Impact of ocean acidification on reproductive output in the deep-sea annelid Ophryotrocha sp. (Polychaeta: Dorvilleidae)

As increasing anthropogenic CO2 emissions are absorbed by the oceans, a decrease in seawater pH is expected to occur, causing what is now termed ocean acidification (OA). Deep-sea species have been greatly understudied with respect to OA, even though their response may differ from those evidenced so far in shallow-water taxa. The polychaete worm Ophryotrocha sp. collected at bathyal depth was held and reproduced for several years, offering a rare opportunity to study environmental effects in a member of a deep-sea community. This hermaphroditic species exhibits well defined seasonality in feeding and reproduction and its development and growth have been characterized. The purpose of the present study was to explore the effects of OA on gametogenesis following exposure to a 0.4 unit pH decrease under realistic conditions over 26 weeks. Opportunistic assessments of spawning and development were also conducted. A flow-through design allowing for natural fluctuations in pH, temperature and salinity was used. Individuals exposed to low pH/high ρCO2 produced larger and more abundant oocytes but fewer spermatozoa, compared to individuals in ambient conditions. However, lower effective fecundity (number of eggs laid) was ultimately recorded under low pH conditions, together with slower development of the embryos and larvae. Microstructure of the body wall, and appearance and elemental composition of chaeta were not affected. Despite its ability to live and reproduce normally for years in the laboratory, a realistic decrease of pH in the environment of Ophryotrocha sp. led to reproductive disruption, highlighting its potential vulnerability to OA.