Aquaculture

Nils Kautsky , ... Jurgenne Primavera , in Encyclopedia of Biodiversity, 2001

Filter feeders such as mussels and oysters have successfully been integrated with fish aquaculture (Jones and Iwama, 1991; Shpigel et al., 1993) and the technique is also being practiced in shrimp aquaculture (Hopkins et al., 1993). Filter feeders in such an integrated system benefit from release of particles, from waste feed and feces from the farm, and from the stimulation of bacterial production and phytoplankton cells. In addition to generating additional income for the farmer, such an integrated system will also secure income by resulting in a more diversified production (Troell et al., 1999). In addition to output of paniculate wastes, aquaculture also releases dissolved nutrients, and generally less than one-third of the nutrients added through feed will be removed through harvest in intensive fish and shrimp farming (Gowen et al., 1991; Primavera, 1994; Briggs and Funge-Smith, 1994). Many studies have verified that wastewater from intensive and semi-intensive tank or pond mariculture systems is suitable as a nutrient source in seaweed production (Neori et al., 1991; Buschmann et al., 1994, 1996; Jimenez et al., 1994; Krom et al., 1995; Neori, 1996; Troell et al., 1999), and that integration with seaweeds can significantly reduce the loading of dissolved nutrients to the environment (Lin et al., 1993; Phang et al., 1996). The choice of commercially attractive seaweed species also increases profitability for the farmer in integrated cultivation. In more open culture systems such as cage farming, the continuous exchange of water makes waste disposal difficult to control, but studies have shown beneficial effects from the integration of seaweeds with such culture (Hirata et al., 1994; Petrell et al., 1993; Troell et al., 1997).

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Sponges

James H. Thorp , D. Christopher Rogers , in Field Guide to Freshwater Invertebrates of North America, 2011

III Ecology, Behavior, and Environmental Biology

Sponges are filter feeders and hosts for symbiotic algae (a relatively uncommon relationship in freshwater taxa). They can filter substantial numbers of bacteria and suspended algae from the water, making them serious competitors with some protozoa, zooplankton, and a few other multicellular taxa. A single finger-sized sponge can apparently filter nearly 125  L of water (˜33 gal) in a day. Aside from the filtration impact, the sponge's algal symbionts contribute to benthic primary productivity in lake and stream ecosystems. Contrasting with their role as important consumers of plankton, their silica "skeleton" and various toxic and pharmacologically active compounds make them relatively invulnerable to predators. Consequently, their direct energy contribution to higher aquatic food webs is essentially blocked, and mostly occurs indirectly through detrital pathways. Only a few insect species prey on sponges, such as spongilla flies (order Neuroptera, which includes the terrestrial antlions and their relatives). Sponges also function as physical habitat for a variety of largely innocuous invertebrates that live on or in the sponge body. These include worms, small clams, mites, and various insects.

Many but not all sponges have evolved a symbiotic relationship with algae (typically green algae but at least in one case with a yellow-green algal species). This relationship ranges from facultative (optional) to obligate and seems to vary with prevailing environmental conditions. In such symbiotic relationships, the host provides shelter, protection from predators, and a source of nutrients (nitrogen and phosphorus) and carbon dioxide for photosynthesis. In return, the symbiont provides photosynthetically-fixed carbon to the invertebrate host. These algal symbionts are known to contribute as much as 50–80% of the energy needed for sponge growth, based on a pond study in the northeastern USA.

Many sponges have a rather cosmopolitan distribution, but almost all require (i) a hard surface on which to grow; (ii) an environment that is not overly disturbed by physical fluctuations; and (iii) water that is neither too polluted nor loaded with large amounts of silt, sand, or other particles that could clog their filtering system. Light availability can also be a limiting factor for species with symbiotic algae.

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Business continuity as a means to strengthen disaster risk reduction in a coastal community of oyster farmers

Raymond S. Rodolfo , Mark R. Lapus , in Strengthening Disaster Risk Governance to Manage Disaster Risk, 2021

4.1 Oyster farming in the Philippines

Oysters are filter feeder, bivalve mollusks that grow in brackish waters of estuaries which are common in the Philippines due to its archipelagic nature. The most common oyster species in the Philippines are the slipper-shaped oyster, Crassostrea iredalei; the palm-rooted oyster, Saccostrea malabonensis; Saccostrea palmipes; and the curly or wild oysters, Saccostrea cucullata (Panggat, 2016). Small-scale oyster farming is widespread in the Philippines due to: (1) The presence of coastal bays and rivers which are natural grounds for oysters; (2) The need for additional sources of income brought about by the dwindling catch of small-scale fishermen; and (3) Oysters have economic potential in both domestic and foreign markets (Samonte et al., 1994).

