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Setting

In a tropical shore, where a great river empties into the sea, there is a zone of very high tides. The water level rises and falls by meters every day. In this challenging ecosystem, alternately baked under the tropical sun and drowned in salt-water, a species of corals has developed that is able to survive. When submerged, the coral polyps filter feed on nutrients washed down by the river. In the sun, symbiotic algae photosynthesize and produce energy.

These corals have built up great structures like termite mounds in the tidal flats. These mounts go no higher than the tides come in, for the corals must be submerged part of the day to survive. But there are great forces at work in this flood plain, forces that would destroy these coral monuments. The combination of rising tide and monsoon-swollen floods from the river create ferocious tidal bores. Cyclones built up over thousands of miles of tropical seas to the north slam into the seashore with sustained 200 kph winds multiple times a year, pushing enormous storm surges with them.

Question

Can corals build up structures fast enough to survive the damage caused by this punishing environment? Or will the forces of wind and flood and tide pulverize the coral's would-be protective skeletons?

Considerations

  • Assume the coral and its symbiotic zooxanthellae are evolved to survive the sun/submerge cycle.
  • Assume the coral is subjected to the same limitations on growth speed as on earth.
  • The 'coral' does not have to be coral! If another species of skeleton forming creature (mussels? barnacles?) is better for this task, that is acceptable.
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    $\begingroup$ Just FYI what you describe ate called tidal flats or marshes not flood plains, flood plains occur inland and are formed by flooding rivers not tidal effects. $\endgroup$ – John Sep 7 '18 at 17:14
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There are tidal corals, and many do take quite a pounding, this is one way atolls form. However they are unlikely to form mounds corals are colonial and will tend to spread out and fill up lower areas (which will stay submerged longer) You will need a reason they form mounds instead of reefs.

You may want to look at Stromatolites they may be closer you what you want. They from all kinds of shapes depending on the local wave/flow conditions, and they are remarkably robust. Because they grow sheet by sheet (bacterial mats), you can get mounds and domes easier. It is the tidal forces themselves that keep them from forming a single giant mat. the spaces between them are subjected to more powerful forces keeping the mat from forming between them. Corals don't do this because high flow is actually better for a filter feeder.

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Some of the freshwater ones form shapes remarkably like termite mounds since they don't have to deal with tidal/wave forces.

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The hydrodynamic environment will discourage "termite mound" structures, but the corals may evolve other mechanisms to deal with the environment.

One of the simplest is simply to "sculpture" the sea floor to mitigate the tidal flows and currents. The corals could build a sort of "waffle pattern" across the seafloor to cause the currents to release their energy over the undulating seafloor surface. This will assist the coals in feeding as well, since silt and organic matter will be deposited as the currents are forcibly slowed down. (The natural reason for this to occur is ridges across the flow will cause the current to slow and dump organic materials, multiple ridges will build across the line of the current, but as the current itself will constantly change, the angles of the coral ridges will also change, resulting in a criss crossing pattern).

The ridges of the "waffle" will preferentially host the photosynthetic elements of the organism, since they will have greater exposure to the sun, while the lower parts of the coral will be domain of the filter feeders. This sort of coral will eventually need to develop some sort of mechanism to pass nutrients from the "top" to the "bottom", perhaps long tendrils or root like structures will evolve.

Other species of coral could also rise in this ecosystem, for example a form which builds its colonies in large cylindrical structures which might serve as "breakwaters" deeper in the ocean, being tall enough to reach for the sunlight, while at the same time being large and strong enough to resist the actions of the waves.

The environment will be constantly changing, since the corals will need to continuously build to rise above the deposits of silt, and the actions of large storms will break off sections of coral. Disease and predations will also affect the health fo the reefs, and so sector will die off, changing the local currents and eddies and forcing surviving corals to rebuild to reflect the new conditions, as well as opening up areas for new colonization to occur. Marine life will be quite varied, and any humans sailing in the region will find the approach to the river quite challenging indeed.

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  • $\begingroup$ Great answer! I imagined human hunter gatherers wandering through coral towers at low tide, retreating to the swampy river margins at high tide. This suggests the corals will form a natural maze...oh the drama! $\endgroup$ – kingledion Sep 7 '18 at 17:57
  • $\begingroup$ Stromatolites might serve as the large cylindrical structures. $\endgroup$ – Willk Sep 7 '18 at 20:00
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I doubt the termite mound shape is a good one to take the stress without fracturing. A semi-spherical shape might, but then you'll have the landscape evolve into sorta undulated "tide plain" over time, advancing the river mouth into the sea/ocean. That's also a way of lowering the environment pressure, a shallower sea water would be capable of storing/transporting less energy.

