There are two laws in fish biology. Your waders will leak, and there will be beer in the cooler.
These rules are common knowledge among anglers and biologists, but also brewers who have enticed many weary fish enthusiast with beers donning a fish-centric name. Some of these beers require no stretch of the imagination- Sculpin IPA (Ballast Point), Striped Bass Pale Ale (Devil’s Backbone), and Wall IPA (Northwoods)- which are all well-known fish species.
But some brewers go a step further, intentionally or not, and are able to sell a little fish ecology on every bottle. While my field work continues to crawl along, I felt inspired by the summer heat to introduce a mini-series that goes behind the names of a few beers that may be sitting in your cooler.
Up first is Bell’s Two Hearted Ale. This is a certain eye-catcher for any trout enthusiast as the label features an elegant brook trout surfacing through a stream. But, what does a brook trout have to do with two hearts? Bell’s likely named the beer after the Ernest Hemingway short story ‘Big Two-Hearted River’ which makes mention of trout fishing on the river in Michigan.
But, I like to think the title has another meaning, one Bell’s likely never saw coming. That is, trout actually have two hearts. The first functions as the normal blood-pumping machine and, in most fish, sits right behind the throat. This four-chambered heart pumps deoxygenated blood to the gills where it fills small capillaries. In the capillaries, carbon dioxide is dumped out of the blood and oxygen is absorbed from the water through a processed called diffusion (which basically just means oxygen molecules move from high concentrations in water to low concentrations in blood). From there, blood circulates up towards the top of the fish, and then heads through the body and towards the tail.
Once in the tail, blood is largely deoxygenated and sitting in many small capillaries where the pressure isn’t high enough to efficiently pump it back to the heart. So, evolution fixed that problem and gave many fish a second heart, the caudal heart. The caudal heart is located near the last few vertebrae in the fish’s backbone, in a region called the caudal peduncle. The caudal heart collects blood from all the small capillaries in the tail and forces it into the higher-pressure caudal vein where it can more efficiently flow back to the heart.
The caudal heart is small (about ½ an inch in trout), two chambers, and is powered by a combination of skeletal muscles and movement of the tail during swimming. Muscle contractions force blood from one chamber of the caudal heart into the other. This second chamber then empties blood into the caudal vein where travels back to the heart. With the first chamber now empty, there is room for many capillaries to each deliver a small amount of blood to the caudal heart and the process starts over again.
Interesting, the caudal heart is believed to only be active when fish are swimming, suggesting it is reserved for times when fish are active and in need of more oxygen. So, the caudal heart is kind of like an oxygen mask dropping down when I crank the treadmill up a little higher. Also, not all fish have caudal hearts, and some fish (like eels and hagfish, which both have snake-like bodies) have somewhat different anatomical structures in their caudal hearts. But, the purpose and function of the caudal heart remains the same.
So, next time you’re at a bar order a Two Hearted Ale and think about the caudal heart. Or, maybe go trout fishing and think about the ale. Whichever your fancy.