The cusk-eel family, Ophidiidae, is a group of marine bony fishes in the Ophidiiformes order. The scientific name is from the Greek ophis meaning "snake", and refers to their eel-like appearance. True eels, however, diverged from other ray-finned fish during the Jurassic, while cusk-eels are part of the Percomorpha clade, along with tuna, perch, seahorses, and others. Distribution Cusk-eels are found in temperate and tropical oceans throughout the world. They live close to the sea bottom, ranging from shallow water to the hadal zone. One species, Abyssobrotula galatheae, was recorded at the bottom of the Puerto Rico trench, making it the deepest recorded fish at . Ecology Cusk-eels are generally very solitary in nature, but some species have been seen to associate themselves with tube worm communities. Liking to be hidden when they are not foraging, they generally associate themselves within muddy bottoms, sinkholes, or larger structures that they can hide in or around, such as caves, coral crevices, or communities of bottom-dwelling invertebrates, with some parasitic species of cusk-eel actually living inside of invertebrate hosts, such as oysters, clams, and sea cucumbers. Cusk-eels generally feed nocturnally, preying on invertebrates, crustaceans, and other small bottom-dwelling fishes. Phylogeny Due to the inconsistencies in specific morphological characteristics in closely related species, attempts to use different characters, such as the position of pelvic fins, to classify Ophiididae into distinct families has proven highly unsatisfactory. Overall, Ophidiidae are classified based on whether or not they practice viviparity and the structures they contain that are associated with bearing life. Characteristics Cusk-eels are characterized by a long, slender body that is about 12-13 times as long as it is deep. The largest species, Lamprogrammus shcherbachevi, grows up to in length, but most species are shorter than . Their dorsal and anal fins are typically continuous with the caudal fin (with exception to a few species), forming a long, ribbon like fin around the posterior of the cusk-eel’s body. This caudal fin will often be seen to be reduced to a fleshy or bony point, especially when confluent with the dorsal and anal fins. The dorsal fin to anal fin ray ratio is approximately 1.5:1, leading to the dorsal fin typically being longer than the anal. The pectoral fins of cusk-eels are typically longer than the length of their head. Unlike true eels of the order Anguilliformes, cusk-eels have ventral fins that are developed into a forked