The Absorption and Excretion of Water and Salts by Marine Teleosts

Abstract

It is a peculiar and interesting fact that marine teleosts excrete urine which is isotonic or hypotonic with respect to the blood, and therefore of considerably lower osmotic pressure than sea water (Rodier, 1899; Bottazzi, 1906; Dekhuyzen, 1905; Burian, 1908, 1910). From this, it appears that these animals are faced with the task of concentrating the surrounding ocean in order to obtain water for the formation of urine, a task which can only be performed at the expenditure of energy in osmotic work.' This circumstance might be expected to result in profound differences in the water economy of marine fish as compared with fresh water and terrestial animals to which water is available ad k&turn. The answer to the question of what tissue or tissues actually do the work of separating from the sea water the osmotically dilute solution destined to be excreted as urine appears to rest in the determination of whether the renally excreted water is initially absorbed from the intestine or enters the body through the gills. The work to be reported here leads us to the conclusion that water absorption occurs by way of the intestine; but, surprisingly, it is not in the intestine that the osmotic work is done, for the residue formed there as absorption proceeds is actually more dilute than the ingested sea water, and tends to approach the blood in tonicity. This osmotic dilution results from the fact that Na, K and Cl are absorbed from the intestine, along with a considerable fraction of the water, leaving behind only the poorly absorbed Mg and SO4 in highly concentrated state. During this process of concentration some Mg and SO4 are abosrbed, to be subsequently excreted in the urine. The bulk of the Na, K and Cl, together with the bulk of the water absorbed from the intestine, never appear in the urine, however; they are excreted by routes other than the kidneys-lThis problem was recognized by Dekhuyzen (1905) who remarks '(.. ,. . la richesse relative en eau de cette urine prove plutot que les poissons marine resorbent de l'eau, on si l'on prefer-e une solution diluie effectuant ainsi un travail contre la pression osmotique en consommant de l'energie chimique ." 480

Relative Potency of Three Homologous Natriuretic Peptides (ANP, CNP and VNP) in Eel Osmoregulation

“…The anterior/middle and posterior intestine were divided at the sphincter separating the two. The fluid was transferred to a chilled tube, and centrifuged at 10,000xg for 5 min at 4 ° C. concentration as a marker since it is known that Mg 2+ ions are scarcely absorbed by the teleost intestine (Smith, 1930;Parmalee and Renfro, 1983).…”

Effect of urea introduced into feed on structural changes and mucopolisacharides content in branchial epithelium of rawbow trout (Salmo gairdneri Richardson)

“…Gill function was numerously tested by Smith (1930), Keys and Willmer (1932), Zaccone (1972) and the authors agreed on gills being, due to their structure and fragile epithelium, deeply involved in ion exchange. Intensive slime excretion is characteristic gill's reaction to any change around.…”

“…The adaptative abilities result from high functioning efficiency and platicity of the osmoregulatory organ -gills, and of its respiratory epithelium, in particular. (Smith, 1930;Cykowska, 1978). Urea within the fish body is being included into aminoacids cycles (Kniazieva, 1970), with ammonium as a by-product of it.…”

“…Smith (1929Smith ( , 1930Smith ( , 1931 pointed out to its role in the osmotic pressure regulation in Teieostei and Selachii, although he admitted the extranephritic salt (NaCl, KCl) excretion through the gill aparratus to be highly energetic. Philpott (1966) suggesterl the acid mucopoli saccharides might take part in ion exchange in fish and Whitelaw (1975) proved its share in the regulation of ammonium excretion.…”

Electrolyte changes of serum and muscle, and related mortalities in maturingAnguilla rostrata migrating down the St. Lawrence Estuary (Canada)

“…Passage to salt water results in the loss of water through the gills. This is balanced by the ingestion of salt water, the production of a scanty and highly mineralized urine (Sharratt et al, 1964) and the excretion of salts through the gills (Bellamy, 1961;Motais, 1966;Smith, 1930). Part of the excess sodium and chloride is excreted through Na/Na and C1/C1 exchanges in the gills.…”