So you know when you see those cave paintings from the ancients of that giant crab? Well this could be a potential concept for it, along with new feature: Reefs.
Wrecker Crabs would be a large crab with a small shipwreck on its back like a hermit crab. Now a little about Reefs: Reefs would be new randomly generated shallow locations (similar to how shipwrecks generate) near or next to islands. Reefs would have a similar loot table to washed up treasure except maybe having some coral treasure. Reefs would have small rocks with mussels/coral jutting out of the water. Reefs would also contain small shipwrecks with rarer loot in them. Not all Reefs would contain a Wrecker Crab, it would be fairly rare making it more of a surprise when pirates encounter one. Back onto Wrecker Crabs: Wrecker Crabs would be in reefs, acting like a shipwreck in the reef, with a varying color of glint in front the shipwreck, depending on the variation of Wrecker Crab. The glint would lure pirates towards the Wrecker Crab, with pirates thinking that its treasure, at which point the Wrecker Crab would rise out of the reef bed and start attacking the pirates/s swimming around. The Wrecker Crab would be about halfway out of the water, pinching at the pirate/s with its claws, and chasing them around the reef. The Wrecker crab would be vulnerable to attacks around its mouth and eyes. It will be invincible to any forms of pirate damage anywhere else unless hit with a hunting spear. If hit with a hunting spear in the face for massive damage. Although the Wrecker crab will try to protect itself with it's claws when being attacked in the face. Not very well at that, being all big and bulky. If exploring reefs with others, Pirates can fire at the Wrecker Crab from the ship to break apart it's Shipshell and when broken, revealing a weak spot, and rare treasure on its back. If fired at enough times, the Wrecker Crab will collapse and Wrecker Crab leg will be dropped along with rare shipwreck loot, and Grisly and Pristine variants of Wrecker Crab Eyes and/or Armor Plates.
Some Attacks: Wrecker Crabs can break off chunks of their Shipshell (such as the mast) and use it as a spear or a javelin, depending on what it's attacking. Wrecker Crabs could perch on top of those rocks jutting out and use a sharp piece of wood as a fishing spear to poke at pirate/s swimming in the water. Wrecker Crabs could grab bits of sharp coral boulders and throw those at pirates or ships as well. And it can just use its claws to attack as well. If a ship is hit with a piece of sharp shipshell, it can be harvested when removing from the ship hull (like a hunting spear) for 1-3 pieces of wood.
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