Some problems though:

  1. high speed tides will wash away the silt. A good thing having a rocky bottom, otherwise new polyp colonies wouldn't be able to anchor on something, but then...
  2. ... a new polyp colony (low height) will spend more time submerged, in less conditions of using photosynthesis. And still, it must thrive, fast, the smaller the more vulnerable. If it can do it submerged most of the time, what's the evolutionary pressure to develop photosynthesis capabilities and grow on height, when it's best abilities would push it to stay as close to the bottom, submerged?

Possible (still fictional) mechanism of establish/growing:

  • thick, gel-like, protein membrane - sticky when secreted (mussel glue like), allows trapping nutrients when submerged. May capture silt as well
  • the membrane dries out at surface when exposed to air, becomes a thin shell sealing the water in. When submerged again, the speed of rehydration is finite, the membrane resists intact for 1-2 hours (so that the speed of water near the bottom slows down) then develops cracks and allows the colony to secrete a new protein membrane. This explains why the growth happens more on the base than on top (the base spends longer time submerged), the result is a rounded mound like appearance of old colonies.
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When bad weather comes, your corals will get out of the way. By riding giant crabs.

https://www.sfbaymsi.org/single-post/2017/10/16/Marine-Science-in-the-News-Hitch-hiking-coral-and-hermit-crab-symbionts

D. heteropsammicola is the only known hermit crab to partner with a living coral. The hermit crab and the coral have a mutually beneficial relationship: the hermit crab prevents the coral from being buried in sea floor sediments by carrying it and providing transportation, and the coral provides shelter and protection for the hermit crab. In this specialized codependence, it is usually impossible to replace either partner with a different species.

I am not sure why they say it is the only one because I also found the "stag horn hermit crab". Maybe its pseudo shell is a hydrocoral, not a true coral. But this is the sort of thing I envision for your river delta: monster crabs with huge corals on their backs.

staghorn hermit crab

Now scale it up. These crabs are big, I tell you. Big. And on their backs - great reefs of coral. The crabs move them from place to place and are not averse to taking a clawful of goodness from their rooftop garden. If the crab finds dead fish, the coral benefits from the messy eating habits of its mount.

When sediments come, the crab moves into faster water to wash things away. If there is harsh weather, crab and coral head down to deep water to ride things out. On a nice sunny day you can see all the crabs gathering in the shallow water to let their symbionts sunbathe.

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    $\begingroup$ I'm ok with large shelled animals carrying worlds on their backs. But I wonder in your use case how the elephants will breathe? Lift their trunks up above the surface? Will an elephant small enough to fit on the crab be able to reach the surface with its trunk? I dunno... $\endgroup$ – ivanivan Sep 9 '18 at 2:14
  • $\begingroup$ I figured the ducks could carry bubbles down for the elephants. The bubbles are small, granted, but there are a lot of ducks. $\endgroup$ – Willk Sep 9 '18 at 3:24
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Start a few millennia ago. Have a cypress swamp near shoreline. Water levels rise, water becomes brackish, kills off cypress. Or land drops for some reason - earthquake or similar.

Mineralification or petrification of the wood and knees starts due to exposure to soaking in salt water.

Water levels continue to rise, what was the swamp is now part of the mud flats of 5-6 feet underwater, depending on stage of tides.

All those cypress trunks and knees that started turning to stone have provided a base for oysters, barnacles, anemones, algaes, and corals to attach to and start growing. Go to any sea side pier or dock and you can see this in action on the wood or cement pilings.

Eventually, you have these pillar like things made of layers of the remains of all those crusty animals and maybe even some of the petrified wood in the center. Even with massive waves and high tides now pushing against them, the ocean side may wear down but the lee side is protected, and grows without issue.

You'll see some "layering" from top to bottom of types of critters that are making up your pillars. At the bottom you'll have more of the things that would prefer to remain completely submerged at all times. They can get by with some splashing and spray/mist from wave impact, etc. when the tides are out, but they aren't happy about it - this is where you may find your corals, and softer creatures that don't leave skeletal structure behind. The higher you go, the more of the creatures you'll see that are evolved to tolerate being out of the water for a few hours at a time - oysters, barnacles, etc. The stationary or attached bivalves that can close up and seal up and wait for a bit for water to return.

Height of the pillars will depend on what structure there is to support vertical growth, and how much of the support stays submerged or at least wet from spray/mist for how long during the tide cycles.

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