Modern fungal systematists have divided the fungal kingdom into six major phyla. The image below shows a simplified fungal phylogeny based on molecular data.
We will start with the most primitive members of the kingdom, the Chytridiomycota. Many chytrids are aquatic, which is why they are often called "water fungi." This phylum has only recently been determined to belong to Kingdom Fungi, therefore, not all textbooks include it in this kingdom. Biologically, the approximately 1000 species are mostly saprobes (decomposers that absorb nutrients from dead organic matter), but some species are parasites of animals, plants, or protists. They are also characterized as having absorptive nutrition and they can have septate or coenocytic mycelium, or be unicellular. Morphologically, they are important because some members possess flagellated spores. Until recently, systematists believed that the absence of flagellated cells was required for placement in the fungal kingdom. Molecular data have shown that chytrids are indeed fungi. Ultrastructural and biochemical data (e.g., chitinous cell walls) support this designation as well. The molecular data also support the theory that they are the most primitive group in the kingdom; they retain the ancestral character of having flagellated cells similar to their protistan ancestors.
The chytrids are an ecologically important group of organisms. While most of them function as decomposers, several species have been implicated in global amphibian declines. For example, the chytrid fungus Batrachochytrium dendrobatidis is responsible for the amphibian disease chytridiomycosis that has contributed to the extinction of an estimated 90 species of frog and the significant population decline of another 400 species.