Abstract
Within the marine bivalve family Thyasiridae, some species have bacterial chemosymbionts associated with gill epithelial cells while other species are asymbiotic. Although the abundance of symbionts in a particular thyasirid species may vary, the structure of their gills (i.e., their frontal-abfrontal thickening) does not. We examined gill structure in a species tentatively identified as Thyasira gouldi from a Northwest Atlantic fjord (Bonne Bay, Newfoundland) and found remarkable differences among specimens. Some individuals had thickened gill filaments with abundant symbionts, while others had thin filaments and lacked symbionts. We could differentiate symbiotic and asymbiotic specimens based on the size and outline of their shell as well as 18S rRNA, 28S rRNA and CO1 sequences. The wide morphological, genetic and symbiosis-related disparity described herein suggests that chemosymbiosis may influence host divergence, and that Thyasira gouldi forms a cryptic species complex.
Highlights
The Thyasiridae is one of five bivalve families that have established symbiotic relationships with chemoautotrophic bacteria [2]
In all but one chemosymbiotic thyasirid species, the bacterial symbionts are extracellular [3,6], residing either in enlarged spaces limited by the microvilli and the cell membrane in the bacteriocyte zone of modified, abfrontally expanded gill filaments, or among the microvilli of abfrontal epithelial cells in gills with shorter filaments [4]
One well-supported clade consists of Thyasira flexuosa Montagu, 1803, T. gouldi Philippi, 1845 and T. polygona Jeffreys, 1864, species with abfrontally thickened gills typically containing large numbers of symbionts [3,4,9]
Summary
The Thyasiridae (superfamily Thyasiroidea [1]) is one of five bivalve families that have established symbiotic relationships with chemoautotrophic bacteria [2]. Symbiont presence in the Thyasiridae has been linked to gill structure [4], and a recent molecular phylogeny suggests that symbiotic species belong to more than one distinct clades within the family [1]. One well-supported clade consists of Thyasira flexuosa Montagu, 1803, T. gouldi Philippi, 1845 and T. polygona Jeffreys, 1864, species with abfrontally thickened gills typically containing large numbers of symbionts [3,4,9]. Both T. flexuosa and T. gouldi reportedly have large geographic ranges [10,11], and specimens examined from various sites show similar features and abundant symbionts [3,4]. Symbiont abundance can change according to particulate food availability [9], the structure of chemosymbiotic bivalve gills, i.e., the degree of frontal-abfrontal elongation of filaments, or ‘gill type’ [4], has not been shown to vary along with symbiont abundance within a host species
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