Morphology

Octopoteuthids are generally large squid with a conical mantle up to 170 cm in length. Bordering the mantle are two very large rhomboidal fins. These are the main source of propulsion for octopoteuthid squid; they relying less heavily on jet propulsion than other taxa.

Octopoeuthis deletron and Taningia danae (Tolweb.org).

At the center of the arm crown are two large chitinous beaks that are used to chew up food. These are indigestible, and the recovery of them from predator stomachs has become a very important source of distribution data for the family.

Series of lower Octopoteuthis spp. beaks (Jesse Kelly).

Arms are what really characterize the family. Some important features include:

• Only 8 arms, no tentacles in adults. Young have two tentacles but they're lost with growth.
• Two rows of hooks on all arms pointing down-arm towards mouth.
• Photophores borne on armtips. Believed to occur in species-specific patterns. Important in species identification.
• Fragile arm tips, often breaking off before examination. Believed to be defensive mechanism: following severance tips twist violently and flash brightly to distract predator. Makes species identifications difficult.

Hooks of O. deletron and a young T. danae in an aquarium (Tolweb.org).

Photophores are also borne on the ventral surface of the mantle and on the visceral mass. Ventral photophore numbers and positions are also used in species identifications.

Tolweb.org

An incredible advancement on the armtip photophores is found in Taningia danae. Borne at the tips of arm pair II are the largest photophores known in the animal kingdom! Equipped with an eye-lid mechanism, T. danae can control the length of flashes simply by opening and closing it. They have been observed to emit light in short flashes or long glows Kubodera et al., 2007. The former is associated with prey capture, and is believed to be a final illumination of the target before seizure. Long glows are thought to be a form of intra-specific communication, such as a means for locating mates.

T. danae terminal photophores (Tolweb.org).

(TONMO.com).