Chances are it probably is actually spider. I love having good sushi every once an a while, they can get some pretty different things in there, thigs that you may not have thought of eating before, but they make it taste so good, you almost forget the strange creatures you're actually eating. If you ate it the first time and enjoyed it, spider included, how does knowing that it HAS spider stop you from eating it like that again. Insects are a common snack around the world, and for the most part just taste like any other protien (if they're prepared properly).
If not actual spider, than it's probably Spider Crab.