Intrasexual variations in arm morphology related to alternative reproductive tactics in Japanese spear squid

The morphology of organisms is closely connected to their ecological features and is adapted to their environments. Arms and tentacles, appendages of cephalopods, exhibit great morphological variations both among and within species based on their ecology, but its intrasexual variations remain unexplored. Males of the Japanese spear squid, Heterololigo bleekeri, in different size categories pursue alternative reproductive tactics (ARTs). Large ‘consort’ males copulate with females in a male-parallel position, while small ‘sneaker’ males copulate with females in a head-to-head position. These ARTs result in differences in mating position and hunting strategy based on body size, leading to a hypothesis that ARTs in H. bleekeri may be related to intrasexual variations in arm morphology. This hypothesis was tested by analyzing stomach contents, arm length, and the number of suckers on each arm with squid collected off Miyagi, Japan, during two spawning seasons from December to May in 2021–2022 and 2022–2023. Stomach contents clearly showed a dietary shift with body growth, with consorts mainly preying on fish, and sneakers preying on crustaceans. Relative to body size, sneakers had longer arm I–IV and hectocotylus (male modified left arm IV), while consorts had shorter tentacles. The number of suckers on each arm tended to be lower in sneakers. These results suggest that, in H. bleekeri, tentacle morphology is related to feeding habits, and possessing longer arms holds some advantage for the sneaker tactic. Detailed behavioral observations will be necessary to further explore the adaptive significance of arm morphology in this species.