Consumers of FPOM

The collector-FPOM linkage (Figure 8.5) depends on FPOM captured from suspension or from substrates. As discussed in Chapter 7, FPOM is a poorly characterized food source, and it originates in a number of ways. Categories considered to be richest in quality include sloughed periphyton and biofilm, and particles produced in the breakdown of CPOM. Morphological and behavioral specializations for suspension feeding including setae, mouthbrushes, and fans are diverse and well studied (Wallace and Merritt 1980), whereas the mechanisms of deposit feeding appear to be less elaborate (Wotton 1994).

Caddis larvae in the superfamily Hydropsy-choidea (comprised of the Philopotamidae, Psychomyiidae, Polycentropodidae, and Hydro-psychidae) spin silken capture nets in a variety of elegant and intricate designs. Most net-spinning caddis are passive filter feeders, constructing nets in exposed locations, but some nets act as snares (Plectrocnemia) or as deposi-tional traps where undulations by the larvae create current (Phylocentropus, Wallace and Malas 1976). Filter-feeding hydropsychids vary considerably in mesh size and microhabitat placement of their nets (Wallace et al. 1977, Wallace and Merritt 1980). Brown et al. (2004) studied the properties of the net silk of Hydropsyche silta-lai, finding that it has a very low tensile strength, placing it among the weakest of natural silks; however, it is strong relative to the forces it experiences and it has high extensibility,

Detrital Breakdown Streams

FIGURE 8.5 The collector-FPOM linkages for a small stream within a temperate deciduous forest. Sources of detrital particles <1 mm include CPOM fragments, terrestrial inputs, animal feces, and sloughed algal cells and biofilm material. FPOM and associated microorganisms are ingested from the water column by filter feeders and from the streambed by collector-gatherers. (Reproduced from Cummins and Klug 1979.)

FIGURE 8.5 The collector-FPOM linkages for a small stream within a temperate deciduous forest. Sources of detrital particles <1 mm include CPOM fragments, terrestrial inputs, animal feces, and sloughed algal cells and biofilm material. FPOM and associated microorganisms are ingested from the water column by filter feeders and from the streambed by collector-gatherers. (Reproduced from Cummins and Klug 1979.)

doubling its length before breaking. There is evidence that catch nets of larger mesh tend to be found at higher velocities and capture larger prey, whereas fine mesh nets occur in microhabitats of low velocity and retain smaller particles (Wiggins and Mackay 1978). Members of the Arctopsychinae spin coarse nets, capture a good deal of animal prey and larger detritus, and tend to occur in headwaters. The Macrone-matinae occur in larger rivers, spin fine nets, and capture small particles. The Hydropsychinae are intermediate in net mesh size, more widely distributed, and perhaps because of the broad range of resources utilized, also are richer in genera.

Edler and Georgian (2004) examined the efficiency of particle capture in Ceratopsyche morosa (net mesh size 160 x 229 |m) and C. sparna (150 x 207 |im) by releasing food items of different sizes including Artemia nauplii (mean length 528 mm), and pollen of corn (Zea mays, mean diameter 84 mm) and paper mulberry (Broussonetia papyrifera, 12.5 mm). Both caddis species ingested more of the largest particles despite the greater availability of smaller particles in suspension (Figure 8.6), but particles

Paper mulberry Corn pollen Artemia nauplii pollen

Particle type

FIGURE 8.6 Particles found in the guts of fifth-instar larvae of Ceratopsyche morosa and C. sparna as fractions of total. For each species, bars marked with the same letter are not significantly different. (Reproduced from Edler and Georgian 2004.)

Paper mulberry Corn pollen Artemia nauplii pollen

Particle type

FIGURE 8.6 Particles found in the guts of fifth-instar larvae of Ceratopsyche morosa and C. sparna as fractions of total. For each species, bars marked with the same letter are not significantly different. (Reproduced from Edler and Georgian 2004.)

smaller than mesh openings were retained as well. Selective capture of larger particles might be expected to be energetically rewarding, and this is supported by the finding that H. siltalai nets retained a larger range of particles size (1-40 mm) than those present in the water (1-25 mm) (Brown et al. 2005). Because some captured particles were smaller than the mesh size of H. siltalai's net, adherence of particles to the silk apparently has some role in overall particle retention.

