The aquaculture and husbandry of exotic marine species is ever evolving, with breakthroughs occurring regularly. Currently, marine species which rely on filter or suspension feeding are not easily maintained in closed aquarium systems. As a result, large quantities of these marine organisms are exported to Western countries annually due to high demands from the ornamental aquarium trade. There is a major incentive to aquaculture these animals, due to their interesting applications in the pharmaceutical industry, demand from the ornamental trade and the need for sustainable sources which do not rely on wild populations. Fortunately, new efforts are starting to show promising results.

Figure 1: Tubastrea coccinea, a well-known Scleractinian coral which does not harbour zooxanthellae. This suspension feeder broods its larvae, and readily reproduces in home aquaria.

Filter and suspension feeders

Filter feeders actively filter dissolved and suspended matter from the water by pumping water through filtration structures, which include zooplankton, tunicates, bivalves and sponges. Suspension feeders actively capture food particles from the water which travel in close proximity, by using (stinging) tentacles. This strategy has been adopted by members from the Anthozoa and Hydrozoa class, such as scleractinian corals, octocorals and crinoids.

The advent of mechanical filtration and protein skimming has provided impressive results in the aquaculture of zooxanthellate corals, fish, some bivalves and mollusks, however other major taxa are still lagging behind. These taxa comprise the organisms which rely heavily on either filter or suspension feeding. The key to successful husbandry of these planktivore animals is proper nutrition, which is currently lacking in most marine aquaria. Particulate organic matter (POM or detritus), bacteria, protozoa such as ciliates, phytoplankton and other microfauna are all part of the diet of both suspension and filter feeders. Physical aspects of these particles which are of vital importance in this respect include nutritional value and particle size.

Diverse plankton

Filter and suspension feeding organisms all have a specialised diet, which may include one or more plankton types. It has been found that azooxanthellate Octocorals such as Dendronephthya spp. mainly feed on phytoplankton, particles which are only between 0.2 and 20 μm. Paramuricea clavata, a Mediterranean gorgonian Octocoral, was found to be mainly feeding on diatom algae, nanoeukaryotes, dinoflagellate algae, protozoa such as cilia and particulate organic carbon (POC or POM). These particles are all less than 200 μm in size, which underscores the importance of pico-, nano- and microplankton in the diet of azooxanthellate corals (figure 2).

The same goes for tunicates, which mainly feed on bacterio- and phytoplankton, particles which are all below 20 μm in size. Crinoids have been found to also ingest larger particles such as zooplankton. These may include rotifers, Artemia nauplii and copepods. Sponges may be the ultimate filter feeders, which mainly feed on DOC; carbon-based molecules which are less than 0.45 μm in size. These still fairly large molecules may comprise about 90% of a sponge's diet.

Figure 2: Plankton size classes, ranging from picoplankton, nanoplankton, microplankton and mesozooplankton, as part of the diet of marine (in)vertebrates. (A, B) Scanning electron micrographs of (A) Prochloroccocus sp. (0.6 μm) and (B) Synechococcus sp. (1 μm). (C) Epifluorescence microscope image showing one nanoflagellate cell indicated by a yellow arrow. Image of (D) ciliates (mean total length is 100 – 200 μm) taken under a phase contrast microscope and (E) crab zoea (mean total length is 1000 μm) (Houlbrèque & Ferrier-Pagès, Biological Reviews, 2009).

"Particulate organic matter, bacteria, protozoa, phytoplankton and other microfauna are all part of the diet of both suspension and filter feeders."

Dosing and mimicking plankton

Several aquarists have succeeded in creating an environment which is rich in food particles, resulting in thriving filter and suspension feeders (figs. 2,3,5-10 and movies 1,2). This has been accomplished by continuous feeding of particles ranging in size from 5 to 800 μm, in addition to regular fish feedings. These tanks harbour azooxanthellate corals such as Menella sp., Swiftia exserta and Tubastrea coccinea, tunicates, bivalves such as Lima scabria (flame scallop), crinoids and sponges. The flourishing sponges may not be able to take up particles between 5 and 800 microns in size, however the bacterial breakdown of large quantities of feeds may have elevated POC and DOC to sufficiently high levels for these animals to survive and grow. 

