Convergence in life-history traits in migratory deep-water squid and fish

The life-history traits of two species of nektonic squid, Onykia ingens and Gonatus antarcticus, and Patagonian toothfish Dissostichus eleginoides, all from the Southwest Atlantic, are reviewed to reveal the extent to which their life cycles have converged. All three follow similar ontogenetic migration, with spawning and egg development in meso-bathypelagic water over the continental slope. Larvae and small juveniles ascend to epipelagic water to feed and grow near the slope. Juveniles and immature adults migrate to the shelf and use it as their feeding grounds. After this period of fast growth on the shelf, the adults descend back to deep water to spawn. The potential advantages that such a life cycle gives to migratory deep-water animals over obligate bathypelagic and benthic fauna are discussed. The most likely cause of ontogenetic migration of juvenile deep-water organisms to the shelf, with the resulting fast growth, is to increase individual fecundity during the adult deep-water phase. Escape from predation and resource utilization at the next trophic level are also examined, but found to be less plausible explanations. Larger, mature animals have higher fecundity, bigger eggs, and larger, better-developed hatchlings, all of which enhance the survival rates of the species at the early stages of their ontogenesis.