Boring Sponges and Bored Oysters – Interactions between the Bioeroding Sponge Cliona sp. and the New Zealand Flat Oyster Ostrea chilensis

Bioeroding sponges are the dominant macroborers in many environments. They can affect growth, condition and potentially survival in shellfish populations and cause economic loss in aquaculture and fisheries. The Foveaux Strait oyster fishery in New Zealand, targeting the native flat oyster Ostrea chilensis, is a nationally important fishery. The boring sponge Cliona sp. has long been recognised to colonise O. chilensis in the Foveaux Strait and in other parts of New Zealand. However, no quantitative data exists about the distribution and abundance of Cliona sp. in New Zealand flat oyster populations. Detailed information about the impact of sponge boring on the physiology of O. chilensis is also lacking. The present study aims to fill these knowledge gaps and understand how the oyster-sponge interaction could change in the future due to ocean acidification. In my first data chapter I investigated the distribution of Cliona sp. and the level of infestation in individual oysters from the main O. chilensis populations in New Zealand with a particular focus on the oyster population in the Foveaux Strait. Apart from one site, Cliona sp. was found in all sampled oyster populations. The highest infestation with Cliona sp. was 100% at one station in the western Foveaux Strait. The proportion of infested oysters differed significantly among locations, while the level of infestation within oysters was unaffected by location. In the Foveaux Strait fishery area, a strong east-west gradient of increasing sponge abundance was reported, confirming previous suggestions of higher sponge abundance in the west. The distribution and abundance of Cliona sp. is likely driven by environmental factors such as salinity, sedimentation, and hydrodynamic conditions.

In my second data chapter, the spread and growth of Cliona sp. in O. chilensis and the effects of sponge infestation on oyster mortality were investigated with field experiments. Over the course of one year no uninfested oysters became noticeably infested with Cliona sp., potentially due to settlement of propagules being hindered or because of lack of propagules in the water column. In infested oysters the relative increase in boreholes was highest in oysters with low initial infestation. In the second field experiment I deployed frames (steel frames with cement boards used for oyster deployment) with infested and uninfested oysters separately and used covers over some frames to reduce predator access. However, mortality of infested oysters was higher on covered frames, which could suggest predators were trapped in frames. Overall, I found no strong evidence for increased mortality in sponge-infested oysters compared to uninfested oysters. Potential negative effects of Cliona sp. on O. chilensis physiology were evaluated in my third data chapter. I tested whether the presence of boring sponges affected meat quality, condition, aspects of reproduction, and disease susceptibility of O. chilensis. There were no negative impacts of sponge infestation on meat quality and condition of oysters. Similarly, sponge infestation was not related to sex of the oysters and had no impact on reproductive capacity. The incidence rate of common disease-causing organisms of O. chilensis was also unaffected by the presence of the boring sponge, further suggesting that sponge infestation does not decrease energy available for oyster defence against pathogens. The lack of impact of sponge infestation on O. chilensis highlights the variability of impact of bioeroding sponges on shellfish, which is likely related to location specific factors. Calcifying organisms are especially vulnerable to future ocean acidification scenarios, due to decreases in CaCO3 precipitation and increases in passive dissolution. Bioerosion, on the other hand, could be enhanced under low seawater pH. In my final data chapter, I investigated the impact of two ocean acidification scenarios on O. chilensis, Cliona sp., and their interaction in a laboratory experiment. Calcification decreased significantly with pH in sponge-infested O. chilensis and turned into net dissolution in the lowest pH treatment. Clearance rate of uninfested oysters tended to decline with pH, suggesting oysters were minimizing contact of low seawater pH with internal tissues. However, sponge infestation and pH had a significant positive antagonistic effect on clearance rate. One possible explanation is that infested oysters increased clearance rates because of higher energy expenditure due to shell repair. Additionally, reduced seawater pH did not negatively affect Cliona sp. but did increase its bioerosion rates in oysters. My results suggest that bioerosion activity of Cliona sp. may outpace shell repair and growth of O. chilensis under continued high CO2 emissions. Overall, my thesis provides new knowledge on the distribution and abundance of Cliona sp. in New Zealand flat oyster populations, on the effects of sponge colonisation on an ecologically and economically important oyster species, and on the possible effects of future ocean conditions on the species interaction of calcifying and bioeroding organisms.