{{Short description|Genus of unicellular protists}} {{Automatic taxobox | taxon = Octactis | authority = Schiller, 1925<ref name="Schiller 1925"/> emend. F.H.Chang, J.M.Grieve & J.E.Sutherland, 2017<ref name="Revision Dictyochales"/> | image = Dictyocha_speculum_Stöhr_1880_Palæontogr_vol_xxvi_Taf_vii_fig_8.png | image_caption = Illustration of ''Octactis speculum'' silica skeletons by Stöhr (1880)<ref name="Stöhr 1880"/> | subdivision_ranks = Species | subdivision_ref = <ref name="Revision Dictyochales"/> | subdivision = * ''O. octonaria'' * ''O. speculum'' | synonyms_ref = | synonyms = | diversity = }}
'''''Octactis''''' is a genus of silicoflagellates,<ref name="Revision Dictyochales"/> marine photosynthetic unicellular protists that take the form of either flagellates or axopodial amoebae.<ref name="IGP Silicoflagellata"/> Described by Josef Schiller in 1925, ''Octactis'' contains various species of marine phytoplankton, some of them responsible for algal blooms that are toxic to fish.<ref name="Prego 2023"/>
== Characteristics ==
''Octactis'' species are unicellular protists that appear as several different multinucleate forms (i.e. with multiple nuclei). The most prevalent form is as amoebae surrounded by an external, basket-like siliceous skeleton, consisting of two conspicuous rings: a smaller apical ring supported by short struts that are attached to a larger basal ring. The skeleton pikes sometimes occur on the surface of the basal ring. The large window that the apical ring encloses is undivided. Other forms also present are naked amoebae and sometimes mucocyst-bearing amoebae. Both skeleton-bearing and naked cells present a long flagellum. The mucocysts appear on the periphery of skeleton-bearing cells, absent in the naked cells.<ref name="Revision Dictyochales"/>
Within each ''Octactis'' cell there is a pyrenoid embedded within a chloroplast. The chloroplast has a short thylakoid intrusion. The skeleton-bearing cells contain between 6 and 8 Golgi bodies. The chloroplasts contain all three accessory chlorophylls, c{{sub|1}}, c{{sub|2}} and c{{sub|3}}, as well as fucoxanthin as the main carotenoid, with diadinoxanthin and one or two oxyfucoxanthin derivatives. They lack lutein.<ref name="Revision Dictyochales"/>
== Ecology ==
''Octactis'' species are unicellular algae found in typically cold marine waters as part of the phytoplankton. Like other silicoflagellates, ''Octactis'' cells are surrounded by a siliceous skeleton composed of various spikes arranged in specific shapes.<ref name="Lee Dictyochophyceae"/> One species, ''O. speculum'', is considered harmful for marine fauna, due to appearing in harmful algal blooms that have caused high mortality of fish such as salmon and trout. The silica skeletons provoke irritation and secretion of mucus in fish gills, which leads to death. It has also been suggested that the mortality was caused by excessive cellular respiration during algal blooms of ''O. speculum'', which decreased the dissolved oxygen in the water.<ref name="Prego 2023"/>
== Systematics ==
=== Taxonomic history ===
The genus ''Octactis'' was described in 1925 by phycologist Josef Schiller as a member of the Silicoflagellata,<ref name="Schiller 1925"/> now known as the order Dictyochales, a group of stramenopile algae with siliceous skeletons.<ref name="Lee Dictyochophyceae"/> Several species have been described, but they have been rearranged between this genus and two more genera, ''Dictyocha'' and ''Stephanocha'' (formerly under the invalid ''Distephanus'' name due to being homonymous with the flowering plant ''Distephanus'')<ref name="Stephanocha"/> multiple times.<ref name="Revision Dictyochales"/>
The first ''Octactis'' species to be described was ''O. pulchra'', proposed by Schiller himself in 1925. It was described from superficial marine samples of the middle to southern Adriatic Sea, and remained its type and only species. In 1985 the entire genus with all its species were synonymised to ''Distephanus''<ref name="Ling & Takahashi 1985"/> (later ''Stephanocha'').<ref name="Stephanocha"/> On the basis of molecular phylogeny, researchers in 2017 emended the definition of ''Octactis'' and transferred two species of ''Dictyocha'', ''D. octonaria'' and ''D. speculum'', to it.<ref name="Revision Dictyochales"/>
