This month's article varies from the subject
of corals, strictly, and is more concerned with coral reefs,
in general. I have fielded many questions in "The
Coral Forum" about feeding corals, and have provided
talks to several groups on coral nutrition. I plan to discuss
this in more depth in upcoming articles, but felt an overview
of nutrition to a coral reef community and coral reef aquarium
would be a beneficial prologue to develop a more complete
view of the subject.
A coral reef supports a tremendous variety
of life, all of which are dependent on energy sources for
their survival, growth and reproduction. There are two basic
types of organisms in terms of their method of gaining energy:
heterotrophs and autotrophs. Autotrophs are the primary producers;
they use sunlight, converting its energy through photosynthesis
into energy rich products (reduced forms of carbon, usually
in the form of simple sugars) that are used by the organism.
In this way, they form the beginning of the food chain, as
they are the original or primary source of dietary energy
for all other organisms. Photosynthetic bacteria or cyanobacteria,
may also be considered to be primary producers, and their
biomass on and near coral reefs, including in the water column,
is enormous. Heterotrophs are those organisms that must attain
at least some nutrition from feeding or absorption to acquire
a reduced source of carbon. Even primary producers need more
than sunlight to survive, and this is part of a great misconception;
that being, that autotrophs can "do it all." Consider
the houseplant that dies without nutrients from soil or fertilizer;
it obviously needs additional nutrients besides light and
water. Consider, as well, that fertilizers and soils are commonly
described by their nitrogen and phosphorus content; these
are also among the most important nutrients required by heterotrophic
organisms. The main difference between autotrophs and heterotrophs
is not that one "eats" and the other "just
needs sun," but that one can provide various amounts
of required carbon by using light energy.
The word "nutrient" is often
misunderstood. The terms "high nutrient" and "low
nutrient" can be taken in many contexts. In general,
nutrients are those organic and inorganic compounds necessary
to sustain life. While this comprises a very large group of
potential compounds, nutrients are often simplified in terms
of those elements that are major "building blocks"
for fats, amino acids, and carbohydrates. Furthermore, they
are frequently those elements which tend to limit further
growth by their availability and ability to be procured. In
general, carbon, nitrogen and phosphorus are often used to
describe the "nutrient" condition of reef organisms
(and others, as well). Plants and animals with photosynthetic
symbionts tend to be nitrogen and/or phosphorous limited under
normal conditions, since photosynthesis usually provides non-limiting
carbon source material. Coral reef waters are typically "nutrient
poor" as they contain very low levels of nitrogen and
phosphorus (they are both precious commodities and any excess
is usually taken up quickly). In nearshore areas where there
is significant organic loading from land runoff, waters tend
to be rather nutrient rich. Both types of environments sustain
their own flora and fauna with varying amounts of habitat
overlap in terms of the organisms that can exploit the continuum
of nutrient conditions. The nutrients available in water to
coral reefs can be dissolved in the water, in the form of
particulate material, or as living biomass.
The coral reef is a place of both high
primary productivity and consumption of nutrients, with a
great deal of nutrients being recycling within the community.
For many years, coral reefs were thought to be "nutrient
poor deserts." In fact, this is not the case. It would
be a very poor assumption to imagine that any species-rich
community was not highly dependent on nutrients. While measurement
of the water column shows it to be relatively devoid of organic
and inorganic dissolved nitrogen, carbon and phosphorous and,
therefore, "nutrient poor," it is largely because
of the efficiency of the reef community that such water conditions
are attained. Waters around coral reefs are rich in nutrients
in the form of various types of microplankton; these are largely
removed by coral reef organisms. It should be noted that most
of the plankton on coral reefs is produced by and lives within
the reef or nearby communities, and is not borne into it in
great quantities by the open ocean.
