Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic multicellular or multinucleate fruiting bodies which may be formed through aggregation or fusion. Slime molds were formerly classified as fungi but are no longer considered part of that kingdom. Although not forming a single monophyletic clade, they are grouped within the paraphyletic group Protista.
More than 900 species of slime mold occur globally. Their common name refers to part of some of these organisms' life cycles where they can appear as gelatinous "slime". This is mostly seen with the Myxogastria, which are the only macroscopic slime molds. Most slime molds are smaller than a few centimetres, but some species may reach sizes up to several square metres and masses up to 20 kilograms.
They feed on microorganisms that live in any type of dead plant material. They contribute to the decomposition of dead vegetation, and feed on bacteria and fungi. For this reason, slime molds are usually found in soil, lawns, and on the forest floor, commonly on deciduous logs. In tropical areas they are also common on inflorescences and fruits, and in aerial situations (e.g., in the canopy of trees). In urban areas, they are found on mulch or in the leaf mold in rain gutters, and also grow in air conditioners, especially when the drain is blocked.
The most commonly encountered are the Myxogastria. A common slime mold that forms tiny brown tufts on rotting logs is Stemonitis. Another form, which lives in rotting logs and is often used in research, is Physarum polycephalum. In logs, it has the appearance of a slimy web-work of yellow threads, up to a few feet in size. Fuligo forms yellow crusts in mulch.
Many slime molds, mainly the "cellular" slime molds, do not spend most of their time in this state. When food is abundant, these slime molds exist as single-celled organisms. When food is in short supply, many of these single-celled organisms will congregate and start moving as a single body. In this state they are sensitive to airborne chemicals and can detect food sources. They can readily change the shape and function of parts, and may form stalks that produce fruiting bodies, releasing countless spores, light enough to be carried on the wind or hitch a ride on passing animals.
Dictyostelium discoideum is another species of slime mold that has many different mating types. When this organism has entered the stage of reproduction, it releases an attractant, called acrasin. Acrasin is made up of cyclic adenosine monophosphate, or cyclic AMP. Cyclic AMP is crucial in passing hormone signals between reproductive cells. When it comes time for the cells to fuse, Dictyostelium discoideum has mating types of its own that dictate which cells are compatible with each other. A scientific study demonstrated the compatibility of eleven mating types of Dictyostelium discoideum by monitoring the formation of macrocysts, concluding that cell contact between the compatible mating types needs to occur before macrocysts can form.
Plasmodial slime molds begin life as amoeba-like cells. These unicellular amoebae are commonly haploid and feed on bacteria. These amoebae can mate if they encounter the correct mating type and form zygotes that then grow into plasmodia. These contain many nuclei without cell membranes between them, and can grow to meters in size. The species Fuligo septica is often seen as a slimy yellow network in and on rotting logs. The amoebae and the plasmodia engulf microorganisms. The plasmodium grows into an interconnected network of protoplasmic strands.
Slime molds share some similarities with neural systems in animals. The membranes of both slime molds and neural cells contains receptor sites, which alter electrical properties of the membrane wh