Spicules are structural elements found in most
sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...
s. The meshing of many spicules serves as the sponge's
skeleton
A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
and thus it provides structural support and potentially defense against predators.
Sponge spicules are made of
calcium carbonate or
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
. Large spicules visible to the naked eye are referred to as megascleres, while smaller,
microscopic
The microscopic scale () is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens (optics), lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded a ...
ones are termed microscleres. The composition, size, and shape of spicules are major characters in sponge
systematics
Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic tre ...
and
taxonomy
Taxonomy is the practice and science of categorization or classification.
A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
.
Overview
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
The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...
.
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
Demosponges (Demospongiae) are the most diverse class in the phylum Porifera. They include 76.2% of all species of sponges with nearly 8,800 species worldwide (World Porifera Database). They are sponges with a soft body that covers a hard, ...
,
Hexactinellida
Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers conside ...
, and
Homoscleromorpha
Homosclerophorida is an order of marine sponges. It is the only order in the monotypic class Homoscleromorpha. The order is composed of two families: Plakinidae and Oscarellidae.
Taxonomy
Homoscleromorpha is phylogenetically well separated from ...
) 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.
[ Material was copied from this source, which is available under ]
Creative Commons Attribution 4.0 International License
Research history
In 1833,
Robert Edmond Grant
Robert Edmond Grant Doctor of Medicine, MD Royal College of Physicians of Edinburgh, FRCPEd Fellow of the Royal Society, FRS FRSE Zoological Society of London, FZS Geological Society of London, FGS (11 November 1793 – 23 August 1874) was a Br ...
grouped sponges into a phylum he called
Porifera
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through th ...
(from the Latin ''porus'' meaning "pore" and ''-fer'' meaning "bearing").
He described sponges as the simplest of
multicellular animal
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
s, sessile, marine invertebrates built from soft, spongy (amorphously shaped) material.
Later, the
Challenger expedition
The ''Challenger'' expedition of 1872–1876 was a scientific program that made many discoveries to lay the foundation of oceanography. The expedition was named after the naval vessel that undertook the trip, .
The expedition, initiated by Wil ...
(1873–1876) discovered deep in the ocean a rich collection of
glass sponge
Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers consider ...
s (class Hexactinellida), which radically changed this view. These glass sponges were described by
Franz Schulze (1840–1921), and came to be regarded as strongly individualised radially symmetric entities representing the phylogenetically oldest class of
siliceous sponge
The siliceous sponges form a major group of the phylum Porifera, consisting of classes Demospongiae and Hexactinellida. They are characterized by spicules made out of silicon dioxide, unlike calcareous sponges.
Individual siliachoates (silica ...
s. They are eye-catching because of their distinct
body plan
A body plan, ( ), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many.
This term, usually applied to animals, envisages a "blueprin ...
(see lead image above) which relies on a
filigree
Filigree (also less commonly spelled ''filagree'', and formerly written ''filigrann'' or ''filigrene'') is a form of intricate metalwork used in jewellery and other small forms of metalwork.
In jewellery, it is usually of gold and silver, ma ...
skeleton constructed using an array of morphologically determined spicules.
Then, during the German Deep Sea Expedition "
Valdivia
Valdivia (; Mapuche: Ainil) is a city and commune in southern Chile, administered by the Municipality of Valdivia. The city is named after its founder Pedro de Valdivia and is located at the confluence of the Calle-Calle, Valdivia, and Cau-Cau R ...
" (1898-1899), Schulze described the largest known siliceous hexactinellid sponge, the up to three metres high ''
Monorhaphis chuni
''Monorhaphis'' is a Monotypic taxon#Zoology, monotypic genus of Silicon dioxide, siliceous deep sea Hexactinellid sponges. The single species is the type species ''Monorhaphis chuni'', a sponge known for creating a single giant basal sponge spic ...
''. 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
Lamella (plural lamellae) means a small plate or flake in Latin, and in English may refer to:
Biology
* Lamella (mycology), a papery rib beneath a mushroom cap
* Lamella (botany)
* Lamella (surface anatomy), a plate-like structure in an animal
* ...
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
waveguide
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
s.
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
demosponge
Demosponges (Demospongiae) are the most diverse class in the phylum Porifera. They include 76.2% of all species of sponges with nearly 8,800 species worldwide (World Porifera Database). They are sponges with a soft body that covers a har ...
s. 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 (SiO
2·nH
2O).
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
Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and physi ...
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 Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...
, the time prior to the
Cambrian Explosion
The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil recor ...
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 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
The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...
and the construction of the
metazoan
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motility, able to move, ca ...
body plan.
Spicule types
Sponge spicules can be
calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.
In zoology
''Calcareous'' is used as an adje ...
or
siliceous
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
. Siliceous spicules are sometimes embedded in
spongin
Spongin, a modified type of collagen protein, forms the fibrous skeleton of most organisms among the phylum Porifera, the sponges. It is secreted by sponge cells known as spongocytes.
