Effects of ocean acidification on the interaction between calcifying oysters (Ostrea chilensis) and bioeroding sponges (Cliona sp.)

Ocean acidification can negatively affect a broad range of physiological
Q9 processes in marine-shelled mollusks. Marine bioeroding
organisms could, in contrast, benefit from ocean acidification
due to reduced energetic costs of bioerosion. Ocean acidification
could thus exacerbate negative effects (e.g. reduced growth) of
ocean acidification and shell borers on oysters. The aim of this
study was to assess the impact of ocean acidification on the oyster
Ostrea chilensis, the boring sponge Cliona sp., and their
host-parasite relationship. We exposed three sets of organisms 1)
O. chilensis, 2) Cliona sp., and 3) O. chilensis infested with
Cliona sp. to pHT 8.03, 7.83, and 7.63. Reduced pH had
no significant effect on calcification, respiration, and
clearance rate of uninfested O. chilensis. Low pH significantly reduced
calcification leading to net dissolution of oyster shells at pHT
7.63 in sponge-infested oysters. Net dissolution was likely caused by
increased bioerosion by Cliona sp. at pHT 7.63. Additionally,
declining pH and sponge infestation had a significant negative
antagonistic effect (less negative than predicted additively) on
the clearance rate. This interaction suggests that sponge-infested oysters
increase clearance rates to cope with the higher energy demand of
increased shell repair resulting from the higher boring activity of Cliona
sp. at low seawater pH. O. chilensis body condition was unaffected by
sponge infestation, pH, and the interaction of the two. The
reduction in calcification rate suggests sponge infestation and
ocean acidification together would exacerbate direct (reduced
growth) and indirect (e.g., increased predation) negative
effects on oyster health and survival. Our results indicate that
ocean acidification by the end of the century could have severe
consequences for marine mollusks with boring organisms.