Category Archives: protists

Friday Fellow: Contractile Gentle-Scaled Centrohelid

by Piter Kehoma Boll

Unicellular eukaryotes, traditionally called protists, come in a variety of shapes and sizes and have a complex classification, especially because many lineages evolved similar features. The centrohelids, for example, have a round cell with several needle-like radially-distributed pseudopods, called actinopods, thus looking like radiolarians, but are only distantly related to them.

A single individual of the contractile gentle-scaled centrohelid. Credits to Wikimedia user NEON_ja.*

A centrohelid that has been considerably well studied recently is Raphidiophrys contractilis, to which I decided to coin the common name contractile gentle-scaled centrohelid. It was described in 1995 from specimens collected from brackish ponds in Hiroshima, Japan. As with other species of the genus Raphidiophrys, the contractile gentle-scaled centrohelid has many structures of silica, called scales, covering the cell and embedded in a gelatinous coat. These scales are more concentrated around the base of the actinopods and extend outward around part of them as well. In the contractile gentle-scaled centrohelid, the scales are oblong, flat and slightly curved, resembling a rubber boat. The convex side of the scale is always directed toward the cell.

The contractile gentle-scaled centrohelid is a predator of other protists, especially flagellates and ciliates. Small protists are captured by the actinopods and pulled quickly toward the cell body by a sudden contraction of the actinopod. This behavior is the reason for the species to be named contractilis. Once the prey is close to the cell body, it is surrounded by pseudopods and stored in a food vacuole, where it gets digested. The capture of prey using the actinopods is aided by a special organelle, called kinetocyst, found in large number below the cell membrane. When the centrohelid touches a prey, it discharges the kinetocysts, which immobilize the prey, working similarly to the cnidocytes of cnidarians.

Raphidiophrys contractilis with several flagellates of the species Chlorogonium elongatum trapped in its actinopods (A) and one of the being swallowed into a food vacuole (B). Extracted from Sakaguchi et al. (2002).

When a gentle-scale centrohelid finds a very large prey, much larger than itself sometimes, such as a ciliate Paramecium, it uses an extreme cooperative behavior. Several individual organisms fuse into a single large cell, pull chunks of the prey off and create a large common food vacuole. Isn’t that bizarre and amazing?

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References:

Kinoshita E, Suzaki T, Shigenaka Y, SugiyamaM (1995) Ultrastructure and rapid axopodial contraction of a Heliozoa, Raphidiophrys contractilis sp. nov. The Journal of Eukaryotic Microbiology 42(3): 283–288. doi: 10.1111/j.1550-7408.1995.tb01581.x

Sakaguchi M, Suzaki T, Khan SMMK, Hausmann K (2002) Food capture by kinetocysts in the heliozoon Raphidiophrys contractilis. European Journal of Protistology 37(4): 453–458. doi: 10.1078/0932-4739-00847

Siemensma FJ, Roijackers MM (1988) The genus Raphidiophrys (Actinopoda, Helozoea): scale morphology and species distinctions. Archiv für Protistenkunde 136(3): 237–248. doi: 10.1016/S0003-9365(88)80023-X

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Friday Fellow: Ehux

by Piter Kehoma Boll

We’ll continue among the unicellular marvels of the sea this week. This time our fellow is another member of a poorly known but hugely important group of protists, the coccolithophores.

The coccolithophores are a group of unicellular algae of the marine phytoplankton that is characterized by a series of calcium carbonate plates, called coccoliths, that cover their body, making them look like cells covered by scales.

Today we’ll know the most widespread and abundant species of this group, Emiliania huxleyi, usually simply called Ehux, which I will use here as its common name.

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Scanning elctron micrograph cell of Emiliania huxleyi covered by coccoliths. Credits to Alison R. Taylor.*

Ehux is found in the oceans all around the world, being absent only close to the poles. According to the fossil record, this species appeared about 270 thousand years ago, but became the dominant coccolithophore only anout 70 thousand years ago. Due to its abundance, Ehux is an important species controling global climate. As a photosynthetic organism, it helps to increase atmospheric oxygen and decrease carbon dioxide. Additionally, the fact that its cell is covered by calcium carbonate plates increases even more its importance in removing CO2 from the atmosphere. By capturing CO2 as calcium carbonate, Ehux send it directly to the ocean floor when it dies and the shell sinks.

