What is the difference between a Whelk and a Nassarius snail? Here's an image below. The creature on the left is a Nassarius, the one on the right is a whelk. Btw, whelks get much larger, while Nassarius don't.

If you look at the texture of their snouts, there is a specific pattern tatooed onto the flesh of the Whelk. Also, the whelk has a protective shell on its tail that it uses to seal itself in its shell when in danger. This gives it a water tight seal.

Whelks don't travel too quickly, ime. However, Nassarius glide across the sand very very quickly, moving on a large foot that allows it to move forward effortlessly.

Also, the feeler or eye stalks appear to be longer on the Nassarius snails, when you compare the Whelk's extended appendages.

Lastly, the shell. Of all the whelks I've caught, ever one of them looked like this one, with the darker coloration with whitish patches. In the past 48 hours, I've caught 7 total. They are all trapped in a box in the sump, awaying my decision. They can be super small or quite large, depending on age and growth.