Water flow is more important for corals than light. Part 1. Introduction to Gas Exchange

Water flow is important for many aspects of coral biology.
Water flow determines how efficiently corals capture food, it
helps corals rid themselves of metabolic waste and it also
determines where corals occur by distributing their spawn and
gametes. Most importantly, water flow is critical in driving the
mechanisms of respiration and photosynthesis. When aquarists
describe their reef aquarium setups they often brag about
lighting first. At some point through an equipment run-down an
aquarist may or may not boast about water flow equipment. This
illustrates a trend in which considerations for water flow are
almost always secondary to other decisions made when assembling a
reef set up when in fact, water flow is paramount to the health
and vitality of a reef system. The purpose of this first part in
the article series is to introduce the mechanisms by which
water flow affects coral health. The next article in the series
will review some of the scientific research describing how water
flow affects coral health, especially how flow speed and lighting
intensity go hand in hand. The last article in the series will
discuss the various approaches for providing water flow with an
emphasis on moving beyond “X” times water volume turn
over and on to “mass water movement” techniques and
the equipment necessary to make this happen.


The following are a few questions to get you thinking. Which
is more important, light or water flow? How many coral species do
you know of that can live without light? How many corals do you
know of that can live without flow? Well, there are hundreds of
soft and stony coral species that live without
light.Tubastrea, Dendrophylia andDendronepthea come
to mind. However, there are no corals that can live without water
flow. Even in the case of a photosynthetic coral, how long can
it live without light? All of us have had power outages,
burned out bulbs and ballast failures, and most aquarists would
agree that a coral can endure a week without light before its
health becomes severely impacted. Even in illuminated
aquaria there are reports of people forgetting frags in the sump
for months at a time where they seemingly survive on the food
that they can trap. But how long can a photosynthetic coral live
without flow? The most catastrophic tank crashes are almost
always due to a lack of circulation. In the case a photosynthetic
coral, it not only has to breathe for itself but it must
also support the respiration of the zooxanthellae living within
it. Water flow, therefore, is more important to coral health than
light, since corals will stress or die much more quickly when
flow is inadequate.


Respiration and Photosynthesis

For physiological purposes, the accepted measures for how well
a coral (or any other organism) “performs” are
usually related to either how much energy the coral consumes or
how much energy it produces. Energy is consumed during
respiration and energy is produced during photosynthesis.
Respiration (R) is the process of combining oxygen
(O2) and sugar to produce energy with carbon dioxide
(CO2) as a byproduct, whereas photosynthesis
(P) is basically the same reaction in reverse: energy and
carbon dioxide are combined to form sugars with oxygen as a
byproduct. The rates of these processes are driven by the
delivery of the input (CO2 for P or O2 for
R) and the removal of the output (CO2 for R or
O2 for P). Respiration and Photosynthesis can be
measured by an increase or decrease of CO2 or
O2. Although most people are aware that photosynthesis
occurs only in the presence of light, it is important to note
that respiration is constantly occurring in all organisms.

Gas Exchange

For the sake of this article I will refer to delivery and
removal of CO2 and O2 as “gas
exchange,” which is the sum of the rate of diffusion into
and out of an organism. The total amount of gas exchange is
dependent on the rates of diffusion and the rates of diffusion
are dependent on the availability of moisture, surface area and
concentration gradients. When we are dealing with corals there is
no need to pay attention to the availability of moisture because
they grow in water, and therefore moisture is not a
limiting factor for diffusion. Surface area is controlled mostly
by the corals themselves; in the short term corals can change
their polyp extension and colony expansion, and in the
long term corals can modify their morphology. The only factor
left for aquarists to control is the concentration
gradient, which is affected by the degree and type of
water motion.

