Caranguejo means "crab" in Portuguese. It's a fun and distinctive word that stands out in these simple, repetitive lyrics.
While not extremely rare, it's not an everyday word you'd find in every song, making it memorable. In the context of this children's song, it's central to the playful idea of a crab pretending to be a fish, adding a touch of whimsy and imagination that will entice learners.
Roda Roda Roda is Galinha Pintadinha's version of "Caranguejo não é peixe", a Brazilian cantiga de roda — a ring-dance song children sing while circling and acting out the words. Every verse is three instructions and a claim: "Palma, palma, palma / Pé, pé, pé / Roda, roda, roda" ("Clap, clap, clap / Foot, foot, foot / Spin, spin, spin"), and then "Caranguejo peixe é" ("A crab is a fish").
A crab plainly isn't a fish, and the middle verse settles the argument: "Caranguejo não é peixe" ("A crab is not a fish"), "Caranguejo só é peixe / Na enchente da maré" ("A crab is only a fish / At high tide"). Enchente da maré is the incoming tide, when the water covers the sand and the crabs are swimming instead of scuttling — so the crab gets to be a fish for exactly as long as the tide is in. The verse after it repeats the same commands with "Ora" ("Now") in front of each one, a small Portuguese nudge meaning go on, do it — and then the whole thing comes round again, which is the point of a ring dance.