The subject of this week's Taxon of the Week post is another example of the under-rated nature of protist diversity. Labyrinthuleans, commonly referred to as 'slime nets' are one of those organisms that, being neither animals nor plants, have been shuffled back and forth between and within the nomenclatural codes over the years, resulting in the same taxon being referred to by multiple different names. Labyrinthulea, Labyrinthista and Labyrinthulomycota are just three options that might be encountered. They are one of the protist groups that have been described as 'slime moulds', though they lack the dramatic life cycles of the Mycetozoa, the slime moulds proper. Most labyrinthuleans are found in aquatic habitats, but some species are terrestrial.
Labyrinthuleans may be divided into three groups, the Thraustochytriales, Labyrinthulales and the Diplophrys group. Note that these may not be phylogenetically separate groups - the molecular analysis of Cavalier-Smith & Chao (2006), for instance, doesn't separate the three - but they are still useful form groups. The Labyrinthulales and Thraustochytriales are united by the possession of an organelle called a bothrosome or sagenogen(etosome) that produces large amounts of filamentous net-like ectoplasmic membrane that the individual cells move along and absorb nutrients through, hence the name of 'slime nets'. The image at the top of this post (from here) shows a colony of Labyrinthula on the left and a close-up of individual cells on the right. Many labyrinthuleans are parasitic and invade and break down cells of host organisms, absorbing nutrients released by the decomposing cells. The individual cells form aggregative masses during reproduction, within which enlarged cells undergo meiosis and release flagellated zoospores (Barnes, 1998). The most obvious difference between the two groups seems to be the mode of colony formation - while Labyrinthulales form dispersed colonies of loosely connected cells surrounded by ectoplasmic matrix as seen in the top photo, Thraustochytriales (as exemplified in the photo of Schizochytrium limacinum below, again from here) form more compact colonies with the ectoplasmic net growing as "roots" from the base of the colony. The small inset photo shows the zoospore of Schizochytrium.
The genera Diplophrys and Sorodiplophrys are associated with the labyrinthuleans by molecular (Cavalier-Smith & Chao, 2006) and ultrastructural (Dykstra & Porter, 1984) data. However, while they do produce and move on ectoplasmic outgrowths, they lack a bothrosome for the production of said ectoplasm. Zoospore production has also never been recorded for these genera. The terrestrial Sorodiplophrys has an aggregative stage in its life cycle, but the aquatic Diplophrys marina does not (the actual type species of Diplophrys, D. archeri, has not been observed since 1902, and D. marina is only tentatively included in the same genus). Interestingly, the analysis of Cavalier-Smith & Chao (2006) places Diplophrys marina within the Labyrinthulales, which if correct implies that it lost the labyrinthulean characteristics during its evolutionary history.
As 'slime moulds', the labyrinthuleans were originally regarded as fungi (hence some publications refer to them as labyrinthulomycetes). However, it is now universally agreed that they are in fact members of the heterokonts (Chromista) based on the ultrastructure of the zoospores, as well as molecular data. Within the heterokonts, Cavalier-Smith & Chao (2006) place labyrinthuleans in a basal heterotrophic clade that also includes bicoecids and opalozoans and is sister to the remaining heterokonts. Labyrinthuleans are therefore not even close relatives of the other "ex-fungal" chromists in the Pseudofungi.
Labyrinthuleans have relatively little economic significance to humans. Some labyrinthuleans attack hosts of economic significance to humans, such as bivalves or golf course turf. Oils from the thraustochytrialean Schizochytrium contain one of the current dietary buzzwords, omega-3 fatty acids, so commercial growth of Schizochytrium is used to produce dietary supplements and alternatives to fish oils. Interestingly, webpages, patents, articles, etc. referring to such uses of Schizochytrium seem to invariably refer to it, somewhat misleadingly, as an 'alga', and the product as 'algal oil'. This strikes me as only a marginal improvement over 'fungus'.