Ion-regulatory and developmental physiology of giant clams (Genus Tridacna) and their conservation status on the island of Mo’orea, French Polynesia.
Marine species face several challenges as a result of human activities including direct effects on populations (e.g. overharvesting) and indirect impacts on individuals (e.g. physiological responses to ocean warming and acidification). Ameliorating these impacts requires successful regulation at both the population and physiological levels. One group of critically threatened organisms are the so called “giant clams” (genus Tridacna) of tropical coral reefs. Like corals, giant clams possess calcifying larvae and host symbiotic microalgae and are therefore susceptible to environmentally-driven failure of biomineralization and symbiotic disruption. In addition, giant clams are economically important fisheries species which have been severely overexploited throughout much of their range. While marine reserves have been established to replenish dwindling giant clam stocks, their efficacy in promoting population recovery remains unknown. Similarly, little is known regarding the magnitude of climate-change associated physiological effects on giant clams. This stems, in part, from a lack of knowledge regarding the response of early life-history stages to warming and acidification and because the molecular mechanisms regulating acid-base homeostasis and symbiont photosynthesis in giant clams remain poorly characterized. I addressed these knowledge gaps in populations of the small giant clam, Tridacna maxima, from across a network of marine protected areas (MPAs) on Mo’orea, French Polynesia. I employed a combined physiological and ecological approach to (1) investigate mechanistic processes underlying acid-base regulation within giant clams, specifically in relation to maintenance of host-symbiont homeostasis, (2) measure the effects of increased temperature and elevated pCO2 on giant clam fertilization success, and (3) assess the efficacy of a recently established Marine Protected Area Network in promoting conservation and recovery of this species in an exploitative environment.
I demonstrate that giant clams regulate symbiont photosynthesis through the activity of an ion-transport protein, vacuolar-type H+-ATPase (VHA), which is strongly localized in close proximity to symbiotic algae. I further show that clam VHA actively promotes algal photosynthesis, increasing rates of O2 production and holobiont metabolic rate, and likely represents a convergent exaptation for carbon concentration shared by reef-building corals. This process has implications for climate-related responses and may offset the negative impacts of future ocean acidification in these species. I also present the first data demonstrating giant clam early life-history responses to climate change drivers and show that syngamy in giant clams is extremely sensitive to environmental warming. Finally, I demonstrate the significant, positive, effect of MPA establishment in permitting recovery of overharvested giant clam populations. T. maxima populations have increased approximately 3-fold in Mo’orea’s protected sites relative to non-protected controls and this rate of recovery is significantly higher than the global average for marine reserves. Taken together, these results suggest that effective regulation at both the population and physiological level may permit the recovery and persistence of giant clams in the face of anthropogenic challenges.