DESCRIPTION OF THE PHYLUM LABYRINTHOMORPHA (LEVINE ET AL. 1980)

EUKARYA> CHROMALVEOLATA> HETEROKONTAE> LABYRINTHULOMORPHA

Labyrinthomorpha (la-ba-RIN-tho-MOR-fa) is derived from a Latin root that means labyrinth (labyrinthum) and a Greek root meaning form (morphe -μορφή). The reference is to the intricate slime nets that they produce and move through.

INTRODUCTION TO THE LABYRINTHULOMORPHA The slime nets are fungus-like organisms that mainly are known for attacking sea grasses. Labyrinthula (Figures 1 and 2) lives as a collective of spindle-shaped cells within an anatomizing web of slime tubes which are formed by specialized organelles called bothrosomes (text with tooltip) Bothrosomes (sagenogen, sagengenetosomes) are organelles on the cells of labyrinthulids which produces the slime web. (sagenetosomes). The cells move freely through the tubes, but cannot move at all if removed from them. Despite their fungus-like appearance, they do produce typical heterokont zoospores (text with tooltip) A zoospore is an asexual spore that is motile. Zoo- (pronoumced zo-o) is a prefix that means moving. (meiospores) that function in dispersal. The slime nets are important because of their negative impact on eel grass (Zostera marina), an important member of Chesapeake Bay and other brackish and marine environments (Raghukumar 2002). More recently, labyrinthulids have begun to be implicated as the causative agents of turf grass diseases, particularly on well-watered golf greens (Craven et al. 2005 and Bigelow et al. 2005). Thraustochytrids (Figure 3) do make an ectoplasmic structure that is far less elaborate than that of the Labyrinthulales. They have been found as parasites of plants, kelps, phytoplankton and some animals (Raghukumar 2002). In addition, thraustochytrids also feed on fecal pellets, dead mangrove leaves, and mollusk shells and might be one of the major contributors to the remineralization of these substrates. Their numbers have been quite high in the fecal pellets (> 106 cells per gram fecal pellet) and sediment (18,000-73,000 cells per liter, Raghukumar 2002). The affinities of the labyrinthomorphs have been misunderstood, mainly because they were defined by essentialist arguments like: they feed like slime molds, therefore they must be related to slime molds (e.g. Alexopoulos and Mims 1979). However, the recognition that they are related to other heterokonts (e.g. Patterson, 1989) finally gave them a home and set them apart from the slime molds. Older manuals like Grell (1973) and Kudo (1966) assigned the labyrinthulids to taxa of uncertain status. However, because of their unusual cellular organelles (e.g. bothrosomes) and odd means of locomotion, later manuals [e.g. Pokorny (1985), Sleigh et al.(1984) and Margulis and Schwartz (1988, Pr-21; 1998, Pr-18)] indicated that the labyrinthulids clearly required phylum-level ranking. Although Pokorny (1985) did not group the thraustochytrids together with the labyrinthulids, Porter (1990) united them within the same phylum because they both produced the same kind of motile cell and both had bothrosomes. Patterson (1989) proposes that the motile cells of the labyrinthulids suggest a connection with the chrysophyte complex. A lipid structure in the zoospore might be homologous to the eyespots of organisms like the eustigs. Porter (1990) simply acknowledges that phylogenitic affinities are not known. Leipe et al. (1994 and 1996) placed the labyrinthulomorphs together with other heterokonts (also acknowledged and followed by Patterson, 1999) in two studies using molecular phylogenetic methods. Sogin and Patterson (Tree of Life Project) and Baldauf (2003a) placed the labyrinthomorphs near the base of the heterokont tree. The analysis of Tsui et al. (2009) confirms the relationship between the labyrinthomorphs (sensu latu) with the Heterokontae, but does indicate that the thraustochytrids, as currently defined, are paraphyletic. We accept that Labyrinthomorpha is a sister to Bicosoecida (see Figure 4, after Andersen 2004).

FIGURE 1. A low power view of Labyrinthula cells within the slime net. Image from http://microscope.mbl.edu/baypaul/microscope/images/t_imgAZ/labyrinthula_epw.jpg	FIGURE 2. SEM micrograph of a spindle-shaped Labyrithula cell outside of its slime tube. Image from http://people.uncw.edu/durakom/FHAP/LabySEM.jpg	FIGURE 3. SEM micrograph of thraustochytrid cells. Note the rhizoidal extensions of the cells that are used to penetrate potential food items (dead and alive). Image from Celeste Leander, Creative Commons

FIGURE 4. Labyrinthomorphs are in a clade with Bicosoecida among the heterotrophic heterokont taxa. The topology of this cladogram is informed by Andersen (2004).

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By Jack R. Holt. Last revised: 02/19/2013