Population-level left-side bias during male-male contests in three species of wild cuttlefish

Lateralized aggression, a population-level bias toward one side during agonistic encounters, is well documented in vertebrates, but its direction varies: tetrapods show a left-side bias, while some teleost fish show the reverse. Whether this pattern extends to invertebrates with fundamentally different neural architecture remains largely untested. Using citizen-science video of 30 wild male-male contests across three cuttlefish species (Sepia officinalis, Acanthosepion pharaonis, Ascarosepion latimanus), we investigated whether cuttlefish show a population-level side bias during contests. Cuttlefish engaged opponents preferentially using their left side, matching the tetrapod direction despite sharing the ecological conditions more directly with fish. Laterality did not predict contest outcome, and the rarer (right-biased) phenotype showed no frequency-dependent advantage. This contrasts findings in the giant Australian cuttlefish (Sepia apama), where minority lateral phenotypes win disproportionately in large, high-density breeding aggregations. This discrepancy may reflect the smaller, more dispersed breeding groups typical of the species studied here, though low statistical power leaves this explanation speculative. We suggest the results are consistent with lateralized threat processing being a general, structure-independent principle rather than one tied to vertebrate brain architecture.