An adaptation that has allowed for such
diversity, to a large degree, is the symbiosis of animal and
plants (algae and cyanobacteria) to make efficient use of
each other's limitations. Such symbioses occur commonly in
sponges, corals, nudibranchs, anemones, clams, hydroids, foraminiferans,
and many other invertebrates that make up a large portion
of the total reef community. These organisms are not autotrophs,
no matter how efficient and substantive the contribution of
their symbionts, and they must be fed. So, what do they eat?
Coral reef inhabitants have widely varied
diets, and most aquarists are familiar with the often highly
specific dietary needs of some of these animals. Motile invertebrates
may be predatory, like fish. Others are scavengers of decomposing
material, or they can be "filter-feeders" by any
number of mechanisms. Some employ numerous methods of nutrient
uptake. Both "filter-feeding" by passive means and
active prey capture are used by many of the sessile invertebrates
commonly maintained in aquaria. At various early stages of
their life, the diet of reef organisms may require planktonic
organisms, and they, themselves, may be planktonic at some
part of their life. Some of these animals (and all algae)
are also capable of acquiring nutrition through the absorption
(or direct uptake) of dissolved organic and inorganic nutrients.
Normally, the levels of these substances on a coral reef are
very low, and such nutrients are often a limiting aspect of
the growth of any one life form. Because of the number of
species present on a coral reef, most any food source is often
a source of fierce competition, even if not directly. Often,
simple competition for space is enough to limit nutrient availability.
Nutrients enter a coral reef from a variety
of sources. They can arrive from freshwater or terrestrial
sources; rivers and rain can both wash land based nutrients
out to sea. Cooler water from deep in the ocean moves upward,
bringing nutrient rich water upwards to the reef. This water
is nutrient-rich because of the "downfall" of organic
material into ocean depths and a comparative lack of planktivory
in the deep ocean compared to that which exists in the upper
photic zones. Currents, tides, storms, and waves bring plankton
and nutrients from various distances to wash back and forth
over the reef. The production and waste material of the reef
organisms also provide important nutrition to other animals
on the reef, and they are part of what is know as the detrital
food chain. Detritus, marine snow, particulate organic material,
and suspended particulate matter are all names for the bits
of "dirt" that flow around the reef; material that
is composed of fecal material, borings, algae, plant material,
mucus, associated bacteria, cyanobacteria and other particles.
Decomposers (mainly bacteria and associated flora and fauna)
break down waste material in the water, on the reef, and,
primarily, in the soft sediments. The result of their presence
and action is not only a food source in and of itself, but
provides raw material for channeling back into the food chain,
largely through the benthic algae and phytoplankton.
Phytoplankton are small unicellular algae,
or protists, that drift in the water column. They may be very
abundant in and around coral reefs, and they are capable of
absorbing large amounts of organic and inorganic nutrients.
When conditions are proper, they can reproduce very quickly,
and areas of high nutrients will often have a greenish, reddish,
or brownish, cast and lower water clarity, mostly resulting
from high phytoplankton populations. Some of the reef animals
can feed directly on phytoplankton; many soft corals, some
sponges, almost all clams, feather-duster worms, and other
filter feeders utilize phytoplankton directly as a food source.
Small animals in the water column, termed zooplankton, also
utilize phytoplankton as a food source. For the smaller zooplankton,
phytoplankton and bacteria are the primary food source.
|Both of the photos above are from
reefs on the Great Barrier Reef, Australia. The left photo
shows the clear "nutrient poor" (oligotrophic)
waters of the outer reefs. The right photo is of an inshore
"nutrient rich" lagoon reef off Townsville.
Notice how coral coverage in both systems is high, and
even though the green phytoplankton-filled lagoonal reef
is nutrient rich, it supports a high density of Acropora.
Photos by Eric Borneman.
Zooplankton are of various sizes. Truly
pelagic zooplankton constitute the vast majority of zooplankton
in the ocean, but not on the reef, and are composed largely
of the calanoid copepods and the larvae of many marine organisms.