Spongin gives a sponge its flexibility. True spongin is found ...
. Spicules are found in a range of symmetry types.
Monaxons form simple
cylinder
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infin ...
s 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, 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.
* Megascleres are large spicules measuring from 60-2000 ]μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
and often function as the main support elements in the skeleton.
** Acanthostyles are spiny styles.
** Anatriaenes, orthotriaenes and protriaenes are triaenes[Boury-Esnault, Nicole, and Klaus Rutzler, editors. Thesaurus of Sponge Morphology. Smithsonian Contributions to Zoology, number 596, 55 pages, 305 figures, 1997. https://www.portol.org/thesaurus] - 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. Exemples are oxyasters (euasters with pointed rays) or sterrasters (ball-shaped euasters).
** 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
Animal biomineralization
Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; ...
is a controlled process and leads to the production of mineral–organic composite materials that considerably differ in shape and material properties from their purely inorganic counterparts. The ability to form functional biominerals, such as endoskeleton
An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal, composed of mineralized tissue.
Overview
An endoskeleton is a skeleton that is on the i ...
s and exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
s, protective shells, or teeth, had been a significant step in animal evolution. Calcium carbonate biomineralization, the most widespread type among animal phyla, 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. 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
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrio ...
, mitochondrial
A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is use ...
, membrane-bound
A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of the c ...
, 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 sponge
The calcareous sponges of class Calcarea are members of the animal phylum Porifera, the cellular sponges. They are characterized by spicules made of calcium carbonate in the form of calcite or aragonite. While the spicules in most species hav ...
s 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 sponge
Demosponges (Demospongiae) are the most diverse class in the phylum Porifera. They include 76.2% of all species of sponges with nearly 8,800 species worldwide (World Porifera Database). They are sponges with a soft body that covers a hard, ...
s or sclerosponge
Demosponges (Demospongiae) are the most diverse class in the phylum Porifera. They include 76.2% of all species of sponges with nearly 8,800 species worldwide (World Porifera Database). They are sponges with a soft body that covers a hard, ...
s. The biomineralizing CAs used by carbonate-producing demosponges are not orthologous
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a spec ...
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 sclerocyte Sclerocytes are specialised cells that secrete the mineralized structures in the body wall of some invertebrates.
In sponges they secrete calcareous or siliceous spicules which are found in the mesohyl
The mesohyl, formerly known as mesenchyme or ...
s, which are derived from archaeocyte
Archaeocytes (from Greek '' archaios'' "beginning" and '' kytos'' "hollow vessel") or amoebocytes are amoeboid cells found in sponges. They are totipotent and have varied functions depending on the species.
The structure of these cells match to ...
s. The sclerocyte begins with an organic filament
The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including:
Astronomy
* Galaxy filament, the largest known cosmic structures in the universe
* Solar 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 most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
body, but remains within the cell's membrane
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. B ...
. 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
Siliceous spicules
The largest biosilica structure on Earth is the giant basal spicule from the deep-sea glass sponge
Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers consider ...
''Monorhaphis chuni
''Monorhaphis'' is a Monotypic taxon#Zoology, monotypic genus of Silicon dioxide, siliceous deep sea Hexactinellid sponges. The single species is the type species ''Monorhaphis chuni'', a sponge known for creating a single giant basal sponge spic ...
''. The diagram on the right shows:
: (a) Young specimens of ''M. chuni'' anchored to the muddy substratum by one single giant basal spicule (gbs). The body (bo) surrounds the spicule as a continuous, round cylinder.
: (b) The growth phases of the sessile animal with its GBS (gbs) which anchors it to the substratum and holds the surrounding soft body (bo). The characteristic habitus displays linearly arranged large atrial openings (at) of approximately 2 cm in diameter. With growth, the soft body dies off in the basal region and exposes the bare GBS (a to c).
: (c) Part of the body (bo) with its atrial openings (at). The body surface is interspersed with ingestion openings allowing a continuous water flow though canals in the interior which open into oscules that are centralized in atrial openings, the sieve-plates.
: (d) ''M. chuni'' in its natural soft bottom habitat of bathyal slopes off New Caledonia. The specimens live at a depth of 800–1,000 m 3 In this region, the sponge occurs at a population density of 1-2 individuals per m2. The animals reach sizes of around 1 m in length.
: (e) Drawings of different glass sponges (hexactinellid
Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers consi ...
s).
Siliceous spicules in demosponges exist in a variety of shapes, some of which look like minute spheres of glass. They are called sterrasters when they belong to the Geodiidae
Geodiidae is a family of sea sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channel ...
family and selenasters when they belong to the Placospongiidae
Placospongiidae is a family of sponges belonging to the order Clionaida.
