To bleach or not to bleach: The symbiotic question. Pigmentation changes and chemical investigation of Scleratinian ectosymbionts

Color mimicry is a common phenomenon in marine species, often used for camouflage. This process may rely on pigments, such as carotenoids, contained in chromatophores in many organisms. This strategy is notably used by symbiotic decapods to camouflage themselves on their hosts in order to reduce their predation rate. In addition, it has also been described that when symbionts are separated from their hosts, the latter can suffer from 'Host separation syndrome', leading to a decline in health and sometimes discoloration of the symbiont. This study explores the effects of the ‘Host separation syndrome’ between ectosymbionts (Alpheus lottini and Trapezia serenei) and their host, the coral Pocillopora acuta in Mo'orea (French Polynesia). Two questions arise: do these ectosymbionts undergo separation-related discoloration? And what impact does this have on their carotenoid content? To answer these questions, the ectosymbionts were placed in different environmental conditions and their color evolution was monitored using standardized photography. Chemical analysis of the carotenoids was carried out by HPLC-MS to identify the nature of the pigments and to discover if the symbionts and the host share share similar pigmented molecules. This analysis was also used to assess potential differences in the quantity of these pigments between control symbionts and those suffering from host separation syndrome. The results revealed significant discoloration patterns after isolation and the presence of similar pigments in both partners, such as astaxanthin. The results of the photographic analyses showed various color changes for the 2 ectosymbiont species studied under the different conditions. Discoloration and a decline in survival were observed in individuals physically isolated from their host. These phenomena could be explained by a trophic link between the symbionts and their host.