Comparisons of two types of teleostean pseudobranchs, silver moony (Monodactylus argenteus) and tilapia (Oreochromis mossambicus), with salinity-dependent morphology and ion transporter expression

There are essentially four different morphological types of pseudobranchs in teleosts, including lamellae-free, lamellae semi-free, covered, and embedded types. In the euryhaline silver moony (Monodactylus argenteus), the pseudobranch belongs to the lamellae semi-free type, which is characterized by one row of filaments on the opercular membrane and fusion on the buccal edge. The pseudobranchial epithelium of the moony contains two types of Na⁺, K⁺-ATPase (NKA)-rich cells: chloride cells (CCs) and pseudobranch-type cells (PSCs). Our results revealed increased expression of NKA, the Na⁺, K⁺, 2Cl⁻ cotransporter (NKCC), and the cystic fibrosis transmembrane conductance regulator (CFTR) for Cl⁻ secretion and CCs profiles in the pseudobranchs of seawater (SW)-acclimated silver moonies, which indicates the potential role of pseudobranchs containing CCs in hypo-osmoregulation. In contrast, the pseudobranch of the Mozambique tilapia (Oreochromis mossambicus) belongs to the embedded type, which is covered by the connective tissues and only contains PSCs but not CCs. No sign of NKCC and CFTR-immunoreactivity (IR) was found in the pseudobranchs of SW and freshwater (FW) tilapia. However, higher NKA protein expression and larger sizes of NKA-IR PSCs were found in the pseudobranchs of FW-acclimated tilapia. Moreover, in the FW-acclimated moony, NKA-IR PSCs also exhibited higher numbers and larger sizes than in the SW individuals. Taken together, similar responses in low-salinity environments in different types of pseudobranchs indicated that the salinity-dependent morphologies of PSCs might be involved in critical functions for FW teleosts.