Abstract

The coelacanth, Latimeria chalumnae, possesses a blood chemistry that is nearly identical to that of the elasmobranch fishes and contrasts with that of the bony fishes and tetrapods. Especially notable is the retention of high concentrations of urea (377 mM) and of trimethylamine oxide (122 mM), which aid in raising the blood osmolarity (942 mosm/l) to close to that of the sea water environment. These features also characterize other coelacanth body fluids, such as the notochordal fluid, aqueous and vitreous humours, ventricular fluid, coelomic fluid and bile. The tissues of Latimeria, such as muscle, are also characterized by high urea concentrations. The osmotic balance between extracellular fluids and tissues seems to be achieved by the presence of very high tissue levels of trimethylamine oxide (ca. 300 mmol/(kg H₂O)), which counteract the low ion concentrations found in tissue. Renal function in Latimeria seems to involve the selective elimination of certain divalent ions (magnesium, phosphate and sulphate) and of organic substances (glucuronate, creatine and some amino acids). Unlike other ureosmotic fishes, the coelacanth does not possess the renal capacity to reabsorb urea. Evidence suggests that the rectal gland, structurally much like those of chondrichthyians, functions to excrete excess sodium chloride. Since the blood osmolarity of Latimeria is somewhat lower than that of sea water (942 cf. 1026 mosm/l), it is in negative water balance. Some evidence suggests that this is overcome by drinking sea water in a manner similar to that of the teleosts. The problem of whether ureosmotic regulation is homologous in Latimeria and the chondrichthyians is moot, although we favour the possibility that it was independently acquired for the following reasons. (1) Renal urea reabsorbtion is absent in Latimeria although it is crucial to ureosmotic regulation in the chondrichthyians. (2) Internal fertilization and development are necessary concomitants of ureosmotic regulation in fishes and internal fertilization in the two groups is achieved by non-homologous mechanisms. (3) Ureosmotic regulation has been evolved independently in a third vertebrate group, the euryhaline amphibian Rana cancrivora.