The Hidden World of Carnivorous Fungi
Rare and Unsuspicious Predators
Many of us learned that one of the main and defining characteristics of fungi is their fundamental role in the decomposition of organic matter and in nutrient cycling and exchange. However, although this true for the vast majority of fungi, there are many species of parasitic fungi that depend on animals and plants for their survival. Many of those fungi not only can cause serious diseases to humans but also they can be serious threats to crops like wheat. Being a very small group within the kingdom of fungi, there is about 200 species of fungi, identified so far, that have specialized on a very efficient yet strange way of getting their nutrients – predation. They are thus predatory fungi that kill and eat small microscopic fauna that live underground mostly.
The study of this kind of pathology in relation to lower life forms was initiated by the Austrian botanist Whilhelm Zopf when came to his attention, in 1888, a fungus which attacked nematodes, eelworms or roundworms of the family Anguillulidae. In the various cultures which he was observing, there were numerous living eelworms and many dead ones tangled with and variously penetrated by hyphae of the fungus. The question then arose in his mind as to whether the fungus is purely saprophytic, penetrating and feeding only on already dead worms, or does the fungus attack and kills the living nematodes. In answering this question experimentally, Zopf made the first discovery of a fungus which traps and preys on a living animal. The fungus in question Arthrobotrys oligospora was first described by the German botanist Georg Fresenius, in 1850, but its predatory nature was made by Zopf. It is found in all kinds of more or less decayed matter and makes a thin veil of mycelium of septate hyphae over the surface. From it there extend slender septate conidiophores (aerial hyphae) bearing pear-shaped two-celled spores rising above the substratum. The peculiar feature of this genus of fungi, Arthrobotrys, is the occurrence of many slings or loops of various sizes on the hyphae, formed by the sharp curving of a growing branch which turns upon itself and fuses by its end with its base. From one loop a second and from this a third one may arise, and thus a tangle of loops is formed lying in all positions, similar to a web-like loose structure. This peculiar structure was already known by scientists at the time, but its nature was a mystery. The fact that when once the worm has by chance inserted one end or the other into a loop, it cannot free itself again, was definitely observed. Zopf observed that the eelworm, on being caught, either by the tail or by the head, struggled violently for a half-hour, then became quieter and finally died in 2.5 hours. After the animal succumbs, hyphae from the loop penetrate and invade the worm body and withdraw nutrient. Currently, there is much interest in the study of nematophagous fungus as they can be useful in controlling nematodes that are serious pests in many crops and thus avoid the use of chemicals.
Several Trapping Mechanisms
The predatory nature of another similar fungus Dactylella bembicoides Drechsler, was explained by the American botanist John Couch (1937). In Dactylella the loops are composed of a short branch of three cells turned upon themselves. Fusion occurs between the end and basal cells, and a neat ring is thus formed. By growing the fungus on agar to allow of clear microscopic observation, Couch observed an astonishing thing, that when an eelworm pokes his head or tail into a ring, the ring immediately clamps on it by the sudden swelling of the three cells. Couch proposed that the fungus responds to a chemical stimulus from the worm's body. Afterwards, when the worm is perfectly trapped and no longer fights against it new hyphal branches penetrate the body of the prey. With similar mechanisms to prey on nematodes, there are many species of fungi of the genera Trichothecium, Arthrobotrys, Dactylaria, Monacrosporium and Dactylella; all firstly described, in early twentieth century, by the American botanist Charles Drechsler, who gave a very important contribution to the study of nematophagous fungi. We owe much knowledge about these predatory fungi to Drechsler, who observed that some species have loops the component cells of which do not swell to constrict the loop.
Instead, these fungi catch their prey by means of a strong adhesive found on the inner surface of the loop. Most probably this was the case in the fungus studied by Zopf who did not observe constriction of the loops. The constricting rings used by some carnivorous fungi are considered as active traps to distinguish from those that do not swell and possess adhesive structures only which are called passive traps. In some cases, loops are borne on a very slender stalk which may be broken off during the struggles of the worm, but this does not obviate death and destruction, as the cells of the loop can still form their penetrating hyphae and invade the prey. Some of these nematophagous fungi, e.g. Dactylaria Candida (Nees) Sacc., have in addition to the loops a second organ for catching the prey, called globular bodies. These are round knobs on short hyphae. On the knob is secreted a patch of strong adhesive, by which the eelworm is caught. In the course of a short time, a penetrating haustorium grows through the adhesive pad and enters the animal's body. In all these cases, after the prey is permeated with haustorial hyphae, and after these have withdrawn all the available nutrients, the fungal hyphae withdraw, leaving an empty shell. A similar method of capture is used by species in which the catching organs consist merely of the ends of hyphal branches, provided, as on the globular organ, with adhesive or burr-like structures like the ones observed in the shaggy ink cap that may release toxins to immobilize the prey. The penetrating haustorial tube swells up after entrance, and from the swelling the haustorial complex of hyphae grows into the prey’s body.
