A decade of reproductive abnormalities in pelagic ostracods observed at the entrance to a changing Arctic

Abstract The rapid warming of the Arctic Ocean is driving significant shifts in ecosystem dyamics, from species-level morphological changes to changes in community composition. Within the Arctic pelagic community, Ostracoda is an understudied zooplankton group that exhibits distinct sexually dimorphic features, including carapace shape, size, frontal and reproductive organs, and antennae. During analyses of a zooplankton time series collected each summer from 2010 to 2019 in the Fram Strait, the main gateway between the north Atlantic and the Arctic Ocean, all four dominant pelagic ostracod species ( Boroecia maxima , B. borealis , Discoconchoecia elegans , and Obtusoecia obtusata ) displayed reproductive abnormalities in the form of pseudopenises that otherwise had distinct female features. Out of the total analyzed material of 4,137 adult and pre-mature A-1 stages individuals, 646 A-1 stage individuals, or 15.6% of the population, exhibited pseudopenises. All observed abnormalities were confined to the A-1 stage with no variant adults recorded. The primary difference between these pseudopenises and normal male penises was the lack of muscle bands and sperm duct required for reproductive functionality. However, a few A-1 females had pseudopenises with signs of these bands and ducts developing, suggesting a potential transition to true intersexuality. Monitoring this phenomenon in ostracods is crucial because it can have population-wide reproductive consequences. Furthermore, widespread intersexuality could complicate the interpretation of ecological trends in the changing Arctic.