Acorn barnacles are an extremely common barnacle that lives in the mid-high intertidal zone. They originated on the west coast of North America, but due to ocean-wide shipping routes, these little guys have been spread all over the world. Invasive species like the acorn barnacle can have devastating effects on local populations of initial organisms. Understanding what environmental factors determine where they can live is very helpful in trying to figure out what will happen when an invader is introduced.
Part of what makes acorn barnacles such an effective invasive species is that they are able to handle a wide range of salinities. However, while wandering the beaches of Vancouver, I noticed that some beaches had more dense populations of barnacles than others, and that these beaches tended to have higher salinities!
Barnacles are amazing because not only do they have internal fertilization, they also brood their eggs until they become larva inside their shell for 6 months. While the larva and adults are very tolerant to a wide range of salinities, I wanted to find out whether the difference in barnacle cover between high and low salinity beaches was a result of differences in reproduction, or this brooding behaviour.
Sacrificing the lives of many barnacles collected from beaches around Vancouver, I found an interesting trend! The barnacles from the higher salinities had bigger egg/larval masses than barnacles of the same size from lower salinities. Also, more barnacles from the higher salinities actually had brood masses!
In biology, a result like this is really exciting! You find a trend, you try to explain it, and things go just the way you expect. Unfortunately, a little more digging showed me that I might not be so lucky. Even though for their size, barnacles from higher salinities have more eggs and larvae than barnacles from lower salinities, surveying the beaches made me realized that the barnacles at the lower salinities were a lot bigger. This is because at high salinities, there were so many barnacles that they were crowded together and weren’t able to get any bigger. And bigger barnacles, have bigger broods!
It seems that when barnacles are more crowded, they can’t grow as large so they put more energy into making eggs. Also, because they are closer together, more are able to reach and fertilize each other. However, even though there were less barnacles at the lower salinity, they were bigger, and therefore had bigger broods. It seems like these two things cancel each other out, and salinity isn’t actually behind the difference in barnacle density at the beaches in Vancouver.
And that my friends, is the circle of science! I set out to try and figure out what was causing differences in density of acorn barnacles, but instead, I found a bunch of things that seem to be caused by density! The only thing this does show is that the more barnacles there are, the more larva they make, which explains how these little creatures are taking over the world!