There isn't a "one answer that fits all" answer to this.
It will depend on which math'hab you follow.
For example as a rough (and I mean rough, not to be acted upon) guide, the position of the Hanafi math'hab is that only that which is considered to be fish is permitted from seafood. Everything else is prohibitively disliked. There is considerable difference of opinion about prawn, though. Some Hanafis gave the fatwa that it is permitted, as there is some evidence that Arabs considered it fish-like.
The Shaafi3i math'hab permits anything that lives exclusively in water (i.e. never comes out of the water), but with certain exceptions.
I cannot tell you about the Maaliki and Hanbali positions.
The best advice I can give you, is to find a scholar/learned person of the math'hab you follow, and ask them.