In the Philippines, the most common culture methods are broadcast, stake and rack. Table 13.1 lists the growing methods along with its advantages and disadvantages. Growers have specific areas to culture oysters within each respective municipalities. These areas are mostly family-run and highly generational unless otherwise, sold or leased to other fishermen in the area (Panggat, 2016). Stakes and spat collectors are installed in the natural oyster spawning grounds at the onset of the rainy season, usually during the months of May to August. These months represent the peak period of oyster spawning in the Philippines when the environmental factors such as salinity and temperature are most favorable (Samonte et al., 1994). The drop in salinity in the estuary due to the influx of rain and flood water add stress to sexually mature oysters, inducing them to reproduce (Angell, 1986). Oysters are then allowed to grow to commercially viable sizes before they are harvested, which takes about 6–9   months (Panggat, 2016; Samonte et al., 1994).

Table 13.1. Summary of common oyster growing methods in the Philippines.

Method Description Advantages Disadvantages
Broadcast method Employed in shallow areas with firm solid bottoms to support the collectors such as empty oyster shells which are scattered over areas where there is natural setting occurs. Simplest and low cost. High mortalities of oysters due to siltation and predation. Difficulty in harvesting.
Stake method Applied in relatively shallow areas with soft, muddy bottoms that would allow the bamboo poles to be driven into the substrate. Higher growth rate and production per unit area. Short life span of bamboo poles (1–2   years). Hinders the flow of water, causes siltation and shallowing of the water body.
Rack method Uses bamboos or wooden frames to serve as substrates for oysters.
Tires are often times used as cultches and spat collectors.		 Higher growth rate and production per unit area. Costly. Hinders the flow of water, causes siltation and shallowing of the water body.
Floating raft method Refers to floating structures usually with hanging strings with cultches made from empty oyster shells, old rubber tires, sacks or coconut husks. Higher growth rate and production per unit area.
Can be towed to safety during typhoons.		 Costly. Prone to poaching.
May be washed downstream if not properly anchored.

Oyster farmers belong to the marginalized sector of society and often they do not own the land on which their houses are built. Oyster farmers also usually rely on their family members to labor for various activities such as the preparation of cultches, planting, harvesting, shucking and selling (Samonte et al., 1994). Additional problems encountered by oyster farmers include poaching, mortality due to siltation or sedimentation and pond effluents, absence of oyster spats, harmful algal blooms or red tide, lack of financing, lack of buyers and damage from river flooding and storm surge (Angell, 1986; Panggat, 2016; Samonte et al., 1994).

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Ecosystem Engineers

Jonathan H. Grabowski , Charles H. Peterson , in Theoretical Ecology Series, 2007

Oysters as a biofilter

Oysters are filter feeders that feed upon suspended particles in the water column, pumping such a high rate of water flow that they are considered an important biofilter that helps maintain system functioning ( Baird and Ulanowicz 1989, Grizzle et al. 2006, Newell 1988). The decline of oyster populations in estuaries along the eastern U.S. has coincided with increased external nutrient loading into these coastal systems (Paerl et al. 1998). Collectively these ecosystem perturbations have increased bottom-water hypoxia and resulted in restructured food webs dominated by phytoplankton, microbes, and pelagic consumersthat include many nuisance species rather than benthic communities supporting higher-level consumer species of commercial and recreational value (Breitburg 1992, Jackson et al. 2001, Lenihan and Peterson 1998, Paerl et al. 1998, Ulanowicz and Tuttle 1992).

