Sponge spicule
Spicules are structural elements found in most sponges. The meshing of many spicules serves as the sponge's skeleton and thus it provides structural support and potentially defense against predators.
Sponge spicules are made of calcium carbonate or silica. Large spicules visible to the naked eye are referred to as megascleres or macroscleres, while smaller, microscopic ones are termed microscleres. The composition, size, and shape of spicules are major characters in sponge systematics and taxonomy.
Overview
Sponges are a species-rich clade of the earliest-diverging animals. They are distributed globally, with diverse ecologies and functions, and a record spanning at least the entire Phanerozoic.Most sponges produce skeletons formed by spicules, structural elements that develop in a wide variety of sizes and three dimensional shapes. Among the four sub-clades of Porifera, three produce skeletons of amorphous silica and one of magnesium-calcite. It is these skeletons that are composed of the elements called spicules. The morphologies of spicules are often unique to clade- or even species-level taxa, and this makes them useful in taxonomic assignments.
Research history
In 1833, Robert Edmond Grant grouped sponges into a phylum he called Porifera. He described sponges as the simplest of multicellular animals, sessile, marine invertebrates built from soft, spongy material.Later, the Challenger expedition discovered deep in the ocean a rich collection of glass sponges, which radically changed this view. These glass sponges were described by Franz Schulze, and came to be regarded as strongly individualised radially symmetric entities representing the phylogenetically oldest class of siliceous sponges. They are eye-catching because of their distinct body plan which relies on a filigree skeleton constructed using an array of morphologically determined spicules.
Then, during the German Deep Sea Expedition "Valdivia", Schulze described the largest known siliceous hexactinellid sponge, up to three metres high Monorhaphis chuni. This sponge develops the also largest known bio-silicate structures, giant basal spicules, three metres high and one centimetre thick. With such spicules as a model, basic knowledge on the morphology, formation, and development of the skeletal elements could be elaborated. Spicules are formed by a proteinaceous scaffold which mediates the formation of siliceous lamellae in which the proteins are encased. Up to eight hundred 5 to 10 μm thick lamellae can be concentrically arranged around an axial canal. The silica matrix is composed of almost pure silicon and oxygen, providing it with unusual optophysical properties superior to man-made waveguides.
Since their discovery, hexactinellids were appraised as "the most characteristic inhabitants of the great depths", rivalling in beauty the other class of siliceous Porifera, the demosponges. Their thin network of living tissues is supported by a characteristic skeleton, a delicate scaffold of siliceous spicules, some of which may be fused together by secondary silica deposition to form a rigid framework. The Hexactinellida together with the Demospongiae forms a common taxonomic unit comprising the siliceous sponges. The spicules, the elements from which their skeletons are constructed, are built in a variety of distinct shapes, and are made from silica that is deposited in the form of amorphous opal.
In evolution, after the Ediacaran period, a third class of Porifera appeared, the Calcarea, which has a calcium-carbonate skeleton.
Sponges have been receiving special attention from researchers since the introduction of molecular biological techniques at the turn of the century, since findings point to sponges as the phylogenetically oldest animal phylum. New information has accumulated concerning the relevance of this phylum for understanding of the dynamics of evolutionary processes that occurred during the Ediacaran, the time prior to the Cambrian Explosion which can be dated back to approximately 540 million years ago. According to molecular data from sponge genes that encode receptors and signal transduction molecules, the Hexactinellida were established to be the phylogenetically oldest class of the Porifera. Based on the discovery that the Porifera share one common ancestor, the Urmetazoa, with the other animals, it was deduced that these animals represent the oldest, still extant animal taxon. Even more, the emergence of these animals could be calculated back to 650–665 million years ago , a date that was confirmed by fossils records. Hence the Porifera must have lived already prior to the Ediacaran-Cambrian boundary, 542 Ma, and thus their elucidated genetic toolkit may contribute to the understanding of the Ediacaran soft-bodied biota as well, as sketched by Pilcher. It was the evolutionary novelty, the formation of a hard skeleton, that contributed significantly to the radiation of the animals in the late Proterozoic and the construction of the metazoan body plan.
Spicule types
Sponge spicules can be calcareous or siliceous. Siliceous spicules are sometimes embedded in spongin. Spicules are found in a range of symmetry types.File:Sizes of different spicule types of marine sponges.jpg|thumb|upright=2| Microsclere of Geodia spp.; Microsclere of Mycale quadripartita; Megasclere of Haliclona epiphytica; Spicule tetralophose calthrop of homoscleromorph Plakina
Monaxons form simple cylinders with pointed ends. The ends of diactinal monaxons are similar, whereas monactinal monaxons have different ends: one pointed, one rounded. Diactinal monaxons are classified by the nature of their ends: oxea have pointed ends, and strongyles are rounded. Spine-covered oxea and strongyles are termed acanthoxea and acanthostrongyles, respectively.
