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If the topic of artificial lighting for reef invertebrates is most likely to start a fistfight among aquarists, then any debate about feeding reef invertebrates is at least worthy of a good food fight. Through the slurry of marketing misinformation, legends, popular traditions and actual hard science, an aquarist must distill a plan for feeding their sessile reef denizens. I can assure you that success in doing so is easily achieved and really quite simple for the overwhelming majority of popular corals and like cnidarians.
The first step towards understanding and addressing the dietary needs of a reef invertebrate is identifying the dominant feeding strategies of a given specimen. Even for the layman, it is not difficult at all to see what a coral is likely to feed upon with the casual observation of polyp behavior and structure (physiology). In terms of natural science, there is a reliable adage that "form follows function". Looking at the form of a given feature, one can reliably surmise its function. In the animal kingdom there are many familiar examples: predators have forward set eyes (focused vision) while prey have widely set eyes (wider defensive scope of vision)… drought resistant animals like camels have means for retaining water <wink> while thirsty animals are ambidextrous for drinking two pints at the same time!
In the context of coral keeping, some smart truisms can be followed with regard for "form following function". Please keep in mind that there certainly are exceptions to the following rules. Aquarium science is not exact, and the very nature of the three-dimensional environment of the sea with its many exciting complexities requires that aquarists avoid clinging to absolutes. Nevertheless, the following guidelines are useful for serving the greater good and are proffered here as but a part of your information gathering process. Disclaimer aside, I trust that this will help you make an informed decision based upon an intelligent consensus of the facts.
Most Corals Need to be Fed!
The crash course on coral feeing is that most symbiotic corals need to supplement the products of photosynthetic activity from hosted zooxanthellae. To be clear, know that these corals will starve, albeit slowly, if they are not fed in captivity. Very few species are truly autotrophic (able to derive all necessary nutrition from symbiosis with zooxanthellae). Unfortunately, the condition is not always apparent to an aquarist when so many corals are surviving at a net daily deficit of only a few percent. How can one tell when a coral has lost ten percent of its mass to attrition when polyp cycles (swelling and retracting with water) can be so variable from day to day? Let me explain further. We might fairly say that most popular coral in the aquarium trade are 75-95% photosynthetic. That is to say that they derive 75-95% of their nutrition from the products of symbiosis with zooxanthellae and the balance through other means (feeding by absorption or organismally, for example). This assumes that water quality and light conditions (age of lamps, delivery of light through potentially discolored water or dusty lenses, etcetera) are optimal always and without interruption to maintain such efficiency. Now even if we take an unfed animal that is satisfied 99% by photosynthesis in an otherwise "perfect" aquarium system, that still leaves a net daily deficit of 1% of its dietary needs that is not met (translocation of carbon by symbiotic activity). Such corals appear to carry on quite well and may even seem to grow as they become established in the aquarium and full polyp extension is realized (tricking one to suspect "growth"). The facts, however, remain unchanged: a one percent shortfall is still a daily deficit and such corals are starving… albeit slowly. Attrition is one of the very plausible explanations for corals that seem to be faring well for many months or even a couple of years and then die "mysteriously". Often times if is a simple matter of consumption from the slow deficit in feeding. Now consider that our aquarium systems do not run at peak performance every day to afford optimal photosynthetic activity. Lamps age (the useful color spectrum strays), saltcreep, dust and debris significantly impede the transmission of light (through bulbs, lenses and canopies), water clarity is often compromised by turbidity or discolorants ("aging" yellow water between water changes), and overall water quality fluctuates such that a captive corals and their symbiotic algae cannot possibly flourish at peak activity every day. Combined with the fact that the overwhelming majority of corals are not even remotely close to being autotrophic, we have a situation where feeding is not a question of "if" but rather "what" to feed.
Polyp Structure and Behavior
Most popular coral in the aquarium trade feed upon zooplankton. Zooplankton predominantly comes out at night on a reef and corals extend their polyps then in suit to capture them. Such organismal feeding necessarily requires larger polyps to capture the larger prey (in particle size relative to phytoplankton and microscopic nanoplankton). And so, if your coral tends to extend its polyps at night and has larger polyps relative to other corals, you likely have a carnivore that feeds on zooplankton.
