Exploring the Link between Otolith Growth and Function along the Biological Continuum in the Context of Ocean Acidification

Oceans are acidifying as atmospheric CO2 is drawn down. This process, known as ocean acidification (OA), is well known and documented. Over the next 100 years, pH of the surface ocean is projected to decrease by up to 0.35 units. This CO2 draw down has a direct effect on dissolved inorganic carbon (DIC) balance in the ocean. OA is expected to impact calcifying organisms that rely on constituencies of the DIC system, specifically carbonate ion [CO32-]. It is clear that externally calcified structures, such as coral skeletons, bivalve shells, etc., will be significantly affected as pH, and consequently [CO32-], of the oceans decline. What is unclear, however, is how these changes will impact internally calcified structures, such as earstones (otoliths) of teleost fish. This dissertation examines the impacts of OA on otolith mineralization in larval reef fish (Amphiprion clarkii and A. frenatus). This research included the development of a laboratory controller system for control of experimental aquaria pH through pCO2 dosing, exposure of larvae from hatch to settlement under various pCO2 treatments and evaluation of otolith structure and morphology across treatments within a single genus. No standard method for pH-stat CO2 dosing controllers existed prior to this study. Incorporating low-cost, flexible hardware allowed high precision and accuracy pH controllers to be designed and implemented. Following system stability studies, we found that our system performed at or beyond the level of control exhibited in the literature. Two species of clownfish, Amphiprion clarkii and A. frenatus, were exposed to different pCO2 conditions, reared to settlement and otoliths extracted and studied. I found that the sagittae (largest of the 3 otolith types) of both species exhibited circularity changes towards more oblong otoliths under increased pCO2. For A. clarkii, I found a significant negative relation between pCO2 and lapilli otolith circularity, indicating a shift toward more circular lapilli under increased pCO2. Since lapilli are critical to gravisensing in teleosts these results explain my anecdotal observations that, at high pCO2, larvae exhibited lethargic, uncoordinated swim patterns. The core development of otoliths (sagittae, lapilli, and asterisci) from both species was analyzed using SEM imagery. Otolith images were scored by 6 independent readers for core development (poorly developed to well-developed). Otolith scores were regressed against aragonite saturation state (ΩAr). Results showed significant and strong relations between ΩAr and development score, indicating a shift toward protruding, unorganized crystal clusters within the core under high pH/low ΩAr. This research is the first to comprehensively examine the impact of OA on the otolith system in larval fish. The research revealed direct impacts on otolith structure and morphology as well as mineralogy. These changes will directly impact survival of the larvae. It remains unknown whether the otolith system recovers post-settlement or whether the anecdotal observations on swimming behavior are directly related to otolith deformation. Future research will include exploration of these relations across genera as well as more deeply examine the recovery of the system and behavioral impacts of otolith deformation across life stages