Genera:
* ''Onotoa'' Laubenfels, 1955
* '' Placospherastra'' van Soest, 2009
* ''Placospongia
''Placospongia'' is a genus of sea sponges belonging to the family Placospon ...
family.[ Material was copied from this source, which is available under ]
Creative Commons Attribution 4.0 International License
Siliceous spicules were first described and illustrated in 1753 by Vitaliano Donati
Vitaliano Donati (8 September 1717 – 26 February 1762), born in Padua in Italy, was an Italian doctor, archeologist, and botanist. He took the degree of Doctor of Philosophy in 1739.
He was interested in the flora and fauna of the Adriatic, and ...
, who found them in the species '' Geodia cydonium'' from the Adriatic Sea
The Adriatic Sea () is a body of water separating the Italian Peninsula from the Balkan Peninsula. The Adriatic is the northernmost arm of the Mediterranean Sea, extending from the Strait of Otranto (where it connects to the Ionian Sea) to t ...
: he called these spicules "little balls". They are later called globular crystalloids, globate spicules, or globostellates by sponge taxonomists, until 1888 when William Sollas[Sollas, W. J. (1888). "Report on the Tetractinellida collected by H.M.S. Challenger, during the years 1873-1876". Report on the Scientific Results of the Voyage of H.M.S. Challenger, 1873-1876. ''Zoology'', 25: 1–458.] 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
''Placospongia'' is a genus of sea sponges belonging to the family Placospongiidae.
This genus is characterized by a high density of siliceous spicule (sponge), spicules. Members of this genus are known to be eaten by hawksbill turtles.
Species ...
'', and these are at first considered as "sterrasters" before Richard Hanitsch
Karl Richard Hanitsch (22 December 1860 – 11 August 1940) was a Germany, German-born entomology, entomologist and museum curator who served as the director of the Raffles Museum in Singapore in the early 20th century.
Hanitsch was born at Grosse ...
coins the term "selenaster" in 1895 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
''Geodia'' is a genus of sea sponge belonging to the family Geodiidae. It is the type genus of its taxonomic family.
This genus is characterized by a high density of siliceous spicules. Members of this genus are known to be eaten by hawksbill t ...
''.
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. Sterrasters/aspidaster spicules are currently the main synapomorphy
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
of the Geodiidae. The family currently includes five genera with sterrasters and several others that have secondarily lost their sterrasters. The ''Geodia'' can be massive animals more than a meter across.
Selenasters are the main synapomorphy
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
of ''Placospongia
''Placospongia'' is a genus of sea sponges belonging to the family Placospongiidae.
This genus is characterized by a high density of siliceous spicule (sponge), spicules. Members of this genus are known to be eaten by hawksbill turtles.
Species ...
'' (family Placospongiidae
Placospongiidae is a family of sponges belonging to the order Clionaida.
Genera:
* ''Onotoa'' Laubenfels, 1955
* '' Placospherastra'' van Soest, 2009
* ''Placospongia
''Placospongia'' is a genus of sea sponges belonging to the family Placospon ...
, order Clionaida
Clionaida is an order of demosponges in the subclass Heteroscleromorpha
Heteroscleromorpha is a subclass of demosponges within the phylum
In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdo ...
), a well-supported monophyletic genus 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
In taxonomy, an undescribed taxon is a taxon (for example, a species) that has been discovered, but not yet formally described and named. The various Nomenclature Codes specify the requirements for a new taxon to be validly described and named. U ...
. Placospongia species are usually small, encrusting, and never occur in high densities.
Sterrasters/selenasters are big enough to examine in some detail their surfaces with an optical microscope. However, the use of the scanning electron microscope (SEM) enabled a significantly better understanding of the surface microornamentations. A few descriptive terms have also appeared to describe and compare in greater detail the microornamentations of these ball-shaped spicules. 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". The " hilum" (Latin for a "little thing" or "trifle" or the "eye of a bean") is a small area without rosettes or any kind of surface pattern. There are no particular terms to describe the surface of selenasters, except for the "hilum", also present. Although there appears to be no significant variation in the size of the rosettes and hilum between species, noticed that rosettes could be smooth or warty and hypothesized that this character could be of phylogenetic value if studied more broadly. Furthermore, the rosette morphology also seemed to be variable between ''Geodia'', ''Pachymatisma'', and ''Caminella'' which suggests that a more detailed study of the sterraster/aspidaster surface would potentially bring new characters for Geodiidae genera identification.
Spicule "life cycle"
''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 and quadrangular in hexactinellids. In calcareans (producing calcareous spicules) the axial canal is not developed. The geometry and the length of the axial filament determines the shape of the spicule. 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.
