Physical and chemical analysis of the siliceous skeletons in six sponges of two groups (demospongiae and hexactinellida).

The siliceous skeletons of six hexactinellids and demosponges were compared using a series of physical and chemical tests. The sponges were two hermit-crab sponges (Class Demospongiae, family Suberitidae), one from Scotland, Suberites domuncula, and the other, Pseudospongosorites suberitoides, from the Gulf of Mexico, and four hexactinellids, Hyalonema sp., Euplectella aspergillum, Rhabdocalyptus dawsoni, and Aphrocallistes vastus. The operating hypothesis was that differences in the amorphous hydrated silica skeletons in Demosponges and Hexactinellids might prove taxonomically useful. Physical properties studied included SEM, glass density, glass transition temperature (Tg), TG/DTA to determine water content, and FTIR spectra. Chemical determinations were made using energy dispersive X-ray fluorescence (EDXRF). Spicular skeletal material is deposited in concentric layers around the axial canal. With the exception of differences at several bands in the IR spectra, the siliceous skeletons in demosponges and hexactinellids are largely indistinguishable. Density was similar in all sponges (range 2.03-2.13 g/cc) and similar to the density of opal (SiO2 x 1.5H2O) (mean = 2.09 g/cc). IR spectra were similar, with prominent absorption bands at 460-470, 800, and 1,090-1,100 cm(-1) (due to different vibrational modes of Si-O-Si linkages) and at 1,650 and 3,450-3,560 cm(-1) (due to water). The skeletons of all six showed similar spectra to that of silica gel both before and after heating to 1,200 degrees C. Water comprised 10-14% of the skeleton by weight, slightly higher in the demosponges. Average spicule chemical composition was 85.2% SiO2, 12.3% water, and 2.5% other elements (mainly S, Al, K, Ca, and Na). The percent amounts of Si did not differ significantly between the demosponges and the hexactinellids.