Causes and consequences of mate limitation in two marine copepod species Eurytemora herdmani and Temora longicornis within the Damariscotta River estuary in Walpole, Maine

Background/Question/Methods Within sexually reproducing zooplankton species, birth rates are controlled by female fecundity as well as fertilization success.  In marine copepods, many studies have focused on the effects of environmental and ecological conditions on female fecundity (in terms of egg production).  However, comparatively fewer studies have focused on determining the factors influence fertilization/mating success.  Coastal marine copepods are likely to be vulnerable to mate limitation as a result of dilute population densities and their need to continuously breed in order to produce multiple fertilized clutches in their lifetime. Despite important implications for predicting copepod population growth, field surveys describing the prevalence of mate limitation in coastal copepod species are lacking. The aim of this study was to determine the prevalence of in situ mate limitation within two dominant marine copepod species Temora longicornis and Eurytemora herdmani that co-occur in the Damariscotta River estuary in Walpole, Maine and differ in their reproductive strategies (T. longicornis; spawns eggs and E. herdmani; broods egg sacs). Monthly zooplankton samples were collected to obtain an ecological snapshot of field-conditions.  From these samples, we analyzed egg clutches from field-caught females using an egg-staining technique to determine the female’s in situ fertilization status (which indicates whether or not the female has recently mated).  

Results/Conclusions Our results indicate both species have reduced fertilization at certain times of the year that correlate with high mysid-predator densities (even when copepod population density and sex ratio is sufficient for successful breeding).  To determine a causal relationship between predator density and reduced copepod mating success, we conducted laboratory experiments to determine the effects of a common mysid shrimp predator on Eurytemora herdmani mating success.  Laboratory results indicate that the presence a mysid predator and a predator cue (in the absence of a predator) reduces copepod mating success.  These results indicate that mysid predators can constrain copepod population growth through non-consumptive processes via reduced mating activity.  This study highlights a need to consider behavior in copepod population growth models.