Reproductive ecology of the circumpolar Antarctic nemertean Parborlasia corrugatus: No evidence for inter-annual variation

Waters around the Antarctic Peninsula are experiencing one of the fastest rates of warming seen anywhere on Earth in the sea in recent times. Species inhabiting nearshore seabed sites in Antarctica have a long history in near freezing temperatures and have poor physiological capacities to cope with change. Reproductive success is a vital characteristic in species survival and evaluation of how reproductive processes vary with time is key to identifying vulnerable species and evaluating the effects of ocean warming. The long-term gametogenic development of the nemertean Parborlasia corrugatus (McIntosh, 1876) was analysed from monthly samples collected between July 1997 and November 2000 from a shallow water site adjacent to the British Antarctic Survey Rothera Research Station (Western Antarctic Peninsula). Spermatogenesis, oocyte size and reproductive condition are described for each month using histological and image analysis techniques. Male and female reproductive condition varied seasonally, with a reduction in the proportion of large oocytes and mature sperm in the gonad during the late austral summer (December–March). We identify three key characteristics in the reproductive ecology of P. corrugatus (1) the timing of reproduction was consistent in the late austral summer of each year and synchronous between males and females (2) oogenesis progressed over a discrete 15–16 month period with a single generation of oocytes in the ovary during most stages of gametogenic development and (3) notable differences in reproductive condition were not apparent from year to year. The majority of long-term studies on reproductive ecology of marine invertebrates at high latitudes, with the exception of one or two species, revealed that 18–24 months are required for gametogenesis compared to 6–12 months in temperate species and there is extreme inter-annual variation in reproductive output. P. corrugatus differs from this in requiring only 15 months for gametogenesis, and has little or no variation between years. This may be because of its predatory lifestyle and that coupling between its gametogenic development and timing, and larval abundance and nutrition with phytoplankton productivity is much less than in the previously studied Antarctic marine herbivores and omnivores. However, the timing of larval settlement and peak juvenile abundance in P. corrugatus does suggest coupling with seasonal productivity through the nemertean's larval and juvenile ecology