Corals that extend their polyps by day, full time or indiscriminately usually do not depend significantly on zooplankton as a staple in their diet. Instead, they are more likely to feed by absorption if the polyps are very small and minimal in design, or upon other nanoplankton (bacteria, floc, mucus, etc) if the polyps have complex polyps structures like feathery pinnules (ancillary side "branches" to the polyp tentacles that help to filter feed and trap finer particles).
Corals feed on a wide variety of prey items: organismally upon zoo- (animal) and phyto- (plant) plankton, on nanoplankton (microscopic matter) by filter-feeding, and directly by absorption. Most corals feed by some combination of more than one strategy.
Plankton is a staple food for corals and includes many things from which they selectively feed: colloidal matter, mucus, bacteria, fish and invertebrate eggs, excrement, worms, larvae, medusae, epiphytic matter, tiny crustaceans, unicellular algae and so much more.
In a very generalized observation, some summaries of the feeding strategies of main coral groups are proffered here:
Large polyped stony corals (so-called LPS species) are some of the hungriest and most carnivorous cnidarians. They include many coral that can digest minced meaty foods (although this is still too large for most) and even eat the popular, prepared invertebrate food suspensions (caution here: bottled food supplements are easily abused). Categorically, most LPS feed on zooplankton, larvae/eggs, feces and are surely nourished by absorption. It is not clear how much if any (gross terms, again) phytoplankton is favored by LPS corals: notably the Euphylliids, Fungiids, Faviids, Mussids and Dendrophylliids.
LPS Recipes: very finely minced meats of marine origin will be accepted by many LPS corals. Note: Although some species will sting and draw whole prey or large pieces of meat (chunks of shrimp, whole small fishes like silversides, etc.) it is not necessarily useful or even safe to do. We must remember that corals are sightless stinging animals that may not be able to initially assess the suitability of prey size. Keep in mind that unnaturally large chunks of food like those occasionally offered by aquarists would never make their way through the water column on a wild reef with so many sighted creatures in wait. Often times, prey that is too large appears to be grabbed and ingested but is later (at night) regurgitated in a mucous ball of waste. An aquarist with the habit of feeding prey that is too large may be led to believe that they are adequately feeding their charge but must ultimately watch the animal starve to death! Again, with all corals, the particle size of prey is a critical matter. For guidance, offer no zooplankton substitutes larger than 6mm. Smaller is most always better as a plankton substitute. Nutritious staples include thawed fresh-frozen gammurus, mysids, pacifica plankton, minced shell-on shrimp (much protein to be had in the chitonous shell), and minced krill. Live natural plankton from a refugium is an excellent supplement as well. Course media in fishless refugia for the cultivation of amphipods and like fauna is highly recommended. Refugiums plumbed upstream will overflow nutritious creatures nightly into reef aquarium displays.
Small polyped stony corals (so-called SPS species) are also largely carnivorous although you wouldn't guess by their name. They present a unique problem for aquarists attempting to feed them. Particle size is quite critical to most SPS and even with sharp discrimination between species. Most require zooplankton prey sizes smaller than prepared foods can conveniently offer and, short of collecting or culturing live plankton, it can be a challenge to feed the small polyped carnivores. Even most live cultured prey is too big for many species. Rotifers are relatively effective but fresh baby brine shrimp is largely unacceptable (fine for many Acropora and some Pocillopora but almost useless for Poritids and most Montipora). SPS corals tend to be more "successfully" photosynthetic and less dependant on supplemental feeding than LPS corals. No doubt, this has contributed to the great success of aquarists keeping and propagating this group of corals in modern, well-lit, heavily skimmed aquarium systems.
