Exploring acid secretion machinery in embryos of marine mollusks cephalopod under hypercapnic environment

Abstract

The present studies investigated effects of elevated seawater pCO2 on acid secretion mechanism in early life stages of squid Sepioteuthis lessoniana. Although cephalopods have already proved as powerful acid‐base regulators, which tightly regulate their pH homeostasis; however the acid‐base regulatory machinery is virtually unexplored. Thus, a major focus of this work has been dedicated to the mechanistic understanding of proton secretion pathways in epithelia of squid embryos during environmental hypercapnia. We cloned a Na+/H+ exchanger 3 (slNHE3), V‐type H+‐ATPase (slVHA) and Rhesus protein (slRhP) from squid, S. lessoniana, which are hypothesized to represent key players in proton secretion pathways of marine organisms. In situ hybridization and immunohistochemical analyses demonstrate the expression of slNHE3 and slVHA in epidermal ionocytes, which are rich in mitochondria, and thus, can be referred to as mitochondria‐rich cells (MRCs). Gene expression analysis demonstrates that putative acid‐base transporters, including slNHE3, slVHA and slRhP are up‐regulated in response to environmental hypercapnia (pH 7.4; 0.46 kPa pCO2) in body and yolk tissues. This observation is further supported by selective ion electrode measurements, which demonstrate increased proton gradients on the yolk epithelium of CO2 treated embryos. The present work suggests that in convergence to teleosts, cephalopod embryos have evolved sophisticated physiological mechanisms to cope with acid‐base disturbances during their oviparous development.