Understanding the Impacts of Sargassum horneri on Rocky Intertidal Communities
“Do you like snails!?” my favorite Bill Hoyer quote. About 30 seconds after he sat down for our flight, Bill turned around and asked a random person if she liked snails. I don’t know why that moment was so great, but it was. Bill is one of our collaborators with the US Navy and he loves his snails and seems to be a walking-talking encyclopedia. Through our partnership with Bill Hoyer and Suzie Graham, we have been fortunate enough to work on the Navy Channel Islands on a really cool project that I will now tell you about!
We are broadly looking at the interactions between a non-native alga Sargassum horneri and the intertidal grazer community. First, let me tell you a little about Sargassum horneri (referred to as just horneri from here on out). Horneri is native to western Japan and southern Korea. Before making its debut in the US, there were no reports of it in other locations. So, how did it end up way over here in southern California? Probably ballast water...
It was first found in the Long Beach Harbor in 2003 but has made its up the coast to central California, down to Baja California and the Channel Islands. Marks et al. 2015 explains the range expansion of this alga in more detail, but just to give you an idea, here is a graphic from Lindsay Mark’s paper. Also, a much rougher graphic I made to give you a better idea of this expansion on a more zoomed out map.
The wiki infographic above shows how organisms hitch rides across the ocean through ballast water.
So, why does this matter? Who cares if this non-native alga is now found throughout subtidal and intertidal zones here in California? Well, we don’t quite know that yet. We are concerned about a few things, one of them being its ability to replace native species such as Macrocystis pyrifera, our precious giant kelp. Now, sure it may sound likes it’s a little early to make that assumption, but I don’t think so. Check out this news clip highlighting an invasion in Mexico by Sargassum species.
We want to know how grazers react to horneri when this non-native species settles in their habitat. Do they eat it? Can rocky intertidal grazers prevent or facilitate an invasion? There is still much to learn about this seaweed and how native species will interact with it. With this project, we hope to not only learn about this alga, but also some really cool intertidal grazers, including giant marine snails!
San Nicolas Island is one of the few remaining homes of black abalone Haliotis cracherodii. Abalone, in general, have suffered precipitous declines over the last few decades due to over-harvesting and Withering Foot Syndrome disease here in California. Some people find abalone to be pretty tasty and at some point, we decided to harvest so many of them that we greatly decreased their populations. If that wasn’t bad enough, remaining abalone suffered from a bacterial infection that causes them to whither and die… Sad, I know. To top it all off, these animals are broadcast spawners, meaning they all have to release their gametes at the same time and hope that they meet, and eggs get fertilized, otherwise they don’t reproduce. See the problem? The probability of those gametes meeting is incredibly low when individuals are far away from each other.
The first big problem is, That it’s not easy to study the interaction between an endangered species and a non-native species. Transplanting experiments are out of the question; We can’t just move endangered species around and we don’t want to help a non-native species spread, instead we are using a really cool method that hasn’t been tried in the intertidal!
We take algae and process is in a way that destroys its reproductive ability, turn it into a fine powder, and make an agar-based food that we can deploy in the field. Let me tell you a bit more about this, because I find it super exciting. We can prepare these petri dishes in any makeshift lab (I am not kidding, we have made these in hotel rooms and picnic benches), we then attach them directly to the substrate in the intertidal and leave them for 24 hours! And what does this tell us? Lots of stuff! using algae we know is tasty to grazers, like kelp, we can measure the community grazing rates!
By using two different species, we can determine if grazers prefer one species over another, so far, we have tested this method several times, and I’m happy to say, IT WORKS! Our next step is to measure the relative importance of black abalone grazers using this minimally intrusive method. We plan on doing this using exclusion cages. To avoid having to move black abalone to measure their grazing rates by excluding all other grazers, we will make cages that allow all other grazers in while keeping most abalone (>30mm) out! Using the magic of math, we can then calculate abalone grazing rates and determine their importance in the grazer community!
While we prep for a big deployment this fall, we have been doing complementary experiments in the lab. We selected the most abundant intertidal species and have been conducting assays to determine their food preferences. So far, we have brought black turban snails Chlorostoma funebralis, blue banned hermit crabs Pagurus samuelis, and striped shore crabs Pachygrapsus crassipies into the lab!
Now, I don’t want to give any premature results here yet, so I will report back when I analyze our data, finish the assays and most importantly, conduct these experiments with abalone! That’s right! We are currently working with some really nice folks to conduct preference assays and long-term performance experiments with abalone! So, if it’s not obvious already, YES, WE LIKE SNAILS!
