Our trip to Cabo Pulmo was the first time we’d dived in the tropical East Pacific. A lot of the fish we saw resembled those we’d seen in Hawaii, Bali, and Samoa, but were just a bit different. Due to the vast, mostly islandless expanse of the tropical Pacific east of the 130th meridian the west coast of the tropical Americas is biogeographically distinct from the Central and West Pacific, so fishwatching in Cabo Pulmo left us feeling like beginners. Fortunately, the Smithsonian Institute offers a free app describing over a thousand East Pacific fish species. It’s aptly named Fishes: East Pacific. I don’t know how we’d have begun to identify the fish we saw without this comprehensive guide.