Structure and characteristics of giant spicules of the deep sea hexactinellid sponges of the genus Monorhaphis (Hexactinellida: Amphidiscosida: Monorhaphididae)

Two classes of sponges (phylum Porifera) are characterized by a siliceous skeleton, the Hexactinellida and the Demospongiae. The phylogenetically oldest sponge taxon, the Hexactinellida, lives in the deep-sea (1 000 m). The species Monorhaphis intermedia and Monorhaphis chuni, comprise with their 3 m long spicules the longest bio-silica structures on earth. These giant basal spicules have been analyzed by electron microscopical techniques. All spicules have a concentric lamellar organization. Cross sections through the spicules show a structural division of the spicules: in the center is an axial canal which harbors the axial filament; around this canal an axial cylinder of 50 to 150 μm width can be distinguished; and finally the lamellar region (300 to 500 lamellae; size between 3 and 5 μm). Biochemical studies revealed that the dominant protein in the spicules is a 35 kDa large molecule; in addition, a 23-24 kDa polypeptide, which is possibly silicatein-related, can be identified. Electron microprobe analysis revealed that three main elements compose the spicules: silicon, potassium and sodium. Furthermore, the giant basal spicules have been used as a waveguide for light within the spectral range 600 nm and 1 400 nm. Light transmission studies revealed that the spicules act as optical fibers (like a high pass filter) cutting off the light of wavelengths below about 600 nm; a similar cut-off of the spicule is observed in the infrared wavelength range above 1 400 nm. Finally the gene encoding a photolyase-related protein from the hexactinellid Aphrocallistes vastus is described. Sequence similarity studies suggested that the protein can be subdivided to the photolyase-related proteins which comprise also the metazoan cryptochromes. Based on these data a schematic outline of the formation of the siliceous spicules of hexactinellids is given; in addition, it is proposed that in Monorhaphis the spicules act as waveguides for the transmission of light/electrical and/or chemical signals, thus providing these animals with a neuronal-like network system[Acta Zoologica Sinica 53(3):557-569,2007].