Suspension Feeding

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The Reef Aquarium Consequences of Suspension-Feeding as a Way of Life
By Ronald L. Shimek

Probably the most common way that organisms make a living in the marine environment is by feeding on particulate material suspended in the water. This way of gathering nutrition is fundamental to marine and aquatic ecosystems and probably due to the density of water and its relationship to the density of living tissue. Simply put, many organisms can float or easily swim in water, and this waterborne life constitutes a potential food source to many animals. The exploitation of floating food has been the driving force of much of the evolution occurring in marine ecosystems. Many of the organisms maintained by reef aquarists are suspension-feeders and an understanding of their morphology and natural history and the constraints this places on the organisms is important to successful husbandry of these organisms.


-The Methodology of Passive Suspension Feeding

There are passive and active suspension-feeding organisms, and these two ways of feeding are fundamentally different, particularly as regards the amount of energy expended in the capture of prey. In passive suspension-feeding, the organism may extend a body part to allow a food item to impinge on it and then expend some energy transferring the food to the mouth, but if so, those activities may be the extent of the energy expenditures of the critter. In active suspension-feeding the organism expends significant energy in procuring food by pumping, secreting filter materials or actively grabbing prey.

Passive suspension-feeding animals generally depend on ambient water motion to bring food to them. While these animals expend very little energy to bring water to themselves, they may expend a lot of energy in the actual capture of the food item once it has reached them. Numerous groups of organisms can be considered to contain passive suspension-feeding organisms. Among them are the cnidaria, such as the soft corals, sea anemones, and corals, many annelid worms and some echinoderms such as sea cucumbers and brittle stars (Lewis, 1977, 1981, 1992; Kozloff, 1990, Ruppert and Barnes, 1994, Nybakken, 1997).

Orientation to currents is important in gorgonians. These animals are dependent on food collection from water flowing over them, and their orientation to that water flow is important. Mobile animals such as sea anemones, annelid worms or sea cucumbers may orient themselves or their filters in the water to maximize their filtering efficiency, but immobile animals such as corals and gorgonians cannot change the orientation. In nature these animals are often found specifically oriented to prevailing currents. They grow in these particular orientations after they metamorphose from the larval state. As they fasten themselves securely to the substrate, they will not be able to change their orientation once they grow into it (Leversee, 1976; Warner, 1976; Lasker, 1981; Patterson, 1984, 1991; Sebens, and Johnson, 1991; Johnson, and Sebens; 1993; Lesser, et al., 1994; Sebens et al., 1996, 1997).

Unfortunately for these animals, after collection we hobbyists are often unable to discern what is the correct orientation for optimal feeding. The animals are placed haphazardly, but perhaps aesthetically, in our systems and we hope they will thrive. Particularly for animals such as gorgonians, the water flow direction and strength are important. Fortunately, finding an acceptable water flow regime is not too difficult, but it does take a little time and patience. After the animal has been placed and acclimated sufficiently to the tank that it extends its polyps, newly hatched brine shrimp should be placed in the tank, and the behavior of the gorgonian closely monitored. Generally, a magnifying glass helps to observe the behavior of the ones with smaller polyps. If the orientation to the tank's current flow is acceptable, it will be possible to watch the polyps catch and eat the small crustaceans. If the water flow is not acceptable, then either few crustaceans will move by the polyps or they will move by in eddies or other mini-currents in such a manner that the polyps simply cannot catch them. If the latter situation occurs, the gorgonian, or perhaps the powerhead that generates the current, will need to be moved until the proper water movement occurs. The cnidarians capture, subdue and adhere to prey by the use of nematocysts, and these can be quite effective in maintaining a grip on the prey, even in strong currents.

Many animals such as sea cucumbers often use mucus to catch and hold on to prey. Mucus is a material made by chemically combining sugars and proteins, and as we all know it is sticky. Suspension-feeding sea cucumbers are able to feed by secreting mucus on expanded tentacles located around the mouth. These tentacles are typically repeatedly branched until they can look like miniature tree branches. This tree-like branching pattern is referred to as dendritic branching, and the suspension-feeding cukes are known as "Dendrochirotes," a name which literally means "tree-like fingers." The dendrochirotes extend their sticky tentacles up into the water and leave them there for a while, then bend them back, one at a time, and stick them in the mouth where the mucus and adherent food are licked off (Bakus, 1973; Kozloff, 1990; Ruppert and Barnes, 1994).

Possibly the most unusual way for organisms to capture food is by the use of aerosol properties. This is the method by which numerous brittle stars capture small particulate materials. The brittle stars extend their arms up into the water currents. As the water flows past the arms, it gives the arms a static electrical charge, much as one can get a static charge by shuffling across certain carpets. Particles in the water are also charged and if they are close enough to the arm they will tend to be attracted to it. These small food particles literally are electrostatically pulled out of the water column to stick to the arm, similar to dust particles in an electrostatic air filter (LaBarbera, 1978). Once on the brittle star's arm, the food particles are passed from tube foot to tube foot down to the mouth.


