Here in Maine, it is open-water fishing season.  Superior fishing opportunities attract many tourists to this area.  Consequently, I decided I should learn a few things about fish.  Other than the fact that some fish are pretty, others are predators, and many are tasty, I confess to not knowing much about them.  

One thing I thought I knew for certain was that fish are cold-blooded.  Didn’t I answer that question correctly in sophomore year biology?

Recent studies have uncovered anomalies about this supposed fact.  The reality is that while the majority of fish are cold-blooded, at least one fish is warm-blooded, and others are hybrids.  

In 2015, the opah or moonfish (Lampris guttatus) was discovered to be a completely warm-blooded creature. This is a fish species of commercial interest found around Hawaii. Researchers with the NOAA Southwest Fisheries Science Center, who first identified this new finding hypothesized that warm-bloodedness gives the fish a competitive advantage in deeper, colder waters.  So far, the opah is the only fully warm-blooded fish identified. 

Opah or Moonfish
Image by Mgs99 on

Next, another class of fish combines the properties of both the cold and warm-blooded.  At last count, there are about 35 different species that fit this description.  These include some sharks and some tuna species.  They can warm, more or less on demand, specific sections of their bodies such as muscles and their brain.  

However, until recently, it wasn’t clear what a competitive advantage to self-warm really means.  A study conducted by a team from Trinity University suggests that the advantage translates into the ability to swim faster than competitors and predators.  Warmer muscles mean stronger muscles. These fish can swim about 1.6 times faster than their cold-blooded cousins.  

The second part of the hypothesis suggested that maybe these fish could live in waters with a broader range of acceptable temperatures, including colder areas.  However, the results suggest that this is not one of the benefits of being able to warm their body parts.  Perhaps being able to dart away is a great enough advantage!

Again, nature shows that variation has purpose and adaptability is indispensable.


More about the opah discovery —

More about the Trinity University study —

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