Silica Deposition in Demosponges
Demosponges are the most widespread class of the phylum Porifera. They secrete siliceous (spicules) and proteinaceous (fibers) elements, which constitute the skeleton that shapes the sponge growth and allow cell organization and establishment of the aquif
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1 Introduction Demosponges are the most widespread class of the phylum Porifera. They secrete siliceous (spicules) and proteinaceous (fibers) elements, which constitute the skeleton that shapes the sponge growth and allow cell organization and establishment of the aquiferous system through which the surrounding water accesses the internal cells. The siliceous spicules of demosponges display very different shapes and sizes, ranging from micrometers to millimeters. They are widespread in the sponge mesohyl or arranged in bundles, tracts or networks tending to form three-dimensional structures as the sponge grows massive, erect or branching (Fig. 1). Spongin, a collagen-like protein, accompanies the siliceous skeletons to a variable extent and may even embody the spicules totally (Fig. 2). The high diversity of spicule shapes was early shown by sponge paleontologists (Hinde and Holmes 1892) and taxonomists and, more recently, has been illustrated by scanning electron microscope images (e.g., Hartman 1981; Simpson 1984) and drawings (Wiedenmayer 1977; Boury-Esnault and Rützler 1997). In short, spicules are conventionally divided into microscleres and megascleres (Fig. 3) according to their shape, size or skeletal function (but see Uriz et al. 2003). Demosponge megascleres show one (monaxons) or four (tetraxons) symmetry axes (Fig. 4). Yet instances of incipient polyaxonid symmetry have been reported in some apparently monaxonid spicules (Rützler and Smith 1993; Uriz and Maldonado 1995), which supports a relationship between the subclasses Tetractinomorpha (with tetraxon spicules) and Ceratinomorpha (with monaxon spicules; Lévi 1973) within demosponges. Microscleres can also range from monaxons to polyaxons (star-like forms; Fig. 5). Hypersilified representatives of both spicule categories, which are named desmas, are present in several genera belonging to different orders of demosponges. Moreover, some genera and families of desma-bearing sponges are conventionally
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Centre d’Estudis Avançats de Blanes (CEAB, CSIC),Accés a la Cala Sant Francesc 14, 17300 Blanes (Girona), Spain 2 Department of Animal Biology (Invertebrates), Faculty of Biology, University of Barcelona, 645 Diagonal Ave, 08028 Barcelona, Spain Progress in Molecular and Subcellular Biology, Vol. 33 W. E. G. Müller (Ed.) © Springer-Verlag Berlin Heidelberg 2003
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M.J. Uriz et al. Fig. 1A,B. Haliclona submonilifera (Uriz 1988). A Spicule network at the sponge surface. B Combination of spicule tracks and uni-spiculated networks at the sponge choanosome. (Modified from Uriz 1988)
included in the “lithistids”, a group with no phylogenetic meaning (Pisera and Lévi 2002). Megascleres and microscleres appear to play several functions in sponges. Megascleres have a clear supportive function (Uriz et al. 2003), whereas the role of microscleres is difficult to ascertain (Fig. 6). Only the anisocheloid microscleres of the carnivorous sponge Abestopluma hypogea (Vacelet and Boury-Esnault 1996), have been shown experimentally to play a role in capturing micropreys
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