Histioteuthis heteropsis

The eyes of H. heteropsis are dimorphic both in size and in lens pigmentation for specialized vision in the ocean's mesopelagic zone (200-1000 meters below the ocean surface).[3] The different properties of the squid's eyes allow it to see a variety of different light sources present in its habitat, primarily downwelling sunlight and bioluminescence.[3] H. heteropsis hatchlings are born with identical eyes of the same size and pigmentation.[3] As they develop, the left eye becomes larger and more pigmented.[3] By adulthood, the left eye can grow to be over twice the size of the right eye and has a distinct yellow lens pigmentation.[3] In a 2017 study conducted by Kate Thomas, Bruce Robinson, and Sönke Johnsen, it was found that the large left yellow eye is oriented upwards for viewing objects in dim sunlight and the smaller right blue eye is oriented downwards for viewing bioluminescence.[3] While the larger eye can detect bioluminescence fairly well, the smaller eye struggles to view black objects in dim light.[3]

Small photophores, which are glandular, light-producing organs, are found in high concentrations along the entire body of H. heteropsis.[7] The photophores are more dense near the anterior head region of the squid and become smaller and farther apart near the posterior end.[7] Apart from  H. meleagroteuthis, H. heteropsis is the only member in the family Histioteuthidae to display such small and highly concentrated photophores.[7] H. heteropsis mainly uses its photophores for two purposes: counter-illumination and concealing prey.[8] In counter-illumination, the squid produces blue light from its photophores so that, when viewed from above, it blends in with the downwelling sunlight and effectively erases its silhouette.[8] This type of bioluminescence is characterized by consistent light production, in contrast to bioluminescence used for concealing prey, which is characterized by short bursts of intense light or "flashes".[8] Although the exact purpose for this particular use of bioluminescence is unknown, the heavily pigmented oral cavity and interbrachial membrane of H. heteropsis and some other squids suggests that luminescence by the prey threatens the predator in some way.[8]

H. heteropsis males do not possess a hectocotylus as is common in other mesopelagic squid species, although they do have a penis which is used to transfer spermatophores to the female during copulation.[4] During maturation, the penis grows until it can extend out of the mantle cavity.[4] The spermatophores of H. heteropsis have short sperm masses and are stored in a looped Needham's sac.[7] Female H. heteropsis squids have a singular ovary where the oocytes are developed.[9] Although not much is known about female reproductive systems in H. heteropsis specifically, other members of the family Histioteuthidae show significant oocyte resorption (oosorption) during maturation.[10]

H. heteropsis searches for prey using visual clues and strikes forward at the prey using their tentacles.[9] The tentacles then bring the prey within range of the arms, which hold the prey in place with suckers.[9] H. heteropsis, like most squids, bite immediately into the fleshy parts of the prey with their beak and release the leftover corpse.[9]

Very little is known about mating and courtship in H. heteropsis, although it is likely that these behaviors occur in deeper waters.[4] Spawning may occur in deep-midwater or near-surface waters, but it is still unclear.[4]

Inking developed as an important protective mechanism for coleoid cephalopods after losing their ancestral external shell.[5] However, inking is metabolically costly since it requires the discharge of both melanin and mucus.[5] Because of this, H. heteropsis typically only inks when approached closely or touched.[5] Researchers Stephanie Bush and Bruce Robinson observed that H. heteropsis most often produces an ink cloud categorized as "pseudomorphs", which are "dense blobs of ink the approximate shape and size of the individual releasing them."[5] In all instances of inking, H. heteropsis specimens lingered in the ink cloud unless further provoked.[5]