The interplay between Microsetella norvegica (Harpacticoida, Ectinosomatidae) and Pleurobrachia bachei (Ctenophora: Pleurobrachiidae) reveals novel trophic interactions

Despite the wide geographic distribution of Microsetella norvegica little is known regarding its interspecies interactions. In this work, we provide the first description of the interaction between the harpacticoid copepod M. norvegica and the ctenophore Pleurobrachia bachei in the southeastern Gulf of California. M. norvegica exhibited two remarkable trophic strategies during the period of this study. A parasitic strategy is proposed since M. norvegica undergoes ontogenetic changes from egg to late-copepodite in the mesoglea of the ctenophore. In contrast, adult females exhibited a short-time interaction with the ctenophore, consistent with a micropredation trophic strategy. Females occurred inside the ctenophore in three stages of maturity: lacking egg-sac (presumably fertilized), ovigerous, and with egg-sacs just detached from the female genital segment. This suggests that females enter into the ctenophore's mesoglea to release egg-sacs, and then leave the host to continue a free life where mating occurs. Accordingly, we propose that M. norvegica is able to exhibit a protelean life history in which larvae are parasitic, but adults are free living. Copepods invaded all stages of ctenophores (i.e. from larval to adult stage ≤ 15.1 mm in sagittal diameter) with low prevalence, ranging from 0.01 to 0.4% and intensity between 1 to 3 adult females. In contrast, developmental stages from egg to copepodites reached up to 18 individuals per host. Density of M. norvegica was relatively high attaining up to 29.2 ind. 100 m⁻³. Positive correlation between abundance of ctenophores and copepods (r²=0.6, p < 0.05) suggests a synchronization of the interaction. Abundances were highest during the coldest season at shallow sampling stations with oligotrophic water characterized by low levels of chlorophyll-a, and moderate oxygenated waters.