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Data from Boenigk and Arndt (2000).

Data from Boenigk and Arndt (2000).

Search time (variable with food abundance), handling time (fixed duration)

Search time (variable with food abundance), handling time (fixed duration)

Capture efficiency (ingestion /evasion), refractory period (fixed duration), processing time (variable)

Retention time (variable), growth rate (assimilated/excreted), maximum food vacuole number

Fig. 4.10. Parameters that regulate the rate of food intake affecting optimal foraging, here represented with a biciliated cell ingesting a bacterium cell. The durations of fixed and variable periods both vary with temperature.

Capture efficiency (ingestion /evasion), refractory period (fixed duration), processing time (variable)

Retention time (variable), growth rate (assimilated/excreted), maximum food vacuole number

Fig. 4.10. Parameters that regulate the rate of food intake affecting optimal foraging, here represented with a biciliated cell ingesting a bacterium cell. The durations of fixed and variable periods both vary with temperature.

Invertebrates

Invertebrate consumers of bacteria are also common in the soil and surface litter (see Chapter 1). These include several families of rotifers (particularly the Bdelloides), gastrotrichs, tardigrades, nematodes and Collembola. Both the enchytraeids and earthworms are bacterivores indirectly, because they ingest whole soil or litter. Many invertebrate larvae in the soil will feed on bacteria directly or indirectly. The majority of bacterivorous invertebrate species lick or suck thin water films on soil particle surfaces. They feed by ingestion of bacteria and soil solution.

Fig. 4.11. Prey selection by protozoa. Contact and initiation of phagocytosis of a bacterial cell, followed by its rejection and evacuation, without phagosome formation or digestion.

The Bdelloides (Rotifera) and vorticellid ciliates depend on water currents created by the oral cilia and require at least capillary water in soil pores or litter surfaces for feeding. They encyst in less moist conditions. Although it is possible theoretically for soil mites to feed on bacterial films, there is scant evidence that it occurs or supports growth (Hubert and Lukesova, 2001). Feeding structures of many species, particularly in the smallest forms, could permit some bacterivory by ingesting soil films and slime layers. Most species probably ingest bacteria indirectly, by feeding on micro- and macrodetritus that would sustain bacterial growth. Small Collembola such as species of Micranurida feed on soil solution and films on particle surfaces through sucking mouth parts, ingesting bacteria, yeasts, protists and similarly sized particles.

Nematodes

Ingestion of bacteria is by sucking in soil solution (Fig. 4.12, Table 4.11). The fraction of cells lysed depends on the bacterial species. Some can even be excreted alive (undigested). A review of nematode selection of bacteria and growth efficiency on different bacteria was provided by Yeates (1998). These were elaborated further (Vanette and Ferris, 1998) by observing that some bacteria did not support growth of nematodes at all. Other bacteria could not support maturation of juveniles or egg-laying by females, although they could support growth of adults. In general, a mixed diet is probably more efficient. This study further points out that a minimum abundance of bacteria is required to sustain growth (~103 bacteria/nematode/day). Optimal growth rates can be obtained with about 105-106 bacteria/nematode/day at 20°C. These values vary between species of nematodes and bacteria available. In particular, the nematode oral morphology, metabolic rate, reproduction rate, nutrient requirement, population size and abiotic conditions contribute to the

Nematode Functional Groups

Fig. 4.12. Functional groups of Nematoda, based on the morphology of oral structures. (A) Bacterivorous without appendages. (B) Bacterivorous with some reinforcement of the mouth cavity. (C) Bacterivorous with denticles and reinforced cavity. (D-F) Tubular stylet for penetrating eukaryotic cells of protists, hyphae, invertebrates, roots or mosses. (G) Predacious with wide tubular stylet. (H) Predacious with denticles and reinforced mouth cavity. Scale bar 25 |im. Based on Yeates and Coleman (1982), Yeates et al. (1993) and Yeates (1998).

Fig. 4.12. Functional groups of Nematoda, based on the morphology of oral structures. (A) Bacterivorous without appendages. (B) Bacterivorous with some reinforcement of the mouth cavity. (C) Bacterivorous with denticles and reinforced cavity. (D-F) Tubular stylet for penetrating eukaryotic cells of protists, hyphae, invertebrates, roots or mosses. (G) Predacious with wide tubular stylet. (H) Predacious with denticles and reinforced mouth cavity. Scale bar 25 |im. Based on Yeates and Coleman (1982), Yeates et al. (1993) and Yeates (1998).

Table 4.11. Nematode functional morphology of stoma with feeding types and selected examples.

