Bar jack

Photo: marie verzaux · CC BY-NC

Bar jacks are among the most behaviorally versatile fish in the Caribbean. In open water they form fast-moving mid-water schools that hunt anchovies and small fish; near seagrass beds they shift to hunting alone, and they are famous for a parasitic-cleaning relationship with the southern stingray — swimming alongside stingrays, coloring their underside dark, and picking up prey disturbed by the ray's excavations. The bar jack's ability to shift color rapidly between silver-and-blue in schools to dark-and-barred in hunting mode is one of the most dramatic color changes in any reef fish.