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 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.
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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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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