Their most distinctive features are their extremely elongate tentacles—which retract into membranous lateral sheaths of the fourth (and largest) arms—and their very large ovate fins, which may occupy up to 80% of the mantle length in some species.

Members of this family have scarcely been observed in life, but at least two species (Mastigopsis hjorti and Magnoteuthis magna) are known to hover above the ocean bottom in a vertical orientation, the head pointing downward.

The squid use their large fins to maintain this orientation and use both their extended tentacles like fly paper, held rigidly at a constant distance apart: this has been termed the "tuning fork" position.

With many species represented by single, damaged, and juvenile specimens, this group is still creating controversy in the taxonomic community.

Salcedo-Vargas and Okutani (1994) reclassified the family and eliminated the genus Echinoteuthis, added two subgenera, and reduced the number of species to eight.