Physiology of seawater acclimation in the striped bass, Morone saxatilis (Walbaum)

Abstract

Several experiments were performed to investigate the physiology of seawater acclimation in the striped bass, Morone saxatilis. Transfer of fish from fresh water (FW) to seawater (SW; 31-32 ppt) induced only a minimal disturbance of osmotic homeostasis. Ambient salinity did not affect plasma thyroxine, but plasma cortisol remained elevated for 24h after SW transfer. Gill and opercular membrane chloride cell density and Na(+),K(+)-ATPase activity were relatively high and unaffected by salinity. Average chloride cell size, however, was slightly increased (16%) in SW-acclimated fish. Gill succinate dehydrogenase activity was higher in SW-acclimated fish than in FW fish. Kidney Na(+), K(+)-ATPase activity was slightly lower (16%) in SW fish than in FW fish. Posterior intestinal Na(+),K(+)-ATPase activity and water transport capacity (Jv) did not change upon SW transfer, whereas middle intestinal Na(+),K(+)-ATPase activity increased 35% after transfer and was correlated with an increase in Jv (110%). As salinity induced only minor changes in the osmoregulatory organs examined, it is proposed that the intrinsic euryhalinity of the striped bass may be related to a high degree of "preparedness" for hypoosmoregulation that is uncommon among teleosts studied to data.

Differential regulation of cystic fibrosis transmembrane conductance regulator and Na+,K+-ATPase in gills of striped bass, Morone saxatilis: effect of salinity and hormones

“…Strong antigen recognition with the anti-CFTR antibody was observed in the same positions in the filament section where Na C ,K C -ATPase immunoreactivity occurred, suggesting that these two proteins are located within the same type of cells, presumably chloride cells. Except for faint staining of lamellar pavement cells, immunocytochemistry using the CFTR antibody confirms that there are virtually no lamellar chloride cells in either FWor SW-striped bass (Madsen et al 1994). The similar CFTR staining pattern and intensity in FW and SW gills is very different from the situation in killifish , Hawaiian goby , eel (Anguilla anguilla; Wilson et al 2004) and tilapia (Hiroi et al 2005), where both pattern and intensity of staining change significantly in response to salinity shifts.…”

“…Previous studies have shown a well-developed ability of striped bass to acclimate to salinities ranging from FW to fullstrength SW. Madsen et al (1994) and Tipsmark et al (2004) reported very small and short lasting (!24 h) deflections of ion-osmotic homeostasis, when fish were transferred between FW and SW in either direction. They concluded that rapid, post-transcriptional regulation of ion-transport function was responsible for this ability.…”

“…Smoltification represents a gradual development of hypoosmoregulatory mechanisms while the fish is still in FW; thus, the environmentally regulated process may be seen as a preparatory development, even though such phenomenon may be difficult to explain in evolutionary terms. Yet other euryhaline fish, such as striped bass, Morone saxatilis and killifish, maintain a high level of preparedness to cope with salinity shifts throughout their life (Madsen et al 1994). This strategy has the advantage that salinity fluctuations may be dealt with a minimum of osmotic stress but presumably requires energy input for simultaneously maintaining hyperas well as hypo-osmoregulatory mechanisms.…”

“…The major growth period occurs in SW or BW and reproduction in FW (Raney 1952), but there is no evidence that the ability to live in those variable environments is correlated with a particular developmental stage as seen in smoltifying salmonids. By contrast, the species displays an excellent ability to acclimate to both FW and SW at all life stages (King & Hossler 1991, Madsen et al 1994, Tipsmark et al 2004. This has led us to speculate that regulation of ion transport in the gill of this species may rely more on rapid modulation than on expressional regulation.…”

“…The endocrine regulation of hydro-mineral balance is largely unexplored in striped bass. Plasma cortisol levels increase after FW-SW transfer, but also show a stress-associated increase in the FW-FW sham group (Madsen et al 1994). Insulin-like growth factor-I (IGF-I), a hormone with strong SW-adaptive effects in salmonids (McCormick 2001), has recently been documented to have negative effects on SW acclimation in striped bass (Tipsmark et al 2007).…”