Age and growth of the colossal squid Mesonychoteuthis hamiltoni

The Southern Ocean is a vital but understudied ecosystem, with cephalopods playing key ecological roles. The colossal squid Mesonychoteuthis hamiltoni, the heaviest known invertebrate, is a major predator and prey species in this region. This study presents the first age estimates of M. hamiltoni across the Indian, Atlantic, and Pacific sectors of the Antarctic. Growth increments were analysed via epi-illumination microscopy in rostrum-sagittal sections of 30 lower beaks (10 per sector) recovered from predator stomachs. A consistent pattern of growth increments was observed in the sections. They revealed that M. hamiltoni reaches estimated ages of at least 3.5 years, and based on the largest recorded beak, we estimate a lifespan of over 5.2 years. Validation of daily periodicity is currently unsuitable for M. hamiltoni, but similar beak increments have been cross-verified with statolith data in other oegopsids. A higher proportion of the oldest and largest individuals was found in the Pacific sector. Our results revealed a reduction in growth rate in larger adults. These findings suggest M. hamiltonilives longer than the Southern Ocean endemic Moroteuthopsis longimana (2.2 years from beaks) and the giant squid Architeuthis dux (3.5 years from beaks), but not as long as the deep-sea vampire squid Vampyroteuthis infernalis, which may reach up to 8 years (using reproductive data). Such extended lifespans appear paradoxical given the traditionally assumed semelparity of coleoid cephalopods, although this paradigm is increasingly being challenged. Longevity in M. hamiltoni may reflect adaptive pressures from deep-sea dispersal and potential mating constraints