Zooplankton are larger than phytoplankton, and compose the
primary diet of many marine and reef organisms, from fish
to corals. Stony corals, for example, rely heavily on the
capture of zooplankton to meet their energy needs. Zooplankton
can be grouped into various categories, depending on size,
location, behavior, and other characteristics. Larger pelagic
marine organisms (such as fish, jellyfish and others), or
those that are not associated with the water column (benthic
animals such as echinoderms, crustaceans, mollusks, and others,
can also be prey, or food, to various organisms. Contrary
to what is commonly believed, there are many small benthic
crustaceans, like some amphipods, that are not considered
zooplankton as they do not migrate into the water column.
However, demersal zooplankton, or those with vertical migration
from the reef benthos into the water column (generally at
night), primarily copepods and mysids, comprises the majority
of zooplankton available to coral reefs.
|Prey capture of polychaetes by Diploastrea
sp. at night at Tomia Island, Indonesia. Photo by Eric
|These coral polyps are feeding on
small crustaceans, probably mysids, which were swarming
in the water column. Several captured crustaceans are
visible. Photo taken in Cozumel. Copyright 1981, Ronald
Coral reef food sources, then, are largely
produced by the ocean. Bacteria, detritus, phytoplankton,
zooplankton, small benthic fauna, mucus, and dissolved organic
and inorganic material of various types and sizes are what
comprise the majority of food on a coral reef.
Are We, As Aquarists, Providing It?
In a word, No.
What we provide to, and what is provided
by, our aquariums are extremely limited in both quality and
quantity. Yet, many of us are troubled by high nitrate and
phosphate readings. As a result, many aquarists resort to
minimal feedings, in an attempt to keep water quality manageable.
In terms of aquaria, which are closed systems, we do not have
the luxury of billions upon countless billions of gallons
of water to dilute and wash away high nutrient loads, nor
do we have the bountiful biodiversity (for the most part)
that maintains the "nutrient poor" water quality
of a coral reef. In return, when our water tests "high"
for nutrients, we are often plagued by those aesthetically
undesirable organisms that are most adept at utilizing such
resources as dissolved organic and inorganic material; the
algae and cyanobacteria.
Filamentous, slime, smear, and macroalgae
are highly efficient at absorbing such material, and they
grow rapidly. In most circumstances, the microalgae and macroalgae,
while very useful as part of turf scrubbers or small algal
communities within a reef, often become problematic as they
overtake the more aesthetically and, in some ways, functionally
desirable crustose red algae (coralline), corals, and other
sessile invertebrates. It should be noted, though, that these
organisms might also be capable of significant nutrient uptake.
Bacteria and phytoplankton are also extremely proficient at
removing this material. All these organisms are quite valuable
to our captive reef communities. They not only "purify"
water by the utilization of nutrients, but also are all part
of a beneficial food web, both in coral reefs and in aquariums.
Many elements are already drastically under-represented
in aquaria, not only because of the limited size and productivity
of the average reef aquaria, but because of the incredibly
high bioload relative to the water mass present in even the
most barren tank. I have used an analogy of how a reasonable
facsimile of true natural bioload could be thought of as a
one inch coral fragment in an Olympic-sized swimming pool
of seawater. Even that example, relative to the oceanic volume,
is probably "overstocked."
As a result of the often unnaturally elevated
nutrient levels in aquaria, we employ a number of nutrient
export devices, such as filters, ozonizers, and protein skimmers
(foam fractionators). We also tend to add these devices to
avoid or limit another common nutrient export mechanism, the
water change. Unfortunately, it is a serious and probably
deleterious compromise in many ways. Such devices actively
strip the water column of the very bacteria, detritus, mucus,
and plankton that exist, limiting the effectiveness of our
captive community to deal with the nutrients and, in return,
providing food sources within the food web. When the water
column is "stripped" of its productive elements,
the populations of filter feeding and predatory sessile invertebrates
are compromised, as is the productivity of the substrate communities
- including the live rock and live sand with their associated
microbial, floral, and faunal components. However, if we do
not "purify" the water, we may encounter nutrient
problems and react with limited feeding schedules. It is quite
literally a Catch-22.