Figure 3: A suspension-feeding Octocoral, an Indo-Pacific Menella sp. Particles are readily trapped by polyp tentacles.

To mimic the plankton-rich waters of the oceans, including coral reefs, (semi-)constant feeding is required. This can be accomplished by utilising feeding timers, able to dispense dry feeds in preprogrammed intervals. A feeding regime may consist of dosing a variety of plankton cultures or dry feeds every 30-60 minutes. These feeds should have a wide range of size classes, especially in the nano- and micro ranges (2 - 20 and 20 - 200 μm in size, respectively). Picoplankton, which is classified between 0.2 and 2 μm, may be dosed in the form of bacterial cultures such as Prochlorococcus and Synechococcus sp. These bacteria are not generally available however, with is why continuous feedings, dosing of carbon sources (such as ethanol) and plankton-saving filtration should allow for a buildup of countless species of bacteria. These may be released into the water column by manually stirring aquarium substrates, or by using biodegradable pellets (figure 4) which are placed inside canister filters. These pellets serve as a feeding ground for bacteria, which convert inorganic wastes such as nitrate and phosphate into biomass. This maintains high water quality, and bacteria which are released by mechanical friction also serve as a food source for many invertebrates. This system basically functions as a nitrate-reducing biofilter, with the addition of greatly stimulating bacterial growth due to the biodegradable substrate being used.

Saving plankton

Unfortunately, the currently applied closed aquaculture systems, including home aquaria, are not yet up to the task of sustaining the "plankton-soup" many (in)vertebrate animals require. Reproduction of many species, including marine fish, is also negatively affected by mechanical filtration as offspring is either killed or starved.

There is however an emerging desire to improve current marine aquaculture systems, which aims to preserve the precious plankton which may build up in any system. This practice already existed in the 1950's, in the form of simple aquaria with sand beds and seaweeds. Prof. Jean Jaubert continued this natural approach to aquarium filtration, with the development of his famous Jaubert system in the 1980's. Deep sand beds (DSB's), which similarly utilise heterotrophic, denitrifying bacteria also came into practice. During the recent years, an improved DSB was made available by EcoDeco in the form of the Dymico (Dynamic Mineral Control) system, which allowed for monitoring the biogeochemical processes occurring in the sand bed. Parameters such as redox and pH of the sand bed are vital to successful operation of the system, and can be manipulated by adjusting the carbon and oxygen input to the sand bed.

Various strategies exist to maintain a plankton-friendly system, ranging from deep sand beds, the Jaubert system, Dymico, large refugia with Chaetomorpha or Ulva macroalgae, algal turf scrubbers to dosing carbon sources (Wodka method). In the end, the take home message is that marine aquaria should be rich in planktonic particles, with low concentrations of dissolved inorganics such as nitrate and phosphate.

Movie 1: Feeding behaviour of a Menella sp., an Indo-Pacific gorgonian. This azooxanthellate coral species thrives in particle-rich waters, with high flow rates and low light levels. This specimen has already been kept alive successfully for 18 months, and does not show signs of degeneration.

Movie 2: Contractile behaviour of two Tunicate species. The second species shown was tentatively identified as Neptheis fascicularis. Tunicates are true filter feeders, which are able to filter hundreds of liters of water per day and remove over 95% of its bacteria. Water is pumped through the branchial siphon, and eventually out of the atrial siphon. Inside the animal, bacteria and phytoplankton are filtered from the water, which are trapped by the mucous layer lining the branchial basket of the tunicate. For a detailed article of these unique creatures, read the article of Dr. Shimek in Advanced Aquarist's Online Magazine.

