ATPase activity, proton gradients and proton secretion inhibition during experiments with Sepia officinalis and Sepioteuthis lessoniana

The constraints of an active life in a pelagic habitat led to numerous convergent morphological and physiological adaptations that enable cephalopod molluscs and teleost fishes to compete for similar resources. Here, we show for the first time that such convergent developments are also found in the ontogenetic progression of ion regulatory tissues; as in teleost fish, epidermal ionocytes scattered on skin and yolk sac of cephalopod embryos appear to be responsible for ionic and acid-base regulation before gill epithelia become functional. Ion and acid-base regulation is crucial in cephalopod embryos, as they are surrounded by a hypercapnic egg fluid with a Pco2 between 0.2 and 0.4 kPa. Epidermal ionocytes were characterized via immunohistochemistry, in situ hybridization, and vital dye-staining techniques. We found one group of cells that is recognized by concavalin A and MitoTracker, which also expresses Na+/H+ exchangers (NHE3) and Na+-K+-ATPase. Similar to findings obtained in teleosts, these NHE3-rich cells take up sodium in exchange for protons, illustrating the energetic superiority of NHE-based proton excretion in marine systems. In vivo electrophysiological techniques demonstrated that acid equivalents are secreted by the yolk and skin integument. Intriguingly, epidermal ionocytes of cephalopod embryos are ciliated as demonstrated by scanning electron microscopy, suggesting a dual function of epithelial cells in water convection and ion regulation. These findings add significant knowledge to our mechanistic understanding of hypercapnia tolerance in marine organisms, as it demonstrates that marine taxa, which were identified as powerful acid-base regulators during hypercapnic challenges, already exhibit strong acid-base regulatory abilities during embryogenesis.

Effects of ocean acidification on the calcification of otoliths of larval Atlantic cod Gadus morhua

The growth and development of the aragonitic CaCO3 otoliths of teleost fish could be vulnerable to processes resulting from ocean acidification. The potential effects of an increase in atmospheric CO2 on the calcification of the otoliths were investigated by rearing Atlantic cod Gadus morhua L. larvae in 3 pCO2 concentrations-control (370 µatm), medium (1800 µatm) and high (4200 µatm)-from March to May 2010. Increased otolith growth was observed in 7 to 46 d post hatch (dph) cod larvae at elevated pCO2 concentrations. The sagittae and lapilli were usually largest in the high pCO2 treatment followed by the medium and control treatments. The greatest difference in mean otolith surface area (normalized to fish length) was for sagittae at 11 dph, with medium and high treatments being 46 and 43% larger than the control group, respectively. There was no significant pCO2 effect on the shape of the otoliths nor were there any trends in the fluctuating asymmetry, defined as the difference between the right and left sides, in relation to the increase in otolith growth from elevated pCO2.