During formation of the siliceous spicules (Calcarea displays different mechanisms of spicule biomineralization), sponges obtain silicon in the form of soluble silicic acid and deposit it around the axial filament,
within a special membrane called silicalemma. Silica is first laid out as small 2 μm granules that are fused to bigger spheres (or fused together within process of biosintering in Hexactinellida. After some time, amorphous silica is added, forming evenly-deposited concentric layers, separated from each other by ultrathin organic interlayers. 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.
After completing the deposition of silica (or during this phase), the spicule is transported to the right place in the sponge body by crawling mesohyl cells, where spongocytes secrete spongin fibrils around them and connect them with adjacent spicules. In some hexactinellids, that are characterized by rigid skeleton, the fusion of spicules appears to occur parallel to spicule secretion.
When sponges are alive, their spicules provide a structural "framework". Following their death, the body and the skeleton structure, especially that of demosponges in which the spicules are connected to each other only by perishable collagen fibres, rapidly disintegrate leaving the spicules "free". Because of this, sponges are rarely wholly preserved in the fossil record. Their spicules, however, are incorporated into sediments, often becoming one of the main components of sedimentary rocks. 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,[Gammon, P.R., Middleton, G.V., Church, M.J., Coniglio, M., Hardie, L.A. and Longstaffe, F.J. (1978) "Spiculites and spongolites}. In: ''Encyclopedia of Sediments and Sedimentary Rocks'', Dordrecht: Springer.] and have been formed through the whole Phanerozoic. 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.
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 sponge bodies disintegrate, spicules become incorporated into marine sediments
Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles have their origins in soil and rocks and have been transported from the land to the sea, mainly ...
and sometimes accumulate into enormous agglomerations called spicule mats or beds, or fossilize to form special type of rocks called the spiculites.
The record of fossil and subfossil sponge spicules is extraordinarily rich and often serves as a basis for far-reaching reconstructions of sponge communities, though spicules are also bearers of significant ecological and environmental information. 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
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
in the marine settings throughout the geological history, which enables the reconstruction of past silica cycle
The silica cycle is the biogeochemical cycle in which biogenic silica is transported between the Earth's systems. Silicon is considered a bioessential element and is one of the most abundant elements on Earth. The silica cycle has significant o ...
and ocean circulation
An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...
.
Spicules provide structural support for maintaining the vertical body position, minimize the metabolic cost of water exchange,
and may even deter predators. They often develop in different sizes and a wide variety of three dimensional shapes, with many being unique to clade- or even species-level taxa. Demosponge
Demosponges (Demospongiae) are the most diverse class in the phylum Porifera. They include 76.2% of all species of sponges with nearly 8,800 species worldwide (World Porifera Database). They are sponges with a soft body that covers a har ...
s are characterized by spicules of monaxonic or tetraxonic symmetry. Hexactinellid
Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers consi ...
s produce spicules of hexactinic or triaxonic (cubic) symmetry or shapes that are clearly derived from such morpohologies. The spicules of homoscleromorph
Homosclerophorida is an order of marine sponges. It is the only order in the monotypic class Homoscleromorpha. The order is composed of two families: Plakinidae and Oscarellidae.
Taxonomy
Homoscleromorpha is phylogenetically well separated from ...
s represent peculiar tetractines (calthrop Calthrop may refer to:
* (1878–1937), English author, illustrator
* Donald Calthrop (1888–1940), English film actor
* Everard Calthrop (1857–1927), British railway engineer and inventor
* Gladys Calthrop (1894–1980), British artist and se ...
s) and their derivatives that originate through reduction or ramification of the clads. Spicules of Calcarea are produced in three basic forms: diactines, triactines and tetractines.
The mineral composition of sponge spicules makes these structures the most resistant parts of the sponge bodies and ensures the ability of spicules to withstand various taphonomic processes, 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. The morphologies of spicules and their arrangement, together with other important sponge features, such as the shape, consistency, and color, are essential when identifying sponges.
In contrast to whole-bodied sponge fossils, spicules are common in many depositional environment
In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be ...
s. 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
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...
. 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
In chemistry and materials science, impurities are chemical substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound. Firstly, a pure chemical should appear thermodynam ...
which improve the refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
. In addition, these spicules have built-in lenses
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
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 (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
(as with ''Rosella racovitzae'') or as an attractor for shrimp
Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some crustaceans outside of this order are refer ...
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
* Microfossil
A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...
References
Further references
* {{cite journal, doi=10.1038/s41598-018-37696-z , title= Soft corals form aragonite-precipitated columnar spiculite in mesophotic reefs, year= 2019, last1= Shoham, first1= Erez, last2= Prohaska, first2= Thomas, last3= Barkay, first3= Zahava, last4= Zitek, first4= Andreas, last5= Benayahu, first5= Yehuda, journal= Scientific Reports, volume= 9, issue= 1, page= 1241, pmid= 30718658, pmc= 6362064, bibcode= 2019NatSR...9.1241S
Sponge anatomy
Skeletal system