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Sulphide mining by the superextensile foot of symbiotic thyasirid bivalves

Abstract

In a symbiotic association between an invertebrate host and chemoautotrophic bacteria, each partner has different metabolic requirements, and the host typically supplies the bacteria with necessary reduced chemicals (sulphide or methane). Some combination of anatomical, physiological and behavioural adaptations in the host often facilitates uptake and transport of reduced chemicals to the symbionts1,2,3,4. We have studied five species of bivalve molluscs of the family Thyasiridae (that is, thyasirids) three of which harbour chemoautotrophic bacteria. Here we show that the symbiotic bivalves extend their feet to form elongated and ramifying burrows in the sediment, most probably to gain access to reduced sulphur. Closely related bivalves (including some thyasirid species) without bacterial symbionts show no comparable foot extension behaviour. The length and number of burrows formed by chemosymbiotic thyasirids are related to the concentration of hydrogen sulphide in the sediment. The burrows are formed by the foot of each bivalve, which can extend up to 30 times the length of the shell, and may be the most extreme case of animal structure elongation documented to date.

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Competing interests

The authors declare that they have no competing financial interests.

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Acknowledgements

We thank J. B. C. Jackson, N. D. Holland and E. Kisfaludy for discussion and advice, and A. Aadnesen, P. Johannessen, T. Sorlie and T. Høisæter for help in collecting thyasirids and running experiments at the University of Bergen Biological Station, Espegrend. We are grateful to O. K. Dalland and A. Lerheim for allowing the use of X-ray equipment at the veterinary clinic at Stend, Norway. This work was supported by the Scripps Institution of Oceanography graduate department, the Baxter and Alma Ricard Foundation, and the National Science Foundation.

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The authors declare that they have no competing financial interests.

Correspondence to Suzanne C. Dufour.

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Further reading

Figure 1: Partial X-radiograph of chemosymbiotic thyasirids burrowing in sediment two weeks after introduction.
Figure 2: Typical structure of sulphide mining burrows formed by chemosymbiotic thyasirids in low and high sulphide conditions.

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