Settlement Of Marine Invertebrates Research Articles

Two Distinct Bacterial Biofilm Components Trigger Metamorphosis in the Colonial Hydrozoan Hydractinia echinata.

ABSTRACTIn marine environments, the bacterially induced metamorphosis of larvae is a widespread cross-kingdom communication phenomenon that is critical for the persistence of many marine invertebrates. However, the majority of inducing bacterial signals and underlying cellular mechanisms remain enigmatic. The marine hydroid Hydractinia echinata is a well-known model system for investigating bacterially stimulated larval metamorphosis, as larvae transform into the colonial adult stage within 24 h of signal detection. Although H. echinata has served as a cell biological model system for decades, the identity and influence of bacterial signals on the morphogenic transition remained largely unexplored. Using a bioassay-guided analysis, we first determined that specific bacterial (lyso)phospholipids, naturally present in bacterial membranes and vesicles, elicit metamorphosis in Hydractinia larvae in a dose-response manner. Lysophospholipids, as single compounds or in combination (50 μM), induced metamorphosis in up to 50% of all larvae within 48 h. Using fluorescence-labeled bacterial phospholipids, we demonstrated that phospholipids are incorporated into the larval membranes, where interactions with internal signaling cascades are proposed to occur. Second, we identified two structurally distinct exopolysaccharides of bacterial biofilms, the new Rha-Man polysaccharide from Pseudoalteromonas sp. strain P1-9 and curdlan from Alcaligenes faecalis, to induce metamorphosis in up to 75% of tested larvae. We also found that combinations of (lyso)phospholipids and curdlan induced transformation within 24 h, thereby exceeding the morphogenic activity observed for single compounds and bacterial biofilms. Our results demonstrate that two structurally distinct, bacterium-derived metabolites converge to induce high transformation rates of Hydractinia larvae and thus may help ensure optimal habitat selection.

An unusual microbiome characterises a spatially-aggressive crustose alga rapidly overgrowing shallow Caribbean reefs

Several species of crustose coralline algae (CCA) and their associated microbial biofilms play important roles in determining the settlement location of scleractinian corals on tropical reefs. In recent decades, peyssonnelid algal crusts (PAC) have become spatial dominants across large areas of shallow Caribbean reefs, where they appear to deter the recruitment of scleractinians. Our genetic investigations of PAC in St. John, US Virgin Islands, amplifying the large-subunit ribosomal RNA and psbA protein D1 marker genes, revealed them to be identical to Ramicrusta textilis previously reported overgrowing corals in Jamaica. Specimens of PAC sampled from the Honduras were likewise identical, confirming that this crustose alga inhabits the easternmost and westernmost regions of the Caribbean. We also analysed 16S rDNA tag amplicon libraries of the biofilms associated with PAC and sympatric CCA, which is favoured for coral settlement. Our results show that the microbial communities on PAC (vs. CCA) are characterized by significantly lower numbers of the epibiotic bacterial genus Pseudoalteromonas, which facilitates the recruitment and settlement of marine invertebrates. From these data, we infer that PAC are therefore unlikely to be attractive as settlement sites for coral larvae. Given the significant ecological change anticipated on these reefs due to increasing cover of PAC, there is an urgent need to further investigate competitive interactions between PAC and scleractinian corals, and elucidate the role of PAC and their associated microbiomes in accentuating phase shifts from coral to algae on tropical reefs.

A new passive collector to assess settlement rates, substrate selection and predation pressure in decapod crustacean larvae

Quantitative sampling and experiments evaluating larval settlement of marine invertebrates require an effective, highly replicable sampling unit. Artificial collectors, cages and solid trays have been used to assess spatial and temporal patterns of postlarval settlement of decapod crustaceans. However, because of problems with sample retention upon retrieval, the interpretation of data from artificial collectors can be problematic. Moreover, the costs and time invested in construction can be high, especially in high-energy subtidal environments where there is a considerable risk of damage and loss. In this study, we describe and test a new passive collector for postlarvae of benthic decapod crustaceans. The device is equipped with a mechanism that covers the collector surface during deployment and retrieval, but exposes it while at rest on the sea bed. The collector design also proved effective in determining settlement patterns, substrate preferences and predation effects. In general, passive collectors are effective in monitoring settlement rates for a diversity of species and therefore will be useful in biodiversity studies. Densities of megalopae closely approximated those in comparable natural habitats collected by diver-based suction sampling. Megalopae exhibited a strong preference for structurally heterogeneous substrates. Megalopal settlement to experimental substrates was statistically the same with and without predator exclusion mesh, indicating that the mesh does not produce undue behavioral artifacts, and that predation pressure was negligible at the experimental sites. These collectors represent a versatile alternative to be applied to a wide variety of questions regarding the settlement and early post-settlement processes in brachyuran crabs, as well as other benthic species and lend themselves well to a variety of manipulative experiments in the subtidal environment.