The ethereal creatures you see above are sea angels, or, more formally, pteropods – a kind of shell-less saltwater snail. They are tiny, graceful, and delicate-looking, and they are voracious eaters of only one thing – sea butterflies, another kind of pteropod that does have a shell (below).
- Sea butterflies, photographed by Nancy Copley.
My favorite description of how the innocent looking sea angels get a meal comes from researcher Miriam Goldstein in the endlessly fascinating Deep Sea News: “When (sea angels) see a pteropod, they shoot tentacles out of their face, grab their unfortunate prey, and wrestle it into position to be slowly eaten.” Check out her blog – there’s actually a video of it!
It really doesn’t get any better than that. We had a very lively dinner table discussion in my house after learning this, and talked about all the different things we would do if we could only shoot tentacles out of our faces!
Daydreams aside, these miniature mollusks play a mighty role in our ocean’s ecosystem – they are one of the foundations of our marine food pyramid. Many animals depend on pteropods for a large portion of their diet. Some of our most ecologically and commercially important fish eat pteropods. They’re not the only ones – whales and sea birds eat them, too. And pteropods are in serious trouble.
All pteropods swim, even the ones with shells – the sea butterflies. The shells on sea butterflies are very thin, according to researcher Gareth Lawson of Woods Hole Oceanographic Institution. This is possibly an adaptation to keep them from sinking as fast as an animal with a thicker shell would, but these fragile shells are not standing up well to the rapidly changing conditions in our ocean.
There has been a lot of news about climate change lately, but not as much about ocean acidification (the increasing acidity of the ocean that results from increased carbon dioxide in our atmosphere). That is probably going to change, though, as some startling new discoveries about the effects of “climate change’s evil twin” become more obvious. The plight of the pteropods is one stark example of this.
Sea butterflies are the subject of a worry-inducing new article in Nature Geoscience. These animals must form a specific kind of calcium carbonate to make their shells, and they need to be in water that has just the right chemistry for doing this. As the ocean becomes more acidic due to increased carbon dioxide in the atmosphere the water chemistry changes, and there is less of what pteropods need in the water to form shells. These incredibly important food animals are becoming less able to make shells. To make matters worse, pteropods that had already formed shells were observed to be dissolving. And the ocean continues to become more acidic.
If the sea butterflies go away, so go the sea angels. Then, what happens to the rest of the food web? This “Sea Butterfly Effect,” as Dr. Lawson calls it, may ripple through our oceans in dramatic ways that are hard to think about.
News like this can provoke a range of responses in people. Personally, I had a minor breakdown when I read about this study from my unheated Massachusetts house – unheated because it was 60 degrees outside. In late December.
Some people will ignore the growing evidence of these big problems. Some people will be too afraid to think about it (understandable!) or have more immediate worries to deal with. Some people will keep doing the good work they are already doing to try and make things better. We all have a choice about what we do next.
As for me, I’m going to keep learning what I can about the changes that are happening, and I’m going to help figure out what we can do to keep our oceans healthy as they become more acidic, warmer, and saltier. I’ll keep you posted.
