Suspension Feeders

B T Hentschel, San Diego State University, San Diego, CA, USA J Shimeta, University of Melbourne, Parkville, VIC, Australia © 2008 Elsevier B.V. All rights reserved.

What Is Suspension Feeding? Organisms That Suspension Feed Mechanisms of Particle Contact Retaining Contacted Particles

Suspension Feeding in More Complicated Flow Regimes

Ecological Interactions Related to Suspension Feeding Further Reading

What Is Suspension Feeding?

Suspension feeding is the capture and ingestion of food particles that are suspended in water. These particles can include phytoplankton, zooplankton, bacteria, and detritus. Some suspension feeders are primarily grazers of planktonic algae, while others are carnivores, and some that feed at the sediment-water interface are primarily detritivores. Some suspension feeders are largely nonselective omnivores, whereas others display strong preferences for certain particles according to size or chemical properties.

Suspension feeders are often described as employing passive or active means to capture particles. Passive suspension feeders depend entirely on ambient water flow to supply particles to their feeding structures (e.g., forami-niferans, corals, and brittle stars). In contrast, active suspension feeders usually create their own feeding current to enhance the local supply of food particles or actively swim or engage in other feeding-related behaviors when they sense the presence of nutritious particles (e.g., ciliates, sponges, crustaceans, and bivalves). Some animals can feed either passively or actively, for example, some barnacles, which wave their feeding appendages in weak flow but hold them steady in stronger flow. Many active suspension feeders are often referred to as 'filter feeders' because they pump water through a structure that functions as a filter, removing particles from suspension (e.g., sea squirts, certain worms that secrete mucus nets, and baleen whales).

Organisms That Suspension Feed

All of the major animal clades include species that suspension feed (Table 1). Most small animals and protozoans that inhabit the plankton employ some form of suspension feeding, as do some larger drifters such as jellies and salps. Some nekton such as clupeiform fishes (herrings, sardines, anchovies, menhaden), manta rays, whale sharks, and baleen whales are suspension feeders. Numerous benthic invertebrates also remove particles from near-bottom waters. Many of these taxa obtain their nutrition almost exclusively by suspension feeding, for example, most species of sponges, hydroids, anemones, bivalve mollusks, bryozoans, phoronids, brachiopods, crinoids, sea squirts, and some polychaetes (such as fan worms) and crustaceans (such as barnacles and mole crabs). There are, however, several species of bivalves, polychaetes, and crustaceans that are known to switch between suspension feeding and deposit feeding depending on the supply of suspended particles in the near-bottom water.

A great variety of morphological structures, as well as secreted mucus structures, are used by suspension feeders to capture particles (Table 1 and Figure 1). The taxo-nomic and morphological diversity of suspension-feeding organisms makes it difficult to draw many ecological generalities about suspension feeding as a process. Nonetheless, much has been learned by focusing attention on the small-scale mechanisms by which suspension

Table 1 Examples of marine suspension feeders and the structures used by adults to capture particles®

Phylum

Examples

Particle-collecting structures

Habitat

(Protists)

Flagellates, ciliates, foraminiferans, radiolarians, heliozoans

Flagella, microvilli, cell surface, cilia, pseudopodia, spines

Planktonic, benthic

Porifera

Sponges

Microvilli of choanocytes

Benthic

Cnidaria

Hydroids, hydromedusae, siphonophores, anemones, zoanthids, corals, sea pens, jellies

Tentacles, mucus nets

Planktonic, benthic

Ctenophora

Comb jellies

Tentacles

Planktonic

Rotifera

Rotifers

Ciliated corona

Planktonic, benthic

Entoprocta

Entoprocts

Ciliated tentacles

Benthic

Cycliophora

Cycliophorans

Cilia

Benthic

Sipuncula

Peanut worms

Ciliated tentacles

Benthic

Echiura

Inn-keeper worm

Mucus net

Benthic

Annelida

Polychaetes

Ciliated tentacles, mucus threads or nets

Benthic

Mollusca

Pteropods, snails, limpets, vermetids, clams, mussels, oysters, scallops

Ciliated gill filaments, ciliated parapodia, mucus threads

Planktonic, benthic

Arthropoda (subphylum Crustacea)

