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I am working on a xenobiology biosphere project, and I have a clade of creatures who evolved in the wetlands. They are disconnected entirely from life on our world, although they follow the same general rules of evolution and don't stray into anything crazy like silicon-based life or entirely new organs. They look similar to eels with no dorsal fin and having long feeding arms with a pseudo-jaw, and inhabit almost every aquatic clade in the wetlands. But the main feature they lack is a tail, causing them to have an anguilliform method of swimming since most of the herbivores prefer relying on defense or camouflage over mobility, and doesn't necessitate more efficient swimming methods.

I am trying to figure out if an anguilliform creature could adapt to an up-and-down motion of swimming like cetaceans rather than the side-to-side motion of swimming like fish, and the advantages and disadvantages of this. For reference, I am mainly looking at one specific family of these creatures which is predatory in nature, is the only one to possess a dorsal fin, and hunts by impaling their prey like a swordfish, with their prey inhabiting all aquatic zones of the wetlands.

In conclusion, can a creature who uses anguilliform swimming use an up-and-down motion of swimming like cetaceans? Bonus for the advantages and disadvantages of this, but I can make it a separate question if need be.

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  • $\begingroup$ Why would they? The anguilliforms are mainly bottom feeders (eels, some catfishes) and there's are some reasons for that form being good at the bottom and suboptimal in open waters (speed is lower, being elongated the size of attack area a has larger spatial extension). An up-down swing is... weird when the down swing gets your head banged on the bottom. $\endgroup$ Nov 15, 2021 at 23:58
  • $\begingroup$ @AdrianColomitchi Anguilliform swimmers just happened to be the main group who acted as pioneers to the wetlands in my biosphere, if all the prey uses armor and opts to be slow and well-defended rather than fast, there's no reason to be faster than one's prey. $\endgroup$ Nov 18, 2021 at 4:05

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I got your up and down anguilliform swimmer right here!

leech

https://www.youtube.com/watch?v=d2tL1AN6fcE

Leeches swim by putting their body through sinusoidal waves, like eels (anguilliforms). But the leeches use vertical waves like a cetacean.

I think this sort of motion makes sense for any very elongated creature and the orientation of the waves has to do with the internal structure of the body.

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