Here's the context:

Even back home, the Paleocene-Eocene Thermal Maximum was not a good time to be a marine organism. Equatorial seas spiked up to 36 degrees Centigrade, or 97 Fahrenheit! And the warmer the water, the less oxygen it can hold, and the less oxygen water can hold, the less life it could hold. Oh, but there's a whole lot worse. The waters had also absorbed so much of the atmosphere's excess carbon dioxide that they had become acidified. As a result, most of the world's supply of carbonates--the elements needed to create shells--had been eaten away. As a result, up to half of all the species of forams (single-celled planktonic organisms) had been wiped out, and major coral reefs had disappeared from the fossil record for millions of years afterwards. So, yeah, not a good time to sleep with the fishes.

But if the Paleocene-Eocene Thermal Maximum were prolonged by three or four times, then the end result would be a marine mass extinction. 99% of the corals went extinct, as did 82% of the bivalves, 83% of the barnacles, 87% of the tubeworms and up to 20% of the sponges. In the millions of years since, the sponges, the bivalves, the barnacles, the worms and the sponges that bounced back since have become the new reefbuilders, filling in half of the ocean floor and even colonizing brackish and freshwater habitats.

In order for reefbuilding sponges, barnacles, bivalves and tubeworms to establish reefs in freshwater ecosystems, how would they compensate for life in freshwater as opposed to saltwater?

  • $\begingroup$ Well for hard shelled creatures it will take much longer to grow shells with a lot less calcium available. $\endgroup$
    – John
    Jul 10, 2021 at 3:26
  • $\begingroup$ I understand the long hot ocean scenario but not how that means reefbuilders colonize freshwater. It seems like freshwater should have all the same problems as the oceans. What is the connection? $\endgroup$
    – Willk
    Jul 10, 2021 at 17:59
  • $\begingroup$ @Willk What do you mean? $\endgroup$ Jul 10, 2021 at 22:37
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    $\begingroup$ I understand the question in bold at the end. I dont understand how that relates to the prologue about prolonged hot ocean. What does prolonged hot ocean have to do with freshwater reefs? $\endgroup$
    – Willk
    Jul 10, 2021 at 23:15
  • 1
    $\begingroup$ If your oceans are warm, your fresh water bodies (not directly feed from deep springs or glaciers, if any exist) will be warm too, as they're much shallower. $\endgroup$
    – rek
    Jul 18, 2021 at 13:31

3 Answers 3


Each one may adapt in a different way

Let´s start with sponges:
They may not have any problem at all. In fact, there are sponges that live happily on freshwater rivers: the family Spongillidae:
So these creatures are already adapted.

There are many families of them. And (surprise) there are also some that also live in freshwater:

One of the largest species of freshwater bivalves is the swan mussel, in the family Unionidae; it can grow to a length of 20 cm, and usually lives in lakes or slow rivers. Freshwater pearl mussels are economically important as a source of freshwater pearls and mother of pearl. While some species are short-lived, others can be quite long-lived with some species registering longevity in the 100s of years.

Those little friends may need more help.
Barnacles are sea arthropods (related to crabs and lobsters). There are no barnacles at all in freshwater. However, their cousins (crabs) could adapt to live on freshwater. The first freshwater crabs appeared in the early Cretaceous:


The origins of the major freshwater crab families, Gecarcinucidae, Potamidae, Potamonautidae, and Pseudothelphusidae are ancient, deriving from the early Cretaceous (∼125 Ma; 95% credibility interval = 113–140 Ma).

And how did the crabs achieve that?

The colonisation of fresh water has required crabs to alter their water balance; freshwater crabs can reabsorb salt from their urine, and have various adaptations to reduce the loss of water.[4] In addition to their gills, freshwater crabs have a "pseudolung" in their gill chamber that allows them to breathe in air.[4] These developments have preadapted freshwater crabs for terrestrial living, although freshwater crabs need to return to water periodically to excrete ammonia.

So, to live in freshwater, Barnacles may need to evolve a way of retain minerals that were abundant in the sea water, and to develop a much more efficient way of gathering calcium for their shells (Yes, freshwater contains calcium):


Animals like fish obtain calcium through their diets, while algae and many invertebrates obtain their calcium directly from the water. Calcium often enters freshwater lakes from the slow weathering of their watersheds, with streams and runoff leaching calcium from soils and rocks.


