Cusk-eel

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 8,370 m (27,460 ft).[1][2]

Cusk-eels are generally very solitary in nature, but some species have been seen to associate themselves with tube worm communities.[3] 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.[3] Cusk-eels generally feed nocturnally, preying on invertebrates, crustaceans, and other small bottom-dwelling fishes.

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.[3]

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 2 m (6.6 ft) in length, but most species are shorter than 1 m (3.3 ft). 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.[4] 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 barbel-like organ below the mouth. In true eels by contrast, the ventral fins are never well-developed and usually missing entirely.[5] Cusk-eels have large mouths relative to their heads, with the upper jaw reaching beyond the eye, and paired nostrils on either side of the head. In cusk-eels, scales are potentially absent, and when present, they are small.[4]

Unlike their close relatives, the viviparous brotulas of the family Bythitidae, cusk-eel species are egg-bearing, or oviparous, organisms. While the specifics of the eggs of the family Ophidiidae are unknown, they are believed to be either spawned as individual, free-floating eggs in the open water or are placed in a mucilaginous raft, which will float for several days until they hatch into cusk-eel larvae. These larvae live amongst the plankton relatively close to the water's surface[1] and are believed to be able to control their metamorphosis into adult cusk-eels, allowing them to disperse over greater distances into less utilized habitats and reduce competition in concentrated areas.[3]

While a few species are fished commercially – most notably the pink cusk-eel, Genypterus blacodes – and several species of the order Ophidiiformes are listed as vulnerable, not enough information has been gathered about Ophidiidae as a whole to determine their conservation status.

The cusk-eel family contains about 240 species, grouped into 50 genera:[6]
Subfamily Brotulinae

• Genus Brotula – typical brotulas

Subfamily Brotulotaenilinae

• Genus Brotulotaenia

Subfamily Neobythitinae

Subfamily Ophidiinae

• 
  Brotula multibarbata (Brotulinae)
• 
  Bassozetus normalis (Neobythitinae)
• 
  Lamprogrammus niger (Neobythitinae)
• 
  Monomitopus agassizii (Neobythitinae)
• 
  Porogadus miles (Neobythitinae)
• 
  Tauredophidium hextii (Neobythitinae)
• 
  Otophidium omostigma (Ophidiinae)