Freshening effect on the osmotic response of the Antarctic spiny plunderfish Harpagifer antarcticus.

Global warming is having a significant impact around the world, modifying environmental conditions in many areas, including in zones that have been thermally stable for thousands of years, such as Antarctica. Stenothermal sedentary intertidal fish species may suffer due to warming, notably if this causes water freshening from increased freshwater inputs. Acute decreases in salinity, from 33 down to 5, were used to assess osmotic responses to environmental salinity fluctuations in Antarctic spiny plunderfish Harpagifer antarcticus, in particular to evaluate if H. antarcticus is able to cope with freshening and to describe osmoregulatory responses at different levels (haematological variables, muscle water content, gene expression, NKA activity). H. antarcticus were acclimated to a range of salinities (33 as control, 20, 15, 10 and 5) for 1 week. At 5, plasma osmolality and calcium concentration were both at their lowest, while plasma cortisol and percentage muscle water content were at their highest. At the same salinity, gill and intestine Na+ -K+ -ATPase (NKA) activities were at their lowest and highest, respectively. In kidney, NKA activity was highest at intermediate salinities (15 and 10). The salinity-dependent NKA mRNA expression patterns differed depending on the tissue. Marked changes were also observed in the expression of genes coding membrane proteins associated with ion and water transport, such as NKCC2, CFTR and AQP8, and in the expression of mRNA for the regulatory hormone prolactin (PRL) and its receptor (PRLr). Our results demonstrate that freshening causes osmotic imbalances in H. antarcticus, apparently due to reduced capacity of both transport and regulatory mechanisms of key organs to maintain homeostasis. This has implications for fish species that have evolved in stable environmental conditions in the Antarctic, now threatened by climate change.

Ammonia excretion and expression of transport proteins in the gills and skin of the intertidal fish Lipophrys pholis.

Intertidal pools are intensely challenging environments, due to rapid and extreme fluctuations in water conditions during the tidal cycle. Emersion is another challenge intertidal fishes may face. Mechanisms of ammonia excretion and ion regulation were studied in the resident amphibious blennid Lipophrys pholis. The ammonia transporters Rhcg1 and Rhcg2 were cloned and characterized. Fish were challenged for 24h to 1) emersion, 2) fresh water (FW), and 3) high environmental ammonia (HEA; 1mM NH4Cl), or 4) ammonia loading (1.5µmol/g NH4HCO3). When air exposed, L. pholis maintained aquatic ammonia excretion rates (JAmm) while branchial Na(+)/K(+)-ATPase (NKA) activity increased, but no changes at the protein or mRNA levels of transporters were noted. In FW, JAmm decreased and osmotic problems were encountered. Skin NKA activity decreased, branchial Rhcg2, and skin Rhcg1 and Rhcg2 increased. Exposure to HEA only increased branchial Rhcg2 levels. Although internal ammonia loading only led to a modest non-significant increase in JAmm, skin NKA (activity and α-subunit), carbonic anhydrase protein levels, and branchial Rhcg1 levels increased. In summary, variable responses were observed involving both gill and skin but given the instability of its habitat, the constitutive expression of transporters is likely also of importance.