Settlement behavior of the megalopa and the early juveniles of Portunus trituberculatus (Miers, 1876) (Brachyura: Portunidae) under laboratory conditions

Abstract

Megalopae and early juveniles of Portunus trituberculatus (Miers, 1876), a commercially important species, are known to utilize drifting seaweeds before transition to benthic habitats, but little is understood about this settlement behavior. Clinging and burying behavior of P. trituberculatus was investigated during nighttime and daylight by culturing megalopae and frst to ffth crab stages in tanks with di?erent bottom substrates (polyethylene, gravel, or sand) and an attachment substrate (artifcial seaweed) in the water column. Clinging behavior on the artifcial seaweed was observed for all growth stages. The proportions of individuals that exhibited clinging behavior increased from megalopae to the frst crab stage, peaked in second and third crab stages, and declined through the fourth and ffth crab stages in all treatment groups. The proportion of crab stages that buried in the substrate was much higher in daytime than in nighttime (P < 0.001). For daytime observations, the burying rates linearly increased from frst or second to ffth crab stages and were higher in tanks provided with sand as opposed to gravel. Our results suggest that early juvenile crabs utilize drifting seaweeds as a vehicle, and they voluntarily shift to benthos after the fourth crab stage when the risk of predation is reduced, as frst to third crab stages are known to be vulnerable to predation by benthic fshes. The juveniles of P. trituberculatus thus appear to "hitchhike" drifting seaweeds before fnal settlement on the bottom. The present study supports the idea that hitchhiking behavior could be an important transport mechanism leading to settlement by larval and juvenile stages in decapod crustaceans.