Perhaps one of the most compelling examples of the consequences of loss of filtration capacity is Newell's (1988) estimate that oyster populations in the Chesapeake Bay in the late 1800s were large enough to filter a volume of water equal to that of the entire Bay every 3.3 days, whereas reduced populations currently in the Bay would take 325 days. Two other examples of the filtration capacity of bivalves include the introductions of the clam (Potamocorbula amurensis) in San Francisco Bay and zebra mussels (Dreissena polymorpha) in the Great Lakes, which have demonstrated how dramatically suspension feeding by bivalves can remove suspended solids and nutrients from the water column (Alpine and Cloern 1992, Carlton 1999, Klerks et al. 1996, MacIsaac 1996). Although the decline in oyster populations undoubtedly contributed to the decline in water quality in the Chesapeake over the past century, the application of quantified changes in water quality as a consequence of small-scale restoration studies to larger-scale, estuarine-wide management of water quality presents some significant challenges.

Experimental manipulation of oyster populations has demonstrated that oysters can influence water quality by reducing phytoplankton biomass, microbial biomass, nutrient loading, and suspended solids in the water column. Other potential water quality benefits could result by concentrating these materials as pseudofeces in the sediments, stimulating sediment denitrification, and producing microphytobenthos (Dame et al. 1989). For example, Porter et al. (2004) manipulated the presence of oysters in 1000-l tanks and found that oysters increased light penetration through the water column by shifting algal production from phytoplankton to microphytobenthos-dominated communities. Microphytobenthos biomass subsequently reduced nutrient regeneration from the sediments to the water column. Cressman et al. (2003) determined that oysters in North Carolina decreased chlorophyll a levels in the water column by 10–25% and fecal coliform levels by as much as 45% during the summer. Grizzle et al. (2006) developed a method to measure seston in situ and subsequently demonstrated that this method more precisely identifies differences in seston than traditional techniques conducted in a laboratory, suggesting that studies relying upon laboratory analyses may underestimate the effects of oysters on seston. Nelson et al. (2004) transplanted oyster beds in small tributaries in coastal North Carolina and noted that some small reefs reduced total suspended solids and chlorophyll a levels. Laboratory studies also have found that bivalvesinfluence local plankton dynamics and reduce turbidity levels (Prins et al. 1995, 1998). On the other hand, Dame et al. (2002) removed oysters from four of eight creeks in South Carolina and noted that the presence of oyster reefs explained little of the variability in chlorophyll a, nitrate, nitrite, ammonium, and phosphorous. In general, bivalve control of phytoplankton biomass is thought to be most effective when bivalve biomass is high and water depth is shallow (Officer et al. 1982), so that small-scale restoration efforts or restorations in deeper water may not necessarily achieve detectable gains in water quality.

Attributing a single value per unit of oyster reef restored may be inappropriate if the relationship between the spatial extent of oyster reef habitat and water quality is nonlinear (Dame et al. 2002). For instance, in some estuaries large-scale restoration efforts may be necessary before water quality is measurably improved because nutrient and suspended solid loading rates currently far surpass the filtration capacity of the oyster populations present in these bays. Future research efforts that provide empirical data on this functional relationship will greatly benefit attempts to model the economic services provided by oyster reefs. Because these inherent difficulties exist in generalizing the economic benefits associated with oyster filtration, one alternative approach would be to quantify the cost of providing a substitute for this service. As a natural biofilter that removes suspended solids and lowers turbidity, oyster reefs are analogous to wastewater treatment facilities. Thus the filtration rate of an individual unit of oyster reef can be quantified and compared to the cost of processing a similar amount of suspended solids and nutrients with a waste treatment facility.

Larger-scale restoration efforts within shallow coastal embayments designed to achieve improvements in water quality could have substantial indirect benefits of great economic value. First, by decreasing water turbidity (i.e., by filtering suspended solids) and suppressing nutrient runoff, oyster reefs can promote the recovery of SAV in polluted estuaries (Peterson and Lipcius 2003). Newell and Koch (2004) modeled the effects of oyster populations on turbidity levels and found that oysters, even at relatively low biomass levels (i.e., 25 g dry tissue weight m−2), were capable of reducing suspended sediment concentrations locally by nearly an order of magnitude. This reduction would result in increased water clarity that would potentially have profound effects on the extent of SAV in estuaries such as the Chesapeake Bay.

