Sponge spicule

[11] Most sponges produce skeletons formed by spicules, structural elements that develop in a wide variety of sizes and three dimensional shapes.

[17] He described sponges as the simplest of multicellular animals, sessile, marine invertebrates built from soft, spongy (amorphously shaped) material.

[17] Later, the Challenger expedition (1873–1876) discovered deep in the ocean a rich collection of glass sponges (class Hexactinellida), which radically changed this view.

[18] 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.

[19] Then, during the German Deep Sea Expedition "Valdivia" (1898-1899), Schulze described the largest known siliceous hexactinellid sponge, the up to three metres high Monorhaphis chuni.

[19] 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.

[20] 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 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).

[19] 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.

[19] According to molecular data from sponge genes that encode receptors and signal transduction molecules,[23][24] the Hexactinellida were established to be the phylogenetically oldest class of the Porifera.

[11] 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.

[27] It was the evolutionary novelty, the formation of a hard skeleton, that contributed significantly to the radiation of the animals in the late Proterozoic [28] and the construction of the metazoan body plan.

They are later called globular crystalloids, globate spicules, or globostellates by sponge taxonomists, until 1888 when William Sollas [60] finally coins the term "sterraster" from the Greek sterros meaning "solid" or "firm" – see diagram on the right.

Finally, an additional term "aspidaster" is created by von Lendenfeld in 1910,[62] convinced that the flattened sterrasters in the genus Erylus are significantly different from those in Geodia.

[58] Today, the Geodiidae represent a highly diverse sponge family with more than 340 species, occurring in shallow to deep waters worldwide apart from the Antarctic.

[65][66][67][58] Selenasters are the main synapomorphy of Placospongia (family Placospongiidae, order Clionaida), a well-supported monophyletic genus [68] from shallow temperate/tropical waters worldwide.

polyaxial spicules such as the sterrasters and aspidasters, are the result of fused "actines" (= branches of asters, from the Greek for "star"), later covered with "rosettes" made of different "rays".

Furthermore, the rosette morphology also seemed to be variable between Geodia, Pachymatisma, and Caminella [71][72] which suggests that a more detailed study of the sterraster/aspidaster surface would potentially bring new characters for Geodiidae genera identification.

[77] The cross-section of the axial canal differs across major sponge clades that produce siliceous spicules (it is triangular in demosponges,[78] irregular in homoscleromorphs [14] and quadrangular in hexactinellids.

[92] Biosiliceous sedimentation occasionally results in the formation of spiculitic cherts (in so called glass ramps) which are recorded from the Permian to Eocene of many parts of the world.

[95][96][16] 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.

Specific requirements and preferences of sponges can be used to interpret the environment in which they lived, and reconstruct oscillations in water depths, pH, temperatures, and other parameters, providing snapshots of past climate conditions.

In turn, the silicon isotope compositions in spicules (δ30Si) are being increasingly often used to estimate the level of silicic acid in the marine settings throughout the geological history, which enables the reconstruction of past silica cycle and ocean circulation.

[16] Spicules provide structural support for maintaining the vertical body position, minimize the metabolic cost of water exchange, [98][14] and may even deter predators.

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.

[16] Research on the Euplectella aspergillum (Venus' Flower Basket) demonstrated that the spicules of certain deep-sea sponges have similar traits to Optical fibre.

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.

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.