In this section, we consider primarily feeding on nematodes. The mechanism of capturing a nematode prey is the most studied; however, the same mechanisms operate on rotifers, tardigrades and similar sized organisms that can be captured or penetrated. It is unclear what the extent of nematotrophy, or feeding on microinvertebrates in general, is in the soil. The phenomenon is common but has been quantified insufficiently. A variety of microarthropods are known to feed on nematodes (or microinvertebrates) or at least to supplement their diet with nema-todes (Walter, 1987) (Fig. 4.12). These include pseudo-scorpions and several families of mites and Collembola. Many soil Collembola probably supplement their diet with nematodes (Hopkins, 1997). Among the Acarina, these include the groups listed below as predatory. However, the nematodes are often the preferred prey, especially for Mesostigmata, Prostigmata (Bdellidae and Cunaxidae) and some Endostigamata (Alycus and Alicorhagia). Certain predacious testate amoebae, such as Nebella, are known to hold nematodes with pseudopodia, immobilize the prey and engulf the whole organism. Sometimes, only the tail is held and the rest of the nematode breaks free (Yeates and Foissner, 1995). In all the above cases, access to nematodes in soil pores is not restricted by the size of the predator as much as by the diameter of the amoeba pseudopodia, the chelicerae of mites or other appendages. Several interactions involving hyphal fungi (Basidiomycetes and mitosporic species), Chytridiomycetes, Zygomycetes and the Oomycetes (chromista and pseudo-fungi) are considered below (for further details and references, see Barron, 1977).
One method of attacking and digesting nematodes involves monociliated dispersal cells which swim towards prey. The dispersal cells do not hold many reserve nutrients and have a limited time of activity to find a new prey. The cells probably track a solute from the prey's path by chemotaxis. Penetration into the prey can be through an orifice or through the cuticle. Once attached to the prey cuticle, the cilium is lost and the cell encysts. From the cyst, cytoplasmic extensions grow into the host and form a mesh. The prey is invaded by the hyphae which secrete digestive enzymes, absorb the solutes and empty the cuticle. When nutrients run out or the hyphae become crowded, spores form which grow an exit tube and swim off. In dry conditions or through tight exit tubes, cells can be amoeboid. This mechanism is common in Chytrids (fungi) such as Catenasia. The chytrids also attacks similar sized organisms such as rotifers, tardigrades, invertebrate eggs or macrodetritus consisting of animal corpses or parts thereof.
A modification of this mechanism occurs in the Oomycetes (chromista) Myzocitium, where the dispersal ciliated cells encyst in the soil and become sticky. Attachment on to the cuticle of a passing prey is the stimulus to extend hyphae into the prey. In Haptoglossa, the cyst discharges a coiled tube into the passing prey. The coil is 5-8 |im long and 0.5 |im wide, and discharged in about 0.1 s. No infection occurs if the coil is discharged into a cuticle which is too thick to traverse. In Zygomycetes, such as Meristacrum, at the end of the growth phase in the prey, some mycelia extend out of the cuticle and form adhesive conidia spores, which are then picked up by passing prey. Some of the spores remain inside the old cuticle. The Basidiomycetes Nematoctonus also form adhesive conidia spores which then attach to the cuticle of a new prey. The protoplasm migrates out of the hyphae inside the prey and accumulates in conidia bearing branches outside the prey. Several mitosporic species form adhesive dispersal spores, such as Cephalosporium, Mesia and Verticillium. Ingestion of infective spores can also stimulate growth. In the case of Harporium species, the shape of the spore prevents passage through the nematode pharynx into the middle intestine. Sporulation occurs in the oesophageal muscle, and invasive hyphae grow through the organism. One difference between nematotro-phy by soil hyphal fungi (such as the Basidiomycetes genera mentioned here) and others such as Chytrids and Oomycetes is that the latter two tend to grow into the prey, with very few vegetative hyphae in the soil. In contrast, Basidiomycetes grow extensive saprotrophic hyphae in the soil and only supplement their nutrition with nematodes, or other microinvertebrates. This is particularly important in some lignin-decom-posing fungi (such as the oyster mushroom, Pleurotus), which are otherwise nitrogen limited. The Pleurotus and Hohenbuchelia genera (family Pleorotaceae, order Agaricales, Basidiomycetes) are nematotrophic white rot fungi (Thorn et al., 2000). They secrete a non-adhesive nema-totoxic substance (trans-decenedioic acid) to immobilize nematodes, which are then invaded with hyphae through the orifices. The Hohenbuchelia also produce adhesive knobs, as for the anamorph Nematoctonus.
Hyphal fungi in the soil and litter rely on adhesive mycelium or rings for capturing nematodes and similar sized prey (Fig. 4.14). Barron (1977) distinguishes between six methods as follows.
1. Adhesive hyphae of the Zygomycetes, such as Cystopage and Stylopage, secrete a sticky substance. Trapped invertebrates will fight it to exhaustion. Branching mycelia grow into the prey. The protoplasm moves out of the hyphae in the prey and grows outside the prey. These vegetative hyphae in the soil near the prey form conidia.
2. Short adhesive branches or rings occur in only a few mitosporic species. For example, Dactylella tylopage captures amoebae on its sticky surfaces, although other species in the genus capture nematodes, which become stuck at several points. Only a few seconds of contact are sufficient, and mycelial growth begins rapidly. Nematodes can withdraw quickly enough to avoid being trapped, often succeeding. Sometime they try to proceed only to be caught again.
3. Adhesive nets are formed by ubiquitous and abundant fungi, such as Arthrobotrys. These form more extensive adhesive hyphae branches and rings. Some species secrete toxins which help to immobilize or kill the prey while mycelia grow into it. As in other groups, the protoplasm retracts from the hyphae in the prey and grows outside the cuticle which is left almost empty. Conidia form outside the cuticle from these later emerging branches.
4. Adhesive knobs are formed by several Basidiomycetes, such as Nematoctonus and Hyphoderma, and mitosporic fungi such as Dactylaria. These consist of single adhesive cells at the end of short branches, which can be broken off by a struggling prey. Each adhesive cell, once attached to a nematode or cuticle, will grow invasive hyphae. This mechanism combines prey invasion with dispersal. The adhesive substance of Nematoctonus is very strong and the knob will not break off. A struggling nematode can escape at the cost of losing some epidermal cells and cuticle (causing a wound).
5. Many mitosporic species that form adhesive knobs also form rings that consist of three cells. These break off, so that the nematode carries a collar of three cells which grow mycelia through its cuticle to digest the nematode.
6. Constricting rings also consist of three cells, with a fourth supporting cell and a fifth which branches from the main mycelium. These also occur in the genera Arthrobotrys and Dactylaria. The collar is about 20 |im in diameter and constricts inward within 0.1 s by rapid uptake of water. The collar cells constrict around the nematode, then grow into the caught prey.
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