Recognized as extremely important nursery grounds for many coastal fish species (Thayer et al. 1978), vegetated habitats such as SAV have been reduced in estuaries such as the Chesapeake Bay by agricultural runoff, soil erosion, metropolitan sewage effluent, and resultant N loading fromall of these sources as well as atmospheric deposition. Nitrogen loading at levels of 30 kg N ha−1 yr−1 within Waquoit Bay, Massachusetts, resulted in the loss of 80 to 96% of the total extent of seagrass beds, and seagrass beds were completely absent in embayments with loading rates that doubled this amount (Hauxwell et al. 2003). They also found that nitrogen loading increased growth rates and standing stocks of phytoplankton, which likely caused severe light limitation to SAV by reducing light penetration through the water. Kahn and Kemp (1985) created a bioeconomic model to estimate damage functions for commercial and recreational fisheries associated with the loss of SAV in the Chesapeake Bay and determined that a 20% reduction in total SAV in the Bay results in a loss of 1–4 million dollars annually in fishery value. If improvements in water quality from oyster reef habitat increase the amount of SAV in the estuary, then the value of augmented fishery resources created by this additional SAV should be attributed to oyster reefs.

Second, improvements in water quality in general are valued by the general public who use estuarine habitats for activities such as swimming, boating, and sportsfishing. For instance, Bockstael et al. (1988, 1989) surveyed residents in the Baltimore–Washington area in 1984 and determined that their annual aggregate willingness to pay in increased taxes for moderate (i.e., ∼20%) improvements in water quality (i.e., decreased nitrogen and phosphorous loading and increased sportsfishing catches) was over $100 million. The National Research Council (2004) used the consumer price index to adjust estimates reported in the preceding studies to 2002 price levels and reported that a 20% improvement in water quality along the western shore of Maryland relative to conditions in 1980 is worth $188 million for shore beach users, $26 million for recreational boaters, and $8 million for striped bass sportsfishermen. Although there are several potential sources of error in these estimates, they may be underestimated given that improvements in the Chesapeake Bay water quality will likely result in increased recreation in the Bay and these analyses did not include the value that U.S. residents outside of the Baltimore–Washington area place on Bay resources despite the nation-wide recognition and utilization of the Chesapeake Bay (Bockstael et al. 1988). Evaluation of ecosystem services provided by oyster reefs should include assessment of this suite of benefits if larger-scale oyster restoration efforts achieve measurable improvements in water quality in estuaries along the Atlantic and Gulf coasts of the U.S.

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Lake and Reservoir Management

E. Jeppesen , ... A.-M. Ventäla , in Encyclopedia of Inland Waters, 2009

Mussels and lake restoration

Mussels are efficient filter feeders in lakes. Large unioids like Anodonta, Unio and Hyridella are sometimes abundant in well-mixed macrophyte-dominated lakes and can filter the entire water volume in a few days. They often disappear, however, in turbid lakes probably due to predation by fish larvae. Re-introduction of these species may therefore be a useful tool but it has so far received little attention. Also, the zebra mussel, Dreissena polymorpha, which colonized Europe in the 19th century and more recently also in Great lakes in the United States, may have a major impact on water clarity when abundant. Significant effects on water clarity have also been shown in enclosure experiments. However, spreading Dreissena to other water systems is problematic as demonstrated in North America where lack of natural enemies in the rapid colonization phase allows Dreissena to reach enormous densities. This has resulted in significantly reduced chlorophyll of the Great lakes, but also in fouling of water intakes in reservoirs and uncontrolled impacts on the entire lake ecosystem. The method is potentially most useful in shallow lakes, but the long term perspective is not clear.

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Nitrogen-Fixing and Nitrifying Symbioses in the Marine Environment

Rachel A. Foster , Gregory D. O'Mullan , in Nitrogen in the Marine Environment (Second Edition), 2008

3.1 Sponge–microbe specificity and phylogenetic diversity

Sponges act as filter feeders capable of circulating thousands of liters of seawater through their osculum per day while feeding on organic particles and microorganisms from the water column ( Pile, 1997; Vogel, 1977). Some sponges, primarily those in the class Demospongia (Vacelot and Donadey, 1977), are populated by microbial symbionts, mostly extracellular, that are able to avoid phagocytosis and digestion while residing in the sponge mesohyl matrix. The microbial density within the host biomass can far exceed that of seawater, reaching concentrations up to 1010 bacteria per gram of sponge wet weight (Hentschel et al., 2006). For organisms that can avoid digestion, the host provides a favorable microbial habitat due to increased nutrient availability from the active pumping of seawater and release of ammonia, urea, and organic carbon as by-products (e.g., Davey et al., 2002).

