warm blood throughout its body, shown as yellow and light blue. This allows it to eat at depths other similar fish can't reach.
This expands on the narrative of A World From Dust in two important ways:
1.) The problem with being warm-blooded is not just making the heat, but keeping it. To insulate its precious heat from the cold waters around it, the opah pumps its blood through intricate and efficient blood vessels in twisted hairpin shapes. This structure is called a rete mirabile and can be built using Adrian Bejan's engineering theories for how heat flows. This hairpin structure is optimal for insulating a circulating fluid, so it is found repeatedly in warm-blooded animals. Bejan's Constructal Law could have been used to predict that a warm-blooded fish would have a complex rete mirabile structure before that structure was found in the fish -- it is a consequence of how heat moves. The opah is generating more heat, so I believe it would have a higher Energy Rate Density and Chaisson's ideas may apply, too. It has a more complex internal structure to match its higher energy throughput.
2.) Other fish that look like the opah and have genes like the opah are not warm-blooded, but a few very different fish (for example, tunas and lamnid sharks) have the biological spaceheaters that are halfway there. These fish obviously have different shapes and different genes, but they have independently developed similar systems for heat generation and insulation. In very different species, evolution has converged to produce similar and predictable warm-blooded temperatures and structures. Which species get it may depend on random rolls of the dice at the gene level, but that some species will get it and fluorish, that is predictable, given enough time.
So, not only is a warm-blooded fish very cool (see what I did there?), it also shows that evolution solves similar problems with similar features (warm blood) and similar structures (rete mirabile), in tuna, lamnid sharks, and opah, repeatedly producing predictable complexity.