Sponge spicule
Spicules are structural elements found in most sponge
Sea sponge
Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...

s. They provide structural support and deter predators. Large spicules, visible to the naked eye are referred to as megascleres, while smaller, microscopic
The microscopic scale is the scale of size or length used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...

 ones are termed microscleres.


Spicules are found in a range of symmetry types.

Monaxons form simple cylinder
Cylinder (geometry)
A cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given line segment, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...

s with pointed ends. The ends of diactinal monaxons are similar, whereas monactinal monaxons have a different ends - one pointed, one rounded. Diactinal monaxons are classified by the nature of their ends: oxea have pointed ends, and strongyles 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, the triaxon has 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 (description of types to be incorporated from ). 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.


Sponges can be calcareous
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

, siliceous
Biogenic silica
Biogenic silica , also referred to as opal, biogenic opal, or amorphous opaline silica, forms one of the most widespread biogenic minerals. Silica is an amorphous metal oxide formed by complex inorganic polymerization processes. This is opposed to the other major biogenic minerals, comprising...

, or composed of spongin
Spongin, a modified type of collagen protein, forms the fibrous skeleton of most organisms among the phylum Porifera, the sponges. They are secreted by sponge cells known as spongocytes...


Taxonomic importance

The composition, size, and shape of spicules is one of the largest determining factors in sponge taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...


Types of spicule

  • Megascleres are large spicules measuring from 60-2000 microns and often function as the main support elements in the skeleton.
    • Acanthostyles are spiny styles.
    • Anatriaenes, orthrtriaenes and protriaenes are 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 microns and are scattered throughout the tissue and are not part of the main support element.
    • Anisochelas are microscleres with dissimilar ends.
    • Chelae are microscleres with shovel-like structures on the ends.
    • Euasters are star-shaped microscleres with multiple rays radiating from a common centre.
    • Forceps are microscleres bent back on themselves.
    • Isochelas are microscleres with two similar ends.
    • Microstrongyles are microscleres with both ends blunt or rounded.
    • Oxeas are microscleres with both ends pointed.
    • Oxyasters are star-shaped microscleres with thin pointed rays.
    • Sigmas are "C" or "S" shaped microscleres.
    • Spherasters are microscleres with multiple rays radiating from a spherical centre.


Spicules are formed by sclerocyte
Sclerocytes are spicule secreting cells, found in sponges. They secrete calcareous or siliceous spicules which are found in the mesohyl layer of sponges. The sclerocytes produce spicules via formation of a cellular triad. The triad of cells then undergo mitosis, creating six sclerocytes...

s, 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
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 body, but remains within the cell’s membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

. On occasion, sclerocytes may begin a second spicule while the first is still in progress.

Interaction with light

Research on the Euplectella aspergillum (Venus' Flower Basket)
Venus' Flower Basket
The Venus' Flower Basket, or Euplectella aspergillum is a hexactinellid sponge in the phylum Porifera inhabiting the deep ocean. In traditional Asian cultures, this particular sponge was given as a wedding gift because the sponge symbiotically houses two small shrimp, a male and a female, who live...

 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
Impurities are substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound.Impurities are either naturally occurring or added during synthesis of a chemical or commercial product...

 which improve the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

. In addition, these spicules have built-in lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 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
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 (as with Rosella racovitzae) or as an attractor for shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...

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.
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