Microscopy and molecular genetic techniques have demonstrated that a single sponge often contains a very diverse microbial assemblage including bacteria (Hentschel et al., 2002), archaea (Margot et al., 2002; Preston et al., 1996) and algae (Usher et al., 2004; Wilkinson and Fay, 1979). These 16S ribosomal RNA surveys have detected microorganisms similar to known heterotrophs, photoautotrophs, and chemolithoautotrophs. The nitrogen transformations attributed to these groups include N2 fixation (e.g., Wilkinson and Fay, 1979), ammonia oxidation (e.g., Hallam et al., 2006a,b), nitrite oxidation (e.g., Hentschel et al., 2002), and nitrogen assimilation (e.g., Davy et al., 2002). The phylogenetic diversity and species richness of the symbiont population found within a single host (Hentschel et al., 2002; Hill et al., 2006; Taylor et al., 2004; Webster et al., 2001) are unlike most known marine invertebrate-microbe symbioses which have comparatively low symbiont diversity (Steinert et al., 2000). Nonetheless, the diversity of the sponge-microbe associations are often referred to as host specific. The bacterial symbionts appear to be distinct from the free-living bacterial populations in seawater and seemingly uniform bacterial populations have been detected in many geographically distant sponge species (Hentschel et al., 2002; Hill et al., 2006).

The diversity maintained in the sponge association may be partially attributed to the occurrence of asexual reproduction in sponges, allowing the establishment of a close association without requiring immediate incorporation of microbial cells into the germ line. Usher et al. (2005) reported that germ line incorporation does occur in at least some species and that cyanobacteria could be detected in both the egg and the sperm of Chondrilla australiensis. Sharp et al. (2007) further demonstrated that phylogenetically diverse, yet sponge specific, microbial lineages including bacteria and archaea could be found in Corticium sp. embryos. The stability of these complex associations over evolutionary time scales has only begun to be explored.

In summary, sponges form symbiotic associations with phylogenetically and metabolically diverse microbes. Although there is very limited evidence to document the direct benefits of the symbiosis, the microbes are thought to receive increased nitrogen for growth and the host to benefit from the removal of potentially toxic metabolites, i.e., ammonia and urea (Davey et al., 2002). The metabolic versatility of these abundant and diverse microbial assemblages in combination with the increased flow rate provided by the filter feeding host creates a bioreactor that can have a large impact on the carbon and nitrogen cycles of a marine habitat.

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Trophic Relationships of Coastal and Estuarine Ecosystems

H.W. Paerl , D. Justić , in Treatise on Estuarine and Coastal Science, 2011

6.03.4.4 'Top Down' Control: Herbivory

Zooplankton, benthic filter feeders, larval, and certain juvenile and adult fish are the primary consumers of coastal phytoplankton. The zooplankton are commonly divided into several size classes, that is, microzooplankton (<200  μm), mesozooplankton, (0.2–2   mm), macrozooplankton (2–20   mm), and megazooplankton (>20   mm). The relative contributions of these size classes to total phytoplankton biomass can vary substantially. In terms of numbers, the most abundant zooplankters in coastal ecosystems are microzooplankton. This category includes all heterotrophic protists and protozoans. The mesozooplankton includes calanoid copepods, cladocerans, and thaliacean tunicates.

Zooplankton grazing represents an important control of phytoplankton biomass and community composition, however, its impact varies with factors such as seasonality, vertical mixing, freshwater flushing, and residence time. Phytoplankton generally have faster growth rates than zooplankton, which can result in phytoplankton blooms that accumulate more quickly than the zooplankton that graze them. As a result, some rapidly growing bloom taxa can proliferate in a seemingly unabated fashion. These blooms are primarily limited by the nutrient supply. This is especially true for phytoplankton species that are capable of blooming during winter and early spring, when water temperatures are too low to support rapid growth of zooplankton grazers. During these periods, phytoplankton biomass and composition are largely controlled by physical–chemical factors such as light and nutrient availability. This type of control is termed 'bottom up'. As water temperatures warm up in spring and summer, zooplankton growth rates are enhanced and biomass can accumulate at faster rates. Also, nutrient supplies to phytoplankton tends to decrease, largely because the main source of nutrients to the estuary, freshwater runoff, often decreases during these drier periods. A result of these combined effects is that herbivorous grazing plays a relatively greater role as a control on phytoplankton biomass. This type of control is termed 'top down'. Due to variable discharge and flow rates as well as other physical factors such as wind-induced mixing, storms, and droughts, there are transitions between periods of low and higher grazing controls.