Morphology

Feeding types

Examples

Armoured pharynx Bacterivory, cytotrophy, Dentate pharynx Fungivory, roots, invertebrates

Lanceolate stylet Tubular stylet

No appendage

Roots, invertebrates Hyphae, roots, invertebrates

Bacterivory, cytotrophy

Rhabditidae,

Diplogasteridae, Mononchoidea, Nygolaimidae

Longidoridae, Trichodoridae, Aphelenchina, Leptonchidae, Seinuridae, Tylenchina Araeolaimidae, Monhysteridae efficiency of grazing on various bacteria. Nematodes vary in their sensitivity to bacteria prey variety and abundances. What is ingested by nematodes depends in part on the diameter of the stoma when stretched open (Yeates, 1998). Some nematodes with more complex or armoured pharynx, such as the dentate Diplogaster, can supplement their diet by feeding on bacteria in film water. When bacteria are scarce and bacterivores become more opportunist, other particles can be ingested, such as microdetritus, cysts, spores, diatoms, small testate amoebae or invertebrate eggs. However, ingestion does not mean that the particles can be digested, or that growth can be supported.

Cytotrophy

Species that feed on non-filamentous unicellular eukaryotes are referred to as cytotrophic. These include small invertebrates that suck in soil solution and capillary water, as well as many species of protozoa and several fungi. Some nematodes and tardigrades can prey on large and small testate amoebae by breaking into the test, or through the pseudo-stome opening. Lytic enzymes from primary saprotrophs, such as proteases and lipases, will damage or lyse cell membranes of protists. The lysate contributes to dissolved organic matter in the soil solution.

In general, most invertebrates that can feed on bacteria based on morphology probably also feed on small unicellular eukaryotes (Tables 4.2, 4.11 and 4.12). However, because protozoa are crushed and lysed

Table 4.12. Orders and suborders of Acarina found in soils, with generalized descriptions.

ORDER/Suborder Description

ASTIGMATA Some free-living species in the soil; though usually associated with other organisms, or found as pests in stored food

HOLOTHYRIDA Adults have sclerotized cuticle with dense setae, 2-7 mm in length, found in the soil litter, on grasses, ferns, vegetation canopy. Considered predacious, feed on solution from decomposing or pierced organisms. About 30 poorly known species

IXODIDA Ticks and ectoparasitic species; mostly 1.7-7 mm unfed, increasing to

2-3 cm in fed state; are vectors for disease, e.g. Sporozoa blood parasites

NOTOSTIGMATA Resemble small Opiliones 1.5-2.3 mm in length, found under stones and in surface litter, in seasonally dry conditions. Hypertrophied chelicerae and serrated setae cut up food. Feed on pollen, fungi, microarthropods by ingestion of solids. Single family with nine genera and 20 species

Continued

Table 4.12. Continued.

ORDER/Suborder Description

PROSTIGMATA A heterogeneous group mostly 100-1600 |im in length, considered mostly predatory or detritivorous on microdetritus; the parasitic larvae of the Parasitengona become cytotrophic on pollen and spores, or predacious, or ectosymbionts on skin/scales/fur as adults MESOSTIGMATA Mostly 200-2500 |im in length and very diverse, 10,000 species.

About half are free-living voracious predators, but ingesting fluids only; a membrane excludes entry of particulate matter Feed on pollen, fungal spores and similar sized particles, including invertebrate cuticle, with the less digestible parts visible in the digestive tract

Found feeding on rotting wood and tree stumps (infested with hyphae) Mostly soil species (?)

Some predacious, others ectosymbionts (commensals) on insects and reptiles

Detritivores common in rich organic matter, such as forest floor organic horizons or animal excreta and composts; also fungivorous and predacious on nematodes Specialized parasites

Omnivorous predators in soil litter and organic horizons Two genera found in forest litter and on decomposing wood Probably predacious in soil

Free-living (predacious) adults with sclerotized cuticle, cosmopolitan distribution in transient litter or detritus rich in organic matter, such as composts, manure

The most species-rich group of the order with free-living, parasitic and ectocommensals (on vertebrates, and arthropods such as Coleoptera and Diptera). Soil species are often predacious on nematodes and microarthropods; with families Rhodacaroidea and Ologamasidae dominating in temperate regions. Other species occur in marine/estuarine sediments

Associated with surfaces (ectosymbiont?) of millipedes and snakes in tropical/subtropical regions

About 150 families. Adults with sclerotized cuticle, 200-1400 |im in length; primarily detritivorous, often supplemented by nematodes and microinvertebrates

Mostly soil-dwelling species, with a majority favouring warm climates Found in forest and grassland soil, particularly in temperate climates Known from soils and tree holes Soil species

Burrow into and feed on twigs and conifer needles Found in forest soils, wet moorlands and bogs CircumdehiscentiaeThe largest suborder with cosmopolitan species, including the more significant portion of forest soil mites; also found in the tree canopy, freshwater vegetation and littoral zones

Sejina

Microgyniina

Cercomegistina

Antennophorina

Uropodina

Diarthrophallina

Zercotina

Epicriina

Arctacarina

Parasitina

Dermanyssina

Heterozerconina

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