-The Methodology of Active Suspension Feeding
Active suspension-feeding animals generate a current of their own to bring food into their traps, and unlike the passive feeders can be located in areas of weak currents. These animals include many larvae, some rotifers, annelid worms, crustaceans, many mollusks, but especially bivalves, and tunicates (Kozloff, 1990; Ruppert and Barnes, 1994). Most of them use some kind of ciliary-mucus type feeding.

Basically the animal makes some kind of mucus net and moves water through the net by the use of cilia. Probably the most familiar of the ciliary-mucus suspension feeders are clams. Almost all shallow water bivalves feed by using a ciliary-mucus feeding process. This process of feeding can be used for deposit feeding on sediments as well as feeding on material suspended in the water column, and many bivalves are sediment eaters (Olaffson, 1986).

Most clams, however, feed on particulate material suspended in the water. The bivalves have modified gills that, in addition to having a respiratory function, are used as filter-feeding structures. The structure of the clam gill varies a bit, but it can be visualized as having filaments with very thin openings between them. In many species there are cross connections between the filaments which creates a sieve-like mesh. Virtually all surfaces of the gills are covered in cilia and bathed in mucus. These cilia move the water into the clam, often through a siphon, and the water is forced through the mesh. Particles collect on the gills and get conveyed to the mouth in a conveyor belt of mucus moved by yet other cilia (Kozloff, 1990; Ruppert and Barnes, 1994).

The gills are generally visible in a clam if it gapes, and look like sheets of tissue located in the middle of the cavity between the shells. An aquarist can see the gills in a tridacnid clam simply by looking into the internal cavity through one of the large siphons. Incidentally, all clams need to feed, including the tridacnids. These species can get a lot of their caloric needs from their zooxanthellae, but particularly when the clams are small they don't have enough tissue to support the large populations of the algae necessary to produce sufficient food. Consequently in tridacnids, feeding is more important for smaller clams than larger ones (Klumpp and Lucas; 1994; Griffiths, and Klumpp, 1996; Hoegh-Guldberg, 1996).

A different type of ciliary-mucous feeding is seen in feather-duster worms. Here there is an elaborate crown of highly-branched tentacles on the head. These tentacles are lined with cilia which pull the water through the tentacle crown. Food is caught on the tentacles and moved to a food groove in the center of each tentacle. Moving mucus in the food groove transports the food to the mouth (Fauchald and Jumars, 1979; Kozloff, 1994; Ruppert and Barnes, 1994).

(CONT)

 

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Suspension Feeding (cont)

One characteristic of the ciliary-mucus feeding process is the presence of elaborate sorting mechanisms to ensure that only appropriate particles are eaten. Generally the particles are sorted on the basis of size and density, and there is a very small range for acceptable particles; both larger and smaller particles are rejected (Kozloff, 1990; Ruppert and Barnes, 1994). From the point of view of the aquarist, this can cause significant problems. In general, ciliary-mucous suspension feeding animals eat particles that are either bacterial aggregates or phytoplankton, primarily unicellular algae. Bacterial aggregates such as small particulate material which gets covered in bacteria are really very abundant in tanks with a good community of organisms in the sand bed with a second community of microcrustaceans on the live rocks. The presence of small crustaceans, in particular harpacticoid copepods, helps provide particles of the appropriate size with their fecal material. In nature fecal pellets and other particles are also important as food sources (Passow and Alldredge, 1994). Small unicellular phytoplankton, however, are seldom found in reef systems.

Crustacean feces, and particularly those of the smaller crustaceans, are in the form of small pellets; which are referred to, with a great degree of clarity, as fecal pellets. These pellets are unlike the feces of most and animals and are formed by the peculiar mode of digestion found in these animals. Unlike most other animals when these microcrustaceans feed, the cells lining their foregut secrete a thin, cellophane-like bag of chitin around the food. The food is thus packaged in discrete, packaged blobs which pass through the gut. During digestion, digestion enzymes pass through this bag to act on the food and break it down totally within the membrane-bound pellet. Nutrients ooze out the bag and are absorbed by the gut lining. At the end of the digestive process the bag contains undigested food and probably small amounts of digestive enzymes and digested material. After this is voided from the copepod, it is referred to as a "fecal pellet." These may drift through the water for quite a while. The chitin that constitutes the bag breaks down very slowly in marine waters and the fecal pellet may remain as a discrete structure for a long time.

This fecal pellet is really a small permeable bag holding nutrients. As these nutrients ooze out of the bag, its surface gets colonized by bacteria which utilize the nutrients. Such a bacterially-covered bag is often prime food for suspension feeders. In many cases, they pass the fecal pellet through their guts, digest the bacteria off the surface, and then expel the bag as their fecal material. If so, the whole bacterial colonization process may start all over again. In some open-ocean situations, it is estimated that the fecal pellet produced by a near surface crustacean may be eaten and re-eaten as many as eight to ten times before the original pellet gets incorporated into the marine sediments (Nybakken, 1997).