Because it is very dynamic and dependent on interacting physical, chemical, and biotic factors, the importance of zooplankton grazing as a control on phytoplankton stocks is a topic that carries a great deal of uncertainty and continues to be hotly debated. Steeman-Nielsen (1958) argued that the commonly observed seasonal patterns of more or less coincidental peaks in phytoplankton and zooplankton abundance supported the hypothesis that grazing maintained algal populations in a steady state, the level of which was determined by the limitations of other environmental conditions (i.e., light, nutrients, temperature). In contrast, Cushing (1959) utilized a simple predator–prey model to conclude that grazing did, indeed, affect the magnitude and timing of phytoplankton stocks, and that a lag between peak abundances of phytoplankton and zooplankton populations was readily observable.

High rates of grazer-induced mortality have been observed in coastal environments (e.g., Dagg and Turner, 1982; Welschmeyer and Lorenzen, 1985), and in some river plumes (e.g., Malone and Chervin, 1979). For example, in the northern Gulf of Mexico, the copepod community ingested 4–62% of the daily phytoplankton production (Dagg, 1995a). In a productive subtropical estuary (Fourleague Bay, Louisiana), ingestion rates of phytoplankton by the microzooplankton community averaged 43–165% of the daily phytoplankton production (Dagg, 1995b). In contrast, the grazing contribution from the mesozooplankton, composed primarily of copepod Acartia tonsa, was negligible, presumably because of high advective losses and predation by zooplanktivorous fish (Dagg, 1995b).

Although zooplankton herbivory may be an important control on coastal phytoplankton production during certain seasons under certain environmental conditions (Martin, 1970), it is not likely to be a severe limitation overall (e.g., Oviatt et al., 1979). Often, grazing is insufficient to balance phytoplankton growth, which leads to the development of phytoplankton blooms. In Chesapeake and Narragansett Bays, high suspension feeding rates of ctenophores and medusae on zooplankton may serve to keep the zooplankton grazing in balance (Heinle, 1974; Kremer, 1979). Other studies show inverse relationships between abundances of herbivorous crustacean and gelatinous zooplankton from field observations and direct relationships between ctenophore abundance and phytoplankton chl a (Lindahl and Hernroth, 1983; Feigenbaum and Kelly, 1984).

In some estuaries, suspension-feeding benthic macrofauna can reduce phytoplankton abundance significantly (Cloern, 1982; Officer et al., 1982). For several estuarine systems, it has been shown that a single dominant suspension-feeding bivalve population was capable of filtering the entire overlying water column in 1–4 days (Cohen et al., 1984; Nichols, 1985; Doering et al., 1986). Such grazing rates can quickly reduce phytoplankton standing stocks. For example, the invasion of zebra mussel (Dreissena polymorpha) caused a massive decline in phytoplankton biomass in the Hudson River Estuary (Caraco et al., 1997). Zebra mussels invaded this estuary in 1992 and became well established in 1993 and 1994. During these 2 years, the grazing pressure on phytoplankton increased 10-fold, leading to an 85% decline in phytoplankton biomass (Caraco et al., 1997).

