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