The Moonfish (Lampris guttatus) sometimes called an Opah, is a large discoid (disk like) and deeply keeled fish with an attractive form and a conspicuous coloration. They can reach a maximum length of 2 m and weight of 270 kg.
The one pictured left was picked up this winter on the beach at Lake Tyers in eastern Victoria by Chris Robertson, the son of a SETFIA member. It was estimated to weigh 20-30 kgs.
Moonfish are the first fish known to maintain their entire body core 5°C above ambient temperature – something known as whole-body endothermy. It does this by generating heat as well as propulsion with continuous movements of its pectoral fins and the vasculature of its gill tissue which is arranged to conserve heat.
Endothermy gives them a major advantage at the depths where they live. Since they are relatively warm-blooded at those depths compared to the water around them, they can move more quickly to hunt prey. Most predators at such low depths do not have the energy to be able to move much and therefore must wait for prey to pass them. These adaptations for high performance predation are similar to those found in tuna and some sharks, which actively chase down their prey.
This species is presumed to live out its entire life in the open ocean, at depths of 50–500 m. Typically, it is found within water temperatures between 8 and 22°C.
For more information watch this video.