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Plankton and Planktivores

Walter H. Adey , Karen Loveland , in Dynamic Aquaria (Third Edition), 2007

Filter-Feeding Arthropod Plankters

Many midwater crustaceans are filter feeders. Prominent, for example, are most cladocerans such as the well-known Daphnia (Figure 17.12), which primarily occurs in fresh waters, as well as many of the abundant marine and freshwater copepods (Figure 17.13). These animals establish water currents for feeding by movement of their numerous appendages, and the filtering is accomplished by "fans" of setae or small spines located on the bases of the appendages. Water is forced through the setal filters, in some cases, such as Daphnia, using the "bivalved" shell cavity. The collected planktonic cells are wiped off the setae and delivered to the mouth parts by specialized appendages. Mucus secretions from glands located at the bases of some appendages assist in forming the collected particulates or algal cells into more easily handled "food balls." Other appendages and the mouth parts are capable of rejecting particles that are too large, unpalatable, or perhaps even toxic. Figures 17.14 and 17.15 graphically illustrate the difficulties of discrimination based on size and shape alone faced by these planktonic herbivores.

FIGURE 17.12. Several genera of cladocerans including Daphnia, the common planktonic filter feeder from lakes and rivers.

 From Parker (1982). Reproduced with permission of The McGraw-Hill Companies. Copyright © 1982

FIGURE 17.13. The marine copepod Calanus showing (A) vortex of water movement and (B) feeding currents through filtering setae on the ventral side.

 After Barrington (1979). Copyright © 1979

FIGURE 17.14. Planktonic copepods and the variety and sizes of phytoplankton on which they would feed: (A) Oithona similis (adult); (B) O. similis (nauplius); (C) Temora longicornis (nauplius).

 After Jorgenson (1966). Copyright © 1966

FIGURE 17.15. Planktonic copepod and the relative size and abundance of its principal algal prey.

 After Barnes (1980). Reprinted by permission of Saunders College Publishing.

Many of these animals and related species are also capable of raptorial feeding if the prey is too large for the filtering system. Some scientists have suggested that the physical characteristics of water at small scale would force considerably more raptorial response for success (Levinton, 1995). However, the efficiency of capture is reduced if the prey must be individually handled. In spite of the difficulties of capture and ingestion, populations of cladocerans and copepods are capable of overgrazing phytoplankton blooms. It is typical of northern waters, marine and fresh, that the spring phytoplankton bloom, often of a few particularly well-adapted, and rapidly growing algal species, is rapidly depleted by zooplankton as the populations of the herbivores quickly develop in response to the presence of abundant plant cells.

A group of very small filter feeders that are almost exclusively fresh water is the rotifers. These animals are equally important in the benthic environment and are discussed in Chapter 18.

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Bacteria, Distribution and Community Structure

A.C. Yannarell , A.D. Kent , in Encyclopedia of Inland Waters, 2009

Predation by Cladocerans

Cladocerans are considered nonselective filter feeders, and they are capable of consuming much larger prey than protists. Most cladocerans, particularly Daphnia, are capable of feeding on bacteria-sized particles and of controlling bacterial population growth. However, cladocerans seem to be less efficient at processing the smallest bacterial cells. Strong grazing pressure by Daphnia can shift the bacterial community toward smaller, but more metabolically active cells. In addition, cladocerans can graze upon and suppress populations of nanoflagellates. HNF predation on bacteria is insignificant when large-bodied cladoceran grazers are present. This is not quite a trophic cascade, since the cladocerans themselves can suppress total bacterial abundance. However, certain portions of the bacterial community may be released from selective protistan grazing, leading to shifts within the community. Because cladocerans consume the filamentous bacteria not consumed by HNFs, the taxonomic shifts in bacterial community composition witnessed under HNF grazing should not occur. More importantly, the fast-growing, medium-sized cells targeted by HNFs may be released from pressure by cladoceran grazing, particularly if they are small enough to be inefficiently handled by the cladoceran feeding apparatus.

Microbial food webs are addressed in more detail in article Microbial Food Webs of this volume.

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MOLLUSCA: BIVALVIA

Robert F. McMahon , Arthur E. Bogan , in Ecology and Classification of North American Freshwater Invertebrates (Second Edition), 2001

4. Digestion and Assimilation

Freshwater bivalves are suspension (filter) feeders, filtering algae, bacteria and suspended microdetrital particles from water flowing through the gill. Filtered material is transported by gill ciliary tracks to the "labial palps" where it is sorted on corrugated ciliated surfaces into food and nonfood particles before food particles are carried by cilia to the mouth. Some freshwater species may also utilize the foot to feed on sediment organic detrital particles (Reid et al., 1992). Filter and pedal detritus feeding are described in Section III.C.2, while this section is devoted to food digestion and assimilation.

