Electrolyte Composition of Anguilliform Leptocephali from the Straits of Florida

Premetamorphic, anguilliform leptocephalous larvae were analyzed for total body content of water, nitrogen, and minerals. Sodium chloride of unusually high concentration is the major inorganic electrolyte of the leptocephalous body. Internal salinity and osmolality of small larvae closely approach the values for sea water. Ionic regulation and the development of an osmotic concentration gradient between body fluids and sea water are evident in large leptocephali near to metamorphosis. We suggest that the newly hatched leptocephalus is capable of minimal ionic regulation and only gradually develops the capacity to maintain the composition of its internal environment distinctly different from sea water. A prolonged period for maturation of osmoregulatory processes may be a physiologic feature that distinguishes the leptocephalus from other larval teleostean fishes.