Date of this Version
Nineteen sharks collected from freshwater at several points on Lake Nicaragua and the Rio San Juan were all of the same species and indistinguishable from marine Carcharhinus leucas of the Atlantic. Minor differences previously thought to separate C. leucas from the lake shark proved to be invalid. Females had slightly longer gill slits, somewhat greater breadth of pelvic fin, and a longer abdominal region than males. Clasper length indicated that the onset of sexual maturity in males occurs when they are between 1,600 and 1,700 mm in total length.
The classical theory or origin of the sharks from Pacific ancestry no longer appears tenable. An Atlantic origin is indicated by (a) strong evidence that the taxonomic affinities not only of the shark, but also of the sawfish and tarpon, are more pronouncedly with Atlantic relatives than with those of the Pacific, (b) recent evidence that Lake Nicaragua may never have been a part of the Pacific but opened originally to the Atlantic, (c) the fact that Lake Nicaragua drains into the Caribbean Sea by a large, broad river, and (d) all three of the large, otherwise marine types of fish occurring in the lake are species that habitually congregate in brackish water and frequently move up rivers.
Evidence that the sharks are not landlocked includes the following facts: (a) they are abundant the full length of the river, (b) barges drawing from one to three feet of water regularly negotiate the river (including all of the rapids) in both directions, (c) sharks were observed both below and above the three major rapids as well as actually in the rapids, most of them headed upstream.