''Octactis'' is a genus of silicoflagellates, marine

photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...

unicellular

protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

s that take the form of either

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

s or

axopodia A pseudopod or pseudopodium (: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and ...

l amoebae. Described by Josef Schiller in 1925, ''Octactis'' contains various species of marine

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...

, some of them responsible for

algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in fresh water or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompass ...

s that are toxic to

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

Characteristics

''Octactis'' species are

unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...

s that appear as several different

multinucleate Multinucleate cells (also known as multinucleated cells or polynuclear cells) are eukaryotic cells that have more than one nucleus, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordinate ...

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 A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

. The mucocysts appear on the periphery of skeleton-bearing cells, absent in the naked cells. Within each ''Octactis'' cell there is a

pyrenoid Pyrenoids are sub-cellular phase-separated micro-compartments found in chloroplasts of many algae,Giordano, M., Beardall, J., & Raven, J. A. (2005). CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution. ''An ...

embedded within a

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

. The chloroplast has a short thylakoid intrusion. The skeleton-bearing cells contain between 6 and 8

Golgi bodies The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...

. The chloroplasts contain all three accessory chlorophylls, c, c and c, as well as

fucoxanthin Fucoxanthin is a xanthophyll, with formula C42H58O6. It is found as an accessory pigment in the chloroplasts of brown algae and most other heterokonts, giving them a brown or olive-green color. Fucoxanthin absorbs light primarily in the blue-gree ...

as the main

carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...

, with diadinoxanthin and one or two oxyfucoxanthin derivatives. They lack

lutein Lutein (;"Lutein"

Ecology

''Octactis'' species are

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

found in typically cold marine waters as part of the

. Like other silicoflagellates, ''Octactis'' cells are surrounded by a siliceous skeleton composed of various spikes arranged in specific shapes. One species, '' O. speculum'', is considered harmful for marine fauna, due to appearing in

harmful algal bloom A harmful algal bloom (HAB), or excessive algae growth, sometimes called a red tide in marine environments, is an algal bloom that causes negative impacts to other organisms by production of natural algae-produced toxins, water deoxygenation, ...

s that have caused high mortality of fish such as

salmon Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...

and

trout Trout (: trout) is a generic common name for numerous species of carnivorous freshwater ray-finned fishes belonging to the genera '' Oncorhynchus'', ''Salmo'' and ''Salvelinus'', all of which are members of the subfamily Salmoninae in the ...

. 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 Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cell ...

during algal blooms of ''O. speculum'', which decreased the

dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can ...

in the water.

Systematics

Taxonomic history

The

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

''Octactis'' was described in 1925 by phycologist Josef Schiller as a member of the Silicoflagellata, now known as the order Dictyochales, a group of

stramenopile The stramenopiles, also called heterokonts, are protists distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, an ...

with siliceous skeletons. 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 Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

'' Distephanus'') multiple times. 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 The Adriatic Sea () is a body of water separating the Italian Peninsula from the Balkans, Balkan Peninsula. The Adriatic is the northernmost arm of the Mediterranean Sea, extending from the Strait of Otranto (where it connects to the Ionian Se ...

, and remained its

type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...

and only species. In 1985 the entire genus with all its species were synonymised to ''Distephanus'' (later '' Stephanocha''). On the basis of

molecular phylogeny Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

, researchers in 2017 emended the definition of ''Octactis'' and transferred two species of '' Dictyocha'', ''D. octonaria'' and ''D. speculum'', to it.

Classification

There are two species of ''Octactis'' currently accepted: * '' Octactis octonaria'' : = ''Dictyocha octonaria'' * '' Octactis speculum'' : = ''Dictyocha speculum'' = ''Distephanus speculum'' = ''Stephanocha speculum''

References

{{taxonbar, from=Q25367454 Ochrophyte genera Taxa described in 1925