Chapter 25: Phylum Labyrinthulomycota: Net Slime Molds

Labyrinthulomycota represents a fascinating group of marine protists whose taxonomic classification has shifted substantially as molecular and structural evidence has accumulated. Historically confused with fungi, protozoa, and algae, these organisms are now recognized as stramenopiles adapted to estuarine and nearshore environments. The chapter examines the defining characteristics of labyrinthulids and thraustochytrids, particularly their distinctive ectoplasmic network system created by organelles called bothrosomes, which allows spindle-shaped cells to move through gelatinous slime-like pathways. Reproduction occurs through biflagellate heterokont zoospores bearing both tinsel and whiplash flagella, structures that reflect their aquatic specialization. Labyrinthula vitellina demonstrates the capacity for sexual reproduction and meiosis, indicating greater complexity than initially recognized. The chapter highlights Labyrinthula zosterae as the definitive causative agent of eelgrass wasting disease, an ecologically significant pandemic that devastated Zostera marina populations and disrupted the marine food web by eliminating crucial nursery habitats for commercially important shellfish including scallops, oysters, and shrimp. In contrast, thraustochytrids such as Thraustochytrium and Schizochytrium display different morphological organization with globose thalli and predominantly asexual reproductive strategies characterized by rapid cell division. These organisms function as key decomposers of marine organic matter and detritus while occasionally acting as parasites on mollusks or macroalgae. Their cell walls contain distinctive Golgi-synthesized scales rather than the cellulose or chitin found in true fungi. The chapter contextualizes Labyrinthulomycota within broader protistan diversity by comparing evolutionary relationships to Oomycota and Hyphochytriomycota, demonstrating how these groups occupy taxonomic positions that challenge traditional fungal boundaries while maintaining essential ecological roles in marine nutrient cycling and food web dynamics.