Editing Actinophryid (section)

==Description==

'''Actinophryids''' are largely aquatic protozoa with a spherical cell body and many needle-like axopodia. They resemble the shape of a sun due to this structure, which is the inspiration for their common name: [[heliozoa]], or "sun-animalcules". Their bodies, without arms, range in size from a few tens of micrometers to slightly under a millimeter across.<ref name="CavSmith2013">{{cite journal|last1=Cavalier-Smith|first1=T|last2=Scoble|first2=JM|title=Phylogeny of Heterokonta: Incisomonas marina, a uniciliate gliding opalozoan related to Solenicola (Nanomonadea), and evidence that Actinophryida evolved from raphidophytes.|journal=European Journal of Protistology|date=August 2013|volume=49|issue=3|pages=328–53|doi=10.1016/j.ejop.2012.09.002|pmid=23219323}}</ref>

The outer region of cell body is often vacuolated. The endoplasm of actinophryids is less vacuolated than the outer layer, and a sharp boundary layer may be seen by light microscopy.<ref name="Barrett1958">{{cite journal|last1=Barrett|first1=J.|title=Some Observations on Actinosphaerium nucleofilum n. sp., a New Fresh Water Actinophryid.|journal=The Journal of Protozoology|date=August 1958|volume=5|issue=3|pages=205–209|doi=10.1111/j.1550-7408.1958.tb02553.x}}</ref> The organisms can be either mononucleate, with a single, well defined nucleus in the center of the cell body, or [[multinucleate]], with 10 or more nuclei located under the outer vacuolated layer of cytoplasm. The cytoplasm of actinophryids is often granular, similar to that of ''[[Amoeba]]''.<ref name="Anderson1960">{{cite journal|last1=Anderson|first1=E.|last2=Beams|first2=H. W.|title=The Fine Structure of the Heliozoan, Actinosphaerium nucleofilum|journal=The Journal of Protozoology|date=May 1960|volume=7|issue=2|pages=190–199|doi=10.1111/j.1550-7408.1960.tb00729.x}}</ref>

Actinophryid cells may fuse when feeding, creating larger aggregated organisms.  Fine granules that occur just under the cell membrane are used up when food vacuoles form to enclose prey.<ref>Patterson, D. J. & Hausmann, K. 1981. Feeding by Actinophrys sol (Protista, Heliozoa): I. Light microscopy. Microbios 31: 39–55.</ref>  Actinophryids may also form cysts when food is not readily available. A layer of siliceous plates is deposited under the cell membrane during the encystment process.<ref>Patterson, D.J. 1979. On the organization and classification of the protozoon Actinophrys sol Ehrenberg, 1830. Microbios 26: 165–208.</ref>

[[File:Acti1000.webm|thumb|Video of a [[contractile vacuole]] collapse in ''Actinosphaerium'']]
Contractile vacuoles are common in these organisms, which are presumed to use them to maintain body volume by expelling fluids to compensate for the entry of water by osmosis. Contractile vacuoles are visible as clear bulges from the surface of the cell body that slowly fill then rapidly deflate, expelling their contents into the environment.

