Sperm motility and fertilisation success in an acidified and hypoxic environment

Graham, H., Rastrick, S. P. S., Findlay, H. S., Bentley, M. G., Widdicombe, S., Clare, A. S., and Caldwell, G. S. Sperm motility and fertilisation success in an acidified and hypoxic environment. - ICES Journal of Marine Science Marine Science Helen Graham 0 Samuel P. S. Rastrick 2 3 Helen S. Findlay 4 Matthew G. Bentley 0 1 Stephen Widdicombe 4 Anthony S. Clare 0 Gary S. Caldwell 0 0 School of Marine Science and Technology, Newcastle University , Ridley Building, Claremont Road, Newcastle upon Tyne, Tyne and Wear NE1 7RU , UK 1 Present Address: Faculty of Science & Technology, Bournemouth University , Talbot Campus, Poole, BH12 5BB 2 Marine Biology and Ecology Research Centre, University of Plymouth , Davy 618, Drake Circus, Plymouth PL4 8AA , UK 3 Institute of Marine Research , PO Box 1870 Nordness, 5870 Bergen , Norway 4 Plymouth Marine Laboratory , Prospect Place, West Hoe, Plymouth PL1 3DH , UK The distribution and function of many marine species is largely determined by the effect of abiotic drivers on their reproduction and early development, including those drivers associated with elevated CO2 and global climate change. A number of studies have therefore investigated the effects of elevated pCO2 on a range of reproductive parameters, including sperm motility and fertilisation success. To date, most of these studies have not examined the possible synergistic effects of other abiotic drivers, such as the increased frequency of hypoxic events that are also associated with climate change. The present study is therefore novel in assessing the impact that an hypoxic event could have on reproduction in a future high CO2 ocean. Specifically, this study assesses sperm motility and fertilisation success in the sea urchin Paracentrotus lividus exposed to elevated pCO2 for 6 months. Gametes extracted from these pre-acclimated individuals were subjected to hypoxic conditions simulating an hypoxic event in a future high CO2 ocean. Sperm swimming speed increased under elevated pCO2 and decreased under hypoxic conditions resulting in the elevated pCO2 and hypoxic treatment being approximately equivalent to the control. There was also a combined negative effect of increased pCO2 and hypoxia on the percentage of motile sperm. There was a significant negative effect of elevated pCO2 on fertilisation success, and when combined with a simulated hypoxic event there was an even greater effect. This could affect cohort recruitment and in turn reduce the density of this ecologically and economically important ecosystem engineer therefore potentially effecting biodiversity and ecosystem services. climate change; fertilisation success; hypoxia; ocean acidification; oxygen saturation; sperm motility Introduction Global climate change, fuelled by enriched atmospheric carbon inventories, is altering the physicochemical status of the global ocean (Diaz and Rosenberg, 2008; Kroeker et al., 2010; Byrne, 2012) . The increasing partial pressure of seawater CO2 ( pCO2) is driving a decline in seawater pH?a process termed ocean acidification (OA). Seawater pH is predicted to drop by 0.3 ? 0.5 units by 2100 (based on pCO2 concentrations of 730 ? 1020 matm, respectively (IPCC, 2013) . The combination of rising pCO2 and increasing sea surface temperature will place an additional burden on marine systems by reducing oxygen solubility (Hofmann and Schellnhuber, 2009) . Increased frequencies of ocean hypoxic events, such as may occur via ocean upwelling, are predicted (Po?rtner and Langenbuch, 2005; Oschlies et al., 2008; Po?rtner, 2008) making it necessary to understand the combined effects of OA and hypoxia on marine species and ecosystems (Reum et al., 2016). Reproductive processes and early ontogenetic stages of marine animals appear particularly vulnerable to changing seawater properties (Po?rtner and Farrell, 2008; Byrne et al., 2010a, b; Cooper et al., 2012) . Broadcast spawning, a reproductive strategy common in many marine animals, exposes gametes directly to the seawater environment (Crimaldi, 2012) . Spawned gametes have therefore been used extensively in attempts to describe the potential impacts of OA on reproductive processes (Havenhand and Schlegel, 2009; Byrne et al., 2010a, b; Ericson et al., 2010; Frommel et al., 2010; Morita et al., 2010; Cooper et al., 2012) . Hitherto, reductions in seawater pH have been shown in several studies to impact sperm swimming ability by causing changes in internal pH (pHi) of sperm and affecting motility of the flagellum (Havenhand et al., 2008; Fitzpatrick et al., 2009; Morita et al., 2010; Caldwell et al., 2011) . These changes in sperm pHi affect fertilisation by slowing the fast block to polyspermy through interfering with the Na+/H+ exchange and preventing the fertilisation membrane being raised (Reuter et al., 2011; GonzalexBernat et al., 2013 ). Despite variable results, the consensus is that OA, as a function of climate change, will negatively impact marine biodiversity and ecosystem function via disruption of reproductive processes (Dupont et al., 2010; Byrne, 2012) . Over the past decade, the dissolved oxygen (DO) content of coastal waters has changed dramatically and this has led to widespread occurrences of hypoxia, especially in coastal areas, which have shown an exponential increase of hypoxic events of 5.54% year21 (Diaz and Rosenberg, 1995; Diaz, 2001; Vaquer-Sunyer and Duarte, 2008) . Normal DO levels range between 5.0 and 8.0 mg O2 l21 in coastal waters, while hypoxic conditions are defined as occurring when levels fall ,2.8 mg O2 l21 (30% oxygen saturation or less) (Diaz and Rosenberg, 1995) . The duration of an hypoxic event can be long term/permanent or short term (incidental or episodic) as investigated in the present study (Middelburg and Levin, 2009) . Hypoxia has been shown to negatively affect reproduction and development of marine invertebrates across a range of reproductive endpoints including gonad growth (Siikavuopio et al., 2007), reproduction (Cheung et al., 2008) , egg production (Marcus et al., 2004) , reproductive output (Spicer and El-Gamal, 1990) , and embryonic development (Chan et al., 2008) . A recent study by Shin et al. (2014) reported that hypoxia, as a single stressor, significantly reduced sperm motility in Hydroides elegans, which compromised fertilisation success. There was also a negative effect of hypoxia on embryonic development with an increase in the number of malformed embryos (Shin et al., 2014) . As elevated pCO2 and hypoxia, when applied individually, are reported to have similar negative effects on reproduction, they may be expected to have synergistic or additive effects when applied together. Consequently, we examined the effects of longterm exposure (6 months) of adult sea urchins to elevated pCO2 before spawning, followed by the exposure of spawned gametes to hypoxia and OA before and during fertilisation. This study was designed to represent the eff (...truncated)