Bacterially covered fecal pellets are very nutritious. Bacteria have a high nitrogen to carbon ratio, and thus provide a lot of material to build proteins and other nitrogen containing compounds. This notwithstanding, fecal pellets with a bacterial frosting do not provide much in the way of carbohydrates such as sugar. For those suspension-feeding animals with zooxanthellae, this lack may not pose much of a problem, as the zooxanthellae often produce significant amounts of the sugars for their host. For proper nutrition, animals without zooxanthellae, such most bivalves, worms, and some soft corals and other suspension-feeders, additionally need to feed on the second category of planktonic food, unicellular algae (Fabricius, et al. 1995). These unicellular algae can be of several different types, but the primary types in natural systems are unicellular green algae, diatoms, and dinoflagellates. Many suspension-feeding animals will survive on a diet consisting of a single type of algae, but seem to do best on a mixture of algal types; presumably to balance the variety of nutrients.

Because of this requirement for unicellular algae, many ciliary-mucous suspension-feeding animals are not able to be kept in reef aquaria. We simply lack appropriate food for them in the system. These unacceptable animals include most bivalves such as flame scallops and thorny oysters. These animals are generally able to be kept for a few months; until the animal uses up its stored energy. Then they die. While many aquarists can successfully keep tridacnids, there still is a significant mortality of these animals probably due, in large part, to the lack of sufficient phytoplankton in our systems. Many aquarists have problems maintaining the large feather duster worms, Sabellastarte magnifica, for the same reasons.

Most reef systems probably lack significant populations of unicellular algae; light intensities in our systems are often low relative to nature and I think aggressive use of foam fraction and granular activated carbon (however beneficial they are otherwise) may remove many of the nutrients necessary for algal growth. As a result of this, many of these animals will slowly die in our reef tanks unless their diet is supplemented, and rather heavily supplemented by the addition of algal cultures such as the so-called "green-water.""Green water" is a unicellular green algal suspension, and may be cultured in variety of ways. However, if enough for good growth of some of the animals that feed on it is put into a reef-aquarium the water will be colored a very light green. Additionally, such an addition introduces a significant amount of mineral nutrients to the system in the form of the culture medium. Both the addition of the color and the nutrients are not desired by most hobbyists, so these materials are unlikely to be added to a reef aquarium system, and as a consequence, many suspension-feeders will not be able to be maintained.

-Other Properties of Suspension Feeders.

We tend to compartmentalize our thoughts about animals and really, seldom consider the whole picture. Although suspension-feeding might seem to relatively isolated from some of the other properties of these organisms, it fundamentally dictates many of those properties. For example, from an evolutionary point of view, plankton are seldom in short supply, so food is not a limiting factor for most of the tropical the suspension feeders, all they have to do is sit in one spot and feed. Thus, appropriate "sitting" space IS critical, and these animals are often very good competitors for space (Peterson and Andre; 1980; Endean, et al. 1997).These animals compete for space in any number of ways. Some, such as tunicates, simply grow over and kill competitors by smothering or starving them. Cnidarians typically sting approaching competitors, and can often either drive them off or kill them. Sponges, many soft corals, and some tunicates release noxious or toxic chemicals on to their approaching competitors which can kill them or inhibit their growth.

The competitive ability of these animals has profound consequences for the reef aquarist. It is simply impossible to put a lot of these animals close together in our systems. Unfortunately, by the nature of our systems, we tend to want to place a lot of animals in them, which means the animals are close together. After all, we are actually working with a very limited amount of surface area. In nature, however, many of these animals have a significant and dramatic spacing. If they are placed closer than this they not only try to kill one another, but even the superior competitors can become significantly stressed. This stress may manifest itself by causing a reduction of the animal's immune system, and many stressed animals may get infections such as RTN (rapid tissue necrosis) in corals. Competitively stressed animals are diverting significant metabolic energy to fighting their neighbors rather than growing, so growth will slow and may even stop completely. Additionally, in our closed aquarium systems, these "competitive" chemicals may accumulate and have effects on more than just the offending competitor. When some of these animals start to compete, the effects may felt throughout the aquarium system. Probably the most important way to reduce this competitive stress in an aquarium, is by aquarist prudence and self-control. The natural attributes of the organism must be taken into account when purchasing or positioning animals.

It may be significantly better for a reef system for the aquarist to resist the urge to purchase "just one more animal" unless the appropriate food is present or can be added to the system on a regular basis, and unless spatial conditions are clearly adequate for it at the time of purchase, and after allowing for future growth. This may limit the aquarist's display, however, it will result in healthy animals and significantly less mortality.

END.

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