===Axopodia===

[[File:Axopodium Mikrotubuli.jpg|thumb|Cross-section of the double spiral [[microtubule]] structure in an axopod]]
The most distinctive characteristic of the actinophryids is their axopodia. These axopodia consist of a central, rigid rod which is coated in a thin layer of ectoplasm. In ''Actinophrys'' the axonemes end on the surface of the central nucleus, and in the multicellular ''Actinosphaerium'' they end at or near nuclei.<ref name="Anderson1960" /> The axonemes are composed of microtubules arranged in a double spiral pattern characteristic of the order.<ref>{{cite book|author1=Gast, R.J.|editor1-last=Archibald|editor1-first=J.|editor2-last=Simpson|editor2-first=A.|editor3-last=Slamovits|editor3-first=C.|editor4-last=Margulis|editor4-first=L.|editor4-link=Lynn Margulis |editor5-last=Melkonian|editor5-first=M.|editor6-last=Chapman|editor6-first=D.|editor7-last=Corliss|editor7-first=J.|title=Handbook of the Protists|pages=1–17|date=2017|publisher=Springer International|location=Cham, Switzerland|isbn=978-3-319-32669-6|doi=10.1007/978-3-319-32669-6_28-1|chapter=Centrohelida and Other Heliozoan-like Protists}}</ref> Due to their long, parallel construction, these microtubules demonstrate strong birefringence.<ref name="Tilney1967">{{cite journal|last1=Tilney|first1=L.|last2=Porter|first2=K.|title=Studies on the microtubules in heliozoa II. The effect of low temperature on these structures in the formation and maintenance of the axopodia|journal=Journal of Cell Biology|date=July 1967|volume=34|issue=1|pages=327–343|pmc=2107222|pmid=6033539|doi=10.1083/jcb.34.1.327}}</ref><ref name="Suzaki1994">{{cite journal|last1=Suzaki|first1=Toshinobu|last2=Ando|first2=Motonori|last3=Inai|first3=Yoko|last4=Shigenaka|first4=Yoshinobu|title=Structure and function of the cytoskeleton in heliozoa|journal=European Journal of Protistology|date=November 1994|volume=30|issue=4|pages=404–413|doi=10.1016/S0932-4739(11)80215-4}}</ref>

These axopodia are used for prey capture, in movement, cell fusion and perhaps division.<ref name="Suzaki1980" /><ref name="Ando1989" /> They are stiff but may flex especially near their tips,<ref name="Barrett1958" /> and are highly dynamic, undergoing frequent construction and destruction. When used to collect prey items, two methods of capture have been noted, termed axopodial flow and rapid axopodial contraction.<ref name="Suzaki1980" /> Axopodial flow involves the slow movement of a prey item along the surface of the axopod as the ectoplasm itself moves, while rapid axopodial contraction involves the collapse of the axoneme's microtubule structure.<ref name="Suzaki1994" /> This behavior has been documented in many species, including ''Actinosphaerium nucleofilum'', ''Actinophrys sol'', and ''[[Raphidiophrys contractilis]]''.<ref name="Suzaki1994" /><ref name="Kinoshita2001">{{cite journal|last1=Kinoshita|first1=E|last2=Shigenaka|first2=Y|last3=Suzaki|first3=T|title=The ultrastructure of contractile tubules in the heliozoon Actinophrys sol and their possible involvement in rapid axopodial contraction.|journal=The Journal of Eukaryotic Microbiology|date=2001|volume=48|issue=5|pages=519–26|pmid=11596916|doi=10.1111/j.1550-7408.2001.tb00187.x}}</ref><ref name="Kinoshita1995">{{cite journal|last1=Kinoshita|first1=Eiji|last2=Suzaki|first2=Toshinobu|last3=Shigenaka|first3=Yoshinobu|last4=Sugiyama|first4=Masanori|title=Ultrastructure and Rapid Axopodial Contraction of a Heliozoa, ''Raphidiophrys contractilis'' sp. nov.|journal=The Journal of Eukaryotic Microbiology|date=May 1995|volume=42|issue=3|pages=283–288|doi=10.1111/j.1550-7408.1995.tb01581.x}}</ref> The rapid axopodial contraction occurs at high speed, often in excess of 5mm/s or tens of body lengths per second.<ref name="Shigenaka1982">{{cite book|last1=Shigenaka|first1=Y.|last2=Yano|first2=K.|last3=Suzaki|first3=T.|title=Biological functions of microtubules and related structures|date=1982|publisher=Academic Press|location=Tokyo|pages=105–114|chapter=Shigenaka, Y., Yano, K., Yogosawa, R. and Suzaki, T., 1982. Rapid contraction of the microtubule-containing axopodia in a large heliozoan Echinosphaerium}}</ref>

