Absence of a Hectocotylus and Associated Mating Behaviour in the Paintpot Cuttlefish Ascarosepion tullbergi

A hectocotylus is a sexually modified arm used by males of some cephalopod species to transfer spermatophores to females. In most cuttlefish (Sepiidae), it corresponds to the left arm IV, in which basal suckers are reduced or absent; this modification is thought to facilitate sperm transfer without premature rupture, which could be caused by friction and pressure from unmodified suckers. In Ascarosepion tullbergi , however, previous descriptions are inconsistent: some report a hectocotylised left arm IV with reduced suckers in the mid‐portion, whereas others report no hectocotylus. To resolve this discrepancy, we examined the holotype and additional museum specimens, quantified sucker size in the mid‐portion of male arms IV, and recorded copulatory behaviour in captive pairs in an aquarium. The putative hectocotylised region of the male left arm IV showed no sucker reduction or specialised structures. During copulation, males did not bring arms IV close to females, but inserted arms III into the female arm crown. After copulation, males rubbed arms III while arms IV remained attached to the substrate. These results indicate that A. tullbergi lacks a hectocotylus and uses arms III for copulation, broadening our understanding of reproductive diversity within Sepiidae.