Comparative oxygen consumption rates of subitaneous and delayed hatching eggs of the calanoid copepod Acartia tonsa (Dana)

Abstract

Life history strategies can be adaptations to existence in e.g. unstable environments. Under stressful environmental conditions mobile species tend to migrate to more suitable places, but migration in time by dormancy is another durable strategy. More than 50 species of calanoid copepods can produce diapause eggs requiring a long refractory phase. Delayed hatching eggs (DHE; maternally determined oligopause eggs hatching within >72h to a month) have been described as a state between diapause and subitaneous (hatching within 24–72h) because of their very short refractory phase. We used nanorespirometry to monitor initial oxygen consumption rate of individual eggs of the ubiquitous neritic calanoid copepod Acartia tonsa to distinguish between subitaneous and DHE. We hypothesized that subitaneous eggs exhibit higher initial oxygen consumption rates than DHE, and that initial egg oxygen consumption rate is correlated to the time for the individual egg to hatch. Subitaneous eggs exhibited higher initial oxygen consumption rates than DHE and there were no pattern in initial oxygen consumption rates vs. time to hatch or die from the eggs. Variability in initial oxygen consumption rates within batches of both subitaneous and DHE, as well as between these egg types, is prevalent. There was a continuum from sluggish- to fast metabolising eggs considering initial oxygen consumption rates most likely reflecting phenotypic variation within cohorts. No matter the individual initial egg oxygen consumption rate, embryogenesis took place with unpredicted rates and most eggs eventually hatched. Based on our observations and theoretical considerations we suggest that DHE follow a U shape metabolism pattern with time as described for true diapause eggs. DHE strategy (oligopause) is an important life history trait allowing the spread of hatching over a period of time (a month or two), increasing the likelihood that some offspring encounter suitable environmental conditions and can contribute to the pelagic population of neritic calanoids in environmental fluctuating environments.