Suggested feeds and filtration method

The main feed which has been used to successfully maintain the described invertebrates is commercially available in the form of agglomerated microcapsules. These are feeding pellets used in the shrimp aquaculture industry, which can be used as plankton analogues. They are produced in various sizes, ranging from 5 to 1000 μm. They do not sink quickly, due to their somewhat neutral buoyancy. They can be dosed to the aquarium easily by feeding timers, greatly reducing husbandry efforts. Such pellets may be combined with regular fish feeds and live plankton cultures. They may also stimulate the development of benthic mesofauna such as copepods, isopods and other small crustaceans. Fish species which depend on live feeds such as dragonets and pipefish will greatly benefit from this. 

Figure 4: Biodegradable pellets allow for the buildup of significant bacterial biomass, which removes (in)organic wastes and provides an additional plankton source.

Several plankton-friendly filtration systems have been described, which may all yield comparable results. The usage of biodegradable pellets may be the most user-friendly and cost-effective method for maintaining oligotrophic aquaria whilst ensuring high particle concentrations.

An additional disturbance for live plankton which should not be disregarded is the usage of aquarium pumps. High pressure and cavitation produced by pumps will likely destroy a significant proportion of the aquarium plankton. Although plankton-friendly pumps have been designed recently, these are not yet available for home aquaria. 

"Marine aquaria should be rich in planktonic particles, with low concentrations of dissolved inorganics such as nitrate and phosphate."

Concluding remarks

The obtained results with filter and suspension feeders are promising, yet preliminary. More research needs to be done by both professional and private aquarists, to ascertain the long-term effects of the applied feeds and filtration methods. However, the survival, growth and regeneration of azooxanthellate corals (including Menella sp and Swiftia exserta), Crinoids, tunicates, bivalves (Lima scabria) and sponges is promising (ten Klooster, pers. obs.).

Although several filtration methods are not yet optimised, including the usage of biodegradable pellets, these may prove to be highly successful in the sustainable aquaculture of filter and suspension feeders.

Figure 5: A tunicate with a thick outer mantle composed of tunicin, a polysaccharide. These animals contract when disturbed, which is accompanied by closing off their branchial and atrial siphons. In the photograph, the incurrent branchial siphon is located on top of the animal, whereas the atrial excurrent siphon can be seen laterally.

Figure 6: Swiftia exserta, an azooxanthellate Octocoral which requires plankton feeding.

Figure 7: Tubastrea coccinea is a hermaphroditic brooder, and reproduces in aquaria by releasing planula larvae. Due to their negative buoyancy, larvae have higher chances of survival. Within 1-2 days, they settle onto live rock and subsequently develop into primary polyps and start forming a new colony. 

Figure 8: A Goniopora sp. Members of this genus often do not thrive in home aquaria. These corals probably require ample particles in the nano- and picoplankton range. By means of cilia and flagella plankton may be transported into the polyps, a process which is undetectable by the naked eye. Notice the flatworms attached to the polyps, which do not seem to harm the colony. The presence of wrasses prevents these commensals from growing into large numbers.

Figure 9: Flame scallops (Lima scabra) have a rough outer shell and a red mantle, which is surrounded by red and white tentacles. The flame scallop's vibrant red colour is due to the large amount of carotenoids found within its body. Gills are used for respiration and filtration. Flame scallops mainly consume phytoplankton. To escape predators or harm, the flame scallop uses its valves to propel itself away from dangerous situations.

Figure 10: This species was tentatively identified as Neptheis fascicularis. Tunicates are true filter feeders, which are able to filter hundreds of liters of water per day and remove over 95% of its bacteria.

For those who would like to learn more about new feeds and filtration systems for the successful husbandry of filter and suspension feeders, go to www.reefinterests.com. To learn more about coral nutrition in general, read our in-depth article How corals feed. All photographs and other media, unless stated otherwise, by the author.

This article is proudly sponsored by: