Salerno, Jennifer L et al.. 2005. "Characterization of Symbiont Populations in Life-History Stages of Mussels From Chemosynthetic Environments." The Biological bulletin 208, 145â155. https://doi.org/10.2307/3593123

The densities of chemoautotrophic and methanotrophic symbiont morphotypes were determined in lifehistory stages (post-larvae, juveniles, adults) of two species of mussels (Bathymodiolus azoricus and B. heckerae) from deep-sea chemosynthetic environments (the Lucky Strike hydrothermal vent and the Blake Ridge cold seep) in the Atlantic Ocean. Both symbiont morphotypes were observed in all specimens and in the same relative proportions, regardless of life-history stage. The relative abundance of symbiont morphotypes, determined by transmission electron microscopy, was different in the two species: chemoautotrophs were dominant (13:1-18:1) in B. azoricus from the vent site; methanotrophs were dominant (2:1-3:1) in B. heckerae from the seep site. The ratio of CH 4 :H 2 S is proposed as a determinant of the relative abundance of symbiont types: where CH 4 :H 2 S is less than 1, as at the Lucky Strike site, chemoautotrophic symbionts dominate; where CH 4 :H 2 S is greater than 2, as at the seep site, methanotrophs dominate. Organic carbon and nitrogen isotopic compositions of B. azoricus (5 I3 C = -30% e ; 5 15 N = -9%r) and B. heckerae (S I3 C = -56% c ; 5 15 N = -2%o) varied little among life-history stages and provided no record of a larval diet of photosynthetically derived organic material in the post-larval and juvenile stages. Received 17 December 2003; accepted 9 February 2005. * To whom correspondence should be addressed. E-mail: c vand@wm.edu Introduction Mussels in the genus Bathymodiolus are biomass dominants at many known deep-sea hydrothermal vent and cold seep habitats, where they are host to endosymbiotic, autotrophic bacteria in their gills. Anatomical and nutritional relationships between the symbionts and their adult hosts is well documented (e.g., Distel el al, 1995; Robinson et ai. 1998; Southward et al.. 2001; Fiala-Medioni et al.. 2002; Raulfs et al., 2004). Although bathymodiolin mussels may be able to obtain some nutrition by suspension feeding (Le Pennecera/., 1990; Page et al., 1991; Fujiwara et al., 1998), most of the nutrition of adult mussels is derived from their symbionts (Fisher et al.. 1988; reviewed in Fisher, 1990; Childress and Fisher. 1992; Kochevar et ai, 1992). Larval stages of vent mussels are pelagic and have been described as planktotrophic (Lutz et al., 1980; Berg, 1985; LePennec and Beninger, 2000), but the distribution of mussel larvae in the water column, the nature of their planktonic diet, and the trophic transition that takes place as they become benthic are all unknown. Some mussel species host only chemoautotrophic (also referred to as thiotrophic), sulfur-oxidizing bacterial symbionts (Nelson et al.. 1995; Fujiwara et al.. 2000); other species host only methanotrophic symbionts (Fujiwara et al., 2000; Barry et al., 2002); and still others host both types of bacteria ("dual symbionts"; Fisher et al., 1993; Robinson et al., 1998; Fiala-Medioni et al., 2002). Dual symbionts