In aquaria, we are faced with several realities.
Our phytoplankton and zooplankton populations are generally
negligible to non-existent in comparison with coral reef communities.
Those which do exist are either rapidly consumed without having
a chance to reproduce, or they are rapidly removed or killed
by pumps and filtering devices or suspension-feeders. Coral
mucus, bacteria, detritus, larval benthos and other "psuedo-plankton"
might be present in a reasonable amount if the water column
were not stripped. On the other hand, dissolved organic and
inorganic material levels are frequently much higher than
they are in the ocean. For an excellent, detailed analysis
of sampled aquarium water refer to It's
(In) the Water and It
Is Still in the Water by Ron Shimek, Ph.D. Even very well
maintained aquaria are generally found with much higher levels
of nitrogen and phosphorous than wild communities. Even though
many desirable organisms are able to utilize these nutrients,
levels in most aquaria are very unnatural, and coral reefs
under such conditions often wane or die - a process known
It is the lack of water column-based food
that results in limited success with the maintenance of some
desirable animals, such as crinoids, flame scallops, clams,
certain corals, sponges, bryozoans, and many other invertebrates.
Even the symbiotic (zooxanthellate) corals suffer, despite
many obvious long-term successes with these animals. However,
sexual reproduction in corals is not common. Some of this
may have to do with the lack of proper spawning cues (moonlight,
temperature, etc.), and some may have to do with the small
sizes of corals not being of sufficient area, age, or polyp
density to be reproductively viable. However, heterotrophic
nutrition, especially in the acquisition of nitrogen, is very
important in gonad development, whereas the nutrition provided
by the symbiotic algae (zooxanthellae) is largely used in
their metabolic needs and growth through the production of
large amounts of carbon. If we fed our corals more often,
and with proper food sources, without the stress of being
in a high nutrient environment, would we see more spawning
events? Quite possibly.
Can We Provide Enough Food In An Aquarium?
Yes and No. To a degree, some of the limitations
of a closed system are insurmountable. In a wonderful analogy
using some feeding rate data of reef communities from scientific
literature, Dr. Ron Shimek (Shimek, WMC 1998) noted how it
would take 250-350 ml of wet food per 100 gallon of water
per day to approximate food availability on a coral reef.
Using a similar analogy, based on nutrient and water dwell
times, I would add that the coral reef gets a 100% water change
2-3 times per day! This degree of nutrient availability and
water exchange is coupled with the fact that we have relatively
little data on the exact feeding requirements of various animals.
However, we do know some specifics, and many generalities.
For many filter feeders, it is not even so much the constituency
of the food, but a requirement based largely on size. In other
words, many filter-feeding and prey capturing animals will
capture whatever particle size is manageable by the mechanics
of water flow and capture mechanism only. Other animals are
far more specific, and may depend on complex chemorecognition.
Simple observation of the life in our tank gives us some clue
that we are not providing the right stuff, or enough of it,
and/or too much of the wrong stuff. Furthermore, we simply
do not have access to many of the species that exist on coral
reef. Yet, we do have access to many (often beautiful) species
that, perhaps, we shouldn't, as they are still too difficult
What can we do to make our situation better?
There are many solutions. One way is to purchase a plankton
net, and perform plankton drags in the ocean. However, this
is not an option for those without easy access to the sea
- and it is not very convenient, either. Still, I have found
occasion to grab a net full of plankton on trips to the beach,
and the animals one finds are simply fascinating. Another
way to provide food sources is to culture plankton. It is
certainly possible to begin producing batch cultures of plankton
and/or plankton substitutes. Culture materials are generally
simple, and various algae, rotifers, Artemia nauplii,
ciliates, mysids, Gammarus, etc. are readily available
and easy to grow. These food sources are not only nutritious
inputs for reef aquaria, but may be enriched with vitamins,
minerals, trace elements, medications, antioxidants, etc.,
and used as biocarriers of such substances. Cultured food
sources, I feel, are far more valuable in both time and expense
than many of the other products and devices we operate and
Our use of "live sand" has provided
another important contribution to food sources. These areas
are breeding grounds for many of the worms, crustaceans, microbes,
and algae that later directly feed grazers and predators,
or add food to the water column with their larvae and gametes.
