A GASTROPOD WITH IRON-SULFIDE MINERALIZED DERMAL SCLERITES FROM A DEEP-SEA HYDROTHERMAL VENT IN THE INDIAN OCEAN—A MODERN ANALOGUE OF CAMBRIAN SCLERITOME-BEARING ANIMALS

A recent dive by the submersible ‘Jason’ at a hydrothermal vent in the Indian Ocean has brought up specimens of a gastropod carrying a mail of imbricating scale-shaped sclerites on its foot. The animal represents a hitherto unknown genus of the Neomphalina, a group which seems to be endemic to vent and seep environments. The sclerite coat recalls the scleritomes appearing at the onset of skeletal biomineralization in various metazoans, including mollusk-like forms. These predominantly Cambrian sclerites were mineralized with aragonite, calcite or calcium phosphate. The sclerite-building minerals in the new gastropod, however, are iron sulfides, the crystalline species being pyrite (FeS₂) and greigite (Fe₃S₄). The minerals are integrated in a conchioline-like laminated tissue recording successive growth stages of the sclerites. In the inner parts of the sclerites the minerals are finely dispersed or form lumps and framboids; these become more dense toward the exterior to form a more-or-less solid outer wall of the sclerite. Because of the presence of greigite, the sclerites are magnetic. Biomineralization of iron sulfides is known from bacteria but has never before been reported from metazoans. Although a variety of mineral species are formed in metazoan tissues as mineral grains or minute spicules, only carbonates and phosphates of calcium (and, in sponges, opaline silica) have hitherto been known to be used to build structurally coherent mineral skeletons, shells, or sclerite coats. The use of iron sulfides as skeletal minerals in the new gastropod is interpreted as an adaptation to the sulfide-rich environment around the hydrothermal vent. The degree and nature of biological control of the mineral formation are currently unknown, but the integration of the crystalline fabric with the organic tissues in localized parts of the body indicates that the mineralization may be biologically mediated as well as induced. The appearance of pyrite/greigite sclerites in a modern gastropod raises fundamental questions about the evolutionary origins of mineralized exoskeletons in molluscs as well as in other metazoans. It also suggests that presumed diagenetic pyrite encrusting shells from fossil hydrothermal vent environments might also include cases of original pyrite biomineralization by metazoans.