The life cycle of Ehux is not yet completely understood, but includes at least two different cell forms. The C form is spherical, nonmotile and covered by coccoliths (hence the name C) and can reproduce asexually by fission. Another form, called S (scaly) lacks coccoliths but is covered by a group of organic scales. This form is motile, swimming using two flagella, and also reproduces asexually by fission. How one form turns into the other is unclear, but there are some evidences that the C form is diploid and the S form is haploid, so C cells could turn into S cells by meiosis and two S cells could act as gametes and fuse to produce a new C cell. A third form, called N (naked) cell is similar to a C cell but is unable to produce the coccoliths. It is assumed that they appear by a mutation of C cells that makes them lose the ability to produce coccoliths, as N cells never change back to the C form.

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A bloom of Ehux south of Great Britain as seen from a sattelite photo. Credits to NASA.

During some special conditions, such as high irradiance, ideal temperatures and nitrogen-rich waters, Ehux populations can cause blooms which extend over large portions of the ocean. This species is known as a producer of Dimethyl Sulphide (DMS), a flammable liquid that boils at 37°C and has a characteristic smell usually called “sea smell” or “cabbage smell”. The release of DMS in the atmosphere interferes in cloud formation, so that this is one more way by which Ehux influences global climate.

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References:

Paasche E (2002) Paasche, E. (2001). A review of the coccolithophorid Emiliania huxleyi (Prymnesiophyceae), with particular reference to growth, coccolith formation, and calcification-photosynthesis interactions. Phycologia 40(6), 503–529. doi:10.2216/i0031-8884-40-6-503.1

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Friday Fellow: Giant Gromia

by Piter Kehoma Boll

Some time ago I introduced a cool unicellular alga, the Sailor’s Eyeball, which can reach about 5 cm in diameter, being one of the largest unicellular organisms known to exist.

Today  we’ll know one more creature of this type, only it is not an alga, but a testate amoeba more closely related to foraminifers. Named Gromia sphaerica, I will here call it the giant gromia.

gromia fig2

Specimens of the giant gromia from the Bahamas. Image extracted from Matz et al. (2008).

The giant gromia was first found in the Arabian Sea at depths of more than 1100 m and was formally described in 2000. It lives lying on the substrate and is usually covered by a thin layer of sediment, appearing as small spheres scattered across the sea floor. The body is spherical or grape-shaped but hollow, with the interior filled with fecal material (called stercomata) or other fluids. This spherical cell is covered by a shell, or test, of organic material which shows several small perforations by which thin expansions of the cytoplasm, forming a kind of pseudopod, can be extended. The size of the test can reach up to 3 cm in diameter, being much larger than that of its best known relative, Gromia oviformis.

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Several specimens of Gromia sphaerica on the sea floor of the Bahamas with the tracks left by their movement. Extracted from Matz et al. (2008).

In 2008, another population of species was found in the waters around the Bahamas. Specimens there are not as spherical as in the population in the Arabican Sea and  were seen associated with tracks that indicate that these organisms slowly move across the sediment. The tracks clearly resemble some fossil tracks from the Pre-Cambrian period, which are usually considered an indication of the early evolution of multicellular animals. However, this discovery of unicellular organisms being able to produce tracks similar to those associated with animals raises doubt about the time of origin of multicellular animals.

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References:

Gooday AJ, Bowser SS, Bett BJ, Smith CR (2000) A large testate protist, Gromia sphaerica sp. nov. (Order Filosea), from the bathyal Arabian Sea. Deep-Sea Research II 47: 55–73.

Matz MV, Frank TM, Marshall NJ, Widder EA, Johnsen S (2008) Giant deep-sea protists produces bilaterian-like traces. Current Biology 18(23): 1849–1854. https://doi.org/10.1016/j.cub.2008.10.028

 

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Friday Fellow: Common giardia

by Piter Kehoma Boll

Parasites are always a group eager to be featured here, and human parasites have a special place in our hearts… sometimes literally. Today’s species, however, has a special place in our small intestine.

