Many benthic marine animal populations are established and maintained by free-swimming larvae that recognize cues from surface-bound bacteria to settle and metamorphose. Larvae of the tubeworm Hydroides elegans, a significant biofouling agent, require contact with surface-bound bacteria to undergo metamorphosis; however, the mechanisms that underpin this microbially mediated developmental transition have been enigmatic. Here, we show that a marine bacterium, Pseudoalteromonas luteoviolacea, produces arrays of phage tail–like structures that trigger metamorphosis of H. elegans. These arrays comprise about 100 contractile structures with outward-facing baseplates, linked by tail fibers and a dynamic hexagonal net. Not only do these arrays suggest a novel form of bacterium-animal interaction, they provide an entry point to understanding how marine biofilms can trigger animal development.
Tubeworms are important marine benthic species that encrust rocks and contribute to fouling of man-made objects, such as ships' hulls and drilling well heads. Like most marine invertebrates, the larval stages of tubeworms are free-swimming, but the cues for larval settlement and the triggers for metamorphosis are mysterious. Shikuma et al. (p. 529, published online 9 January) experimented on larval settlement by the tubeworm, Hydroides elegans, which needs to associate with a biofilm-forming bacterium, Pseudoalteromonas luteoviolacea, before settlement can occur. The bacterium was found to express metamorphosis-associated contractile structures (MACs) in large and structurally elaborate arrays that allow the tubeworm larvae to develop.