From the designer....
"In the summer of 1997, I saw two horse mackerels that had somehow managed to get into the sardine tank at the aquarium. The sight of these two fish being unable to blend in with the shoal of sardines made them seem as if they were on an expedition of self-discovery. What sort of adventures will they have? Imagining this led to the conception of six Cast Puzzles based on oceanographic subjects."
A husband and wife pair of gobies living in a coral reef. Separate then from the coral, and then put them back together again.
Free the fish from the reef and put them back!