Reproductive strategies in vent shrimps shaped by feeding ecology with intriguing seasonality unlinked to surface productivity

Variations in offspring production according to feeding strategies or food supply have been recognized in many animals from various ecosystems. Despite an unusual trophic structure based on non-photosynthetic primary production, these relationships remain largely under-studied in chemosynthetic ecosystems. Here, we use Rimicaris shrimps from deep-sea hydrothermal vents as a study case to explore relations between reproduction, diets and food supply in these environments. For that, we compared reproductive outputs of three congeneric shrimps differing by their diets. They inhabit vents located under oligotrophic waters of tropical gyres with opposed latitudes, allowing us to also examine the prevalence of phylogenetic vs environmental drivers in their reproductive rhythms. For this we used both our original data and a compilation of published observations on the presence of ovigerous females covering various seasons over the past 35 years. We report distinct egg production trends between Rimicaris species relying solely on chemosymbiosis - R. exoculata and R. kairei - and those relying on mixotrophy - R. chacei - where R. chacei produces broods with higher numbers of smaller eggs. Besides, our data and historical records suggest a reproductive period with substantial proportions of brooding females mostly between January and early April for all examined species whatever the region. Intriguingly, this periodicity does not correspond to seasonal variations in surface production with presence of brooding females during either boreal winter or austral summer. These observations contrast with the long-standing paradigm in deep-sea species for which periodic reproductive patterns have always been attributed to seasonal variations of photosynthetic production sinking from surface. Our results suggest the presence of intrinsic basis for biological rhythms in the deep sea, and bring to light the importance of having year-round observations in order to understand life history of vent animals.