Corals are colonial animals, with each polyp sort of acting like its own animal.
In fact, no matter how large that colony is, in the wild it all starts from a single polyp.
In reality though, like most things, it’s not that simple - corals can move nutrients
and things between polyps via a complex system of cavities, currents, and mucus which moves
across their surface and inside their tissues. Just under the surface of a coral is a complex
network of cavities and canals lined by cells with little hairs which stick out - ciliated cells.
Those cells wave their tiny hair around, called a cilia,
and that moves fluid through these canals between polyps and across the surface of the coral.
Hi guys and girls, I’m ReefMan, and today we’ll be talking about the
invisible conveyor belts of mucus that cover our corals inside and out.
There’s an incredible paper all about this titled “Surface flow for colonial
integration in reef-building corals” and it’s published in Current Biology in June of 2022.
It’s open access, and the link is down below if you want to check it out.
I’d recommend it - it’s a great read and full of incredible imagery.
Alright, mucus. We know it, we may not love it, but mucus is used in feeding,
it’s used to grow microbes that benefit the coral, it’s used in defense, and even
just to flush the surface of the colony clean from dirt and sand. Corals are able to create
currents moving in complex networks across their surfaces, allowing them to move food
trapped in mucus over to their mouths and to sweep debris towards the edges of the colony.
To learn how this works, the team from the paper squirted a fluid containing
tiny fluorescent beads at different corals, recording the movement that the beads took.
On Agaricia lamarcki you can see that the beads move in streams along the surface of the coral
flowing up and away from each polyp. See how the beads are moved along the valleys in the
surface of the coral from the polyps and then along the surface of the coral? This coral is
flushing the contamination from its surface. In fact, the surface geometry of this sort of coral
creates a low pressure zone directly above the polyps. This low pressure zone creates a tiny
column of water that moves directly into the polyp’s mouth before being transported
back up along the ridges surrounding the polyp out from the surrounding coralite.
Some corals create a lot of mucus as soon as they’re disturbed. I’m
sure those of you with Acropora are familiar with this - it’s secreted from each polyp,
and in some corals it’s produced in volume and can even drip off the colony.
It does serve a purpose though, and it’s not just to be gross.
Mucus helps trap food during feeding, and even assists in the surface flow of water around
the colony. You can distinctly see this when the team applies the fluorescent beads in this video.
See on the left in the low mucus area how the polyp boundaries are clearly outlined?
This coral produces so much mucus that it’s not even possible to see the surface flow
until later when you can see the polyps bringing in their mucus,
containing all those fluorescent particles, back into their mouths.
Corals do not produce mucus consistently across their entire surface, and different
corals keep mucus around for different times. Some corals clear their mucus within minutes,
while others like Montipora foliosa maintain high mucus levels for more
than thirty minutes. This gives the coral a much greater chance at capturing food.
Why would this be? What purpose could some corals have for making so much mucus?
Some corals, like we saw, use it to trap food before pulling it all into their mouths.
Others use water currents and mucus along their surfaces to form vortices above their polyps.
You can sort of see that here where there is a concentration of beads over each polyp
on this coral and a network of moving currents between the polyps. Contrast that to the dead
coral on the other side, where the beads don’t even spread out on the surface of the coral.
Acropora do both of these, like you can see in their Acropora muricata
video here from the Great Barrier Reef. To avoid microplastic contamination,
they used charcoal instead of fluorescent beads - but you can clearly see the charcoal stuck in
the Acropora’s mucus and being transported over time into the mouths of the polyps.
Think about this the next time you’re feeding your Acropora - don’t blast them so much that
you’d remove their surface mucus - they’ll get a lot more food if you let the mucus do its thing.
In the second part of the video, you can see the particles tracked. See how each
polyp is collecting particles from around it, but not from further out?
Each polyp sort of competes for its own food, trying to grab particles from as far
away as possible - using mucus and water currents to move it all toward its mouth.
After finding this behavior in an Acropora on the Great Barrier Reef, the team reproduced this
test in several corals kept in reef tanks. It’s really interesting to see these videos, with the
particles tracked so that we can see the complex currents generated on the surfaces of the corals.
You can see how the particles travel along the surface of the coral, and in some concentrate
near the polyps themselves. You can see that there are clearly defined surface currents being
generated - several particles track along the same curved paths throughout the videos taken.
Imagine that’s your coral food. This might not be exactly why so many of our pumps have
feeding modes - I think those are more to prevent your foods from going through your overflow
and into filter socks - but those feeding modes also slow down the water in the tank
letting these surface currents move food around as the coral wants. The end result of all this
is that more polyps get more food when you feed your corals with reduced water flow in the tank.
Multiple polyps actually work together to form these currents
and no single polyp can do it all on their own. The flow patterns created in the water
are actually specific to each species - something that I think is pretty cool.
Maybe we could use this to help identify species in the hobby one day.
