Topic: Carnivorous fungi: a diet of worms (and other animals)
Fungi have learnt how to trap living prey, notably nematodes but also a range of other animals include rotifers, tardigrades and even springtails.
Fungi are like us, in as much as they are heterotrophic, that is they breakdown complex molecular compounds. This is in contrast to the autotrophs (such as green plants) that employ photosynthesis to synthesize complex compounds from simple ones. Fungi, of course, do not have guts, and generally they absorb nutrients using the usual enzymes; in some cases the capacity to absorb material is impressively developed as in the arbuscular myccorhiza. One might think, therefore, that any living animal might face the problem of pathogenic attack by fungi (as is the case in many immuno-compromised patients), but otherwise they should be quite safe. Not so. Fungi have learnt how to trap living prey, notably nematodes but also a range of other animals include rotifers, tardigrades and even insects in the form of the springtails. And of course it is convergent.
This group is most familiar in the form of mushrooms, but a number have also developed nematophagy, as in the pleurotaceans (which include the edible oyster mushroom). The trapping occurs in the underground hyphae, and typically the nematode is trapped on adhesive knobs, before enzymes attack the cuticle. Interestingly toxins are employed, and even more extraordinarily some forms such as Coprinus also bear minute spiny balls that serve to damage the surface of the nematode. Another fungus, Stopharia, is even more dangerous because it has cells with spiny projections (acanthocytes) that can penetrate the body wall of the luckless nematode.
Often referred to as the hypomycetes, this group is the most important in terms of zoophagy and enjoys not only nematodes, but rotifers, tardigrades and insects, as well as protistans. Five basic mechanisms for entrapment have evolved, which mostly depend on adhesive knobs or a network, but extraordinarily some have a lasso like arrangement formed in a circle of cells. In some types this can constrict, trapping the nematode. Molecular evidence suggests that this group of nematophagus fungi are monophyletic, but interestingly examples from 100 Ma-old Cretaceous amber show the trapping ring but it is only composed of a single cell. In addition this fungus has an associated yeast phase, which is not known in living hypomycetes. It is likely, therefore, that this is an independent lineage of ascomycetes.
Not surprisingly the zoophagy of these fungi depends on enzymes, and the proteases employ the classic aspartic acid-histidine-serine catalytic triad for binding to the substrate, which is of course rampantly convergent. Similar enzymes are used by animals, and also carnivorous plants. Several workers have pointed out that this zoophagy in fungi has important similarities to the carnivorous plants, and again represents a response to nutrient shortages, especially nitrogen.
Not to be left out, some zygomycetes are also convergently nematophagus, including the appropriately named Zoophagus which employs adhesive pegs.
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Map of Life - "Carnivorous fungi: a diet of worms (and other animals)"
December 3, 2020