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Members of the genus are mostly small to medium sized squids of under 20 cm in mantle length as adults. The largest, however, O. borealijaponica, reaches a size of 350 mm ML (Kubodera et al. 1998). Species are found mostly in tropical and subtropical waters throughout the world's oceans although they are also common in high latitudes of the North Pacific where O. borealijaponica reaches subarctic waters. One of the diagnostic features of Onychoteuthis species is the presence of a bilobed photophore on the ventral surface of each eyeball as seen in the photograph below right. Another diagnostic feature is the presence of two photophores (arrows) on the viscera as can barely be seen through the mantle in the photograph below left.
Figure. Ventral views of Onychoteuthis sp. showing photophores. Left - Arrows indicate visceral photophores (ocular photophores also visible), juvenile. Right - Ocular photophores of a different squid. Both squid from Hawaii waters. Photographs by R. Young.
Species of Onychoteuthis are commonly seen in surface waters at night and are often collected by dipnet at nightlight stations. Only young squid are normally captured in standard midwater trawls; apparently older squids avoid the trawls. One non-standard method of collection results from individuals being able to leap high out of the water and, sometimes, landing on the deck of a ship.
An onychoteuthid ...
- with photophores (unique character).
- with gladius visible beneath skin in dorsal midline.
- with 8-13 occipital folds.