The first record of the genus Lamellibrachia (Siboglinidae) tubeworm along with associated organisms in a chemosynthetic ecosystem from the Indian Ocean: A report from the Cauvery–Mannar Basin

Here, we report for the first time, the genus Lamellibrachia tubeworm and associated chemosynthetic ecosystem from a cold-seep site in the Indian Ocean. The discovery of cold-seep was made off the Cauvery–Mannar Basin onboard ORV Sindhu Sadhana (SSD-070; 13th to 22nd February 2020). The chemosymbiont bearing polychaete worm is also associated with squat lobsters ( Munidposis sp.) and Gastropoda belonging to the family Buccinidae. Relict shells of chemosynthetic Calyptogena clams are ubiquitous at the seep sites. The Lamellibrachia tubes were found to be firmly anchored into the authigenic carbonate crusts. The authigenic carbonate crusts (chemoherm) are packed with large Calyptogena shells (whole shell and fragments). Very high concentrations (3800–12900 µM) of hydrogen sulfide (H 2 S) in the interstitial waters (40 cmbsf) is responsible for the sustenance of chemosymbiont bearing tubeworms. The posterior end of the tube penetrates downwards into the H 2 S-rich zone. The high concentration of H 2 S at ~40 cmbsf is attributed to sulfate reduction via anaerobic oxidation of methane (AOM) pathway. Methane hydrate was observed within the faults/fractures in the sediments. The presence of ethane and propane along with methane in the headspace gases and δ 13 C CH4 values (–28.4 to –79.5‰ VPDB) suggest a contribution of deep-seated thermogenic methane.