Copepods, krill, crabs, shrimps, cephalocarids, branchiopods, leptostracans, mysids, cumaceans, tanaids, barnacles, amphipods, ostracods

Setae, cirri

Planktonic, benthic

Bryozoa

Bryozoans

Ciliated tentacles

Benthic

Phoronida

Phoronids

Ciliated tentacles

Benthic

Brachiopoda

Lamp shells

Ciliated tentacles

Benthic

Echinodermata

Sea stars, brittles stars, basket stars, sea cucumbers, sea urchins, sand dollars, crinoids

Tube feet, spines, pinnules, tentacles, pedicellariae, mucus threads

Benthic

Hemichordata

Acorn worms, pterobranchs

Ciliated proboscis, ciliated tentacles, mucus nets

Benthic

Chordata

Sea squirts, salps, larvaceans, lancelets, fishes, baleen whales

Mucus nets, gill rakers, filter plates, baleen plates

Planktonic, nektonic, benthic

aMost of these animals also have a suspension-feeding larval stage in which particles are captured by setae (arthropods) or cilia (other taxa).

aMost of these animals also have a suspension-feeding larval stage in which particles are captured by setae (arthropods) or cilia (other taxa).

(b)
(d)

Figure 1 Photographs of some of the more common feeding structures of benthic suspension feeders. (a) The two tentacles of a spionid polychaete extending from the worm's tube into the benthic boundary layer in a coiled pattern downstream. (b) Magnified image of a bivalve gill that shows the stacks of lamellae that filter particles; a mucus string that aids in transporting captured particles to the mouth is visible at the bottom of the image. (c) The cirri appendages of a barnacle extending above its shell. (d) Magnified image of the lophophore tentacles of a colonial bryozoan. (a) Photograph by J. Shimeta; (b-d) Photograph by B. T. Hentschel.

feeders capture particles. Capture is a two-step process that involves contacting and retaining particles.

Mechanisms of Particle Contact rate at which particles are contacted by direct interception. Direct interception is a common contact mechanism for animals that suspension feed with tentacles or setae, for example, polychaetes, echinoderms, crustaceans, etc. (Table 1).

The simplest models of particle contact have considered nonturbulent flows that have Reynolds numbers much less than one, where the Reynolds number is a dimension-less ratio of inertial to viscous forces on a fluid. Four general mechanisms (Figure 2) have been described and related to the form and function of animals' particle-collecting structures such as tentacles or setae, referred to here as particle collectors.

'Direct interception' occurs when particles follow the streamlines of the flow field and the center of a particle comes within one particle radius of the organism's collector. The area of the fluid that is sampled by direct interception at low Reynolds numbers is approximately twice the radius of the particle. The fluid velocity near the collector and the size of the particles determine the

Direct interception

Flow

Inertial impaction

Gravitational deposition Diffusional deposition

Figure 2 Mechanisms of particle contact for a cylindrical collector (large circle, shown in cross section), such as a tentacle or seta.

'Inertial impaction' can bring more distant particles in contact with a collector when the specific gravity of the particle exceeds the specific gravity of the fluid. The momentum of a relatively heavy particle can transport it to the collector even if the streamlines of the flow will not bring the particle to the collector via direct interception. The rate at which particles are contacted by inertial impaction is affected by the local velocity, the size of the particles, and the specific gravity of the particles relative to that of the fluid. Inertial impaction is especially important for animals that suspension feed on organic-mineral aggregates near the sediment-water interface, such as various worms, crustaceans, and echinoderms.

'Gravitational deposition' can also cause particles that have a relatively high specific gravity to contact the collector from above. Unlike direct interception and inertial impaction, the rate at which settling particles are contacted by gravitational deposition is not affected by the local fluid velocity at low Reynolds number. Gravitational deposition is especially important for drifting protozoans, pteropods, and benthic cnidarians.

