Centrolophus niger, known commonly as the black ruff or rudderfish, is a deep-water, predatory marine fish belonging to the family Centrolophidae. It is a cosmopolitan species, found in the temperate waters of the Atlantic, Indian, and Pacific Oceans, typically residing at depths between 300 and 700 meters, though it often migrates toward the surface at night to feed. The genus name Centrolophus comes from the Greek kentron ("spine" or "center") and lophos ("crest"), referring to the spiny structure of its dorsal fin, while the species epithet niger is Latin for "black," describing the fish's dark, slate-grey to black coloration. Historically, it has been known by fishermen as a "rudderfish" because of the species' documented behavior of following ships and drifting objects, acting like a rudder behind the vessel.

The fish possesses a moderately elongated, compressed body covered in small, cycloid scales and is equipped with a distinct, long dorsal fin that runs nearly the length of its back. It can reach lengths of up to 100 centimeters but is commonly encountered at smaller sizes. Centrolophus niger is a highly mobile, opportunistic hunter that feeds primarily on jellyfish, salps, small cephalopods, and crustaceans. Due to its life cycle in the "twilight zone" of the ocean, it has adapted to survive in lower-oxygen, high-pressure environments, resulting in a physiology characterized by a high lipid content within its muscle tissue, which aids in buoyancy control.

The black ruff is highly regarded in specific regional cuisines, particularly in the Mediterranean, the Azores, and parts of Japan, where it is often sold as a high-quality "whitefish." Its flesh is exceptionally delicate, succulent, and firm, with a clean, mild flavor that is often compared to butterfish or swordfish. Because of its high natural oil content, it is ideally suited for grilling, pan-searing, or smoking. It is a prized catch for commercial longline fisheries, though its deep-water habitat makes it a less common market item compared to surface-dwelling species. However, black ruff contains high levels of wax esters called gempylotoxins can cause keriorrhea, oily orange stool, when consumed excessively.

Nutritionally, Centrolophus niger is an excellent source of high-biological-value protein and is exceptionally rich in omega-3 polyunsaturated fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The lipid profile of the fish is notable for its high concentration of monounsaturated fats, which contribute to its buttery texture. It also provides significant amounts of fat-soluble vitamins, such as Vitamin A and Vitamin D, and minerals including selenium and iodine. In the context of gastronomic ethnobiology, the rudderfish is a prime example of a "deep-sea luxury" that has gained traction as interest in sustainable, deeper-water resource utilization has grown.