back prior to the “Precambrian-Cambrian” boundary [Vendian (610–545 Ma)/Ediacaran (542–580 Ma)]. A further molecule required for the formation of a hard
skeleton was collagen, fibrous organic filaments that need oxygen for their formation. Silicatein forming the spicules and collagen shaping their morphology are the
two organic components that control the appositional growth of these skeletal
elements. This process starts in both demosponges and hexactinellids intracellularly
and is completed extracellularly where the spicules may reach sizes of up to 3 m.
While the basic strategy of their formation is identical in both sponge classes, it
differs on a substructural level. In Hexactinellida, the initial silica layers remain
separated, those layers bio-fuse (bio-sinter) together in demosponges. In some
sponge taxa, e.g., the freshwater sponges from the Lake Baikal, the individual
spicules are embedded in an organic matrix that is composed of the DUF protein.
This protein comprises clustered stretches of amino acid sequences composed of
pronounced hydrophobic segments, each spanning around 35 aa. We concluded
with the remark of Thompson (1942) highlighting that “the sponge-spicule is a
typical illustration of the theory of ‘bio-crystallisation’ to form ‘biocrystals’ ein
Mittelding between an inorganic crystal and an organic secretion.” Moreover, the
understanding of the enzymatic formation of the spicules conferred sponge biosilica
a considerable economical actuality as a prime raw material of this millennium.
9.1 Introduction
Sponges are sessile filter-feeding organisms with an extremely effective and complex network of water-conducting channels and choanocyte chambers lined with
flagellated choanocyte cells. Until not too long ago the “ground” material, the
mesohyl, between the external pinacoderm and the internal choanoderm
(endopinacoderm), the two cell layers that seal sponges against the environment,
was thought to consist of mostly functionally independent cells (Pechenik 2000).
Such a setup would result in the formation of amorphous, unorganized creatures
(Pechenik 2000). However, during the last few years, the existence of cell surfacebound receptors and their extracellular as well as intracellular segments could be
verified: this led to the conclusion that also sponges possess molecules that allow
the establishment of a distinct body plan. The discovery of the metazoan novelties
first developed during evolution in sponges, comprising cell-cell/-matrix, signal
transduction-, immune-, neuronal-, and morphogenetic molecules, helped to overcome the long-standing debate whether sponges are specialized protists or true
Metazoa (Hyman 1940). The phylum Porifera is subdivided into three classes,
Hexactinellida, Demospongiae, and Calcarea. Until very recently, the phylogenetic
positions of these classes remained unresolved. Like any other metazoan, also
sponges have a defined Bauplan; this has most artistically been illustrated by
Haeckel (1872a). But unlike other Metazoa, adult sponges are considered to have
no pronounced anterior/posterior polarity; surely they do not have a dorsal ventral
axis. In higher metazoans, the patterning along the anterior–posterior axis is
252
W.E.G. M€ uller et al.
skeleton was collagen, fibrous organic filaments that need oxygen for their formation. Silicatein forming the spicules and collagen shaping their morphology are the
two organic components that control the appositional growth of these skeletal
elements. This process starts in both demosponges and hexactinellids intracellularly
and is completed extracellularly where the spicules may reach sizes of up to 3 m.
While the basic strategy of their formation is identical in both sponge classes, it
differs on a substructural level. In Hexactinellida, the initial silica layers remain
separated, those layers bio-fuse (bio-sinter) together in demosponges. In some
sponge taxa, e.g., the freshwater sponges from the Lake Baikal, the individual
spicules are embedded in an organic matrix that is composed of the DUF protein.
This protein comprises clustered stretches of amino acid sequences composed of
pronounced hydrophobic segments, each spanning around 35 aa. We concluded
with the remark of Thompson (1942) highlighting that “the sponge-spicule is a
typical illustration of the theory of ‘bio-crystallisation’ to form ‘biocrystals’ ein
Mittelding between an inorganic crystal and an organic secretion.” Moreover, the
understanding of the enzymatic formation of the spicules conferred sponge biosilica
a considerable economical actuality as a prime raw material of this millennium.
9.1 Introduction
Sponges are sessile filter-feeding organisms with an extremely effective and complex network of water-conducting channels and choanocyte chambers lined with
flagellated choanocyte cells. Until not too long ago the “ground” material, the
mesohyl, between the external pinacoderm and the internal choanoderm
(endopinacoderm), the two cell layers that seal sponges against the environment,
was thought to consist of mostly functionally independent cells (Pechenik 2000).
Such a setup would result in the formation of amorphous, unorganized creatures
(Pechenik 2000). However, during the last few years, the existence of cell surfacebound receptors and their extracellular as well as intracellular segments could be
verified: this led to the conclusion that also sponges possess molecules that allow
the establishment of a distinct body plan. The discovery of the metazoan novelties
first developed during evolution in sponges, comprising cell-cell/-matrix, signal
transduction-, immune-, neuronal-, and morphogenetic molecules, helped to overcome the long-standing debate whether sponges are specialized protists or true
Metazoa (Hyman 1940). The phylum Porifera is subdivided into three classes,
Hexactinellida, Demospongiae, and Calcarea. Until very recently, the phylogenetic
positions of these classes remained unresolved. Like any other metazoan, also
sponges have a defined Bauplan; this has most artistically been illustrated by
Haeckel (1872a). But unlike other Metazoa, adult sponges are considered to have
no pronounced anterior/posterior polarity; surely they do not have a dorsal ventral
axis. In higher metazoans, the patterning along the anterior–posterior axis is
252
W.E.G. M€ uller et al.