Ionocytes as an Early Life Mechanism of High CO2 Tolerance.

Schwemmer, Teresa G.¹*, Hannes Baumann² , Christopher S. Murray³, Janet A. Nye⁴ ,¹School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, ²University of Connecticut, Department of Marine Sciences, Groton, CT, ³Woods Hole Oceanographic Institution, Woods Hole, MA, ⁴Institute of Marine Sciences, University of North Carolina at Chapel Hill, Morehead City, NC

An important part of understanding how fish survival, growth, and reproduction are affected by ocean acidification and co-occurring stressors is to quantify physiological mechanisms by which fish respond to them early in life. Embryos and larvae have ionocytes, cells that use ATP to transport ions, on the skin surface of the body until gills develop. We hypothesized that early life stages of the estuarine fish Menidia menidia respond to elevated carbon dioxide (CO2) by increasing the density of ionocytes on epithelial surfaces for enhanced acid-base balance. To test our hypotheses, we reared M. menidia embryos and larvae in factorial CO2 and temperature treatments and stained their ionocytes to quantify ionocyte density per unit of skin surface area. A significant interactive effect was found in which embryonic ionocyte abundance increased with temperature at high CO2 but was unaffected by temperature at ambient CO2. In addition to highlighting the importance of multistressor experiments, this suggests that warmer temperatures could enhance internal pH regulation, while also increasing energy spent on acid-base balance. In larvae sampled the day of hatching, the temperature and CO2 interaction remained but ionocytes decreased, rather than increased, with increasing temperature under high CO2. When larvae reached about 10mm total length, and gill development had progressed, elevated CO2 slightly increased ionocyte density but temperature had no effect. Interestingly, results varied between experiments conducted at various times throughout the spawning season with different wild-caught parents, suggesting that environmental conditions before spawning may influence acidification tolerance of offspring.