Outer organic layer and internal repair mechanism protects pteropod Limacina helicina from ocean acidification

Chair: Janice Lough

Vicky Peck(1), Liz Harper(2) Geraint Tarling(1), Eithne Tynan(3), Clara Manno(1),

1 British Antarctic Survey, High Cross, Madingley Rd, Cambridge, CB5 8NP, UK
2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge,
3 Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, UK

Pteropods, Limacina helicina, collected under sea ice off East Greenland in June 2012 exhibit varying degrees of shell dissolution. Localised areas of dissolution are clearly visible under light microscope in about half of the specimens collected, while others maintained wholly ‘pristine’ shells. In specimens exhibiting dissolution, the dissolution was localised to discrete sites within an otherwise pristine shell where the protective organic coating of the shell, the perisotracum, had been compromised. Damage to the periostracum occurred mechanically, such as where the shell has been broken and regrown and the old and new periostracum did not form a perfect seal or the shell surface has been scratched or punctured. Microbial or epibiont activity on the shells surface may also have damaged the periostracum in some cases. Specimens that appeared to be pristine on collection were subjected to 8 day incubations in conditions simulating subsurface waters at pCO2 >750 ppm and also began to exhibit dissolution on historical and/or newly acquired sites of periostracum damage at the end of the incubation. SEM images confirm that dissolution was found exclusively in areas where the periostracum had been damaged and the underlying aragonite shell exposed. Several specimens exhibited discrete sites of dissolution so extensive that the full depth of the original shell had been eroded. In these cases, additional aragonite was secreted internally to ‘patch up’ the damage and maintain a fully intact, rigid shell to protect the animal.
At the time of collection, water down to 300m was supersaturated with respect to aragonite, suggesting that dissolution should not have been active at that time. SEM analysis clearly demonstrates that dissolution of the original shell in specimens exhibiting damage had occurred earlier in the animal’s life. Whether these specimens were subject to undersaturated waters during the winter months at this site or within waters, perhaps to the north, where they may have spent their earlier lives is unknown, but exposure of these pteropods to undersaturated waters on one or more occasions over the last year or two has revealed their scars from a violent past. Although these specimens exhibit some shell deterioration they remain robust and demonstrate the ablity of L. helicina to maintain their shell and continue to calcify in undersaturated waters. What our results confirm is that for L. helicina, as with all molluscs, the periostracum is crucial for secreting and protecting the shell, even in undersaturated conditions and that net dissolution of their shells is an unlikely consequence of ocean acidification within the coming century