'Diffusional deposition' occurs when the random motion of a particle causes it to cross fluid streamlines that would otherwise prevent it from contacting the collector by direct interception or inertial impaction. The rate at which particles are contacted is affected by the surface area of the collector, the concentration gradient of the particles, and the diffusivity constant of the particle which describes its rate of random motion. This mechanism can be especially important for contacting living, motile particles, for example, protozoa feeding on bacteria and cnidarians feeding on zooplankton.

These four mechanisms of particle contact can, and usually do, act in combination when suspensionfeeding organisms live in natural mixtures of particles that have different sizes, shapes, concentrations, and specific gravities. The relative contributions of each contact mechanism can also vary due to fluid velocity. Together, these contact mechanisms can account for selective feeding due to differential contact rates among particle types.

When the Reynolds number of the collector approaches unity, which is the case if the diameter of the collector is approximately 0.01 cm and the local velocity is roughly 1 cms _1, streamlines become compressed near the sides of the collector and separate in the collector's lee (Figure 3). The streamline compression along the sides of the collector results in sampling particles from a greater area of the fluid than when it occurs without streamline compression at low Reynolds number. Streamline compression also enhances particle contact by inertial impaction, gravitational deposition, and diffusional deposition.

Figure 3 Flow patterns and particle contact by direct interception for a cylindrical collector at various Reynolds numbers.

When the Reynolds number of the collector is greater than 10, which can occur if the diameter of the collector is approximately 0.1 cm and the local velocity is roughly 1 cm s _ , streamlines around the collector can separate and form downstream vortices (Figure 3). These vortices allow for particle contact on the downstream side of the collector. Organisms themselves usually obstruct the flow and create downstream vortices. Many benthic suspension feeders, especially colonial ones like bryozoans and corals, capture particles in the wake of upstream neighbors. Capture rates can be highest on the downstream edge of such colonies.

Retaining Contacted Particles

Particles contacted by the collector(s) of a suspension feeder are not necessarily captured and ingested. They must be retained during transport to the organism's mouth. To successfully retain a particle after its initial contact, some type of adhesive force is necessary to overcome the force of drag and the particle's inertia. Drag and inertia increase with particle size and with increasing velocity. The inertia of a particle also increases with its specific gravity. Therefore, retention is greater at slower velocities and for smaller particles that have a low specific gravity. Because of this, most passive suspension feeders experience a maximal capture rate at an intermediate flow strength.

A variety of mechanisms serve in retaining particles. Mucus and other organic coatings secreted on the surface of feeding structures can enhance retention efficiency. For example, many benthic suspension feeders retain particles on strings or sheets of mucus that cover their tentacles, gills, or pharynx and are then transported to the mouth (e.g., various worms, bivalves, echinoderms, hemi-chordates, and sea squirts, Table 1). Similarly, particles that have sticky organic coatings are more likely to be retained than are relatively clean particles. The electrostatic charge or hydrophobicity of particles also influences their retention. In cnidarians, the nematocysts retain zooplankton prey with barbs and toxins. In filter feeders, networks of collectors form a sieve that retains all particles larger than the sieve's pore size (e.g., the overlapping setae on crustacean appendages, and the mucus nets of worms, sea squirts, and other invertebrate chor-dates, Table 1 ).

Suspension Feeding in More Complicated Flow Regimes

Although much has been learned from relatively simple modeling of the mechanisms underlying particle contact and retention, suspension feeders often live in flow environments that are much more complicated than steady, nonturbulent conditions. In some active suspension feeders, particle capture depends on strong velocity gradients produced by the feeding currents. For example, ciliary capture occurs in ciliates, poly-chaetes, entoprocts, bivalves, bryozoans, phoronids, brachiopods, and many invertebrate larvae (Table 1 ). In this capture mechanism, cilia or cirri redirect approaching particles onto a ciliary tract, often by a reversal of the ciliary beat direction. The particles are retained in mucus strings or within the currents of the ciliary tract, without necessarily contacting the cilia. As another example, copepods intercept and sieve particles with complex appendage motions that isolate and trap desired particles from the suspension (see Estuarine Ecohydrology and Hydrodynamic Models).

