The very first animal that I saw upon landing in the Galapagos was also one that I found to be the most beautiful of all the organisms I encountered in the archipelago. Every rock within the splash zone of the shore was dotted with vermilion red, large crabs that moved with a slow, deliberate gait of an animal always ready to sprint at the slightest provocation. These were the famous Galapagos Sally Lightfoot Crabs (Grapsus grapsus), which I soon discovered to be a permanent fixture on nearly every piece of volcanic rock throughout the islands.
Sally Lightfoot crabs are not endemic to the Galapagos, and they can be found along the Pacific shores from Baja California to Chile. They are also known from the tropical waters of the Atlantic, although some taxonomists suggest that the Atlantic populations represent different species. But it is in the Galapagos where they reach the highest density, and their flamboyant bodies are as emblematic of the islands as the iguanas and giant tortoises.
Sally Lightfoot crabs got their name from their ability to move with lightning-fast speed across the the surface of the water, without sinking under. They do it in a way similar to that of basilisk lizards, by using their strongly flattened legs to increase the area of contact with the surface of the water, and if there is a crab species that deserves to be called Jesus Christ Crabs, it is them.
Sally Lightfoots feed mostly on algae growing within the splash zone, and only very reluctantly enter the ocean. I had caught one large individual while snorkeling along the shore and released it under water a few meters from the shore – the crab immediately ran towards the rocks and climbed above the surfaces. Because of their mostly vegetarian nature, their chelipeds (“pincers”) are rather weak and designed for grazing soft plant tissue, and only rarely do they resort to preying on other animals.
Their bright coloration has baffled zoologists for quite some time. Both males and females are equally brightly red and blue, which seems to preclude the use of these colors as sexual attractants. Young crabs are cryptically dark, and almost invisible on rocks, which is a typical kind of coloration found in grapsid crabs living above the water level. My personal suspicion is that their bright colors have a similar function to that of brightly colored hind wings seen in many otherwise cryptically colored insects, such as noctuid moths or oedipodine grasshoppers. These animals display flashy, bright patterns while flying away from a predator, but immediately hide them the moment they land, seemingly disappearing and confusing the predator. Crabs cannot fly, but they run towards narrow crevices in the volcanic rocks with such an amazing speed that the effect is quite similar; the presence of similarly colorful individuals all over the rocks is sure to make a bird hunting for crabs confused about which individual he was following.