According to young-earth creationism, there was no animal death before the fall. No predation or carnivory. In order to deny prelapsarian animal mortality, YECs restrict the definition of nephesh chayyah to vertebrates rather than invertebrates.
In addition, they distinguish between aquatic and terrestrial species, based on the flood account. That was designed to destroy "all flesh," and only terrestrial animals were taken abroad the ark. But there are problems with this argument:
i) To my knowledge, no Hebraist would define nephesh chayyah as a synonym for vertebrate. That invests nephesh chayyah with a more specialized meaning that the term can bear.
ii) It would be arbitrary to posit that terrestrial vertebrates are nephesh chayyah ("living creature," animal), but aquatic vertebrates are not. For instance, it wold be arbitrary to posit that land snakes are nephesh chayyah, but sea snakes are not.
iii) If you deny that invertebrates are nephesh chayyah, then that would be consistent with invertebrate predation and carnivory.
iv) The classification results in hairsplitting distinctions, where a frog or lamprey is a living creature or animal but an octopus, colossal squid, giant squid, or giant centipede (Scolopendra) is not. What's the principled distinction?
For instance, an octopus is a very impressive piece of bioengineering. Is it less advanced, less sophisticated, than a lamprey or frog?
v) In evolutionary theory, as I understand it, the basic distinction is that invertebrates are more primitive than vertebrates.
But isn't there are more obvious reason for the distinction? Isn't the distinction between vertebrates and invertebrates related to gravity? Because water supports body weight, there's less need for aquatic species to have a backbone. There are, of course, aquatic vertebrates, but my point is that their natural element allows for design plans that would be infeasible for land animals.
To take another example, due to their small size and light weight, insects don't need a backbone. By the same token, I don't think an ant the size of a horse would be feasible.
Likewise, it's not coincidental that the largest animals are aquatic animals (e.g. whales). It's not coincidental that the largest snakes are aquatic snakes (Anaconda).
It's not that vertebrates are more advanced than invertebrates. Rather, their respective natural element places an upper limit on size and structure.
vi) There are, of course, bony fish and marine mammals. However, an octopus can go places a vertebrate cannot. Likewise, the exoskeleton of a crab enables it to function both in and out of water. So vertebrate and invertebrate designs have respective advantages and disadvantages.
vii) Finally, there's the principle of plenitude. God rings the changes on different possibilities and combinations. Different design strategies.