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Communities of microorganisms form thin coats across solid surfaces in the sea. Larvae of many marine invertebrates use biofilm components as cues to appropriate settlement sites. Research on the tube-dwelling polychaete worm Hydroides elegans, a globally common member of biofouling communities, is described to exemplify approaches to understanding biofilm bacteria as a source of settlement cues and larvae as bearers of receptors for bacterial cues. The association of species of the bacterial genus Pseudoalteromonas with larval settlement in many phyla is described, and the question of whether cues are soluble or surface-bound is reviewed, concluding that most evidence points to surface-bound cues. Seemingly contradictory data for stimulation of barnacle settlement are discussed; possibly both explanations are true. Paleontological evidence reveals a relationship between metazoans and biofilms very early in metazoan evolution, and thus the receptors for bacterial cues of invertebrate larvae are very old and possibly unique. Finally, despite more than 60 years of intense investigation, we still know very little about either the bacterial ligands that stimulate larval settlement or the cellular basis of their detection by larvae.
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Supplemental Figure 1. Sensory organs on marine invertebrate larvae. A, larvae of a polyclad flatworm (Ph. Platyhelminthes), arrow points to thin apical sensory tuft; B, larva of a nemertine worm (Ph. Nemertea), inset shows the anterior sensory organ and its nerves in higher magnification; C, cyphonautes larva of a marine bryozoan (Ph. Bryozoa), arrow points to the eversible vibratile plume; D, veliger larva of the marine limpet Lottia digitalis (Ph. Mollusca, Cl. Gastropoda); E, veliger larva of a chiton (Ph. Mollusca, Cl. Polyplacophora); F, trocohphore larva of the polychaete worm Hydroides elegans (Ph. Annelida, Cl. Polychaeta); G, larva of the brachiopod Terebratalia transversa (Ph. Brachiopoda); H, planula larva of the sea anemone Metridium senile (Ph. Cnidaria, Cl. Anthozoa). A - G are from members of the superfphylum Lophotrochozoa. In all except C, the larva bears an apical tuft of sensory cilia, derived embryologically from the first quartette of micromeres. It is likely that this tuft of cilia senses environmental cues — many from surface biofilms — that stimulate settlement and metamorphosis. The cyphonautes larva, C, bears a vitbratile plume of cilia (arrow) which is everted when the larva contacts a surface, presumable testing it. While not the larva of a lophotrochozoan, the planula illustrated in H also bears an anterior tuft of cilia that is probably sensory and may be used in determining settlement sites. Given the basal position of the Cnidaria in animal evolution, it is possible that this tuft is a homologue of that seen on other larvae in this illustration. All of the larvae are in a size range of 70 - 150 µm in length. The sources of the larval photos are: A, Mark Q. Martindale (University of Hawaii); B, Richard Emlet (Univ. Oregon); C, Friday Harbor Laboratories embryology class summer 2005; D, Richard Emlet; E, Friday Harbor Laboratories embryology class summer 2005; F, Brian Nedved, (University of Hawaii); G, Friday Harbor Laboratories embryology class summer 2005; H, Richard Strathmann (University of Washington).
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