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Mastigoteuthis agassizii is a species of whiplash squid in the Mastigoteuthidae family.

Distribution
Mastigoteuthis agassizii can be found in the bathypelagic zone throughout the Northern Atlantic Ocean to the Subarctic. Mastigoteuthidae are among the most abundant family of bathyal squid in the world, and Mastigoteuthis agassizii is the most abundant species of Mastigoteuthid to occur in the temperate North Atlantic Ocean. Typically, this species is distributed from about 30° N to 50° N in the Atlantic Ocean. It was thought that the northernmost range of Mastigoteuthis agassizii was at 60° N, but two specimens caught north of this border at around 65° N suggests that these squid occupy a wider range. These two specimens were caught at bottom depths around 1300 meters and bottom temperatures of around 3° C. Due to bottom trawls catching more specimens than pelagic trawls, along with submersible observations, Mastigoteuthis agassizii is suggested to be a “bottom-associated” squid, drifting just above the ocean floor ,assisted by their large fin. As this species matures, they tend to move deeper into the ocean.

Their main predators in the North Atlantic include Cuvier’s beaked whales and northern bottlenose whales, while predators in the mid Atlantic include sperm whales.

Species
Despite being such an abundant species, there is still much that is unknown about Mastigoteuthis agassizii. MAR-ECO, an international study aimed to describe animals inhabiting the northern mid-Atlantic region, provided valuable information regarding cephalopods in a two-leg cruise along the Mid-Atlantic Ridge in 2004. Unfortunately, most Mastigoteuthids collected from the cruise were badly damaged, making identifying species as Mastigoteuthis agassizii a taxonomic challenge. Based on MAR-ECO material, four similar species of Mastigoteuthis (M. agassizii, M.grimaldi, M.schmidti, and M.flammea), were all synonymized under M. agassizii.

Morphology
There is speculation that there are boreal and tropical forms of Mastigoteuthis agassizii. No specimens with a mantle length of 80 mm or larger with its tentacles intact have been discovered in northern temperate to boreal waters. Additionally, the presence of a weak antitragus is more common in boreal specimens compared to tropical specimens, which do not have them at all.

In two specimens found in the Subarctic Atlantic, the anterior part of the mantle was cylindrical, while the posterior formed a tapered narrow cone. Their fins were heart-shaped and rounded anterior lobes were present. In the collected samples, the integument, while damaged, was covered by small, white photophores with an average diameter of 0.25 mm. Chromatophores formed rings with a mean diameter of 0.07 - 0.08 mm. Both specimens had funnel pockets, but only one specimen, a female, had its head intact. The head was cylindrical with no eyelid photophores. Suckers on its arms had a diameter of 0.75 mm in proximal parts and 0.69 mm distally. On the suckers, there were 11 - 12 teeth and 14-16 teeth respectively. Teeth on the proximal suckers were blunter than those on the distal suckers. Its 2.9 mm lower beak had a slightly curved rostrum while the slightly longer 3.0 mm upper beak had a curved rostrum that curved more at the tip. The radula had seven teeth per transverse row as well.

Mating
For the Subarctic specimens, males were found to have the distal part of their penis to be enlarged, divided into two valves, and pigmented. While spermatophore transfer is still obscure in deep sea squids, morphological similarities with families of squid such as Pholidoteuthidae, a family of deep sea squid, which can elongate their penis, suggest that Mastigoteuthis agassizii can have their reproductive organ protrude out of their mantle cavity and transfer spermatophores directly through the penis. This method could potentially substitute their lack of a hectocotylus.

Reproductive Strategy
Potential fecundity was estimated as 8,000 to 16,000, although a specimen collected in the Subarctic Atlantic was found with fecundity of approximately 23,000 oocytes. Mastigoteuthis agassizii have a synchronous ovulation pattern, which is associated with spawning of a single egg mass which is then brooded. However, due to studied females indicating living for an extended amount of time after a spawning event as well as no evidence of egg mass brooding being found on the arms, egg masses and hatchlings in Mastigoteuthid families may be pelagic. Despite having synchronous ovulation patterns, egg release may occur over an extended period of time.