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Bioluminescence relies on the oxidation of a luciferin substrate catalysed by a luciferase enzyme. Luciferins and luciferases are generic terms used to describe a large variety of substrates and enzymes. Whereas luciferins can be shared by phylogenetically distant organisms which feed on organisms producing them, luciferases have been thought to be lineage-specific enzymes. Numerous light emission systems would then have co-emerged independently along the tree of life resulting in a plethora of non-homologous luciferases. Here, we identify for the first time a candidate luciferase of a luminous echinoderm, the ophiuroid Amphiura filiformis. Phylogenomic analyses identified the brittle star predicted luciferase as homologous to the luciferase of the sea pansy Renilla (Cnidaria), contradicting with the traditional viewpoint according to which luciferases would generally be of convergent origins. The similarity between the Renilla and Amphiura luciferases allowed us to detect the latter using anti-Renilla luciferase antibodies. Luciferase expression was specifically localized in the spines which were demonstrated to be the bioluminescent organs in vivo. However, enzymes homologous to the Renilla luciferase but unable to trigger light emission were also identified in non-luminous echinoderms and metazoans. Our findings strongly indicate that those enzymes, belonging to the haloalkane dehalogenase family, might then have been convergently co-opted into luciferases in cnidarians and echinoderms. In these two benthic suspension-feeding species, similar ecological pressures would constitute strong selective forces for the functional shift of these enzymes and the emergence of bioluminescence.

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Marine organisms have developed a high diversity of chemical defences in order to avoid predators and parasites. In sea cucumbers, saponins function as repellents and many species produce these cytotoxic secondary metabolites. Nonetheless, they are colonized by numerous symbiotic organisms amongst which the Harlequin crab, Lissocarcinus orbicularis, is one of the most familiar in the Indo-Pacific Ocean. We here identify for the first time the nature of the molecules secreted by sea cucumbers and attracting the symbionts: saponins are the kairomones recognized by the crabs and insuring the symbiosis. The success of this symbiosis would be due to the ability that crabs showed during evolution to bypass the sea cucumber chemical defences, their repellents becoming powerful attractants. This study therefore highlights the complexity of chemical communication in the marine environment.C hemical sensing is considered as the most ancient and the most ubiquitous mode of communication in the biosphere 1 ; all living organisms are able to detect chemical cues in their environments 2,3 . These cues allow different types of intra-and interspecific interactions between organisms, including, for example, mate recognition, prey-predator interactions and symbiotic associations. Indeed, communication between symbionts and their hosts is needed to insure appropriate host selection and the maintenance of the symbiotic relationship through time. Chemical signals emitted by hosts are named kairomones when they elicit a commensal or a parasitic symbiosis and synomones for mutualistic ones. Symbiotic associations are very common in echinoderms 4 and it is well known that echinoderms scent can attract various symbionts. This has been highlighted through different behavioural experiments on symbiotic polychaetes 5-7 , bivalves 8 , fishes 9 , crabs 10 , brittle stars 11 and shrimps 12,13 . All these studies showed that odours emitted by echinoderms in surrounding seawater are detected by symbionts and trigger an attractive chemotaxy. However, the exact chemical nature of the olfactory signals involved in the selection of an echinoderm host has never been discovered. In fact, up to now, only one semiochemical involved in a marine symbiosis has been identified: the amphikuemin, a synomone secreted by some species of sea anemones and recognized by clown fishes 14 .On the other hand, echinoderms are also well known for possessing defensive chemicals 15,16 that deter predation or prevent organisms to establish on their body. These allomones, semiochemicals providing a benefit to their producers, have been described in many representatives from the five echinoderm classes, but the most studied are the noxious saponins produced by sea stars (asteroids) 17,18 and sea cucumbers (holothuroids) 19 . Saponins are secondary metabolites first discovered in higher plants 20 that are also present in several marine sponges 21 . In sea cucumbers, saponins are found in the body wall and in the viscera, including the adhesive Cuvieran tubules. Structurally,...

Spinochromes are principally known to be involved in sea urchin pigmentation as well as for their potentially interesting pharmacological properties. To assess their biological role in sea urchin physiology, experiments are undertaken on crude extracts from four species and on four isolated spinochromes in order to test their antibacterial, antioxidant, inflammatory and cytotoxic activities. First, the antibacterial assays show that the use of crude extracts as representatives of antibacterial effects of spinochromes are inaccurate. The assays on purified spinochromes showed a decrease in the growth of four strains with an intensity depending on the spinochromes/bacteria system, revealing the participation of spinochromes in the defense system against microorganisms. Secondly, in the 2,2-diphenyl-1-picrylhydrazyl antioxidant assays, spinochromes show an enhanced activity compared to the positive control. This latter observation suggests their involvement in ultraviolet radiation protection. Third, spinochromes present a pro-inflammatory effect on lipopolysaccharide-stimulated macrophages, highlighting their possible implication in the sea urchin immune system. Finally, cytotoxicity assays based on Trypan blue exclusion, performed in view of their possible future applications as drugs, show a weak cytotoxicity of these compounds against human cells. In conclusion, all results confirm the implication of spinochromes in sea urchin defense mechanisms against their external environment and reveal their potential for pharmacological and agronomical industries.