Creature Feature: the Opah
Eelgrass is a type of seagrass, a group of flowering plant species that thrive in saltwater. Often confused with seaweed, eelgrass has tiny flowers and is thought to be more closely related to lilies than to grasses. Eelgrass flowers bloom underwater and have filamentous pollen strands that drift onto stigmas of nearby flowers by flowing through the water column. The evolution of filamentous pollen in seagrasses is an adaptation to submarine pollination, which is quite uncommon. Even less common among plants is the adaptation to saltwater.
When envisioning the ocean’s ferocious fish predators, what often would come to mind is a sleek, fast body. The Opah does not match this description. Often called the Moonfish for its notably circular shape and vibrant color, this fish is a predator throughout the open ocean, though one wouldn’t guess it by looking at this large and humorous ocean dweller.
Weighing over 100 pounds and reaching over six and a half feet in length, this striking fish preys in cold, deep waters of the mesopelagic ocean, typically from 150 to 1,300 feet. Though it is known as a mysterious, solitary animal, very little is known of its behavior, habits and biology. Squid and krill make up their general diet, and large sharks are their primary predator, aside from humans.
The Opah has most recently become well known for research done this year by the National Oceanic and Atmospheric Administration, revealing that this creature is the only fish capable of whole-body endothermy; it retains heat to keep its body temperature above the temperature of its environment. In other words, it’s the only warm blooded fish in the world.
While some fish have developed the ability to keep certain parts of the body warm, the Opah is the first to achieve entire body regulation at around five degrees above the water it swims in.
To accomplish this unique talent, the Opah propels itself forward with its pectoral fins to produce body-warming heat. The heat is kept within its body by its specialty gills, which have an arrangement of blood vessels unlike any other fish to retain warmth through “counter-current heat exchange.” This distinguishes it from other predators in deep, cool waters, who tend to swim slowly and ambush prey. Whole-body endothermy allows the Opah to migrate long distances, swim quickly, see better, and bea fast hunter.
Unique appearance and taste has made the Opah increasingly popular in the fishing world for both trophies and food. Often caught unintentionally by commercial or recreational fisherman, NOAA research fisherman have reported catching more Opah in recent years, possibly due to changes in current conditions or a population increase. The exact population of the Opah is unknown.
Despite its mystery, the Opah has continued to amaze its observers. Nicholas Wegner, biologist for NOAA and lead researcher behind the paper published on the Opah, stating, “Nature has a way of surprising us with clever strategies where you least expect them.”