Furthermore, the action of the sand and live rock communities
as decomposers and consumers of organic and inorganic material
is invaluable. Live rock is also an important source of detritus
and other reef food. We have also begun to make use of refugia,
small areas or separate tanks separated from, but connected
to, the main tank. Refugia provide areas where continual cultures
of small flora and fauna can be produced without the intrusion
of predators. I find refugia to be both fascinating sub-communities
and very important for the main community.
|Notice the high level of particulate
material in the water column; a perfect food source for
this Sarcophyton sp. soft coral. Photo by Eric
Finally, we can feed the tank more often
with conventional foods. This is the area where the most care
must be taken. One of the biggest problems with early aquariums
was overfeeding, as there were not significant or sophisticated
means of nutrient export, uptake, or recycling. Today, with
reef systems and natural fish systems, there is significant
decomposition occurring in the tank, without solely aerobic
breakdown. Furthermore, nutrient export mechanisms, like protein
skimmers, along with the "many mouths to feed" (in
terms of the abundance of life forms other than fish), make
overfeeding a less troublesome occurrence in today's aquaria.
It is not, however, absent; overfeeding and poor nutrient
management is still an area that could stand improvement.
Often, conventional food is too large to be utilized by most
reef organisms, except larger predators (brittle stars, fish,
anemones, etc.) and large-polyped corals. Since the food is
not alive, it starts to decompose immediately after being
added to the aquarium, and will eventually be reduced into
its constituent organic and inorganic components - substances
of which we already have enough. Some of this material does,
in fairness, contribute to larger populations of beneficial
microbes and deposit feeders. But, it is a food source that
is not self- limiting, and it is less desirable than live
In terms of previously mentioned export
mechanisms, it really does little good to be cultivating or
adding more food material in the water column if it is all
being rapidly removed by filtration devices. Live rock and
sand provides abundant filtration, and some of the articles
in past issues describing the set-up and use of unskimmed
tanks are, in my experience, something that should be seriously
considered. Algae Turf Scrubbers are also viable systems that
provide low ambient water nutrient levels while maintaining
higher amounts of food and particulate matter in the water.
I also feel that if protein skimmers are used, they should
probably be used in an intermittent fashion. I realize this
is contrary to the advice that many others may offer, and
it may sound like a reversal of thought and progression over
the past year's trends towards increasingly efficient protein
skimmers. However, I feel today's powerful skimmers are certainly
able to provide adequate nutrient removal to maintain aquariums
with very low nutrient levels without running "around
the clock." However, I also think that as our understanding
of the biology of captive systems and natural communities
has increased, and our experiences have accumulated, that
some important contributions may no longer be quite as important
as they once were.
In fact, I feel the most beneficial nutrient
export mechanism is the "old-fashioned" water change.
Not only does this simple procedure remove excess nutrients
and toxins, but also provides a more balanced replacement
of water constituents to a baseline level. Yet, most of the
"food items," such as plankton and particulate matter,
are conserved in the remaining water, continuing to exist
as both immediate food and as reproducing plants and animals.
Filtering devices are not so gentle, as they process all of
the tank's water over and over again. Most aquarists dread
water changes, but they are simple, effective, and inexpensive.