The bivalve mouth is a simple opening flanked laterally by left and right pairs of labial palps, whose ciliary tracks deliver filtered food from the gills to the mouth as a constant stream of fine particles. After ingestion, food particles pass through a short, ciliated esophagus where mucus secretion and ciliary action bind them into a mucus string before entering the stomach. The stomach, lying in the anterio-dorsal portion of the visceral mass (Fig. 4), is a complex structure with ciliated sorting surfaces and openings to a number of digestive organs and structures. On its ventral floor, posterior to the opening of the midgut, is an elongated, evaginated, blind-ending tube, the "style sac." The distal end of the style sac secretes the "crystalline style," a long mucopolysaccharide rod that projects from the style sac into the stomach. Style-sac cells secrete digestive enzymes into the style matrix and have cilia which slowly rotate the style. Stomach and style pH ranges from 6.0–6.9, acidity depending on phase of digestion (Morton, 1983).

The rotating style tip projects against a chitinous plate or "gastric shield" on the dorsal roof of the stomach. The gastric shield is penetrated by microvilli from underlying epithelial cells, believed to release digestive enzymes onto the shield surface (Morton, 1983). Style rotation mixes stomach contents and winds the esophageal mucus food string on to the style tip where slow release of its embedded enzymes begins extracellular digestion. Abrasion of the style tip against the gastric shield, triturates food particles and allows their further digestion by enzymes released from the eroding style matrix and from digestive epithelial cells underlying the shield.

After initial trituration and digestion at the gastric shield, food particles released into the stomach may have several fates. Particles are size-sorted. A ciliated ridge, the "typhlosole," running the length of the midgut returns large particles to the stomach for further digestion and trituration and eventually selects and passes indigestible particles to the hindgut/rectum for egestion. Ciliated surfaces on the posterior "sorting caecum" of the stomach direct sufficiently small food particles into tubules of the "digestive diverticulum" for final intracellular digestion (Fig. 4). The diverticula have the lowest fluid pH of the gut (Morton, 1983). Larger food particles falling on the caecum are recycled into the stomach for further trituration and enzymatic digestion, thus particles may pass over its ciliated sorting surfaces several times before acceptance for intracellular digestion or rejection into the rectum to form feces.

Digestive cells lining the lumina of the tiny, blind-ending, terminal tubules of the digestive diverticula take up fine food particles by endocytosis into food vacuoles for final digestion and assimilation. After completion of intracellular digestion/assimilation, the apical portions of the digestive cells, which contain food vacuoles with undigested wastes and digestive enzymes, are shed into the tubule lumina as "fragmentation spherules" which are returned to the stomach on ciliated rejection pathways. Breakdown of fragmentation spherules in the stomach releases their food vacuole contents, hypothesized to be a major source of stomach acidity and extracellular digestive enzymes (Morton, 1983).

Rejected indigestible matter passes through the short hindgut into the rectum for egestion from the anus, opening into the epibranchial cavity on the posterior face of the posterior shell adductor muscle just upstream from the exhalant siphon or opening, allowing feces expulsion on the exhalant current. Undigested particles are consolidated into discrete, dense, fecal pellets by mucus secreted by the hindgut/rectum, preventing their uptake on the inhalant current.

The cerebropleural and visceral ganglia release neurohormones which influence glycogenesis (Joosse and Geraerts, 1983). The vertebrate glycogenic hormones, insulin and adrenalin, have similar effects on unionoideans. Insulin injected into the Indian unionid, Lamellidens corrianus, resulted in declining hemolymph glucose concentration and increase in pedal and digestive diverticular glycogen stores, while adrenaline injection induced glycogen store breakdown and increased hemolymph sugar concentration (Jadhav and Lomte, 1982b). Elevated hemolymph glucose concentrations stimulates gut epithelial cells to produce an insulin-like substance in the unionoideans, Unio pictorum and A. cygnea, which stimulated activity of glucose synthetase, increasing uptake of hemolymph glucose into glycogen stores (Joose and Geraerts, 1983). Cerebropleural ganglionic neurosecretory hormones regulate pedal and digestive diverticular accumulation and release of protein and nonprotein stores in L. corrianus (Jadhav and Lomte, 1983).

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