Postponing development: dormancy in the earliest developmental stages of a high-latitude calanoid copepod

Abstract

Abstract Large lipid-rich copepods inhabiting high-latitude environments synchronize growth with the annual spring phytoplankton bloom. However, reproduction often precedes the bloom, raising the question of how the nauplii survive long enough to encounter the bloom. Are their energy stores sufficient to maintain an active life until then, or do they occupy a state of inactivity or dormancy, postponing development until food appears? These alternative hypotheses were tested in nauplii of Neocalanus flemingeri using gene-expression profiling. Stage NII and NIII nauplii were incubated for three days in either the presence or absence of food. Gene-expression differences between developmental stages and between food/no food treatments in individual nauplii were assessed using RNASeq. In the absence of food, nauplii exhibited transcriptional profiles typical of a dormant state. Similar to diapausing N. flemingeri females, genes involved in anaerobic metabolism, chromatin silencing and longevity were highly expressed. Nauplii of both stages responded to the food with the up-regulation of genes associated with diapause termination in copepods and insects, as well as genes involved in transcription and energy metabolism. The ability of nauplii to remain dormant could be a key adaptation that allows them to delay development and conserve energy, while they await the phytoplankton bloom.