Microbial Bioerosion of Erratic Sub-Fossil Nautilus Shells in a Karstic Cenote (Lifou, Loyalty Islands, New Caledonia)

AbstractIn a cenote formed in the limestone karst of Lifou (New Caledonia), more than 35 shells of Nautilus macromphalus were discovered in 35–40 m water depth. Seven shells were recovered and subsamples of two shells were used for a study on microbial bioerosion. Both shells were intensively bioeroded and SEM-analyses revealed a total of six ichnotaxa with a dominance of the cyanobacterial trace Scolecia filosa. Such a low diversity ichnocoenosis and the strong dominance of a single ichnotaxon indicate that the environment in the cenote was hostile for many euendolithic organisms. The unfavourable conditions were caused by (1) restricted water-mass exchange with the open ocean, limiting the chance of larvae being transported into the cenote; (2) intense fluctuations in fresh water influx in the karst system promoting traces of euryhaline euendoliths (Ichnoreticulina elegans, S. filosa, Scolecia serrata); and (3) very limited light availability, allowing the development of traces of organotrophs (S. serra...