, hatching success and egg production of the small copepods <i>Oithona similis</i> and <i>Microsetella norvegica</i> (sac spawners) and <i>Microcalanus pusillus</i> (broadcast spawner) between March and August. Incubations were performed at ecologically relevant temperatures between 1.3 and 13.2°C, and egg production rates were calculated. All egg hatching rates were positively correlated to temperature, although with large species-specific differences. At the lowest temperatures, <i>M. pusillus</i> eggs hatched within 4 days, whereas the eggs from sac spawners took 3–8 weeks to hatch. The egg hatching success was ≤25% for <i>M. pusillus</i>, >75% for O. similis and variable for <i>M. norvegica</i>. The maximum weight-specific egg production rate (μg C μg⁻¹ C d⁻¹) of <i>M. pusillus</i> was higher (0.22) than O. similis (0.12) and <i>M. norvegica</i> (0.06). <i>M. norvegica</i> reproduction peaked at 6–8°C, the prevailing in situ temperatures during its reproductive period. The difference in reproductive rates indicates species-specific thermal plasticity for the three copepods, which could have implications for present and future population dynamics of the species in arctic fjords.