Freshwater tubeworms also exist in lakes and rivers.
Here is a beautiful example:


So, basically, only the barnacles may need some evolutional changes (or genetic engineering) to adapt to freshwater and be part of the reef community.

TL, DR 1: Reefs require photosynthesis.

2: Giant clams have got photosynthetic symbionts.

3: In a world where corals are extinct, freshwater (and saltwater) reefs will be built of giant clams.

Photosymbiosis: The Driving Force for Reef Success and Failure

Photosymbiosis has been an important process in the evolution of ancient reef systems and in reef success today. Modern reefs and many of those in the geologic past inhabited nutrient-depleted settings. The complete collapse of some ancient reef ecosystems may be attributed to the breakdown of the ecologic and physiologic relationships between symbiont and host. Many algal groups developed symbioses with calcifying metazoans and protists and live with them, but the most common of these today are dinoflagellates in the genus Symbiodinium, sometimes called zooxanthellae. This photosymbiotic relationship conferred important metabolic advantages to both partners, allowing exploitation of tropical, shallow-water oligotrophic settings. In addition to improved metabolism, a by-product was rapid calcification which increased the growth of reefs and provided advantages to the hosts through larger and stronger skeletal support. Strong evolutionary pressures exerted by the symbiont-host relationship created bonds and favored longevity and adaptive novelty. Photosynthesis accounts for the remarkable reef growth and carbonate sedimentation in the tropics. Photosymbiosis gave reef organisms an adaptive edge to develop new life strategies that could not be developed by organisms which did not foster this relationship.

Current reefs are build by corals with photosymbionts. Until their extinction at the end of the Cretaceous, ancient reefs were built by rudist bivalves also suspected to have harbored photosymbionts.

Freshwater reefs existed in recent history.

Freshwater (phytoherm) reefs: the role of biofilms and their bearing on marine reef cementation

256 M PI Dl [:k" Growth of the phytoherms appears rapid un- der ideal conditions (Kemp and Emeis, 1985; Srdo~ et al., 1985). Individual structures can attain heights in excess of 20 m in the Plitvice region of Yugoslavia where they frequently dam extensive river courses, Phytoherm development demands that living surfaces be submerged or at least continuously kept wet. A continuous (but not agressive) water circulation is necessary in order to bring in nutri- ents for the biota and to provide replenishment of CaCO 3 for the cementation processes. Growth is encouraged under humid temperate conditions, consequently the Quaternary Mediterranean ex- amples are mostly extinct under the present seasonally arid climate. The Holocene Atlantic phase (esp. 6000 to 8000 years B.P.) was most favourable to tufa development in NW Europe. The generalized phytoherm construction Freshwater reefs share many similarities with their marine counterparts. The principal constructional differences lie in the dominance of frame-building vegetation and cements in the phytoherm and the subordinate role played by invertebrates in the constructional process.

My takeaway: "modern" freshwater reefs are plant based, and so depend on photosynthesis. Although their remnants remain they seem less robust than marine reefs.

Of candidates for freshwater reefbuilders, only the bivalves have the potential to harbor photosynthetic symbionts.

The evolution of molluscan photosymbioses: a critical appraisal

Living photosymbiotic molluscs represent a small and atypical sample of all the photosymbiotic clades that have evolved.

Established: we need bivalves with photosynthetic symbionts to make invertebrate dominated freshwater reefs. The question - is it easier for a freshwater mollusc to acquire photosynthetic symbionts, or for a saltwater mollusc to move into freshwater?

I will assert the latter because GIANT CLAMS ARE AWESOME

GIANT CLAM https://www.hakaimagazine.com/videos-visuals/raising-giants/


Algae provide giant clams with a supplementary source of nutrition.[8] These plants consist of unicellular algae, whose metabolic products add to the clam's filter food.[4] As a result, they are able to grow as large as one meter in length even in nutrient-poor coral-reef waters.[8]

Giant clams are the largest bivalve ever and there have been bivalves a long time. They are evolutionarily recent, as bivalves go. They have photosynthetic symbionts and lots of them. They live in nutrient poor reefs, competing with (or cooperating with?) the corals. They are ready to take over reefbuilding if something happens to the corals, the way the corals took over reefbuilding after the rudist Cretaceous reef-building bivalves went extinct with the dinosaurs.