Called Giardia lamblia, sometimes also identified under the outdated synonyms Giardia duodenalis or Giardia intestinalis, our species has not a common name, but as it is the most popular and widespread species of the genus Giardia, I decided to call it simply the common giardia.

The common giardia is a flagellated unicellular organism that affects not only humans but several other mammal species. In the wild, the common giardia exists in the form of an inert cyst that can survive for prolonged periods and under different environmental conditions.

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A cyst of Giardia lamblia. Credits to Centers for Disease Control and Prevention.

When the cysts are ingested by humans or other mammals, they develop into the active stage, called trophozoite, once they reach the small intestine. The trophozoite is a flagellated cell with two well-developed nuclei that make it look like a smiling face. In this stage, the common giardia reproduced by simple binary fission. For a long time, it was thought that sexual reproduction did not occur at all in this species, but some recent evidence indicate that recombination may occur, although it is not very clear yet how it happens.

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Two trophozoites of the common giardia under the microscope. Credits to Josef Reischig.*

The ventral surface of the trophozoite is concave, forming an adhesive disk that attaches the cell to the wall of the intestine, preventing it to be transported downward the intestinal tract. Although not invading the intestinal cells, the infection of Giardia lamblia usually causes diarrhea and malabsorption. When exposed to biliar secretions, the common giardia may develop into a cyst and is thus eliminated with the feces, allowing the cycle to begin again.

Humans are very often contaminated by several means, such as by ingesting contaminated water, which may include both urban untreated water or clear water in the wild where other mammals may have defecated. It is, therefore, a common infection among hikers, people living under poor sanitary conditions and so on.

The common giardia has some peculiarities, such as the lack of mitochondria, which for some time led to the assumptions that they may belong to a very primitive group of Eukaryotes. Recently, however, a vestigial organelle that likely derived from mitochondria, named mitosome, has been found in this species, suggesting that this feature is a secondary loss caused by its parasitic life in an anoxic environment.

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References:

Adam, R. D. (2001) Biology of Giardia lambliaClinical Microbiology Reviews 14(3): 447–475.

Wikipedia. Giardia lamblia. Available at < https://en.wikipedia.org/wiki/Giardia_lamblia >. Access on 28 June 2018.

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Friday Fellow: Common Pelomyxa

by Piter Kehoma Boll

It’s time to dig deep into the mud again and bring up a peculiar protist, the third species of the clade Amoebozoa to be featured here. Its binomial name is Pelomyxa palustris, and I decided to call it the common pelomyxa.

Measuring up to 5 mm in length, although usually having less than 1 mm, the common pelomyxa is considered a “giant amoeba”. In fact, the Giant Amoeba Chaos carolinense, previously featured here, was once classified in the genus Pelomyxa, but currently we know that they are not closely related at all. While the true giant amoebas of the genus Chaos are closely related to the common amoebas of the genus Amoeba, the species in Pelomyxa belong to a completely different group of amoebas.

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A specimen of the common pelomyxa. The large pseudopod is to the right and the small uroid can be seen projecting from the cell at the upper left. Credits to Proyecto Agua.*

The cell of the common pelomyxa has a somewhat cylindrical shape with a single, large, semicircular pseudopod at the front, thus moving basically always in the same direction, forward, differently from the more classical amoebas with several pseudopods and the ability to move in any direction. At the opposite side of the cell, the common pelomyxa has a small, also semicircular appendix called the uroid that is covered by several small non-motile flagella. The flagella are surrounded by small cytoplasmic projections (villi) that are easily seen under the microscope.

The common pelomyxa lives buried in the sediments of freshwater lakes throughout the northern hemisphere, especially those rich in decaying organic matter. It slides through the mud while feeding on smaller microorganisms and organic debris. Such an environment is characterized by the complete absence or extremely low concentrations of oxygen. As a result, the common pelomyxa is anaerobic and even lacks mitochondria. For this reason, it was once considered part of a very primitive group of eukaryotes that diverged before the incorporation of the endosymbiotic bacteria that would evolve into mitochondria. Currently, however, it is known that their lack of mitochondria is actually due to a secondary loss and they seem to be related to true amoebas and slime molds.

