While I often focus on intertidal species for this column, this week I wanted to write about a marine animal that lives in the subtidal. In fact, it can go into impressively deep subtidal (beneath the lowest tide line) habitats. That means it is sometimes difficult to see since it spends most of its life under the water. But, like intertidal species, these animals also cross over the boundary between air and sea. They are one of the many marine mammals that live in Maine and one of the most graceful if you happen to see them – or at least a bit of their dorsal fin. 

Harbor porpoises are small when it comes to marine mammals. They are similar in scale to a harbor seal, measuring up to six feet or so in length. Phocoena phocoena means “pig fish” in Latin, which seems less than fitting for a species I think of as quite elegant. The reason for the name isn’t their habits of cleanliness, but rather it refers their pig-like “snout” and somewhat snorty breathing.  

While we see seals fairly often along the coast, porpoises are a rare treat – their small triangular dorsal fins gently breaking the surface as they travel along in groups or pods. You can watch them travel along in one direction for a little while before they dive down beneath the surface, arching their backs as they go to signal that you won’t see them again for a few minutes.  

During a recent visit from my family, my nephew asked whether porpoises were the same as dolphins. This is something I knew the answer to somewhere in the depths of my brain, but I had to refresh myself. While both dolphins and porpoises are both toothed whales, they are parts of different families (Delphinidae and Phocoenidae) that evolved separately. However, at this point, they are quite similar and often mistaken as one and the same. They both use their teeth to grip onto and eat small fish along with a variety of other sea creatures.

They both have a “melon” in the front of their heads that makes sound waves called sonar to help them find their way around the ocean. They both give birth to live young under the water that they nurse with fatty milk. And they both need to come to the surface of the water to breathe. And they both sleep half-awake. They have a cool system called bihemispheric slow-wave sleep that allows them to turn off half their brain but remain conscious by using the other half so that they don’t forget to breathe. Unlike us, they must be partly awake to breathe. 

When trying to identify them, they are a couple of tell-tale differences. First, dolphins are typically a bit bigger and more often found in deeper water than harbor porpoises. Aside from habitat and size, in general, dolphins are more distinctly shaped. To break it down, porpoises have a more pig shaped snout and dolphins have a longer snout that is often called a beak.

In terms of their dorsal fin, a porpoise has a more triangular fin that allows it to steer easily in shallow water as compared to the more curved fin of a dolphin. Even dolphin teeth are shapelier; they are little cones versus the small spade-like teeth of porpoises. Finally, dolphin bodies are leaner and longer than “portly” porpoises.  

There are some differences in their behavior as well. Dolphins tend to be more playful and leap up out of the water more frequently. Although, porpoises can be fun to watch as well as they play in the waves. One behavioral difference you wouldn’t know about unless you could hear under water is that you won’t hear porpoises.

Their vocalizations are at a higher frequency than humans can hear – unlike those of a dolphin. Scientists use neat tools called hydrophones to listen under water which is how we are able to know of the whistles and clicks dolphins regularly make.  

Whether you are able to get close enough to see these impressive marine mammals first-hand or just take the time to learn more about them, it is fun to think of them making the connection between air and water so smoothly as they traverse our coastal waters.  

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