The longfin escolar, Scombrolabrax heterolepis, also known as the black mackerel, is a widespread but uncommon deep sea fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 that presents some difficulties for taxonomy

Taxonomy

Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

.

It is placed in its own family Scombrolabracidae, but the family's placement in the suborders of Perciformes

Perciformes

The Perciformes, also called the Percomorphi or Acanthopteri, is one of the largest orders of vertebrates, containing about 40% of all bony fish. Perciformes means perch-like. They belong to the class of ray-finned fish and comprise over 7,000 species found in almost all aquatic environments...

 has included Scombroidei

Scombroidei

Scombroidei is a suborder of the Perciformes, the largest order of fish. The suborder includes the barracuda, tuna, mackerel and swordfish....

, Percoidei

Percoidei

Percoidei is one of eighteen suborders of bony fish in the order Perciformes. Many commercially harvested fish species are contained in this suborder, including the snappers, jacks, whitings, groupers, bass, perches and porgies.-Divisions:...

, and Trichiuiroidei, while some authors place it in its own suborder Scombrolabracoidei.

The fish bears some resemblance to members of Gempylidae, but has protrusible premaxilla

Premaxilla

The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....

, serrated opercle and preopercle, and a spur on the lowest principal caudal ray, all of which are characteristic of percoids. Its color varies from black to dark brown. This fish is known to grow to 30 cm in length. The body is covered in soft scales which easily slough off when handled. The eyes are large and there is usually a single pair of elongated teeth in the middle of the top jaw.

The longfin escolar is unique among fishes for having several of its vertebrae hollowed out and filled by evaginations of the gas bladder

Gas bladder

The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming...

. It is most often encountered as a bycatch species in pelagic longline fisheries.