I get it. Some of you are skeptics. After shooting it back and forth with someone, here I am writing this blog. I am by no means a seasoned marine biologist, but this one had me laughing. I was asked, "What's the only organ system that aquatic animals have, that terrestrial ones don't?"
Weird question. I heard the terms "organ system and aquatic", so I immediately thought of clams, more specifically mussels. Clams and mussels have something that many animals don't. Before I answer that, I need to address the flaws in this question. A lot of aquatic animals are also terrestrial. (Hippos)
Some animals can be both. These animals are defined as 'semiaquatic.' Mussels are the only animal that I could think of that are fully aquatic. Right?
Well, I learned something new throughout all of this, clams are in fact gastropods. Many other animals are gastropods, including but not limited to, the snail and sea slug. These animals are full-well capable of being what you would define as "terrestrial," while still being aquatic. This question had no actual basis.
When I was first asked this question, I answered, "a foot" to throw them off.
Not many people know that a foot is actually a muscle in osmoregulatory systems. Clams have what's called a foot. What I did not know was that some snails and other gastropods also have it. Don't worry, these animals don't need shoes.
Mussels and clams live most of their lives in the water and only wash ashore in wet environments. Mussels or clams are the only animal with a foot that are fully aquatic. Freshwater mussels are fully aquatic bivalves, a kind of mollusk that's encased in a shell made of two valves or hinging parts. Source: Google
Note: Not all gastropods are bivalve mollusks.
Incomplete digestive systems are not found in mammals.
Thanks for reading!
I may not have all the answers, but I know what questions to ask.