The Polykrikaceae (also known as Polykrikidae) are a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

of athecate

dinoflagellate The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...

s of the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

Gymnodiniales The Gymnodiniales are an order of dinoflagellates, of the class Dinophyceae. Members of the order are known as gymnodinioid or gymnodinoid (terms that can also refer to any organism of similar morphology). They are athecate, or lacking an armore ...

. Members of the family are known as polykrikoids. The family contains two

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

: ''

Polykrikos ''Polykrikos'' (from Greek “poly” - many, and “krikos” – ring or circle) is one of the genera of family Polykrikaceae that includes athecate pseudocolony-forming dinoflagellates. ''Polykrikos'' are characterized by a sophisticated bal ...

'' and '' Pheopolykrikos''.

Characteristics

The most distinctive feature of polykrikoids is their formation of multinucleate "pseudocolonies" consisting of an even number of subunit

zooid A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooi ...

s. The two genera differ in number of nuclei; possessing two nuclei regardless of the number of zooids is a synapomorphy for ''Polykrikos'', whereas ''Pheopolykrikos'' possess equal numbers of nuclei and zooids. Along with the

Warnowiaceae The Warnowiaceae are a family of athecate dinoflagellates (a diverse group of unicellular eukaryotes). Members of the family are known as warnowiids. The family is best known for a light-sensitive subcellular structure known as the ocelloid, a hi ...

(warnowiids), polykrikoids are known for possessing unusually complex subcellular structures. In particular, an

extrusome Extrusomes are membrane-bound structures in some eukaryotes which, under certain conditions, discharge their contents outside the cell. There are a variety of different types, probably not homologous, and serving various functions. Notable extru ...

complex of two

organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...

s called the

nematocyst A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this ce ...

and taeniocyst is considered a synapomorphy for ''Polykrikos''.

Molecular phylogenetics 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 ...

studies suggest some inconsistency in the

taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

of this group, particularly in the assignment of species to one of the two genera.

Habitat and life cycle

Most polykrikoids are

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

ic, although one species - '' P. lebourae'' - is

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

. The family includes

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...

, heterotrophic, and

mixotrophic A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...

species. Some species, such as ''

P. kofoidii ''Polykrikos kofoidii'' is a species of phagotrophic marine pseudocolonial dinoflagellates that can capture and engulf other protist prey, including the toxic dinoflagellate, ''Alexandrium tamarense''. '' P. kofoidii'' is of scientific interest du ...

'', are of scientific interest due to their status as

predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

s of other dinoflagellates, a behavior that is significant in the regulation of algal blooms. Others, such as '' Ph. hartmanii'' (which has been reclassified '' P. hartmanii'') are themselves causes of ichthyotoxic algal blooms. ''P. hartmanii'' is capable of both heterothallic (outcrossing) and homothallic (self-fertilizing)

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

.Z Chai, Z Hu, Y Liu, Y Tang 2020. Proof of homothally of Pheopolykrikos hartmannii and details of cyst germination process Journal of Oceanology and Limnology 38 (1), 114-123 The reproductive behaviors of polykrikoids are mostly not well understood, although ''

'' has been studied and found to have a complex life cycle of both vegetative (asexual) and sexual reproduction complicated by its pseudocolonial structure.

Evolution

The family demonstrates a complex evolutionary history indicating multiple instances of loss of photosynthetic

plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosy ...

s in different lineages. The distinctive pseudocolonial structure may have arisen in multiple evolutionary lineages from ancestors capable of forming chains of distinct individual cells.

References

{{Taxonbar, from=Q20801627 Dinoflagellate families Gymnodiniales