Laboratory Culture of the Sepiolid Squid Euprymna scolopes: A Model System for Bacteria-Animal Symbiosis

The small Hawaiian sepiolid Euprymna scolopes, with its symbiotic luminous bacterium Vibrio fischeri, was cultured through one complete life cycle in 4 months. Paralarval squid hatchlings were actively planktonic for the first 20-30 days, after which they settled and assumed the typical adult mode of nocturnal activity and diurnal quiescence. Squids were aggressive predators that preferred actively swimming prey up to 2-4 times their size; the only diet that yielded good survival and rapid growth for paralarvae was large adult mysids. Survival to settlement was 73% on this diet, whereas it was 0%-17% on controls and three other diets. Paralarvae initially lacked both detectable luminescence and V. fischeri cells in their incipient light organs; all remaining stages produced luminescence, and their light organs were colonized by apparently pure cultures of > 105 V. fischeri typical of E. scolopes symbiont strains. Survival from settlement to sexual maturity was 76%. Mating and egg laying commenced at 2 months, yet attempts to culture the next laboratory generation of hatchlings were not as successful. The results indicate that the host organism of this symbiosis can soon be cultured with consistency through its brief life cycle, thus opening new avenues of research into developmental aspects of this symbiosis.