SPS Recipes: Perhaps the most successful strategy for feeding planktivorous SPS corals is the employment of mature refugiums. Fishless rubble zones and seagrass refugia provide copious amounts of zooplankton and epiphytic material. Occasional feedings of live rotifers and/or baby brine shrimp would be ideal (weekly to begin with) and a slight source of nitrogen will be necessary as well (to nourish zooxanthellae). A reasonable fish load and regular fish feedings may provide adequate supplies of nitrogen and ammonia for corals and their symbiotic algae through feces and other dissolved organics. Pale pigmentation and "bleaching" in coral tissue (when not caused by elevated temperatures or salinity shock) may indicate a lack of nitrogenous "fertilizer" for zooxanthellae. Phytoplankton has been observed to elicit a weak-stinging response in some SPS corals but few if any corals are believed to consume much plant matter. In fact, many species observed to grasp phytoplankton in their polyps have been witnessed to release said plant matter shortly afterwards. Rely on high quality illumination and zooplankton for most SPS diets. Feeding by absorption is inevitable with a reasonable bio-load.
Soft Corals: The Octocorallians are the most difficult to make a gross generalization for regarding feeding protocol. Families within this group run the gamut from being almost fully autotrophic (some Pulse corals and Star Polyps are suspected) to aposymbiotic and fully dependant on supplemental feedings (some Neptheids). More than any other group of corals, soft corals will require that an aquarist thoroughly research a given species or at least its family for some indication of necessary feedings. Alcyoniids including the popular Leather corals are believed to derive most of their nutrition from photosynthetic activity and dissolved organic matter (absorption). We know from practical applications that many Leather corals like Sarcophyton sp. will grow very well (and to great sizes quickly!) under intense lighting and without target feeding. Many Neptheids however, are believed or known to feed heavily on phytoplankton and like substitutes. The non-photosynthetic species, so-called "Colored Cauliflower" corals, are perhaps the most mysterious and demanding. They depend entirely on their environment for nutrition and sustenance. For these animals the purchase or construction of a good live phytoplankton reactor will be priceless. Regularly stirring the sand substrate (begin weekly) in Neptheid displays has been demonstrated to be a very useful feeding strategy, liberating bacteria, organic slime/mucus, mulm, plankton and much more for hungry filter feeders. Seagrass refugiums may also be measurably instrumental in providing nutrition for Octocorals.
Soft Coral Recipes: A case-by-case basis… mostly light (symbiosis) for Alcyoniids, mostly phytoplankton for Neptheids, some sand stirring for many species in general, mature fishless refugiums and dissolved organics for all. Very little target feeding with large particulate food (bottled supplements) is required or even possible for branching or toadstool leathers.
Gorgonians: In many ways, the current feeding methodologies for Gorgonians mirrors that of Neptheids. Most need or seem to favor nanoplankton and phytoplankton. Very few can be target fed with large particulate foods (bottled supplements). Seagrass refugiums are likely quite beneficial for providing natural plankton and epiphytic material. Casual aquarists with mixed garden reefs should avoid all aposymbiotic species (best for species-specific displays), but symbiotic species (including encrusting forms) are some of the hardiest and most rewarding Gorgonians to keep.
Zoanthids and Corallimorphs: By nature, Zoanthid polyps are quite hardy and durable. As a rule, the smaller "buttoned" species (like Zoanthus) require little or no feedings. They do not seem able or inclined to feed upon large particulate matter. They are mostly photosynthetic enjoying moderate to bright illumination and clearly benefit from the dissolved organic matter of a healthy bio-load. Larger polyped species (Palythoa and Protopalythoa) tend to require somewhat lower light and larger feedings including deliberately targeted particles. Some species like the deepwater (and toxic!) Protopalythoa grandis demand frequent and large feedings to stay in optimum health. For species that can feed organismally, finely minced meats of marine origin are appropriate. Corallimorphs, the mushroom false-corals, are rather variable like Zoanthids but feed similarly. They tend to favor low to moderate light and higher levels of dissolved organics or target feedings. Pimpled or Hairy varieties are more likely to need and accept particulate feedings in contrast to species with a smooth crown (capitulum).
* Anthony Calfo a lifelong aquarist, pond-keeper, and industry professional residing in Pennsylvania, USA. He is the author of the Book of Coral Propagation, Volume 1 (2001) and Volume 2 (2003). He has also authored a series of three texts with Bob Fenner and WetWebMedia on the Natural Marine Aquarium (the first volume on Reef Invertebrates to be released March 2003)
Questions for the author may be sent via e-mail to firstname.lastname@example.org or navigated through www.readingtrees.com or www.wetwebmedia.com