Sponge symbiosis is facilitated by adaptive evolution of larval sensory and attachment structures in barnacles

Symbiotic relations and range of host usage are prominent in coral reefs and crucial to the stability of such systems. In order to explain how symbiotic relations are established and evolve, we used sponge-associated barnacles to ask three questions. (1) Does larval settlement on sponge hosts require novel adaptations facilitating symbiosis? (2) How do larvae settle and start life on their hosts? (3) How has this remarkable symbiotic lifestyle involving many barnacle species evolved? We found that the larvae (cyprids) of sponge-associated barnacles show a remarkably high level of interspecific variation compared with other barnacles. We document that variation in larval attachment devices are specifically related to properties of the surface on which they attach and metamorphose. Mapping of the larval and sponge surface features onto a molecular-based phylogeny showed that sponge symbiosis evolved separately at least three times within barnacles, with the same adaptive features being found in all larvae irrespective of phylogenetic relatedness. Furthermore, the metamorphosis of two species proceeded very differently, with one species remaining superficially on the host and developing a set of white calcareous structures, the other embedding itself into the live host tissue almost immediately after settlement. We argue that such a high degree of evolutionary flexibility of barnacle larvae played an important role in the successful evolution of complex symbiotic relationships in both coral reefs and other marine systems.

Evaluation of Larval Medium in the Controlled Current Toxicity Test

ABSTRACT The Controlled Current Toxicity Test (CCTT) is a protocol used by the University of Georgia Black Fly Rearing and Bioassay Laboratory to evaluate the efficacy of larvicides based on Bacillus thuringiensis subsp. israelensis (Bti) against Simulium vittatum larvae. A standard CCTT provides a larval medium with suspended organic nutrients, a sustained current, and a clean surface for black fly larval attachment in each exposure vessel. In an effort to streamline the CCTT and eliminate a potential source of variability, 3 types of larval medium were evaluated: deionized water, distilled water, and moderately hard water, which were compared to the currently used protocol. A statistical significance in mean lethal concentration (LC50) rates was observed between the CCTT (deionized water with suspended organic nutrients) medium and the other 3 experimental media. The presence of suspended organic nutrients in the CCTT significantly improved the efficacy of the Bti-based larvicide. The interaction of the suspended organic nutrient particles and the Bti-based particles in the larvicide appears to produce a mixture that is more efficiently captured and ingested by the black fly larvae than the larvicide formulation particles alone.

Biofilms and Extracts from Bacteria Producing "Quorum Sensing" Signaling Molecules Promote Chemotaxis and Settlement Behaviors in Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa) Larvae

Many sessile marine invertebrates have life cycles involving the development of larvae that settle on specific substrates to initiate metamorphosis to juvenile forms. Although is recognized that bacterial biofilms play a role in this process, the responsible chemical cues are beginning to be investigated. Here, we tested the role of substrate-specific bacteria biofilms and their Quorum Sensing Signaling Molecule (QSSM) extracts on chemotaxis and settlement of larvae from Hydractinia symbiolongicarpus, a hydroid that grows on gastropod shells occupied by hermit crabs. We isolated and taxonomically identified by 16S rDNA sequencing, 14 bacterial strains from shells having H. symbiolongicarpus. Three isolates, Shigella flexneri, Microbacterium liquefaciens, and Kocuria erythromyxa, were identified to produce QSSMs using biosensors detecting N-acyl-L-homoserine lactones. Multispecies biofilms and QSSM extracts from these bacteria showed a positive chemotactic effect on H. symbiolongicarpus larvae, a phenomenon not observed with mutant strains of E. coli and Chromobacterium violaceum that are unable to produce QSSMs. These biofilms and QSSMs extracts induced high rates of larval attachment, although only 1 % of the attached larvae metamorphosed to primary polyps, in contrast to 99 % of larvae incubated with CsCl, an artificial inductor of attachment and metamorphosis. These observations suggest that bacterial QSSMs participate in H. symbiolongicarpus substrate selection by inducing larval chemotaxis and attachment. Furthermore, they support the notion that settlement in cnidarians is decoupled into two processes, attachment to the substrate and metamorphosis to a primary polyp, where QSSMs likely participate in the former but not in the latter.