I like horseshoe crabs (Limulus polyphemus). They're such bizarre little crustaceans, which can get to be an impressive size, and despite their evil glaring eyes on top, they're totally innocuous. They have no dangerous pincers like many other crabs and are quite easy to handle. Their main defense is the hard domed shell that fits flush to the bottom surface, preventing predators from getting at their semi-vulnerable undersides.
They also haven't changed much in millions of years. This seems to give the impression of them being 'slow' or resistant to evolving, but when you think about it, it's actually an indication that they're so well-suited to their environment.
They seem to appear in the river in cycles, probably connected to breeding season, so it's no mistake I spotted these two comporting in the shallows near a boating dock. What's interesting to note is that this is one of the few times I've seen horseshoes of this size (the largest being about 18 cm, or 7 inches, wide) that were unadorned by either slipper snails or barnacles. Horseshoes often play host to a large variety of hitchhikers, some innocuous, some not.
Take it from me, too: They are not easy creatures to disentangle from lost casting nets, and offer no gratitude when you do so. But I don't suppose I'm in it for the recognition. [So, naturally, I manage to mention it on my website]