Complexities in the ambient flow also have strong impacts on suspension feeders. The feeding rates of passive suspension feeders depend entirely on variability in the surrounding flow regime. Even the feeding rates of some active suspension feeders such as sponges can be enhanced by ambient flow. Strong flow can deform an animal's feeding structures or otherwise interfere with the animal's ability to create an effective feeding current. Bivalves and other active suspension feeders are known to alter their pumping rates in response to ambient velocities and the concentration of food particles. The growth form or orientation of some benthic suspension feeders is adjusted to maximize exposure to flow, for example, gor-gonian corals, crinoids, brachiopods, and scallops.

Most benthic and planktonic suspension feeders experience fluid turbulence. Turbulence can affect the local velocities and the concentration gradients of food particles near suspension feeders. Turbulent pulses of increased velocity reduce particle retention due to greater drag on contacted particles. Under nonturbulent conditions, colonial or aggregated suspension feeders can deplete particle concentrations before the fluid reaches downstream regions of the colony. Under turbulent conditions, however, local depletion of particles is less likely to occur. Many planktonic protozoans were once thought to be smaller than turbulent eddies and, therefore, not affected by turbulent variability in fluid motions. Recent studies have, however, found that the feeding rates of some protozoans can increase or decrease significantly in response to moderate levels of turbulence.

Suspension feeders living in the benthic boundary layer face strong vertical gradients in velocity, turbulence, and particle concentrations. Many passive suspension feeders have a stalked morphology or build tubes that elevate their feeding structures to regions of enhanced particle supply (e.g., foraminiferans, sponges, hydroids, corals, polychaetes, crinoids, sea squirts). If the concentration and horizontal flux offood particles reach a local maximum at some height above the bottom, many passive suspension feeders such as tube-building polychaetes can optimize the height at which they feed by varying the height of their tube or the extension of their feeding tentacles.

Many suspension feeders inhabiting shallow, coastal areas experience flow that oscillates in time due to wave motion. The behaviors of many benthic suspension feeders have been observed to differ between steady, unidirectional flows and oscillatory flows. Quantitative measures of particle contact, retention, and capture in oscillatory flows are, however, poorly understood relative to those in steady, unidirectional flows (see Waves as an Ecological Process).

Ecological Interactions Related to Suspension Feeding

Like all trophic processes, suspension feeding is integral to many ecological interactions. For example, bacterivory by suspension-feeding protozoans and grazing on those protozoans by larger zooplankton are major linkages in pelagic food webs. Unlike many other predator-prey interactions, however, the activities of most suspension feeders extend beyond biotic interactions to affect a wide range of biogeochemical processes.

The vast majority of sessile invertebrates that colonize hard substrata are suspension feeders. These organisms include bryozoans, ascidians, hydroids, encrusting sponges, mussels, and barnacles. Dense assemblages of these sessile suspension feeders often form what are termed 'fouling communities' that create unique microhabitats for other organisms. Suspension-feeding corals create an even more extensive habitat that supports diverse communities (see Coral Reefs).

Another obvious impact that suspension feeders have on the environment involves the aggregation and removal of many small particles from suspension. Pelagic grazers such as copepods and ciliates process thousands of microalgal and bacterial cells every hour. The capture and ingestion of these small, dilute food items usually results in aggregation in the form of fecal pellets that sink more rapidly out of the water column and increase the export of organic material from the photic zone to deeper depths.

Benthic suspension feeders can also remove vast quantities of phytoplankton and other particles from suspension. Bivalve mollusks typically pump on the order of 10 l d~\ but some large mussels and oysters have filtration rates as high as 1000 ld_1. The unintentional introduction of zebra mussels (Dreissena polymorpha) into North American lakes and rivers has greatly altered the ecosystem because dense populations can effectively clear the entire bodies of water they inhabit of phytoplankton and other particles every few days (see Invasive Species).

In soft-sediment habitats, some infauna suspension feed by pumping water through their burrows or tubes and capturing food particles with a mucus net. The echiuran 'inn-keeper worm', Urechis campo, typically irrigates its burrow with 50-70 l d~\ Chaetopterid polychaetes typically pump 10-20 l d_1 through their tubes. In addition to removing particles from suspension, this type of infaunal pumping irrigates subsurface, anoxic layers of sediment, creating suitable habitat for many small metazoans.

