THE CHITIN SYSTEM IN THE TUBES OF DEEP-SEA HYDROTHERMAL VENT WORMS

The ultrastructure of the protective tubes secreted by the vestimentiferan Tevnia jerichonana has been investigated. The tube wall is made of a hierarchy of structures and substructure which are visualized by scanning and transmission electron microscopy (SEM and TEM). When the tubes are observed at a low magnification, a layering of the wall is observed. The layers consist of a crisscrossed arrangement of flat straight ribbons, each of them having widths of 10-50 μm and thicknesses ranging from 0.1 to 1 μm. When viewed at higher magnification, the ribbons themselves are seen to consist of a substructure of endless parallel chitin microfibrils embedded in a protein matrix (estimated weight ratio: 55% protein and 45% chitin). The structure of the chitin microfibrils was investigated by diffraction contrast electron microscopy (DCTEM) together with electron diffraction analysis. These techniques, applied to either tube wall cross sections or microfibrils that were unhinged after protein removal, reveal that the chitin microfibrils have unusually large widths, reaching up to 50 nm in some cases. Each microfibril is a β chitin whisker-like single crystal having a four-sidedpolygonal cross section. Within a given ribbon, all crystals have the same polarity. The Tevnia chitin microfibrils are susceptible to intracrystalline hydration: this is denoted by an increase in one of the lattice parameters when they are subjected to hydration under temperature and pressure conditions. Upon dehydration, the anhydrous β chitin unit cell is restored, but a cracking of the crystalline microfibrils observable in their cross sections seems to be connected with this dehydration. The overall organization of the microfibrillar chitin network within the tube wall is reminiscent of the continuous twisted arrangement found in many cholesteric arthropod cuticles. However in the Tevnia tubes, the microfibrillar twist is discrete and irregular as there is an aperiodic variation of the fibrillar orientation within the tubes. Their overall organization which is still of the "plywood type" cannot therefore be described as cholesteric but rather as "random twisted nematic.". © 1992.