Induction of sea cucumber (Apostichopus japonicus) larval settlement by bacterial biofilms and its application of hatchery rearing

Natural biofilms induce the settlement of sea cucumber Apostichopus japonicus larvae and have been applied to polyethylene corrugated plates (PCP) as settlement substrata in Chinese hatcheries. The method is technologically simple, but typically results in low efficiency of larval settlement, while dense natural biofilms are formed over relatively short periods. Moreover, the roles of biofilm-forming bacterial species in larval settlement preferences are unclear and the application of these biofilms to hatchery settings requires further investigation. In this study, PCP surface characteristics were improved by coating with different sized oyster shell powders (OSP) that promoted the formation of dense natural biofilms. Laboratory bioassays and hatchery applications indicated that improved PCP (i.e., IP) significantly increased larval settlement efficiencies, especially IP with OSP sizes of 250–330 μm (i.e, IP (S)), which increased efficiencies by 38.3–49.1%. Bacterial isolates recovered from natural biofilms belonging to three genera were investigated to evaluate their inductiveness of larval settlement via formation of monospecific biofilms. Three patterns of induction activities were observed, including increasing, stable, and decreasing inductiveness, that coincided with increasing cell densities. Further, five categories of induction activity properties were observed including strongly, moderately, inhibitive, strongly inhibitive, and completely absent inductive activities. These results indicate that bacterial inductiveness of A. japonicus larval settlement is species-specific and independent of taxonomic associations. Further, the inductive characteristics of isolates were not affected by substratum surfaces (i.e., glass, polyethylene, or OSP) nor growth media (i.e., sterile seawater or diluted broth). OSP application with monospecific biofilms also exhibited similar advantages of as natural biofilms. The application of Alteromonas sp. 1 biofilms as substrates in a hatchery increased the inductive efficiency of PCP and IP (S) significantly by 32.0–54.7%, indicating that inductive bacterial biofilms are applicable for sea cucumber larval breeding. This study is the first to demonstrate the inductiveness of bacterial species-specific biofilms for A. japonicus larval settlement. Further, these results provide a framework for applying such designs to natural and bacterial biofilms in hatcheries. Moreover, this is the first study to apply bacterial biofilms to marine invertebrate hatcheries.