Behavioural response of bivalve molluscs to calcium hydroxide

Blue mussels (Mytilus edulis) that are cultivated in the marine area around Prince Edward Island, Eastern Canada, are susceptible to the heavy biofouling of their shells by an invasive solitary tunicate, Styela clava, which rapidly proliferates. To mitigate this issue, mussel farmers periodically lift the longlines on which the mussels are suspended out of the water to spray a highly alkaline (~ 12.7 pH units) calcium hydroxide solution onto fouled individuals. Here, we tested the hypothesis that calcium hydroxide exerts behavioural stress on mussels and other bivalves. Field surveys revealed that the alkalinity of the seawater in the vicinity of longlines increased (9.3-11.7 pH units) immediately after treated mussel sleeves were returned into the water column. Thereafter, pH values declined rapidly, and met federal water quality guidelines (7.0-8.7 pH units) within 3.1 ± 0.5 min (range 0.3-10.5 min, n = 31 sleeves). Cultivated mussels challenged to both emersion and calcium hydroxide closed their valves for 14.0 ± 3.3 min (n = 18) compared to 6.5 ± 1.6 min (n = 17) by control mussels (emersion only). We subsequently assessed how three benthic bivalve species (M. edulis, Crassostrea virginica (eastern oyster) and Argopecten irradians (bay scallop)) respond to weak (pH ≤ 9.2) but sustained (3-h daily for 3 days) alkalinity conditions. All three species consistently responded by completely or partially closing their valves. However, all behavioural responses were short-lived (0.2-4.7 h), and were generally confined to the treatment period. In conclusion, spraying calcium hydroxide onto cultivated mussels has limited impact on seawater alkalinity and the behaviour of nearby bivalves.