Not Just Eelworms
A similar, very striking case of a fungus which catches armoured Rotatoria, rotifers or wheel animals as they are commonly called, the first of its kind known, was described in 1911 by the German Hermann Sommerstorff under the name Zoophagus insidians, following Zopf’s original discovery of a carnivorous fungus in the genus Arthrobotrys. This water mould is fairly common in pond water where there are abundant water weeds and aquatic rotifers and it grows epiphytically on green alga Cladophora. Zoophagus insidians consists of a network of septate hyphae which bear short branches scattered at irregular intervals along them. These short branches have dense glistening contents, like sticky lollipops, and are the organs of capture. Rotatoria feeding among the threads of the algae and associated fungus, take hold of the ends of the fungus short branches, and remain attached, unable to break loose. By pulling off a newly captured animal Sommerstorff was able to determine that the end of the hypha had enlarged, apparently by the swelling of the membrane and invaded the small rotifers’ bodies. Sommerstorff concluded that the swelling of the hyphae takes place on the stimulation occurring when the animal takes the short branch into its mouth. Generally the prey cannot escape, despite his size. But as it has no other organs of locomotion besides its cilia, the tail is available for struggling only.
If rotifers can get leverage with this on a neighbouring algal filament, they may and sometimes do escape to a fatal destiny. After struggling ceases and death is intervening, the capturing branch grows into a penetrating tube which then sends numerous thin-walled haustorial hyphae to withdraw nutrients. The oval lollipop-like glistening body, observable in the short hyphae, is a discrete body of mucilage capable of great swelling and is always present in organs ready for capture as a definite plug. After a short time following capture (10-30 minutes) the callus plug is emptied into the mouth of the prey and spreads out entangling the whole of its ciliary mouth apparatus. It seems that Zoophagus insidians has specialized on certain sorts of Rotatoria only and never those which are supplied with a strong ciliary apparatus. The short hypha now begins to send out haustoria which penetrate throughout the body of the animal. Even at the end of digestion, the mucilage plug, which can still be seen, is found to have hardened and become yellow in color, holding the shell of the animal in position. The growth of the haustoria proceeds till the interior of the body is a mass of hyphae which send out conidiophores projecting from the animal and in swarm-spores produced in a sac which escape through the mouth end.
- Carnivorous fungus prey on microscopic animals
- They have two types os traps: active traps (constricting rings) and passive traps (adhesive web- or knob-like structures)
- Fungus that prey on nematodes can be used to control nematodes populations that affect crops.
A fungus with a similar method of capturing its prey as that employed by Zoophagus insidians is Sommerstorffia spinosa Arnaudow, described by the Bulgarian botanist Nicola Arnaudow in 1923. An extraordinary group of fungi, of genera Acaulopage, Pedilospora and Dactylella, which preys upon species of Amoeba and shelled rhizopods was studied by Drechsler. They are nearly all fungi with septate hyphae. The method of capture is quite similar in all cases. The species of amoeba appear to be large, Amoeba terricola or related species being often the victim. In some fungi adhesive hyphae have been observed, in others not, leaving it for conjecture that a non-visible adhesive occurs. There is seldom any preformed structure with the function of capture, but this occurs in Dactylella tylopaga Drechsler. In this species ellipsoidal protuberances are provided with an adhesive. An animal sticks to one of these, which then sends out a tube of penetration. This grows inside the animal into a branching mass of short hyphae which absorb the body of the animal. An amoeba after capture is always to be seen attached whether to a mycelial element or, as is often the case in some species, to a fallen conidium (non-motile spores of fungi) by means of a minute mass of golden yellow adhesive material. From the mycelial element or the conidium is thrust forth a narrow process which gives rise to a more or less characteristically branched haustorium or haustorial system. When the protoplasmic contents of the amoeba are nearly exhausted, the protoplasm of the haustorium begins to withdraw back into the parent mycelial filament.
A excerpt about nematophagous fungi from the famous TV series The Private Life of Plants
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