What's going on? Answer below the fold.
It is definitely a goby with eggs. A proper kitchen knocks each oyster and listens to the thud that let's you know the oyster is alive and sealed in the shell. This one would have sounded hollow and likely would have broken open with the slightest tap. Agree with the comment below that this likely would've been caught in a nicer restaurant. The occurrence of animals nesting in a partially open oyster shell is incredibly high, hence the reason the watermen put so much time into being "hands-on" with the bivalves to do quality control.It's not clear whether the oyster was served or whether the surprise was found by the person processing the oysters in the kitchen.