Defensive Ball Formation in Oceanic Squid Paralarvae

ABSTRACT The head withdrawal reflex is a behavior that has been reported in young pelagic squids (paralarvae) and involves retraction of the head, arms, and proboscis (fused feeding tentacles) into the mantle cavity when the animals are disturbed. The present study investigated which artificial stimuli (mechanical, chemical, or light) trigger this behavior in three oceanic ommastrephid squid species: Sthenoteuthis oualaniensis, Eucleoteuthis luminosa, and Todarodes pacificus. All stimuli triggered ball formation and chromatophore expansion. During the early paralarval stage, the head was completely withdrawn into the mantle cavity, and the squid formed a ball posture with expanded orange chromatophores. This response might make it difficult for predators to recognize and consume the squid or it might facilitate feeding.