Non-parasitic symbioses between echinoderms and bacteria

During the last several decades, symbiosis has been described as a pervasive and evolutionarily significant biological phenomenon that has played a fundamental role in the evolution of life on Earth (Margulis 1976, 1991, Margulis & Fester 1991). The term symbiosis was first defined by the German mycologist Anton De Bary (1879) as “the living together of differently named organisms”. Such a broad definition naturally encompasses a wide variety of neutral, beneficial or detrimental interactions between heterospe-cific individuals and may thus refer to plant pollinisation by insects as well as to plant-herbivore or to prey-predator relationships. By insisting on the duration and contact, several authors sharpened the definition: symbiosis is an interspecific association in which the partners are in intimate contact during an appreciable time relative to their life-spans (e.g., Smith & Douglas 1987). This is the meaning used here. Sapp (1994) reviewed the history of the concept. In symbiosis, the larger partner is usually referred as the host and the smaller one as the symbiont. According to their external or internal position to the host, the symbionts are termed ectosymbiotic or endosymbi-otic, respectively. Endosymbionts can be located intracellularly or extracel-lularly. Extracellular endosymbionts occupy a cavity (e.g., lumen of organs) or intercellular spaces (e.g., intraepithelial spaces). The degree of dependence of the partners (obligate vs facultative symbiosis), their taxonomic specificity (specificity of the symbiosis), the transmission of the symbiont to its host (perpetuation of the symbiosis) and the balance of advantages obtained by the partners (functional significance of the symbiosis) are additional characteristics. In an obligate symbiosis, host and/or symbiont are physiologically (or morphologically) linked to such an extent that they cannot survive outside the association. Alternatively, the symbiosis is facultative (for one or both partners). The degree of specificity of a symbiosis refers to the range of hosts taxa and/or of symbionts taxa involved in a particular symbiosis: the specificity grades from low to high with the decreasing number of taxa involved. The transmission of symbionts from one generation of hosts to the next follows two main modes: the transovarian (or vertical) transmission and the cyclic (or horizontal) transmission. In transovarian transmission, the symbionts are transmitted via the eggs; in cyclic transmission, the symbionts are acquired anew each generation from the environment (indirect transmission) or from conspecific individuals e.g., via contacts between parents and offsprings (direct transmission or reinfection) (Stanier et al. 1986, Douglas 1994, McFall-Ngai 1998). Symbioses encompass various types of interactions between the partners which may derive benefit or harm or be unaffected. Although the interpretation of what is advantageous or disavantageous (cf. limits between benefit and neutrality or between neutrality and pathogenicity) is usually difficult to assess (Smith & Douglas 1987, Morton 1989), the fact that two organisms have evolved a symbiosis implies that at least one partner gains some advantage from the association; the balance of advantages within a symbiosis can be roughly summarized as shown in Table 1. Mutualism refers to a symbiosis involving reciprocal benefit (host and symbiont benefit from the association). Parasitism refers to a symbiosis affecting negatively (more or less severely) the host and positively the symbiont. Another set of symbioses shares the tendency to be harmless for the host and unilaterally beneficial for the symbiont. Giving the nature of the benefit obtained by the symbiont, these symbioses are named: commensalism when the benefit is trophic, phoresis when it refers to provision of transport, epizoism when it refers to provision of surface/support, endoecism when it refers to protection through shelter/burrow sharing and inquilinism when it refers to protection through sheltering within the host body itself. The limits between these categories are not always sharp (see e.g., Stanier et al. 1986, Smith & Douglas 1987, & Morton 1989).