Water Flow > Concentration Gradient > Diffusion (Gas
Exchange) > rate of R and P

We take for granted that we have very specialized breathing
structures: our lungs are internalized and actively create
favorable concentration gradients by forcing air into narrow
passage ways so that we can breathe properly. Fish have external
gills, crustaceans have internal gills, insects have trachae and
even nudibranchs have external branchae. All of these structures
feature a very high surface area and a good deal of
vascularization. Corals, on the other hand, have no specialized
respiratory structures. Their external anatomy only features
tentacles, a mouth, some tissue in between the polyps and,
in the case of soft corals, they also have some pinnules
along the sides of the tentacles. None of these are
differentiated into specialized respiratory structures even
though they have to rely on these anatomical features for gas
exchange. If you had to breathe (respire) as a coral
breathes,this would be the equivalent of holding your
lungs outside of your body, inside out, and just hoping that the
wind would blow hard and long enough for you to be able to
breathe. This is how corals breathe in their environment and the
scenario above illustrates the importance of water flow for
adequate gas exchange in corals. This perspective might make you
think twice about reducing your aquarium’s flow at night.

Surface Area

As mentioned earlier, one of the only things corals can do to
alter rates of gas exchange is to change their surface area
either through polyp extension or morphology. Having a higher
surface area increases the amount of gas exchange which can take
place. The image below (figure 1) features an example of a sample
of P. damicornis in which two images are juxtaposed. On
the left side, the coral displays polyps which are well extended
and therefore have a higher surface area. On the right side, the
coral has its polyps mostly retracted and it therefore has a much
smaller surface area.



Figure 1. This image demonsrates how the surface area of a
coral can change with the degree of its polyp

The other thing that corals can modify to change surface area
is their morphology. The image below (figure 2) shows two
specimens of P. damicornis which are from the same mother
colony. The specimen on the left was grown under lower water flow
conditions and moderate intensity lightin (75 gallon tank, 4
Maxijet 1200 and 6 X 4ft vho bulbs). The specimen on the right
was grown under a very high energy environment (180 gallon tank
multiple, 3 X 400w iwasaki bulbs, 1200gph eductor closed loop and
opposing this flow is a 45 gallon carlson surge device with a
period of 3 minutes). It should be noted that even though the
coral on the right was grown in higher water flow, it took about
twice as long to grow to the same occupied volume but it had more
than twice the mass of the lower flow specimen.


Figure 2. These two specimens of Pocillopora damicornis
are from the same colony but they were grown in different tanks.

Concentration Gradients

Rates of diffusion are determined by concentration gradients
which can be manipulated by water motion. A large difference in
concentration yields a higher concentration gradient. The effect
of concentration gradients on the rate of diffusion is analogous
to the effect that the steepness of a slope will have on an
object which is moving downhill: the greater the slope the
faster the object will move. The following schematic is an
attempt to illustrate how diffusion rates are affected by
concentration gradients.


Figure 3. In this illustration there is a high
concentration of a substance within the coral.


Imagine that there is a high concentration of a solute within
the coral. The solute can be gas, nutrients, or minerals. On the
left side there is a high concentration of solute in the water
surrounding the coral. If the coral is trying to rid itself of a
solute, this side has a low concentration gradient because there
is not much of a difference in the concentration between the
coral and the outside environment. In this scenario it will be
more difficult for the coral to rid itself of a particular
solute. On the right side, there is a low concentration of solute
in the water column surrounding the coral. Since there is a high
concentration within the coral, this scenario is an example of a
high concentration gradient: there is a large difference
between concentrations within and outside the coral.

The example on the left of figure 3 is characteristic of a low
to moderate flow environment where the available water motion is
not sufficient to dilute the solute that the coral is trying to
release. Whether a coral is absorbing or releasing solutes,
greater water motion will always produce a concentration outside
the coral which is favorable for creating a high concentration
gradient. A high concentration gradient will lead to higher
diffusion rates which in turn will support higher rates of
respiration and photosynthesis.


  1. Corals are dependent on diffusion for gas and nutrient
    exchange across their tissue layers.
  2. The rate of diffusion is determined by concentration
  3. High gradients yield high diffusion rates and
  4. The concentration gradient of solutes can be manipulated by
    water motion

Hopefully this introduction to the mechanisms of how corals
interact with water flow has been useful to you. I know that some
of the details in this introduction may seem a little heavy but
in the next part you will be able to read along and press the
“I believe” button in regards to scientific findings
related to corals and water flow. For those of you who are vying
for more, I have saved the fluid dynamics for the third part of
this article series when I discuss how we can all deliver better
and more relevant flow to our reef aquaria.

  Advanced Aquarist
Jake Adams

 Jake Adams

  (26 articles)

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