Evidence in support of a novel symbiotic relationship in a “bottom-dwelling” bivalve.....

Following a taxonomic review in 2012, Oliver described a tiny bivalve mollusc and named it Syssitomya pourtalesiana, owing to its commensal association with the deep-sea echininoid species, Pourtalesia Miranda.  This minute bivalve, measuring up to 4 mm in length, has the enviable habitat choice of living attached to the anal spines of the urchin (Fig. 1).   In this paper, Oliver et al. (2013) describe the apparent bacterial symbiosis that occurs in this species.

Figure 1.  Syssitomya pourtalesiana attached to the anal spine of Pourtelasia miranda.

A food accepting tract is found along the lower edge of the ctenidial gill, with extensive bacteriocyes lining the surfaces.  The bacteriocytes are densely packed with mainly rod shaped bacteria and coccoid cells to a lesser extent, with pili connecting bacteria in places (see fig. 2).  There is also a layer of diverse bacteria on top of the ctenidial bacteriocytes that are not embedded into the cell walls or within the bacteriocytes, but are interspersed by many haemocytes.   The authors suggest that this association between bivalve and bacteria may represent an intermediate stage to an oblgate symbiotic association, where the bacteriocytes do not enclose the bacteria but instead, phagocytosis occurs by roaming haemocytes.

Figure 2. E. Rod-shaped bacteria within bacteriocyte; pili indicated with arrows; a few filamentous bacteria are also visible. F. Hyphomicrobial cells with paddle-like mother cells (arrowed).

The anatomy and stomach contents of the bivalve indicate that S. pourtalesiana is able to filter feed.  In previously described bacterial symbiotic relationships with deep-sea bivalves, the bivalves appear to have an obligate relationship, relying on the bacteria for nutrients.  S. pourtalesiana however, appears to have a mixotrophic strategy, combining filter feeding with bacterial symbiotic sustenance.  Given the preferred positioning of the bivalve at the anal opening of the urchin, the bacterial metabolites are likely to originate from urchin faecal matter.  Many of the filamentous bacterial cells found atop the bacteriocytes are Hyphomonas-like, with paddle shaped swellings at the tips of many (fig 2.).  Hyphomonas bacteria are known to be a primary food source of a hydrothermal vent limpet, Lepetodrilus schrolli and this bacterial genus utilises DOM.  The exterior positioning of the filamentous bacterial cells on the ctenidium indicates that they are unlikely to be a source of food for the bivalve, giving further support to the idea of symbiosis.

One of the key driving forces of chemosymbiosis is the response to nutrient poor conditions, such as those commonly found in the deep ocean.  Furthermore, the presence of an apparently diverse microbial community associated with the bivalve gill may indicate utilisation of a wide range of trophic pathways, as seen in the magnificent gutless worm, Olavius algarvensis.  Unfortunately however, the authors were unable to isolate any of the bacteria or carry out any molecular analyses, limiting the weight that can be attributed to the observations made.  The evidence is compelling however and genomic analyses of the bacteria is imperative to be able to understand the processes involved.   Given the recent taxonomic description of the bivalve and apparent novel symbioses in this family, I hope that follow up work can ascertain the nature of the symbiosis, whether bacteria are acquired or inherited and how well the bivalve can continue with biological processes in the absence of the bacteria.

Oliver, P. G. (2012). Taxonomy of some Galeommatoidea (Mollusca, Bivalvia) associated with deep-sea echinoids: a reassessment of the bivalve genera Axinodon, Verrill & Bush, 1898 and Kelliola Dall, 1899 with descriptions of new genera Syssitomya and Ptilomyax. European Journal of Taxonomy, 12, 1-24.

Oliver, P. G., Southward, E. C., & Dando, P. R. (2013). Bacterial symbiosis in Syssitomya pourtalesiana Oliver, 2012 (Galeommatoidea: Montacutidae), a bivalve commensal with the deep-sea echinoid Pourtalesia. Journal of Molluscan Studies, 79(1), 30-41.