Water Moulds — Phylum Oomycota
Oomycetes, commonly known as water moulds, resemble fungi in appearance because their body consists of narrow, strand-like hyphae, which are collectively called a mycelium. However, these hyphae differ from those produced by fungi because their walls contain cellulose; fungal hyphae are composed of chitin.
Oomycota means "egg fungi", a reference to their mode of sexual reproduction, in which an egg cell is fertilized by either a sperm cell or an antheridium (male sex organ containing male gametes). Asexual reproduction also occurs in oomycetes, but, in this case, motile zoospores with two flagella are produced.
As their name suggests, these organsims require water to survive. Some arctic water moulds grow as cottony masses on dead algae and animals, while others are important decomposers in aquatic ecosystems. A few species are even parasitic on the gills of fish!