Chromatophore arrangement and photophore formation in the early development of swordtip squid Uroteuthis (Photololigo) edulis

The commercially valuable swordtip squid Uroteuthis (Photololigo) edulis has recently experienced a decline, indicating a need for its management. However, very little is known about its morphology during early developmental stages. Therefore, in this report, we describe the chromatophore arrangement and photophore formation in laboratory-reared specimens ranging from 2.2 to 13.6 mm in dorsal mantle length (DML). We observed seven characteristic chromatophore arrangements and a pair of photophores on the ventral surface of the ink sac in the five developmental stages after hatching, which allow U. edulis hatchlings and paralarvae to be distinguished from other sympatric loliginid squids around the coastal area of central Japan. We also found an increase in the number of other chromatophores and rapid growth of the photophores between 7 and 9 mm in DML, which marks the transition between the paralarva and juvenile stages. These findings will be useful for identifying U. edulis hatchlings and paralarvae from natural plankton samples, thus contributing toward research on the spatial distribution, seasonal migration, and predicted spawning grounds of this species.