The impressive nets of caddis larvae are but one of the many specialized adaptations for capturing particles from suspension that have arisen frequently and repeatedly among aquatic invertebrates (Wallace and Merritt 1980). Larval black flies (Diptera: Simuliidae) are highly specialized suspension feeders (Figure 8.7). They have been studied extensively because the adults include important disease vectors as well as nuisance pests (Malmqvist et al. 2001). Black fly larvae attach to the substrate in rapid, often shallow, water and extend their paired cephalic fans into the current (Chance 1970, Currie and Craig 1988). Particles apparently are snared by sticky material on the primary fans, which are the main suspension-feeding organs, while secondary and medial fans act to slow and deflect the passage of particles. Food items are removed by the combing action of mandibular brushes and labral bristles, further adaptations to a filtering existence and lacking in some black fly species that scrape substrates instead. Fans are opened when feeding and closed at other times (Crosskey 1990). The four species studied by Chance (1970) ingested particles from <1 to >350 |im. Field studies generally report the majority of ingested particles to be <10 |im in diameter (Merritt et al. 1982).

Visualization of the fields of flow surrounding individual simuliid larvae indicates that they position their fans for maximum filtering effectiveness, and may be able to manipulate flow vortices to enhance feeding (Chance and Craig 1986, Lacoursiere and Craig 1993). Palmer and

Rheotanytarsus Tube

FIGURE 8.7 (a) The typical filtering stance of a black fly larva (Simulium vittatum complex). The larval body extends downstream at progressively greater deflection from vertical with increasing current velocity, and is rotated 90° to 180° longitudinally as can be seen by following the line of the ventral nerve cord. The position of the paired cephalic fans is upper and lower, rather than side by side. The boundary layer (depth where U falls below 90% of mainstream flow) begins at roughly the height of the upper fan (Chance and Craig 1986). (b) Details of cephalic fans: left: head of a normal larvae seen from beneath, with cephalic fans fully open; middle: Simulium atlanticum with uniform fringe of microtrichia; right: S. manense with long and short microtrichia. (Reproduced from Crosskey (1990) and SEM photographs of DA Craig.)

FIGURE 8.7 (a) The typical filtering stance of a black fly larva (Simulium vittatum complex). The larval body extends downstream at progressively greater deflection from vertical with increasing current velocity, and is rotated 90° to 180° longitudinally as can be seen by following the line of the ventral nerve cord. The position of the paired cephalic fans is upper and lower, rather than side by side. The boundary layer (depth where U falls below 90% of mainstream flow) begins at roughly the height of the upper fan (Chance and Craig 1986). (b) Details of cephalic fans: left: head of a normal larvae seen from beneath, with cephalic fans fully open; middle: Simulium atlanticum with uniform fringe of microtrichia; right: S. manense with long and short microtrichia. (Reproduced from Crosskey (1990) and SEM photographs of DA Craig.)

Craig (2000) suggest that black fly larvae occurring in fast-flowing, particle-rich water will tend to have strong fans with a porous ray structure, whereas larvae found in slow-flowing, particle-poor water will tend to have weak fans with a complex structure. Despite the evident elegance of the adaptations of larval simuliids for suspension feeding, this is by no means the only feeding mode employed. Currie and Craig (1988) state that scraping the substrate using mandibles and labrum is the second most important method of larval feeding, not including species that lack cephalic fans and are obligate scrapers. In addition, black fly larvae occasionally ingest animal prey, and Ciborowski et al. (1997) demonstrated that black fly larva grow when supplied only with DOM. This diversity is a useful reminder that even those taxa displaying great specialization for a particular trophic role also may be capable of great versatility.

As mentioned in Section 7.2.1, larval black flies not only are important for their ability to filter very fine particles, but also for their production of fecal pellets (Wotton and Malmqvist 2001). In northern rivers and particularly at lake outlets where very dense black fly aggregations occur, fecal pellet loads of several tons of C per day have been reported (Malmqvist et al. 2001). These pellets are available to filter feeders when in suspension, and to deposit feeders after they have sedimented. When Wotton et al. (1998) induced black fly larvae to produce labeled fecal pellets by adding paint to a lake outlet stream, the guts of midge larvae, oligochaetes, and black fly larvae contained abundant label, and lesser amounts were found in baetid mayfly larvae and the isopod Asellus.

Fecal pellets likely are an under-appreciated source of FPOM. Feces usually contain undigested food items and often are bound into discrete pellets although some are diffuse (Wotton and Malmqvist 2001). Pellet size varies with the size of the animal that produced them, and can be as small as 6 x 9 |m in protozoans. Although most organisms produce fecal pellets that are smaller than the food they consume, some suspension feeders such as larval black flies can ingest very small food items and so produce fecal pellets larger than the food they ingest.

