Effect of near-future seawater temperature rises on sea urchin sperm longevity

Global warming has and will continue to warm the world's oceans, which may have detrimental consequences for marine life. Studies assessing the impact of climate-change stressors on early life-stages of marine invertebrates have focussed on immediate fertilisation success or larval development, but have so far not considered gamete longevity. Recent studies have suggested that sea urchin fertilisation can take place for several hours, as dilute spermatozoa can travel to fertilise distant eggs, making gamete longevity an important factor in fertilisation success for some species. The longevity of spermatozoa from Heliocidaris tuberculata was assessed over a 3-h exposure to current ambient (20 degrees C), near-future (24 degrees C) and future (26 degrees C) ocean-temperature scenarios. Sperm mitochondrial activity was also measured throughout the 3-h exposure using the stain Rhodamine 123 (Rh123) and flow cytometry. Sperm longevity, based on fertilisation success, significantly decreased following a 1-h exposure at 26 degrees C, or a 3-h exposure at 24 degrees C, relative to the 20 degrees C treatment. However, sperm mitochondrial activity did not correlate with fertilisation success. Even when fertilisation success was below 20%, Rh123 uptake remained above 80%, indicating the presence of active mitochondria in non-viable spermatozoa. Our results suggested that at projected sea-surface temperatures, the longevity of sea urchin spermatozoa is reduced, which may have consequences for sea urchin population dynamics.