It would be easy enough to squirt some tiny food particles on a coral and then compare
the patterns produced on its surface to some library of patterns online all using our phone
and machine learning. Perhaps this could even feed into some sort of AI system
that identifies the patterns in the flow along with visual cues from the coral itself.
These surface currents are produced by tiny hairy - called ciliated - cells
on the surface of the coral. We know this because the currents that are produced on
a specific coral are stable over time, over hours or even weeks.
If this was not something produced by the coral itself,
maybe an interaction of just water flow over the geometry of the coral,
then we’d expect that to be different depending on the particular ocean currents at the time of the
test. In reality, the flow is stable in direction and speed over a range of water conditions.
A single coral also produces multiple currents that don’t always
intersect. These currents link some polyps in the colony together into a
larger region of polyps that work together to distribute food particles amongst themselves.
The speed of the water flow seems to be controlled by the nervous system of the coral. The team was
able to affect the speed of the particle transport in the currents by adding neurotransmitters like
adrenaline and serotonin to the water. Serotonin produced the greatest max speed, but it didn’t
change the overall pattern of flow on the coral. This means that the direction and patterns of flow
on a coral are likely set during its growth, those ciliated cells all point in whatever direction
they’re going to point in, but the overall health of the coral and it’s nervous system can
change the speed at which those cells beat their little hairs which are actually moving the water.
When partially exposed to air in the lab, corals used their surface currents to move water to the
exposed parts of the coral - a behavior which may allow corals growing high on a reef to survive
being exposed during the lowest tides. This is actually pretty common on some reefs, particular
in the Indo-Pacific region - and in our aquariums sometimes when we’re doing water changes.
In the video here, the coral is underwater on the left, and in the air on the right bottom.
You can see how the beads are slowly transported from the water towards the exposed section on
the right of the screen in the same way they’re transported when the entire colony is underwater.
This is something that warrants further study - can a partially submerged colony keep its entire
exposed surface wet this way? It might be able to! It would be interesting to
explore the limits of this though. How far up can corals keep wet like this?
Of course, it’s not just on the surface of a coral that water is moving around on. We
don’t see it in our reef tanks, but coral colonies also have a complex system of tunnels
just under their skin serving as a gastrovascular system. You can see this here in this CT scan of
Stylophora pistillata. The gastrovascular system is shown in blue, while each individual polyp’s
gastric cavity is in yellow. The gastrovascular system in our corals is quite extensive!
This system’s walls are very rich in the symbiotic algaes that coral gets so much food from.
The movement of fluid around this cavity not only moves food around,
but also oxygen and other dissolved chemicals needed by the algaes living inside the coral.
It might also provide a mechanism for the coral
to use in transporting these algal cells around after part of the colony bleaches.
This system transports fluids all over the coral very effectively. You can clearly see
the transport of the beads after having been injected into the polyp’s gastric cavities.
This is how nutrients and fluids are moved around the entire colony of a coral.
The flow is bidirectional as well, so no single polyp gets left out of this transport
network simply by being at the start of the system - or the oldest polyp in the colony.
That’s not to say that the polyps are all entirely altruistic - particles have a good
chance of staying in the system near the polyp that actually ingested them, giving that polyp
a greater chance at using the nutrients contained in the particle. If it’s not used, or maybe not
needed, then it can eventually be transported elsewhere in the coral for use by other polyps.
Chemical signals can also be passed around this network. Think about sending a signal that there’s
a predator - all the polyps in an Acropora close up seemingly at once when you disturb one area.
That signal may be sent chemically via this network of interior channels.
It doesn’t seem to matter if a coral is growing wild on a coral reef
or in captivity in our reef tanks. Both exhibited these distinct flow patterns,
and they’re distinct per species of coral. As the coral grows, the ciliated cells that
produce these currents grow in patterns that are stable for that species. We see this in
some of our corals as ridges and valleys on the surface of the coral, but on others like
smooth skin Acroporas, the surface currents are still there if not invisible to our eyes.
Each polyp on our coral has a little dedicated territory on the surface of the coral surrounding
it from which it alone feeds. We saw that in the videos showing mucus being pulled
in - there’s no competition between coral polyps except at the outer edges of their territories.
I think that the currents produced on the surfaces of our corals cannot possibly be all that strong,
and so strong flow from powerheads or even strong currents in the wild
might very well outcompete it. Because of that, I’d recommend taking advantage of
feeding modes that many of our DC return and flow pumps have. This would give our coral
the chance to funnel food more effectively, even if we don’t see it with our own eyes.
Mucus certainly plays other roles for our coral, but collecting food is one of the jobs it proves
quite good at. The next time you’re noticing the slime on one of your corals keep that in mind -
it does serve a purpose in otherwise healthy corals. In some species, the mucus even forms
something akin to a conveyer belt bringing food to the waiting mouths of the coral.
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Leave a comment, let me know what you thought - and what sort of stuff you might like in the
future so I can watch for papers on it. Stay safe out there with all these viruses going around,
be kind to each other, and have a fantastic day!
Bye!