There may be an exception to every rule, but the bull shark, Carcharhinus leucas, can claim to be the exception to at least three. Long famous among ichthyologists for its fierce aggressiveness (bull sharks resemble short, wide great whites; the five shark attacks on humans that occurred on the Jersey Shore between July 1 and July 12 of 1916, and inspired the book and film Jaws, are now thought to be the work of a single C. leucas), it is also the only shark to retain the elasmobranchial ability to not just survive but hunt and thrive in both marine and freshwater environments. C. leucas accomplishes this neat trick through an impressive grab-bag of adaptations, including decreased urea production by the liver, diffusion of urea by the gills, the ability to increase its urine output by twenty-fold, and the ability to switch between active and passive transfer of electrolytes, via Na⁺, K⁺-ATPase, in both the distal tubules and rectal glands. The third unique distinction of C. leucas is its range: bull sharks have been found as far north as Massachusetts and as far south as the Cape of Good Hope, in a band that circumnavigates the globe.

Despite being geographically widespread, however, individual bull sharks are sufficiently rare that in the past they’ve been thought to incorporate over a dozen different species. Specimens from places as diverse as the Ganges, Zambezi, and Mississippi rivers (bull sharks have been found as far up the Mississippi as Illinois) have been subsumed into C. leucas only gradually, usually on the basis of anatomical comparison. For example, the Nicaragua Lake shark, or Carcharhinus nicaraguensis, was declared C. leucas by taxonomical agreement in 1961.

One holdout to this process, because of its particular rarity and presumed population fragility, has been the Vietnamese river shark, Carcharhinus vietnamensis. Gordon et al. now use dye-terminator sequencing to compare the genome of C. vietnamensis sampled in the wild with that of C. leucas, and find that the two are the same. The authors theorize that the Mekong Delta may be the northernmost passage available for bull sharks to cross between the Indian Ocean and the Pacific.

Journ. Exp. Mar. Bio. and Eco. 356, 236 (2008)