More than only lacking mitochondria, the cell of the common pelomyxa has a lot of peculiar features. Depending on the size of the cell, it may contain a few to several hundred nuclei. The cytoplasm also appears to lack several typical eukaryotic organelles, such as the Golgi apparatus and the endoplasmic reticulum. There are, however, many, many small vacuoles, so many that the cell usually has a foamy appearance.

Endosymbiotic bacteria are also found in great numbers in the cytoplasm of the common pelomyxa. After several years of research, it seems that these bacteria are obligate symbionts. Perhaps they help this strange amoeba to perform some of the tasks that should be done by the several organelles that it lacks.

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References:

Goodkov, A. V.; Chistyakova, L. V.; Seravin, L. N.; Frolov, A. O. (2004) The concept of pelobionts (Class Peloflagellatea): Brief history and current stateEntomological Review 84(Suppl. 2): S10–S20.

Wikipedia. Pelomyxa. Available at < https://en.wikipedia.org/wiki/Pelomyxa >. Access on May 26, 2018.

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Friday Fellow: Handsome Asterisk-Diatom

by Piter Kehoma Boll

It’s time for the next diatom to be featured here. Differently from the previous ones, today’s diatom is a freshwater species commonly found in lakes of North America and Eurasia. It has also been reported for South America and Africa, but it is likely that these individuals actually belong to another, closely related species.

asterionella_formosa

Asterionella formosa from a lake of the Rocky Mountain National Park, United States

Named Asterionella formosa, this diatom has small rod-shaped cells that are 60 to 85 µm long and only 2 to 4 µm wide. The individuals usually organize themselves in colonies linked by one of the ends in a star fashion. Most colonies include eight organisms and look somewhat like an asterisk, hence I chose to give the common name asterisk-diatom to the genus, this species then being called the “handsome asterisk-diatom”, from the translation of the specific epithet formosa. However, some colonies may have up to 20 individuals and organize in a more spiral fashion.

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A spiral-shaped colony from a small lake in Spain. Credits to Proyecto Agua.*

Originally found and described from water supplies used in London, the handsome asterisk-diatom has a preference for cold waters, occurring commonly in temperate lakes under temperatures between 0 and 15 °C. During summer, when temperatures get too high and the light intensity also increases, its photosynthesis is inhibited by these two factors as well as by the increase in oxygen caused by the metabolism of the species itself as well of other algae from the phytoplanktonic community.

Sexual reproduction is not well known in the handsome asterisk-diatom, but must certainly occur, as asexual reproduction alone leads to a continuous decrease in cell size in all diatoms. Studies on genetic diversity show that this species is very genetically diverse, which proves that sexual reproduction indeed occurs and in a apparently high rate, contributing for the dominance of this species in many of the ecosystems of which it takes part.

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References:

AlgaeBase. Asterionella formosa Hassall. Available at < http://www.algaebase.org/search/species/detail/?species_id=31441 >. Access on May 25, 2018.

Belay, A.; Fogg, G. E. (1978) Photoinhibition of photosynthesis in Asterionella formosa (Bacillariophyceae). Journal of Phycology14(3): 341–347. https://doi.org/10.1111/j.1529-8817.1978.tb00310.x

De Bruin, A.; Ibelings, B. W.; Rijkeboer, M.; Brehm, M.; Van Donk, E. (2004) Genetic variation in Asterionella formosa (Bacillariophyceae): is it linked to frequent epidemics of host-specific parasitic fungi? Journal of Phycology40(5): 823–830. https://doi.org/10.1111/j.1529-8817.2004.04006.x

EOL – Enclyclopedia of Life. Asterionella formosa. Available at < http://eol.org/pages/917771/details >. Access on May 25, 2018.

Lund, J. W. G. (1950) Studies on Asterionella formosa Hass: II. Nutrient depletion and the spring maximum. Journal of Ecology38(1): 15–35.