=== Classification ===
There are two species of ''Octactis'' currently accepted:<ref name="Revision Dictyochales"/>
* ''Octactis octonaria'' {{au|(Ehrenberg, 1844) Hovasse, 1946}} : = ''Dictyocha octonaria'' {{au|Ehrenberg, 1844}} * ''Octactis speculum'' {{au|(Ehrenberg, 1839) F.H.Chang, J.M.Grieve & J.E.Sutherland, 2017}} : = ''Dictyocha speculum'' {{au|Ehrenberg, 1839}}{{br}} = ''Distephanus speculum'' {{au|(Ehrenberg, 1839) Haeckel, 1887}}{{br}} = ''Stephanocha speculum'' {{au|(Ehrenberg, 1839) McCartney & R.W.Jordan, 2015}}<ref name="Stephanocha"/>
== References == <references>
<ref name="Stöhr 1880">{{cite journal | title = Die Radiolarienfauna der Tripoli von Grotte Provinz Girgenti in Sicilien | first1 = Emil | last1 = Stöhr | journal = Palaeontographica | volume = 2 | pages = 69–124 | url = http://aquaparadox.obs-vlfr.fr/html/PFD/Taxonomic%20Monographs/Stohr1880.pdf }}</ref>
<ref name="Schiller 1925">{{cite journal | title = Die planktontischen Vegetationen des Adriatischen Meeres. B. Chrysomonadina, Heterokontae, Cryptomonadina, Eugleninae, Volvocales. 1. Systematischer Teil. | first1 = Jos. | last1 = Schiller | journal = Archiv für Protistenkunde | volume = 53 | pages = 59–123 | url = https://www.zobodat.at/pdf/Archiv-fuer-Protistenkunde_53_1926_0059-0123.pdf }}</ref>
<ref name="Revision Dictyochales">{{cite journal | first1 = Fook Hoe | last1 = Chang |first2 = Judy | last2 = Sutherland | first3 = Janet | last3 = Bradford-Grieve | title = Taxonomic revision of Dictyochales (Dictyochophyceae) based on morphological, ultrastructural, biochemical and molecular data | journal = Phycological Research | date = 2017 | volume = 65 | issue = 3 | pages = 235–247 | doi = 10.1111/pre.12181| s2cid = 89818534 }}</ref>
<ref name="IGP Silicoflagellata">{{cite book | chapter = Class Silicoflagellata Lemmermann, 1901 | first1 = Øjvind | last1 = Moestrup | first2 = Charles J. | last2 = O'Kelly | title = An Illustrated Guide to the Protozoa | edition = 2nd | editor-first1 = John J. | editor-last1 = Lee | editor-first2 = Gordon F. | editor-last2 = Leedale | editor-first3 = Phyllis | editor-last3 = Bradbury | publisher = Society of Protozoologists | publication-place = Lawrence, Kansas | date = 2000 | url = https://protistologists.org/publications/illustrated-guide-to-the-protozoa/ | volume = II | pages = 775–782 | chapter-url = https://protistologists.org/wp-content/uploads/2023/07/17SILICOFLAGELLATA.pdf}}</ref>
<ref name="Prego 2023">{{cite journal | last1 = Prego | first1 = Ricardo | first2 = Rafael | last2 = Carballeira | first3 = Yolanda | last3 = Pazos | first4 = Roberto | last4 = Bao | date = 2023 | title = Oceanographical Context of the First Bloom of the Silicoflagellate ''Octactis speculum'' (Ehrenberg) Recorded to Cause Salmon Mortality in a Galician Ria: Was This Bloom a Rare Event in the Iberian Coast? | journal = Toxins | volume = 15 | issue = 7 | pages = 435 | doi = 10.3390/toxins15070435 | pmid = 37505704 | pmc = 10467100 | doi-access = free }}</ref>
<ref name="Stephanocha">{{cite journal|doi=10.11646/phytotaxa.201.3.1 | title = ''Stephanocha nom. nov.'', a replacement name for the illegitimate silicoflagellate genus ''Distephanus'' (Dictyochophyceae) | first1 = Richard | last1 = Jordan | first2 = Kevin |last2 = McCartney | journal = Phytotaxa | volume = 201 | issue = 3 | date = 2015| page = 177 }}</ref>
<ref name="Lee Dictyochophyceae">{{cite book | first = R.E. | last = Lee | chapter = 14. Heterokontophyta: Dictyochophyceae | title = Phycology | chapter-url = https://books.google.com/books?id=gfoIAFHgusgC&pg=PA359 | date = 2008 | publisher = Cambridge University Press | isbn = 978-1-139-46987-6 | pages = 359–364}}</ref>
<ref name="Ling & Takahashi 1985">{{cite journal | title = The silicoflagellate genus ''Octactis'' Schiller 1925: A synonym of the genus ''Distephanus'' | first1 = Hsin Yi | last1 = Ling | first2 = Kozo | last2 = Takahashi | journal = Micropaleontology | volume = 31 | issue = 1 | date = 1985 | pages = 76–81 | doi = 10.2307/1485583 | jstor = 1485583 | bibcode = 1985MiPal..31...76L | url = https://www.jstor.org/stable/1485583 | url-access = subscription }}</ref>
</references> {{taxonbar|from=Q25367454}} Category:Ochrophyte genera Category:Taxa described in 1925