Ah well, this has dragged out for a while. Sea horse is one of my favorites to pick, but it has been eliminated this time as the animal is an invertebrate (page 2 or 3 of this thread). That would rule out eel, terrapin, sea horse. As noted, not being a crustacean rules out some guesses as well.
I suppose I should just reveal the answer, but I'll throw out a few hints and give it a little more time.
1- There are four species of it worldwide. One of these species resides in the Atlantic Ocean.
2- Females are significantly larger than males at maturity. (1/3 to 1/2 larger in terms of length and weight.)
3- This is a saltwater animal. (I'm surprised nobody has asked this!)
4- It is a scavenger.
5- [This one is the giveaway.] While not typically consumed by humans, it provides nourishment to shorebirds, particularly in the late spring and early summer months.
These are all the hints for now, happy guessing!