If you don’t think sea urchins are dangerous, you have never been an echinoderm, a barefoot snorkeler, or a Pathfinder player. This is a game where you run a real risk of being devoured by a ravenous urchin swarm. So of course the man-sized hunter urchins and ogre-sized spear urchins are going to be a threat. That’s just science.
A band of youths aren’t even adventurers yet—they’re just a handful of friends who get asked by Father Michaels to check in on an old hermit. Instead they find his cabin ransacked, with a note telling them where they should look for the codger. If they follow up (perhaps prodded by Father Michaels and the jingle of the pastor’s gold, if necessary), they find the old man. Or rather, they find his corpse, wedged into a deep tide pool and currently feeding hunter urchins. To get the pastor’s blessing (and his gold) they need to risk the urchins’ poisonous spines to retrieve the body.
The libertine nobles of Fever Bay delight in games of one-upmanship. This season the fashion is gourmet cookery, the more outlandish the better…and if one of the ingredients killed someone along the way, that’s even better still. Thus are some down-on-their luck adventurers sent after “rose petal uni”…without necessarily being informed ahead of time that rose petal uni are the tasty gonads of the deadly spear urchin.
Most river giants live deep inland, traveling the continent’s wide waterways. But a few outcasts eke out existences along the coasts, hoping to avoid the attention of humanoid pirates, the perverse marsh giants, and the far-more-powerful ocean giants. Since even outcast river giants spend a lot of time traveling, they use spear urchins to guard their treasure troves and supply caches while they are away.
—Pathfinder Adventure Path #37 84–85 & Pathfinder Bestiary 5 223