Why aren't there any freshwater cnidarians/echinoderms?

[Colin_T]

Pretty sure there are some freshwater corals, as well as sponges, jellyfish and I think even starfish, but they are rare and only found in a couple of freshwater pools in the middle of nowhere. I think Lake Tanganyika has some stuff like sponges.

 

[JackGulley]

The jellyfish species I linked is apparently
-Native to the Yangtze
-Occasionally found as an introduced/invasive species in temperate lakes all over the world, usually in a sudden bloom
-About an inch in diameter
-Eats zooplankton like cyclops and daphnia

 

[JackGulley]

I'm trying to hunt it down, but someone in a YouTube comment described a freshwater jellyfish that feeds by rising to the surface for light, then sinking to the bottom for fresh nitrogen, in order to maintain a symbiotic algae that produces its food.

 

[Uberhoust]

There are a number of locations in British Columbia where I have found fresh water sponges, but I have never seen any jelly fish or polyps (Cnidarians) in our temperate waters. It is interesting that the more complex Sticklebacks have re-colonized our streams and lakes after the ice ages but not the simpler jellyfish. I wonder if we only get sponges because they don't need as much food as jellyfish. There is only a fraction of the food available in the freshwater column (Plankton) as compared to what is found in the marine waters.

 

[Colin_T]

I'm trying to hunt it down, but someone in a YouTube comment described a freshwater jellyfish that feeds by rising to the surface for light, then sinking to the bottom for fresh nitrogen, in order to maintain a symbiotic algae that produces its food.

I saw that documentary. It might be another BBC David Attenborough documentary or something else. But I would check the BBC ones first and see if they have anything. It might be Planet Earth 3, that has a freshwater video.

 

[Seisage]

Pretty sure there are some freshwater corals, as well as sponges, jellyfish and I think even starfish, but they are rare and only found in a couple of freshwater pools in the middle of nowhere. I think Lake Tanganyika has some stuff like sponges.

I'm an invertebrate biologist who specifically works with sea stars and no, I can confirm that there are zero freshwater sea stars (starfish). In fact, you'd be extremely hard-pressed to find any echinoderm (sea stars, sea urchins, sea cucumbers, brittle stars, crinoids, etc.) in an estuary, let alone full freshwater. They lack mechanisms to osmoregulate, so they can't handle fluctuations in salinity, which means there hasn't been a way for them to make an evolutionary transition to freshwater. Where I live (right on an estuarine bay) the farthest into the bay I've ever seen a sea star is about a half mile from the mouth, where there's almost no freshwater at all, even during ebb tide.

There aren't any freshwater corals either, and I'm not sure what's limiting them from freshwater, but they're more likely than echinoderms to be able to make that transition in the future given that other cnidarians have already done so.