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Friday Fellow: Pink Miniacina

by Piter Kehoma Boll

It’s time  for the next foraminifer, which is always a problematic time, but I managed to find a suitable fellow for this Friday. Called Miniacina miniacea in the scientific community, it obviously lacks a common name, so I decided to call it the pink miniacina.

Differently from the previously introduced foraminifers, the pink miniacina is a sessile and colonial species. It usually grows attached to other lifeforms, especially algae and corals. Due to its colonial nature, added to the already bigger-than-average size of foramnifers when compared to other unicellular organisms, the pink miniacina is easily visible to the naked eye and can be seen as a series of small branched organisms with an intense pink color. It is particulary common in the Mediterranean Sea, although it can be found in other places as well.

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Several pink colonies of Miniacina miniacea growing in the Mediterranean Sea. Photo by Stefano Guerrieri.

Due to its habit of living on the surface of other sessile organisms, the pink miniacina competes with many other organisms that have the same behavior. As a result, its abundance tends to increase in deeper water, where many of such organisms find the conditions too unsuitable to live. In a few areas, the abundance of the pink miniacina may be high enough to create a “pink sand” from the shells of dead specimens.

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References:

Di Camillo, C.; Bo, M.; Lavorato, A.; Morigi, C.; Reinach, M. S.; Puce, S.; Bavestrello, G. (2008) Foraminifers epibiontic on Eudendrium (Cnidaria: Hydrozoa) from the Mediterranean Sea. Journal of the Marine Biological Association of the United Kingdom88(3): 485–489. https://doi.org/10.1017/S0025315408001045

Milliman, J. D.(1976) Miniacina miniacea: modern foraminiferal sands on the Outer Moroccan shelf. Sedimentology23: 415–419. https://doi.org/10.1111/j.1365-3091.1976.tb00059.x

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Friday Fellow: Sea Sparkle

by Piter Kehoma Boll

If you live near the sea or visit it often, you may sometimes have seen the waves glowing while breaking on the shore at night. This beautiful phenomenon is caused by the presence of bioluminescent microorganisms, the most famous of which is our newest Friday Fellow. Scientifically known as Noctiluca scintillans, it is populary known as the sea sparkle.

Bioluminescent_sea

Waves glowing blue at Atami, Japan. Photo by Kanon Serizawa.*

The sea sparkle is a dinoflagellate and is common worlwide. It is an heterotrophic flagellate and feeds on many other small organisms, such as bacteria, diatoms, other dinoflagellates and even eggs of copepods and fish. Having only a small tentacle and a rudimentar flagellum, the sea sparkle is unable to swim, making it a very unusual predator. Studies have suggested that it preys by bumping into the prey during water flow or by ascending or descending in the water column due to density differences. It can also produce a string of mucus attached to the tentacle that entagles prey and brings them to their horrible end.

noctiluca_scintillans_unica

A single Noctiluca scintillans. Photo by Maria Antónia Sampayo, Instituto de Oceanografia, Faculdade Ciências da Universidade de Lisboa.**

In temperate waters, the sea sparkle is an exclusive predator, but in tropical water it may maintain some of the ingested algae alive and use them in a symbiotic association to receive nutrients from photosynthesis. Diatoms of the genus Thalassiosira appear to be one of its favorites.

The most striking feature of the sea sparkle, however, is its bioluminescence, from which it receives its names. The light that it emits is produced by a chemical reaction between a compound called luciferin and an enzyme, called luciferase, that oxidizes it, causing it to emit light. The phenomenon is clearly visible on the sea during blooms of the dinoflagellate, which usually happen right after a bloom of its food.

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References:

Kiørbe, T.; Titelman, J. (1998) Feeding, prey selection and prey encounter mechanisms in the heterotrophic dinoflagellate Noctiluca scintillansJournal of Plankton Research 20(8): 1615–1636.

Quevedo, M.; Gonzalez-Quiros, R.; Anadon, R. (1999) Evidence of heavy predation by Noctiluca scintillans on Acartia clausi (Copepoda) eggs of the central Cantabrian coast (NW Spain). Oceanologica Acta 22(1): 127–131.