Monactical monaxons always have one pointed end; they are termed styles if the other end is blunt, tylostyles if their blunt end forms a knob; and acanthostyles if they are covered in spines.
Triaxons have three axes; in triods, each axis bears a similar ray; in pentacts, triaxons have five rays, four of which lie in a single plane; and pinnules are pentacts with large spines on the non-planar ray.
Tetraxons have four axes, and polyaxons more. Sigma-C spicules have the shape of a C.
Dendroclones might be unique to extinct sponges and are branching spicules that may take irregular forms, or may form structures with an I, Y or X shape.
- Megascleres are large spicules measuring from 60-2000 μm and often function as the main support elements in the skeleton.
- * Acanthostyles are spiny styles.
- * Anatriaenes, orthotriaenes and protriaenes are triaenes - megascleres with one long and three short rays.
- * Strongyles are megascleres with both ends blunt or rounded.
- * Styles are megascleres with one end pointed and the other end rounded.
- * Tornotes are megascleres with spear shaped ends.
- * Tylotes are megascleres with knobs on both ends.
- Microscleres are small spicules measuring from 10-60 μm and are scattered throughout the tissue and are not part of the main support element.
- * Chelae are microscleres with shovel-like structures on the ends. Anisochelas are microscleres with dissimilar ends. Isochelas are microscleres with two similar ends.
- * Euasters are star-shaped microscleres with multiple rays radiating from a common centre. Examples are oxyasters or sterrasters.
- * Forceps are microscleres bent back on themselves.
- * Microstrongyles are small rods with both ends blunt or rounded.
- * Microxeas are small rods with both ends pointed.
- * Sigmas are C- or S-shaped microscleres.
Calcareous spicules
Among these genes, members of the alpha carbonic anhydrase gene family are essential for biomineralization. CAs are zinc-binding enzymes that catalyze the reversible conversion of carbon dioxide and water to bicarbonate and one proton. The zinc-binding is mediated by three histidine residues essential for the protein's catalytic function. CAs are involved in many physiological processes requiring ion regulation or carbon transport, both of which are crucial for the controlled precipitation of carbonate biominerals. In mammals, where they are best studied, 16 different CAs are expressed in specific tissues and active in defined subcellular compartments. Cytosolic, mitochondrial, membrane-bound, and secreted CA forms can be distinguished, and these groups got expanded and reduced in different animal groups. Specific CAs are involved in the carbonate biomineralization in distinct animal lineages, including sponges.
Among extant sponges, only the calcareous sponges can produce calcite spicules, whereas other classes' spicules are siliceous. Some lineages among demosponges and a few calcareans have massive calcium carbonate basal skeletons, the so-called coralline sponges or sclerosponges. The biomineralizing CAs used by carbonate-producing demosponges are not orthologous to the CAs involved in the spicule formation of calcareous sponges, suggesting that the two biomineralization types evolved independently. This observation agrees with the idea that the formation of calcitic spicules is an evolutionary innovation of calcareous sponges.
Spicules are formed by sclerocytes, which are derived from archaeocytes. The sclerocyte begins with an organic filament, and adds silica to it. Spicules are generally elongated at a rate of 1-10 μm per hour. Once the spicule reaches a certain length it protrudes from the sclerocyte cell body, but remains within the cell's membrane. On occasion, sclerocytes may begin a second spicule while the first is still in progress.
The shapes of calcareous sponge spicules are simple compared with the sometimes very elaborate siliceous spicules found in the other sponge classes. With only a few exceptions, calcareous sponge spicules can be of three basic types: monaxonic, two-tipped diactines, triactines with three spicules rays, and four-rayed tetractines. Specialized cells, the sclerocytes, produce these spicules, and only a few sclerocytes interact in the formation of one specific spicule: Two sclerocytes produce a diactine, six sclerocytes form a triactine, and seven a tetractines. A pair of sclerocytes is involved in the growth of each actine of these spicules. After an initial phase, the so-called founder cell promotes actine elongation, the second, so-called thickener cell in some, but not all species deposit additional calcium carbonate on the actine, as it migrates back toward the founder cell. Calcareous sponges can possess only one or any combination of the three spicule types in their body, and in many cases, certain spicule types are restricted to specific body parts. This indicates that spicule formation is under strict genetic control in calcareous sponges, and specific CAs play an essential role in this genetic control