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Deep-sea research utilizing hyperbenthic sampling devices has revealed unexpected high densities and diversity of sponges, and many of the recently collected deep-sea sponges are new species. Selected case studies in the Antarctic, North Atlantic and Mediterranean Sea show that this is true not only in hydrothermal environments but is a general feature of bathyal and abyssal areas in our oceans. Some groups of siliceous sponges are known to be a significant part of benthic life in the deep-sea, but the large populations of extremely small sponges and especially the documentation of rich associations of calcareans are new to science. So far, only five species of calcareous sponges have been described from depths below 2000 m, and the Calcarea were considered as a group of primarily shallow water animals. Organic sheats that cover the spicules of calcarean sponges, probably offers some protection and enables them to mineralize in spite of the high CO2 pressure. Special adaptations to soft bottom environments and alternative feeding strategies allow the sponges to thrive in deep-sea environments. The fossil resord of sponges goes back to the Late Proterozoic and also molecular data confirm that the Porifera are the oldest known metazoans. Palaeontological investigations have shown that sponges have been living in deep water at least since the Early Cambrian. The radiative success of sponges, particularly after extinction events in Earth's history, may be linked with their ability to survive in the deep-sea.

Histological investigations of animals with hard skeletons represent a challenging problem in histotechnology. Siliceous and calcareous sponges commonly are treated with acid to remove the spicules prior to embedding and cutting. Methods for the investigation of spiculated sponge tissue are poorly known. Furthermore, fluorescence in situ hybridization (FISH), a key method to study symbiotic microbes, is not possible after acid treatment. For a broad range of siliceous sponge species, we developed and evaluated methods for embedding in paraffin, methylmetacrylat resins, LR White resin and cryomatrix. Different methods for cutting of tissue blocks as well as mounting and staining of the sections also were tested. Our aim was to enable histological investigations and FISH without prior removal of the spicules. To obtain an overview of tissue and skeleton arrangement, we recommend embedding of tissue blocks with LR White resin combined with en bloc staining techniques for large specimens with thick and numerous spicules, but paraffin embedding and subsequent staining for whole small specimens. For FISH on siliceous sponges, we recommend Histocryl embedding if the spicule content is high, but paraffin embedding if it is low. Until now, a standard method suitable for all histological attempts is not available.