body (Fig. 9.4b, c), decorated on their ragged edge with coarse coronal spicules
which they used as pillar/support to remain freely exposed on the seafloor. The
contact to the substrate on which they lived was held with their basal spicules,
without digging into the seafloor. We consider this morphology – free resting on the
seafloor – as an ancient living form. Surely, these fossil sponges already produced
free-floating larvae (Steiner et al. 2005). Whereas the taxon Choia was not firmly
attached to the seafloor, other fossil demosponges from that period were sessile, like
the species Paraleptomitella dicytodroma (Wang et al. 2010a).
Specimens of Choia sp. are occasionally highly abundant within the limestone
(Fig. 9.4a). They were settling with their basis on the seafloor and left the oscule
open at the opposite side (Fig. 9.4b, c). EDX (energy dispersive x-ray) analyses
revealed that the regions within the limestone that do not contain fossils (Fig. 9.4d
and f1) show low signals for carbon, in contrast to regions with fossils (Fig. 9.4e
and f2).
9.4 Morphology and Synthesis of Spicules in Demosponges
In the last few years, motivated by previous ultrastructural analyses (reviewed in
Uriz 2006), the development and the morphology of the spicules have been studied
thoroughly in the demosponge Suberites domuncula (reviewed in M€ uller et al.
2007c); Fig. 9.5a. The skeleton of S. domuncula is composed of only two types
of megascleres, monactinal tylostyles and a smaller fraction of diactinal oxeas. The
spicules reach lengths of up to 450 mm and diameters of 5–7 mm (Fig. 9.5c); they
grow through apposition of lamellar silica layers. While the two ends of the oxeas
are pointed, the tylostyles have one pointed end and one swollen knob (Fig. 9.5b).
Microscopic analyses showed that all spicules have a 0.3–1.6-mm wide axial canal
in their center (Fig. 9.6e–i). Applying the primmorph system (the established 3D
cell culture of sponges), it became possible to follow the different steps of spicule
formation (M€ uller et al. 2005). These studies establish unequivocally that the initial
steps of spicule formation occur intracellularly in the sclerocytes (M€ uller et al.
2005); Fig. 9.6a–d. The 15-mm large sclerocytes produce one to three of up to 6-mm
long spicules (Fig. 9.6c).
The formation of – at least – the first silica layer around the axial filament starts
within the sclerocytes. In the primordial stage, spicule growth begins around the
axial filament (Fig. 9.6a–d). Clods with highly electron dense material which
represent the first deposits of silica become visible. During growth in the extracellular space, the spicules reach up to 450 mm in length with a diameter of 5 mm.
Initially, the 1.6 mm wide axial canal is filled primarily with the axial filament and
additional membrane structures, while in the final stage it is almost completely
filled with the axial filament, which is homogenous (Fig. 9.6e) and displays the
characteristic triangular axial form (Fig. 9.6h, i).
Toward a further understanding of the synthesis of spicules, immunogold labeling/TEM studies with antibodies against silicatein were performed (M€ uller et al.
9 The Unique Invention of the Siliceous Sponges
261
which they used as pillar/support to remain freely exposed on the seafloor. The
contact to the substrate on which they lived was held with their basal spicules,
without digging into the seafloor. We consider this morphology – free resting on the
seafloor – as an ancient living form. Surely, these fossil sponges already produced
free-floating larvae (Steiner et al. 2005). Whereas the taxon Choia was not firmly
attached to the seafloor, other fossil demosponges from that period were sessile, like
the species Paraleptomitella dicytodroma (Wang et al. 2010a).
Specimens of Choia sp. are occasionally highly abundant within the limestone
(Fig. 9.4a). They were settling with their basis on the seafloor and left the oscule
open at the opposite side (Fig. 9.4b, c). EDX (energy dispersive x-ray) analyses
revealed that the regions within the limestone that do not contain fossils (Fig. 9.4d
and f1) show low signals for carbon, in contrast to regions with fossils (Fig. 9.4e
and f2).
9.4 Morphology and Synthesis of Spicules in Demosponges
In the last few years, motivated by previous ultrastructural analyses (reviewed in
Uriz 2006), the development and the morphology of the spicules have been studied
thoroughly in the demosponge Suberites domuncula (reviewed in M€ uller et al.
2007c); Fig. 9.5a. The skeleton of S. domuncula is composed of only two types
of megascleres, monactinal tylostyles and a smaller fraction of diactinal oxeas. The
spicules reach lengths of up to 450 mm and diameters of 5–7 mm (Fig. 9.5c); they
grow through apposition of lamellar silica layers. While the two ends of the oxeas
are pointed, the tylostyles have one pointed end and one swollen knob (Fig. 9.5b).
Microscopic analyses showed that all spicules have a 0.3–1.6-mm wide axial canal
in their center (Fig. 9.6e–i). Applying the primmorph system (the established 3D
cell culture of sponges), it became possible to follow the different steps of spicule
formation (M€ uller et al. 2005). These studies establish unequivocally that the initial
steps of spicule formation occur intracellularly in the sclerocytes (M€ uller et al.
2005); Fig. 9.6a–d. The 15-mm large sclerocytes produce one to three of up to 6-mm
long spicules (Fig. 9.6c).
The formation of – at least – the first silica layer around the axial filament starts
within the sclerocytes. In the primordial stage, spicule growth begins around the
axial filament (Fig. 9.6a–d). Clods with highly electron dense material which
represent the first deposits of silica become visible. During growth in the extracellular space, the spicules reach up to 450 mm in length with a diameter of 5 mm.
Initially, the 1.6 mm wide axial canal is filled primarily with the axial filament and
additional membrane structures, while in the final stage it is almost completely
filled with the axial filament, which is homogenous (Fig. 9.6e) and displays the
characteristic triangular axial form (Fig. 9.6h, i).
Toward a further understanding of the synthesis of spicules, immunogold labeling/TEM studies with antibodies against silicatein were performed (M€ uller et al.
9 The Unique Invention of the Siliceous Sponges
261