After having come full circle, I have found small water changes
to be less work than what is involved with performing the
additions, purchases, and maintenance of so many products
and equipment available in the market. Furthermore, I have
found mature, well managed, diverse reef communities to be
A Word of Advice and Experience
It has been my experience that the following
pattern emerges among aquarists that begin "upping the
volume" of food to their aquarium: Increased addition
of prepared foods begins, followed with a concomitant and
fairly rapid increase in measurable nutrient levels in the
tank water. Soon thereafter, the aquarium begins to experience
blooms and growth of cyanobacteria and filamentous algae.
At this point, the aquarist typically ceases feeding at the
increased rate, worried that the nutrient level will remain
elevated and cause the demise of the health of the tank inhabitants
at the expense of the algae. I stress that this is in all
likelihood not the case. When first setting up an aquarium,
levels of uptake and decomposition are low. As live rock "cycles,"
and dead plants and animals decompose, a nutrient spike is
seen in all cases. Following this, various algal successions
occur, usually in the order of diatoms, cyanobacteria, filamentous
algae, and finally crustose coralline algae. Nutrient levels
drop over time and the reef becomes a stable low nutrient
place. The same process is occurring with increasing food
sources to an aquarium. The nutrient levels spike, and various
algal successions occur, until a new steady state is reached
with a larger number and diversity of life than at the previous
level. This process can take time, and food can be slowly
increased over longer periods of time, allowing for such development
to occur and bring measurable nutrient levels down to previous
water column levels. It is my experience that perfectly "obscene"
levels of food can be added to well stocked and diverse reef
aquariums over time without high nutrient levels in the water
column. To be sure, algae growth will also increase even over
the long term with the added nutrient inputs, even though
measurable levels are low. This is easily countered with the
addition of more herbivores. Grazing has been shown to be
the primary means of both filamentous and fleshy algae control
on reefs. Ambient nutrient levels are far less important in
algal-dominated reefs than the lack of herbivory. Even if
a reef aquarium is highly mismanaged and has aberrantly high
nutrient levels that result in prolific and undesirable algae
growth, it can be controlled with additional grazing. However,
I stress that such conditions may also act to the detriment
of other organisms and is not encouraged. I make the point
simply to illustrate the importance of adequate grazing.
Do We Need to Provide All This Food?
I think we do. There are many ways to do
be a successful reefkeeper. I think such a diversity of thought
and method should be encouraged. I also think the understanding
and provision of proper food sources is an important and relatively
recent school of thought in keeping aquaria; one that is just
beginning to be realized by many. It is a key aspect of natural
communities, and it has provided me with visible and tangible
evidence of its importance in aquariums. I have crystal clear
water and no problem algae with healthy fish and thriving
corals. "So what," the reader may say, "Certainly
the same can be said for those keeping stony coral galleries
with powerful foam fractionators." Yes, it could. Indeed,
I was once one of those people and I considered myself to
have a very successful aquarium. But now, I have "reef
snow" in my tanks, I have copious natural sponge growth,
and I have communities of animals that never existed (or did
not thrive) in the absence of these food sources. I also feel
it is important to utilize food sources that provide maximal
nutrition with minimal volume or unused components. In other
words, high protein sources (e.g. "Golden Pearls")
live or cultured live sources (e.g. Artemia, Mysis,
rotifers), unicellular algal cultures (or live phytoplankton
products such as DT's phytoplankton), and fresh whole food
products (e.g. blenderized seafoods and algae), along with
the intentional growth of a biodiverse community acting together
as predators, prey, producers, and decomposers, is vital to
success in keep coral reef communities in aquariums.
It is my personal belief that reef aquaria
should be a thriving community of biodiversity, representative
of their wild counterparts, and not merely a collection of
pretty specimens growing on tidy clean rock shelves covered
in purple coralline algae. By intentionally depriving many
of these animals of natural food sources, I think we become
lax in our responsibility, even if we did not spend money
to acquire them. Dinnertime is a happy time for all, and nutrition
is a universal requirement for survival. We may never be able
to duplicate the coral reef, but we can get closer and closer
as we learn more about closed systems and the natural communities.