Other dipteran families with representatives adapted to a suspension-feeding existence in running waters include the Culicidae, Dixidae, and Chironomidae (Wallace and Merritt 1980). Some Chironomidae construct tubes or burrows with catchments and create current by body undulations; others such as Rheotanytarsus passively suspension-feed by means of a sticky secretion supported by rib-like structures on the anterior end of the case.

Bivalved mollusks are effective filter feeders, capable of removing very small particles (10 | m and smaller) from their respiratory water current using sieve-like modified gills and mucus to filter and trap particles. Bivalves can remove large amounts of FPOM from the water column, including detritus, bacterioplankton, phyto-plankton, and zooplankton (Strayer et al. 1999). Roditi et al. (1996) reported that zebra mussels removed phytoplankton and nonfood particles at the same rate, but other studies suggest that mussels can be selective within the FPOM pool. Based on stable isotope analyses, Nichols and Garling (2000) determined that unionids, which are the dominant group of freshwater mollusks, used bacteria as their main C source, although algae were found in the gut and provided vitamins and phytosterols. Christian et al. (2004) also found that mussels were using a bacterial fraction of FPOM as their food source based on stable isotope and digestive enzyme analyses. Although bivalves are traditionally seen as suspension feeders, Raikow and Hamilton (2001) reported that stream unionids obtained 80% of their food from deposited material versus 20% from suspended material. These unionids were probably assimilating the microbi-al and algal components of the suspended or benthic organic matter rather the bulk material.

Mechanisms of FPOM feeding by collector-gatherers either are less diverse in comparison with suspension feeding, or less is known about the subject. Nonetheless, this feeding role is well represented in most stream ecosystems in numbers of both individuals and species. Among the macroinvertebrates in swifter streams, representatives of the mayflies, caddis flies, midges, crustaceans, and gastropod mollusks are prominent deposit feeders. In slow currents and fine sediments one would also expect to find oligo-chaetes, nematodes, and other members of the meiofauna. It would be surprising if these animals all fed in the same way and consumed the same food. In addition to their particular food-gathering morphologies, these taxa differ in their ability to produce mucus, in mobility and body size, in their digestive capabilities and in whether they are surface dwellers or live within the sediments.

Browsing on easily assimilated biofilms may allow consumers to meet their energy needs without having to ingest large quantities of material. This is not the case for animals that ingest low-quality POM mixed with sediments. Many deposit feeders "bulk-feed," processing each day from one to many times their body mass of sediments and assimilating a low fraction of what they ingest. The burrowing mayfly Hexa-genia limbata ingests more than 100% of its dry mass daily (Zimmerman and Wissing 1978). Estimates of assimilation efficiencies for FPOM deposit feeders in streams are scant, but numerous studies of leaf-shredding insects document assimilation efficiencies in the range of 10-20% (range: 1-40%, McDiffett 1970, Golladay et al. 1983) and daily ingestion rates in excess of one body mass per day. The assimilation efficiency of FPOM collectors in Sycamore Creek, Arizona, was estimated at 7-15%, and they consumed the equivalent of their body weight every 4-6 h (Fisher and Gray 1983).

Under the reasonable assumption that detritus varies widely in food value, one may ask whether deposit feeders adjust to different feeding opportunities. Taghon and Jumars (1984) argue that selection can be accomplished by either differential ingestion, which usually involves some method of particle rejection in the buccal region, or differential digestion, based on digestive physiology and gut retention time. High-quality foods that can be absorbed rapidly should favor high feeding rates and short retention times, whereas feeding should slow to allow longer digestion of poor-quality foods. Calow (1975a) demonstrated an inverse relation between ingestion rate and absorption efficiency in two freshwater gastropods. When starved, snails slowed the rate of passage of food through the hepatopancreas, the main site of absorption and digestion. The effect of changing food quality on gut retention time apparently varies with the quality of the food. Calow (1975b) found that the herbivorous limpet Ancylus fluviatilis increased its retention time for poor-quality food (the expected result), but the detritivorous snail Planorbis contortus did the opposite. It may be that whenever the food carrier is highly refractory, as in the case of lignin, it pays to process material rapidly for easily removed microbes rather than attempt to extract energy from nearly indigestible substrate.

In summary, deposit feeders are among the least understood of FFGs in running waters, partly for lack of analysis of feeding mechanisms, and partly because of shortcomings in understanding the sources and pathways of FPOM. Some taxa may shift opportunistically between this role and shredding (e.g., Gammarus), and others between the collecting of FPOM and grazing of easily removed periphyton (the "brusher" category discussed below). There is evidence to suggest that deposit feeding is common in early instars that will occupy other, more specialized guilds as they grow larger. Clearly, there is room for further research into this feeding role.

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