Ontogeny of the Osmoregulatory Capacity of Teleosts and the Role of Ionocytes

Sophie Fridman

doi:10.3389/fmars.2020.00709

Abstract

Whilst osmoregulation in the adult teleost fish has been extensively studied and significant advances have been made in recent years, much less information exists regarding osmoregulation during the early stages of development of teleosts. Adult fish maintain their blood osmolality in a narrow physiological range, i.e. ≈ 280 - 360 mOsm kg-1, through the combined osmoregulatory capabilities of several sites i.e. branchial chambers, skin, digestive system and urinary organs. However embryonic and post-embryonic stages maintain their blood osmolality in a less narrow range of ≈ 240 - 540 mOsmol kg-1 and osmoregulatory capacity is restricted to the cutaneous ionocytes located on the tegument with a transference in osmoregulatory function occurring during the early life stages to the developing digestive tract, the urinary organs and the developing branchial tissues and the ionocytes which they support. This review will discuss the development of osmoregulatory capacity that occurs throughout early life stages of teleosts and its role in conserving physiological homeostasis, focusing on the form and function of related mechanisms, i.e. the ionoregulatory cell or ionocyte, outlining the different roles and functions of different ionocyte types relative to their environment, i.e. freshwater or seawater, their plasticity and discuss spatio-temporal changes in ionocyte distribution that occur during ontogeny.

Highlights

Prunet and Bornancin (1989; p. 92) describe teleost fishes as “an open system in dynamic equilibrium with aquatic surroundings.” As osmoregulators, teleosts are homeo-isosmotic, i.e., able to regulate the concentration of solutes and their total osmolarity of their internal fluids at levels different to their external environment
The ontogenetic development of osmoregulatory capacity, moving from a somewhat limited trans-membrane particle exchange at a cellular level in the embryonic blastular stage, to the fullyfunctioning regulatory tissues in juvenile and adult, such as the renal complex, the gut and the branchial epithelium, is described succinctly by Alderdice (1988; p.225) as a process which displays “continuity, with increasing complexity.”. As part of this ontogenic process, teleost embryos and post-embryonic larvae are able to maintain osmotic and ionic gradients between their internal and external environments due to the presence of numerous extrabranchial, cutaneous ionocytes commonly observed on the abdominal epithelium of the yolk-sac and other body surfaces of fish embryos and larvae
Ayson et al (1994) using transmission electron microscope to examine ionocytes in the yolk-sac membrane of freshwater and seawater-adapted O. mossambicus tilapia embryos and larvae noted a similarity with ionocytes in branchial and opercular epithelium of the adult fish; the cytoplasm of the ionocytes was seen to contain numerous mitochondria and Na+/K+- ATPase located on the extensive and well-developed tubular system

Summary

Introduction

Prunet and Bornancin (1989; p. 92) describe teleost fishes as “an open system in dynamic equilibrium with aquatic surroundings.” As osmoregulators, teleosts are homeo-isosmotic, i.e., able to regulate the concentration of solutes and their total osmolarity of their internal fluids at levels different to their external environment.

Results

Conclusion