Suspension feeders that inhabit soft sediments tend to be more common in sandy substrates than in mud. Mud and silt particles accumulate only in regions of reduced water flow, which is not conducive to passive suspension feeding. In addition, fine-grained mud and silt can clog the filter elements of some suspension feeders.

Many benthic suspension feeders live in dense aggregations. In fact, many suspension feeders such as corals, bryozoans, and ascidians are colonial. Nearby neighbors can alter local flow fields and particle concentrations. Downstream members of a colony often experience reduced velocities and particle concentrations due to the 'current shading' of upstream neighbors. When roughness elements that obstruct flow (e.g., whole organisms or their feeding appendages) have diverse sizes and shapes, as typically occurs in a mixed-species assemblage, the topographic complexity can lead to enhanced local velocities and feeding rates. When the density of roughness ele ments exceeds approximately 8% of the bottom area, the wakes surrounding individual organisms interact to create what is termed 'skimming flow' around the entire aggregation. This can lead to reduced local velocities and depleted particle concentrations within the aggregation, for example, over stretches of coral reefs or mussel beds.

See a/so: Connectance and Connectivity; Coral Reefs; Detritus; Food Chains and Food Webs; Optimal Foraging Theory; Pelagic Predators; Trophic Structure.

Further Reading

Cardinale BJ, Palmer MA, and Collins SL (2002) Species diversity enhances ecosystem functioning through interspecific facilitation. Nature 415: 426-429.

Eckman JE and Duggins DO (1993) Effects of flow speed on growth of benthic suspension feeders. Biological Bulletin 185: 28-41.

Frechette M, Butman CA, and Geyer WR (1989) The importance of boundary-layer flows in supplying phytoplankton to the benthic suspension feeder Mytilus edulis L. Limnology and Oceanography 34: 19-36.

Hentschel BT and Larson AA (2005) Growth rates of interface-feeding polychaetes: Combined effects of flow speed and suspended food concentration. Marine Ecology Progress Series 293: 119-129.

Johnson AS (1990) Flow around phoronids: Consequences of a neighbor to suspension feeders. Limnology and Oceanography 35: 1395-1401.

Koehl MAR and Strickler JR (1981) Copepod feeding currents: Food capture at low Reynolds number. Limnology and Oceanography 26: 1062-1073.

Miller DC, Bock MJ, and Turner EJ (1992) Deposit and suspension feeding in oscillatory flows and sediment fluxes. Journal of Marine Research 50: 489-520.

Muschenheim DK (1987) The dynamics of near-bed seston flux and suspension-feeding benthos. Journal of Marine Research 45: 473-496.

Okamura B (1985) The effects of ambient flow velocity, colony size, and upstream colonies on the feeding success of bryozoa. II. Conopeum reticulum (Linnaeus), an encrusting species. Journal of Experimental Marine Biology and Ecology 89: 69-80.

Patterson MR (1991) The effects of flow on polyp-level prey capture in an octocoral Alcyonium siderium. Biological Bulletin 180: 93-102.

Riisgard HU and Larsen PS (2001) Mini review: Ciliary filter feeding and bio-fluid mechanics - Present understanding and unsolved problems. Limnology and Oceanography 46: 882-891.

Sebens KP, Witting J, and Helmuth B (1997) Effects of water flow and branch spacing on particle capture by the reef coral Madracis mirabilis (Duchassaing and Michelotti). Journal of Experimental Marine Biology and Ecology 211: 1-28.

Shimeta J and Jumars PA (1991) Physical mechanisms and rates of particle capture by suspension feeders. Oceanography and Marine Biology: Annual Review 29: 191-257.

Shimeta J and Koehl MAR (1997) Mechanisms of particle selection by tentaculate suspension feeders during encounter, retention, and handling. Journal of Experimental Marine Biology and Ecology 209: 47-73.

Wildish D and Kristmanson D (1997) Benthic Suspension Feeders and Flow. Cambridge: Cambridge University Press.

Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Responses

Post a comment