Flocculation of Artemia induced by East Asian common Octopus octopus sinensis paralarvae under culture conditions

•Octopus sinensis paralarvae induced flocculation of coexisting Artemia nauplii or metanauplii.•Proportion of flocculated Artemia was higher in 3-day-old metanauplii than in newly hatched nauplii.•Flocculated Artemia had deformed setules with hook-shaped tips and adhesion of neighbouring tips on setae.•Flocculation is likely occurred via a mechanism similar to the ‘hook-and-loop fastener’.•Causative matter of Artemia flocculation secreted by O. sinensis paralarvae may not accumulate in culture water. Artemia are potential food organisms for the mass culture of common octopus paralarvae but cause poor paralarval growth and mortality. To understand problems arising from Artemia use, we focused on Artemia flocculation in paralarval culture tanks; Artemia get caught up with each other, exhibit disrupted swimming, are deposited on the tank bottom and eventually die. To clarify whether paralarvae induce the flocculation of food organisms or not, we cultured newly hatched Artemia nauplii, 3-day-old metanauplii and decapod crustacean zoeae with or without paralarvae at different growth stages (weight). Flocculation occurred only when Artemia were cultured with paralarvae; metanauplii had a higher susceptibility for flocculation than nauplii. Flocculated Artemia proportion increased with increasing paralarval weight. Scanning electron microscopy revealed that flocculated metanauplii had deformed setules on their setae, with hook-shaped tips and adhesion of neighbouring tips, suggesting that flocculation may occur via a mechanism similar to the ‘hook-and-loop fastener’. As octopus paralarvae exhibit external digestion, digestive enzymes secreted by paralarvae may deform Artemia setules and result in flocculation. As flocculation did not occur when metanauplii were cultured in water in which paralarvae were cultured and then removed, causative enzymes were probably rapidly inactivated after secretion.