Abstract
Tuleariocaris holthuisi and Arete indicus are two ectocommensal shrimps closely associated with the tropical sea urchin Echinometra mathaei. This study provides a comparison of these two E. mathaei symbiotic crustaceans and particularly focuses on the relationship between T. holthuisi and its host’s pigments (i.e. spinochromes), and its dependency on its host. While all the analyses underline a close association between A. indicus and E. mathaei, they reveal a particularly close interaction between T. holthuisi and its host. Chemical analyses reveal that these shrimps present the same spinochrome composition as E. mathaei, and have similar colouration, allowing camouflage. Isotopic composition and pigment loss after host separation suggest that these pigments are certainly assimilated upon feeding on the urchin. Moreover, symbiont isolation experiments demonstrate the high dependency of T. holthuisi on its host and the importance of the host’s pigments on their survival capacity. Finally, some host recognition mechanisms are investigated for T. holthuisi and show the probable implication of spinochromes in host selection, through chemical recognition. Hence, all the results suggest the essential roles of spinochromes for T. holthuisi, which, in turn, suggests the potential implication of these pigments in the shrimps’ metabolism.
Highlights
Symbioses are intimate associations between two heterospecific organisms that are generally classified into three categories: parasitism, commensalism and mutualism[1,2,3]
E. mathaei are infested by T. holthuisi (Fig. 1B,C) and 43% by A. indicus (Fig. 1A)
Two ectocommensal symbioses associated with echinoderms and decapods had previously been investigated in order to identify the nature of the chemical signals allowing host selection
Summary
Symbioses are intimate associations between two heterospecific organisms (commonly called symbiont and host) that are generally classified into three categories: parasitism, commensalism and mutualism (these categories are themselves split into subclasses)[1,2,3]. The sea urchin Echinometra mathaei (Blainville, 1825), which is widespread in the Indo-Pacific Ocean, is the host for two crustacean symbionts: Tuleariocaris holthuisi Hipeau-Jacquotte, 1965 (Palaemonidae) (Fig. 1B,C) and Arete indicus Coutière, 1903 (Alpheidae) (Fig. 1A) (Cimino and Ghiselin 2001; Hipeau-Jacquotte 1965; Gherardi 1991). The two species may be found simultaneously on the same individuals, raising the question of the differences in their respective ecological niche, as well as the question of potential competition between the two species It is presently unknown if the two symbionts feed on their host (parasitism) or if they are true commensalists. Carbon and nitrogen stable isotopes have recently been used to study the diets of various echinoderm invertebrate[13,14] or vertebrate[15] symbionts These analyses revealed that this method is a powerful tool to determine the food sources that are assimilated over a longer period of life. The polyhydroxynaphthoquinones (PHNQ), known as spinochromes or echinochromes[30,31,32], provide sea urchins with their coloration, and are involved in their protection through antibacterial[33,34,35], antioxidant[35,36,37,38] and potential immune activities[39,40,41,42]
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