I have a type of coral that feeds off of bacteria that love bird feces (or something involving a bird species). It grows large rocky spires out of the water to create a habitat for birds. I need to know how high these spires can get realistically and if coral could even grow above the water's surface.

  • They grow in ocean water about 8 meters deep
  • Rising and falling of ocean levels is not possible

I do not like leaving this without some cool way to accomplish what the OP requests: a living thing, feeding on bird feces which grows in such a way as to host a bird colony.

Living with part of the body out of the water requires vascular channels. What has those? Multicellular life: vascular plants and animals. Vascular plants that do this exist: mangroves. But let us be awesome. What about an animal that does this?

I have two dreamt-up inventions to offer.

The Shell.

Mollusks can get big. Giant clams are big but prehistoric bivalves got much bigger.
huge clam https://en.wikipedia.org/wiki/Inoceramus

And the mollusk body plan is amenable to scaling up even more - giant squids are mollusks. A clam would be delighted to /feed off of bacteria that love bird feces/ as the OP states. That is the sort of thing filter feeding clams eat. Your bird island is comprised of a colony of these. They are big, with the bodies below water feeding on what dribbles down from the tall Murex-type spires of shell they extend up above the waterline for the birds.

https://en.wikipedia.org/wiki/Murex_altispira Murex shell with spires

Small clams nestle among the big ones, shell spires reaching for the surface. These small ones also feed on what comes down from the birds and grow as they do, ready to take the place of their elders when those die.

The star.

Echinoderms are durable and cool ocean creatures. They can put up with some time out of water and it is not uncommon to see a sea star making a foray into the air at low tide. Your bird host could be comprised of huge, sessile echinoderms. It is easy to picture a clump of mighty sea urchins, their ramifying spines jutting above the surface. But I like the crown of thorns starfish as the model because these things are so burly and huge. Echinoderms are very ancient, and your creature can be a survivor from the Ordovician.

crown of thorns starfish https://www.advancedaquarist.com/blog/bounty-called-for-on-deadly-crown-of-thorns-starfish

Your bird host star can be mobile when young, foraging and eating as starfish do. Then as it gets large and less mobile, it seeks the company of its kind on the bird roost. It sends its spikes above the surface to host the birds and feeds on what washes down.

  • $\begingroup$ Using a mollusk is an AMAZING idea! I wanted to use something besides trees. Thank You so much! @Willk $\endgroup$
    – Thalassan
    Jul 4 '18 at 22:52

I need to know how high these spires can get realistically and if coral could even grow above the water's surface.

Corals do not grow outside water. If they don't have water, they die.

Corals grow on top of the deceased ones, not on their bottom. Being enclosed between a rocky/sandy bottom and a layer of deceased con-specific is really a poor choice if your nutrition model is based on capturing food particles suspended in water. It means that only the outer ring will get some nutrition, you would grow on rings, not on pillars.

  • 5
    $\begingroup$ Dutch is right. Corals stop at the top. In a coral reef you will see nothing sticking out of the water all the time - at least at high tide stuff must be submerged. There is nothing else which grows up out of the water except plants. To live above of the water you need vascular channels to transport the water up to those parts. That evolutionary accomplishment is the claim to fame of the green plants and allowed them to leave the water and take over the land. $\endgroup$
    – Willk
    Jul 4 '18 at 16:27

What you describe is more like a salt-water tolerant tree with limestone for wood, if it has something akin to a coralline algae as a cambiun layer then such a thing might be possible. The bark will be a flaky layer of mineral salts that the tree can't use while the core "wood" is largely similar to modern coral structures. Dead trees would look a lot like coral sticking up out of the water.

The problem is rigid structures like coral don't last very well near the surface due to wave action, have a look at the third paragraph of the answer to this question to get an idea of the hurdles involved. If they pierce the surface the erosive forces are even stronger so your pillars won't last very long and will have to grow very fast to get above the surface at all.


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