Ocean acidification is not happening in isolation but against a background of chronic low-level pollution for most coastal marine environments. The reproductive and larval stages of marine invertebrates can be highly sensitive to the impacts of both environmental pollutants and ocean acidification, but very little is currently known regarding the potential impacts of combined contaminant and high C[O.sub.2] exposures on the health of marine organisms. Ocean acidification research to date has focused heavily on the responses of calcifying marine invertebrate larvae and algae, and as such the polychaetes as a group, despite their ecological importance, remain understudied. Here, we investigate the effects of elevated seawater C[O.sub.2] (pH range 8.1-7.4, plus an extreme pH of 7.2 in the sperm motility experiments), in combination with the environmental pollutant copper (0.002 µM), on the early life history stages of the intertidal polychaete Pomatoceros lamarckii from two populations. P. lamarckii sperm appear to be robust to elevated seawater C[O.sub.2]. Whilst all three of the sperm motility end points measured showed a response to elevated C[O.sub.2], these responses were small and not linear. The percentage of motile sperm and sperm curvilinear velocity were significantly reduced in the lower pH treatments of 7.4 and 7.2, whereas sperm straight-line velocity (VSL) was mostly unaffected except for an increased VSL at pH 8.0. Fertilisation success was investigated using two populations from the South West (UK), one from Torquay and one from Plymouth Sound. Fertilisation success was slightly but significantly reduced at the 7.6 and 7.4 pH treatments for both populations (a 9.0% reduction in fertilisation success from pH 8.1 to 7.4 for Torquay), but with a greater effect observed in the population from Plymouth Sound (a 13.33% reduction in fertilisation success). No additional impact of 0.002 µM copper exposure on fertilisation success was found. Larval survival was found to be much more sensitive to elevated C[O.sub.2] than sperm function or fertilisation, and a significant interaction with copper exposure was observed. These results demonstrate the potential for polychaete larvae to be affected by predicted ocean acidification conditions and that chronic coastal pollutants, such as copper, have the potential to alter larval susceptibility to ocean acidification conditions., Introduction There is now compelling evidence that the early life history stages of calcifying marine invertebrates are particularly sensitive to the C[O.sub.2]-induced reduction in seawater pH, termed ocean acidification (OA), [...]