In the freshwater world of this question, giant clams establish first in brackish waterways, carried by seawater rise caused by melting of the icecaps during the prolonged Eocene heat states in the OP. The reefs that the giants build produce serve as dams, flooding large inland areas with freshwater lakes. Rivers do not flow in this world, becoming instead a series of lakes choked by clam dams.

The specific adaptations of a bivalve to freshwater involves serious biochemistry. Suffice it to say that bivalves can do well in freshwater environments as evidenced by the many species.

Thinking about a bivalve dominated freshwater reef, I can imagine a system where small purely filter-feeding bivalves (like zebra mussels - and tubeworms?) keep water clarity high by removing suspended algae and planktonic life. Clear water is good for photosynthesizers but the only ones safe from the zebra mussels will be the photosymbionts in the giant clams. Populations of the smaller mussels will boom and crash with food availability, with the long lived giant clams and their photosynthetic symbionts forming the long lasting backbone of the reef.

  • $\begingroup$ "Reefs require photosynthesis." Since when???? $\endgroup$ Jul 15, 2021 at 3:58
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    $\begingroup$ @JohnWDailey - since the Ordovician, more or less. That is all laid out in depth in the first source I linked. If you hit a paywall you can get it for free by joining researchgate. $\endgroup$
    – Willk
    Jul 15, 2021 at 22:04
  • $\begingroup$ Were the rudists photosynthetic? $\endgroup$ Jul 17, 2021 at 3:25
  • $\begingroup$ Probably they were because 1: they were huge, which characterizes things from various phyla with photosymbionts in common (see that first cite) and 2: they had clear shell windows to let in light! from Vogel, K., 1975. Endosymbiotic algae in rudists? "These radiolitids may have lived in association with zooxanthellae which absorbed the necessary light through the oscula as well as through the shell by the help of a light-transmission system". There are modern (freshwater!) bivalves with such windows though they are not huge. $\endgroup$
    – Willk
    Jul 17, 2021 at 14:59

Fresh water, meaning runoff from the land, is different in many ways besides the drastically lower salt content.

Rivers are often full of a huge variety of suspended materials. Rock dust, dissolved minerals, organic materials, chunks of organic substances, decayed versions of all the organic material, etc. and etc. The result is, rivers are frequently turbid. However, this is intermittent in many locations, following seasons and storms. The pH of the water, the temperature, the flow velocities and even direction of currents, all will have large variations affected by season and storm. The content of such things as calcium (or, if it's a freaky sci-fi alternative metabolism, whatever is required to make shells or coral or whatever makes up the reef) is going to fluctuate madly.

Imagine autumn, and the trees drop their leaves, many falling in water and decaying. A huge variety of bacteria, fungi, algae, and all the stuff that feeds on those, has a feast. The water fills with small bits of decayed leaf and various metabolic products. The runoff causes the pH to go nuts. So calcium-based structures have a tendency to dissolve. Filter feeders have a tendency to choke.

The best adaptation of "reef builders" would be for them to stop being reef builders. By the time they developed ways to cope with all of the chaotic "soup" at the mouth of a river, they would be very unlikely to look much like their coral-like ancestors.

And that's what we tend to see. Some reef-dwelling life has made it. Fresh water clams, crayfish, and certain things like zebra-muscles have managed to transition. But the actual reef builders, not so much. Coral has not colonized Lake Michigan.

  • $\begingroup$ isn't it more likely that the downstream currents prevent colonization by coral (as difficulty heading upriver) vs inability to adapt? If we planted our own reef beginnings could they potentially take hold? $\endgroup$
    – IT Alex
    Jul 13, 2021 at 19:29
  • $\begingroup$ @ITAlex We'd see adapted corals where rivers reverse (tidal rivers) in that case. $\endgroup$
    – rek
    Jul 18, 2021 at 13:35

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