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Abstract BackgroundThe biofouling marine tube worm,Hydroides elegans, is an indirect developing polychaete with significance as a model organism for questions in developmental biology and the evolution of host‐microbe interactions. However, a complete description of the life cycle from fertilization through sexual maturity remains scattered in the literature, and lacks standardization. Results and discussionHere, we present a unified staging scheme synthesizing the major morphological changes that occur during the entire life cycle of the animal. These data represent a complete record of the life cycle, and serve as a foundation for connecting molecular changes with morphology. ConclusionsThe present synthesis and associated staging scheme are especially timely as this system gains traction within research communities. Characterizing theHydroideslife cycle is essential for investigating the molecular mechanisms that drive major developmental transitions, like metamorphosis, in response to bacteria. 

Summary Pseudoalteromonas luteoviolaceais a globally distributed marine bacterium that stimulates the metamorphosis of marine animal larvae, an important bacteria–animal interaction that can promote the recruitment of animals to benthic ecosystems. Recently, differentP.luteoviolaceaisolates have been shown to produce two stimulatory factors that can induce tubeworm and coral metamorphosis; Metamorphosis‐Associated Contractile structures (MACs) and tetrabromopyrrole (TBP) respectively. However, it remains unclear what proportion ofP.luteoviolaceaisolates possess the genes encoding MACs, and what phenotypic effect MACs and TBP have on other larval species. Here, we show that 9 of 19 sequencedP.luteoviolaceagenomes genetically encode both MACs and TBP. WhileP.luteoviolaceabiofilms producing MACs stimulate the metamorphosis of the tubewormHydroides elegans, TBP biosynthesis genes had no effect under the conditions tested. Although MACs are lethal to larvae of the cnidarianHydractinia symbiologicarpus,P.luteoviolaceamutants unable to produce MACs are capable of stimulating metamorphosis. Our findings reveal a hidden complexity of interactions between a single bacterial species, the factors it produces and two species of larvae belonging to different phyla.