Sponge spicule

Sponges are a species-rich clade of the earliest-diverging (most basal) 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 (Demospongiae, Hexactinellida, and Homoscleromorpha) produce skeletons of amorphous silica and one (Calcarea) 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 (from the Latin porus meaning "pore" and -fer meaning "bearing"). He described sponges as the simplest of multicellular animals, sessile, marine invertebrates built from soft, spongy (amorphously shaped) material. They are eye-catching because of their distinct body plan (see lead image above) which relies on a filigree skeleton constructed using an array of morphologically determined spicules. 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 (SiO2·nH2O). 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 [Ma], a date that was confirmed by fossils records. 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. spp.; (B) Microsclere (sigma) of Mycale quadripartita; (C) Megasclere (oxea) of Haliclona epiphytica; (D) Spicule tetralophose calthrop of homoscleromorph Plakina'' (A) 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. 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. evolved several times independently, resulting in multiple recruitments of the same genes for biomineralization in different lineages. Among these genes, members of the alpha carbonic anhydrase gene family (CAs) are essential for biomineralization. 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, 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. 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 controlShown left: The largest biosilica structure on Earth is the giant basal spicule from the deep-sea glass sponge Monorhaphis chuni. The largest biosilica structure on Earth is the giant basal spicule from the deep-sea glass sponge Monorhaphis chuni. Siliceous spicules were first described and illustrated in 1753 by Vitaliano Donati, who found them in the species Geodia cydonium from the Adriatic Sea: he called these spicules "little balls". They are later called globular crystalloids, globate spicules, or globostellates by sponge taxonomists, until 1888 when William Sollas finally coins the term "sterraster" from the Greek sterros meaning "solid" or "firm" – see diagram on the right. Meanwhile, similar ball-shaped spicules are observed in another genus, Placospongia, and these are at first considered as "sterrasters" for these different spicules (coming from the Greek selene for "moon", referring to the "half-moon" shape). Finally, an additional term "aspidaster" is created by von Lendenfeld in 1910, convinced that the flattened sterrasters in the genus Erylus are significantly different from those in Geodia. The Geodia can be massive animals more than a meter across. from shallow temperate/tropical waters worldwide. It is not a very diverse genus with only 10 species currently described (WPD) and a handful of undescribed species. which suggests that a more detailed study of the sterraster/aspidaster surface would potentially bring new characters for Geodiidae genera identification. ==Spicule "life cycle"==

; (AI) Formation of spicule axial filament (AF); (AII) Spicule (SP) growth within the sclerocyte; (AIII) Spicule growth with two sclerocytes (SC) on spicule tips; (AIV) Transport of mature spicule within the sponge body; (B) Sponge death and body decay; (C) Detached sponge fragment with spicules; (D) Disassociated spicules. (C and D) From formation to deposition The formation of spicules is controlled genetically. In most cases, the first growth phase is intracellular; it starts in sclerocytes (amoeboid cells responsible for spicule formation) in mesohyl and is mediated by silicatein, a special enzyme that initiates formation of the axial filament (harboured by the axial canal) which provides the vertical axis of the spicule. The axial canal is filled with organic proteinaceous material which usually extends to the tip of the newly-formed spicule. The cross-section of the axial canal differs across major sponge clades that produce siliceous spicules (it is triangular in demosponges, irregular in homoscleromorphs In calcareans (producing calcareous spicules) the axial canal is not developed. In desmoid spicules of 'lithistids' (an informal group of demosponges with articulated skeletons), however, the axial filament is shorter than the spicule arms and it is possible that only organic molecules are involved in the spicule-forming process. Silica is first laid out as small 2 μm granules After some time, amorphous silica is added, forming evenly-deposited concentric layers, At this stage, immature spicules are secreted from the sclerocyte and covered by pseudopodia of one to several cells, and the process of silica deposition and spicule growth continues. Sometimes spicules accumulate into enormous agglomerations called spicule mats or beds. These accumulations are characteristic for polar waters. Spicules can fossilize to form special type of rocks called the spiculites ("spongillites" for freshwater sponge spicules); these types of rocks are known globally, and have been formed through the whole Phanerozoic. == Locomotion ==

In 2016 a newly discovered demosponge community living under arctic ice were found to have moved across the sea floor by extending their spicules and then retracting their body in the direction of motion. ==Spiculites==

When dead sponges disintegrate and disarticulate into discrete spicules, the sponge spicules become incorporated into marine sediments. They sometime accumulate as layers (beds) of sediment composed mostly of spicules, called spicule mats or spicule beds. After burial, these beds often lithify to form a special type of sedimentary rock called spiculite. Spicules of Calcarea are produced in three basic forms: diactines, triactines and tetractines. resulting in that they often constitute the only evidence of the presence of some sponges in an ecosystem. Even though sponges are often known from rich assemblages of bodily-preserved specimens, a significant part of their fossil and subfossil record is also represented by their spicules. Having that in mind, spicules can be of crucial importance for reconstructions of extinct or cryptic (hiding in cervices and caves) sponge communities; and, indeed, they have been investigated especially with respect to their taxonomic significance. Their significance, however, is often underestimated, which is mostly due to the difficulties in assigning disassociated spicules to sponge taxa or due to the scarcity of the material. == Interaction with light ==

Research on the Euplectella aspergillum (Venus' Flower Basket) demonstrated that the spicules of certain deep-sea sponges have similar traits to Optical fibre. In addition to being able to trap and transport light, these spicules have a number of advantages over commercial fibre optic wire. They are stronger, resist stress easier, and form their own support elements. Also, the low-temperature formation of the spicules, as compared to the high temperature stretching process of commercial fibre optics, allows for the addition of impurities which improve the refractive index. In addition, these spicules have built-in lenses in the ends which gather and focus light in dark conditions. It has been theorized that this ability may function as a light source for symbiotic algae (as with Rosella racovitzae) or as an attractor for shrimp which live inside the Venus' Flower Basket. However, a conclusive decision has not been reached; it may be that the light capabilities are simply a coincidental trait from a purely structural element. Spicules funnel light deep inside sea sponges. File:MicroscopicSpiculesfromPachastrellidSponge.jpg| Sponge spicules File:Spicules of sponge (SEM).JPG| Spicules of sponge (SEM) File:Sponge Spicules.jpg| Network of sponge spicules ==See also==