Why aren't there any freshwater cnidarians/echinoderms?

[JackGulley]

I've always been low-key jealous you can't have corals in a freshwater tank. I know hydras are a cnidarian species, so my question is why there aren't any macroscopic cnidarians? Corals? Anemones? Jellyfish? I can see with corals it could have something to do with the minerals they make their body out of, but there are freshwater snails. What's the scientific reason?

 

[GaryE]

There are freshwater jellyfish,
(Craspeducusta sowerbii) though they are structurally different from marine ones. My simple answer is the family didn't evolve many freshwater versions because they didn't. Lack of buoyancy, minerals etc combined with random luck probably explains it. There are freshwater sponges, and as you noted, Hydra, but their lifestyle/form gave them an advantage and they survive. Nothing happens for a reason, but if it happens and survives, it exists for a while.

 

[gwand]

There are freshwater jellyfish,
(Craspeducusta sowerbii) though they are structurally different from marine ones. My simple answer is the family didn't evolve many freshwater versions because they didn't. Lack of buoyancy, minerals etc combined with random luck probably explains it. There are freshwater sponges, and as you noted, Hydra, but their lifestyle/form gave them an advantage and they survive. Nothing happens for a reason, but if it happens and survives, it exists for a while.

Spoken like a true neo-Darwinian.

 

[FranciscoB]

Also hydra (freshwater) are cnidarians and are quite common.
There are, however, no freshwater corals (cnidarians), nor any freshwater echinoderms of any kind. For echinoderms, there is also no evidence that ever there were any in the past (geological history). Why? it appears they simply never evolved outside the marine realm.

 

[GaryE]

Spoken like a true neo-Darwinian.

Actually, a neon Darwinian in an LED world...

 

[Uberhoust]

Are there not some freshwater hydrozoans. You would have to have a magnifying glass to make them out.

 

[JackGulley]

Are there not some freshwater hydrozoans. You would have to have a magnifying glass to make them out.

Yes, hydras.

 

[Uberhoust]

Was not thinking about the hydras but more about some of the other groups such as.

Craspedacusta sowerbii - Wikipedia

en.wikipedia.org

Cordylophora caspia - Wikipedia

en.wikipedia.org

There are others but these where the quickest for me to find.

 

[gwand]

Was not thinking about the hydras but more about some of the other groups such as.

Craspedacusta sowerbii - Wikipedia

en.wikipedia.org

Cordylophora caspia - Wikipedia

en.wikipedia.org

There are others but these where the quickest for me to find.

Are Cordylophora caspia available for purchase?

 

[Uberhoust]

Are Cordylophora caspia available for purchase?

You might be able to get them from a biological supply but I doubt it. I only remember a prof of mine discussing a few hydrozoan from fresh water and telling use he found some near Victoria BC, not the hydras we are familiar with.

 

[Uberhoust]

https://nas.er.usgs.gov/queries/FactSheet.aspx?SpeciesID=1060 The link shows that this is an invasive species that can actually cause fouling of equipment.

Quotes from Link:
Means of Introduction: Possibly introduced by aquarium release (Mills et al., 1993), or through ballast water exchange (Seyer et al., 2017)
There is a negative economic impact associated with the biofouling caused by Cordylophora caspia (Folino 2000).

If you were to keep it you would have to be very careful on water changes etc. You should also euthanize it rather than release.

 

[Uberhoust]

https://www.int-res.com/articles/ab2021/30/b030p069.pdf This one is found local to me but could not find reference to it being located in the US.

 

[OnlyGenusCaps]

I've always been low-key jealous you can't have corals in a freshwater tank. I know hydras are a cnidarian species, so my question is why there aren't any macroscopic cnidarians? Corals? Anemones? Jellyfish? I can see with corals it could have something to do with the minerals they make their body out of, but there are freshwater snails. What's the scientific reason?

This is an excellent question! Unfortunately, questions about why did X lineage never evolve to tolerate Y condition are virtually impossible to answer. We can make hypotheses based on current metabolic or physiological conditions of the lineages. But they are all subjective once it comes to asking about potential futures for any lineage. Mutations are random by their vary nature, and sometimes that is the very unsatisfying answer.

Corals not being in freshwater is also why I now have a marine tank! More work? Yes! Worth it? Another emphatic: yes!

 

[JackGulley]

I wonder if there's some specific reason that body plan doesn't work as well in freshwater. Maybe freshwater environments tend not to have as much plankton for sessile filter feeders to eat? I also imagine maybe the calcium that corals build their structures out of is more easily procurable in saltwater than freshwater.

 

[JackGulley]

TIL that there are, in fact, both freshwater jellyfish and sponges