The axopodial contractions have been shown to be highly sensitive to environmental factors such as temperature and pressure<ref name="Tilney1967" /><ref name="Tilney1969">{{cite journal|last1=Tilney|first1=Lewis G.|last2=Byers|first2=Breck|title=Studies on the Microtubules in Heliozoa V. Factors Controlling the Organization of Microtubules in the Axonemal Pattern in Echinosphaerium nucleofilum|journal=The Journal of Cell Biology|date=1 October 1969|volume=43|issue=1|pages=148–165|pmc=2107851|issn=0021-9525|pmid=5824062|doi=10.1083/jcb.43.1.148}}</ref> as well as chemical signals like Ca<sup>2+</sup> and colchicine.<ref name="Kinoshita2001" /><ref name="Tilney1968">{{cite journal|last1=Tilney|first1=L.|title=Studies on the microtubules in heliozoa. IV. The effect of colchicine on the formation and maintenance of the axopodia and the redevelopment of pattern in Actinosphaerium nucleofilum (Barrett).|journal=Journal of Cell Science|date=December 1968|volume=3|issue=4|pages=549–62|doi=10.1242/jcs.3.4.549 |pmid=5707852}}</ref>

===Reproduction===

[[File:Heliozoen.jpg|thumb|''Actinophrys'' undergoing multiple plasmotomy]]

Reproduction in actinophryids generally takes place via fission, where one parent cell divides into two or more daughter cells. For multinucleate heliozoa, this process is [[plasmotomy|plasmotomic]] as the nuclei are not duplicated prior to division.<ref name="Barrett1958" /> It has been observed that reproduction appears to be a response to food scarcity, with an increased number of divisions following the removal of food and larger organisms during times of food excess.<ref name="Johnson1894">{{cite journal|last1=Johnson|first1=Herbert P.|title=The plastogamy of actinosphaerium|journal=Journal of Morphology|date=April 1894|volume=9|issue=2|pages=269–276|doi=10.1002/jmor.1050090206|hdl=2027/hvd.32044107306375|hdl-access=free}}</ref>

Actinophryids also undergo [[autogamy]] during times of food scarcity. This is better described as genetic reorganization than reproduction, as the number of individuals produced is the same as the initial number. Nonetheless, it serves as a way to increase genetic diversity within an individual which may improve the likelihood of expressing favorable genetic traits.<ref name="Grell1973">{{cite book|last1=Grell|first1=Karl Gottlieb|title=Protozoology|date=2013|publisher=Springer Berlin Heidelberg|location=Berlin, Heidelberg|isbn=9783642619588|pages=178–181}}</ref>

Plastogamy has also been extensively documented in actinophryids, especially in multinucleate ones. ''Actinosphaerium'' were observed to combine freely without the combination of nuclei, and this process sometimes resulted in more or less individuals than originally combined. This process is not caused merely by contact between two individuals but can be caused by damage to the cell body.<ref name="Johnson1894" />

===Cyst function and formation===

Under unfavourable conditions, some species will form a [[cyst]]. This is often the product of autogamy, in which case the cysts produced are [[zygote]]s.<ref name="Grell1973" /> Cells undergoing this process withdraw their axopodia, adhere to the substrate, and take on an opaque and grayish appearance.<ref>{{cite journal|last1=MacKinnon|first1=D. L.|title=A few Observations on the Encystation of Actinosphaerium eichhorni under different conditions of Temperature|journal=Quarterly Journal of Microscopical Science|date=1906|volume=52|pages=407–422|url=http://jcs.biologists.org/content/joces/s2-52/207/407.full.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://jcs.biologists.org/content/joces/s2-52/207/407.full.pdf |archive-date=2022-10-09 |url-status=live|access-date=2 January 2018}}</ref> This cyst then divides until only [[uninucleate]] cells remain. The cyst wall is thickly layered 7–8 times and includes gelatinous layers, layers of silica plates, and iron.<ref name="Patterson1981">{{cite journal|last1=Patterson|first1=D.|last2=Thompson|first2=D.|title=Structure and Elemental Composition of the Cyst Wall of Echinosphaerium nucleofilum Barrett (Heliozoea, Actinophryida)|journal=The Journal of Protozoology|date=May 1981|volume=28|issue=2|pages=188–192|doi=10.1111/j.1550-7408.1981.tb02831.x}}</ref>