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Friday Fellow: Lyre ship diatom

by Piter Kehoma Boll

It’s time for the next diatom, and just as with the radiolarian from the last week, it’s a hard task to find good pictures and good information of any species to present here.

Today I’m introducing a species of the most diverse (I guess, or at least one of the most diverse) genus of diatoms, Navicula, a name that means “little ship” in Latin due to the shape of the cells. There are more than 1200 species in this genus, and one of them is called Navicula lyra, which I decided to call the lyre ship diatom. I have also seen it with the name Lyrella lyra, being the type-species of a genus Lyrella (little lyre) that was split from Navicula. I don’t know which one is the official form today, but it seems that Lyrella is sometimes something like a subgenus of Navicula, although sometimes both genera are not even in the same family!

Navicula_lyra

Navicula lyra, a lyre little ship. Photo by Patrice Duros.*

Anyway, the lyre ship diatom is a planktonic species that is found in all the oceans of the world, being present in species lists everywhere. It measures about 100 µm or less, a typical size for a diatom.

As with other diatoms in the genera Navicula and Lyrella, the lyre ship diatom has different varieties, which may eventually be revealed to be separate species, I guess. See, for example, the variety constricta shown below. It looks considerably different from the picture above, which appears to be from the type variety.

Navicula_lyra

Lyrella lyra var. constricta. Extracted from Siqueiros-Beltrones et al. (2017)

Despite being a widespread species, little seems to be known about the natural history of the lyre ship diatom. Aren’t you interested in studying the ecology of these tiny little glass ships?

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References:

Nevrova, E.; Witkowski, A.; Kulikovskiy, M.; Lange-Bertalot, H.; Kociolek, J. P. (2013) A revision of the diatom genus Lyrella Karayeva (Bacillariophyta: Lyrellaceae) from the Black Sea, with descriptions of five new species. Phytotaxa 83(1): 1–38.

Siqueiros-Beltrones, D. A.; Argumedo-Hernández, U.; López-Fuerte, F. O. (2017) New records and combinations of Lyrella (Bacillariophyceae: Lyrellales) from a protected coastal lagoon of the northwestern Mexican Pacific. Revista Mexicana de Biodiversidad 88(1): 1–20.

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Friday Fellow: Twisted-Spined Sponge Radiolarian

by Piter Kehoma Boll

Oh, it’s time for our next radiolarian. As as usual, it’s hard to find good information on any species. (If you work with radiolarians and have good available resources and nice species to suggest, please contact us!)

It’s hard to find pictures of live radiolarians, especially those identified to the species level, but one that I found is seen below and is called Spongosphaera streptacantha, or the twisted-spined sponge radiolarian, as I decided to call it.

4xspongospaerastreptacantha2014oct27

A nice photo of a liveSpongosphaera streptacantha. Extracted from Galerie de l’Observatoire Océanologique de Villefranche-sur-Mer.

The twisted-spined sponge radiolarian is found in warm waters in the Atlantic and Pacific oceans (perhaps the Indian too?) and, as one can notice, may have a diameter of more than 1 mm if we count the longest spines. As with most radiolarians, the cell of this species has two concentric shells and a set of spines, which are 6 to 15 in number.

The food of the twisted-spined sponge radiolarian consists of smaller organisms, such as bacteria and algae, which it captures with the long rod-like pseudopods called actinopodia.

As with most radiolarians, the twisted-spined sponge radiolarian is understudied regarding its ecology. Let’s hope more people get interested in studying this fascinating group of organisms.

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References:

Kurihara, T.; Matsuoka, A. (2004) Shell structure and morphological variation in Spongosphaera streptacantha Haeckel (Spumellaria, Radiolaria). Science Reports of Niigata University (Geology), 19: 35–48. http://hdl.handle.net/10191/2141

Matsuoka, A. (2007) Living radiolarian feeding mechanisms: new light on past marine ecosystems. Swiss Journal of Geosciences, 100: 273-279. https://dx.doi.org/10.1007/s00015-007-1228-y

Radiolaria.org: Spongosphaera streptacantha. Available at: < http://www.radiolaria.org/species.htm?sp_id=90 >. Access on August 8, 2017.

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