One Fish, Two Fish…

Ana Silverio, The University of Texas at Austin

The Approach: In my previous post, I explained how important small fishes are to the food web and how their new found interaction with Gracilaria vermiculophylla came about. Now, measuring something such as diversity and abundance may sound confusing but it’s as simple as one, two, three!

Abundance is the number of individuals per species in an ecosystem and relative abundance is the overall evenness of those individuals. Diversity is more of a measurement of variation or how many different species are counted in a designated area/habitat.

Fine mesh seine net being dragged over the 15-meter transect to capture our fish.
Photo Credit: Norma Salcedo

Now that we understand what we are measuring… what’s next? As mentioned before, the Charleston harbor has been introduced with an invasive species of seaweed, but it has served as a home for the juvenile fish. To measure diversity and abundance we have to take samples from two different sites affected by this invasive species. Luckily, it’s a short stroll over to Grice Beach behind our marine lab to find a section of Gracilaria with 20% coverage for our sparse site and one with 80% coverage for our dense site. After establishing our sample sites, we take a 15-meter transect which we will pull our fine-mesh seine net through at about knee-deep water. We quickly but gently pull the net up to the beach and start sorting through our samples placing the fish in a half-gallon jar while discarding any invertebrates. We repeat this at our second site and voilà we have our samples!

Initial sorting process for our samples
Photo Credit: Norma Salcedo

Are we done yet? Of course not! Once we collect both of our samples from the different patches of Gracilaria, we take them back to the lab to set in preservatives for about a week and begin the sorting process. While we sort each jar, we try to identify each fish down to the lowest classification if possible (in a perfect world we would have all of our critters down to species). After identification is complete, we start our measurements of diversity and abundance by counting our fish. When we are finished counting, we organize our data and use statistical analyses to see if there is a significant difference in diversity and abundance in our two sample sites. We have followed procedures from the past two summers and each time we have sampled this summer to make sure we can compare our data at the end.

And now for the big reveal… Drumroll please! Will we find a difference in diversity? In abundance? In neither or both? Will we finally win a battle against the dreadful pluff mud? Although the last part seems unfortunately unlikely, join me next time to finally find out what secrets Gracilaria has tangled up in the Charleston Harbor!


Special thanks to my mentor, Dr. Harold for his support and guidance throughout this project. Also, to Dr. Podolsky and Grice Marine Lab for giving me the opportunity to conduct this research. This project is supported by the Fort Johnson REU program, NSF DBI-1757899.

Gracilaria: A dynamic habitat

Nick Partington, St. Olaf College

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Findings: In my previous post, I described the methods we would be taking this summer to explore how the biodiversity of fishes differ among dense and sparse patches of the invasive alga Gracilaria vermiculophylla. We followed these methods, and we produced some interesting results!

We finally sorted and identified all of the fishes we collected from our samples this summer, and were able to measure the biodiversity between dense and sparse habitats. In particular, we were interested in four measurements of biodiversity. The first, abundance, is simply the overall number of fishes collected from each habitat type. The second, species evenness, measures how evenly individual fishes

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Some of the fishes we collected this summer, separated by species and sample.

are distributed among the different species collected in each habitat type. Finally, diversity took into account species richness, which counts the total number of species collected, and the Simpson’s Diversity Index, which quantifies diversity based on the number of species and the relative abundance of each of those species.

These measurements provided us with some interesting results. In the end, we collected a greater abundance of individuals in sparse sites than in dense sites. We also saw both greater species evenness and greater species richness in dense sites. Additionally, the Simpson’s Diversity Index showed a greater diversity of fishes in dense sites.

As I mentioned, abundance of individuals and species richness were both calculated by simply counting the overall number of individuals and species, respectively, collected in each site. Species evenness, on the other hand, required a bit more analysis. Figure 1 shows rank abundance curves for both sparse and dense patches of G. vermiculophylla. These curves tell us how evenly individuals are distributed among the species collected from each site. For each habitat type, species are ranked from 1 to 10 in decreasing order of abundance. That rank is then compared with the abundance of each species. The slope of the resulting line is what we are interested in. Basically, the flatter the line, the greater the species evenness. In our analysis, the line representing dense sites had a flatter slope, signaling greater species evenness in dense sites than in sparse sites.

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Figure 1. Rank abundance curves for both dense and sparse habitats. The slope of the curve representing dense habitats is closer to 0, indicating greater species evenness in those sites.

As I mentioned, overall we found a greater abundance of individual fishes in sparse sites, while we had greater species evenness, species richness, and diversity in dense sites. These differences between sites are very interesting in themselves. But what is even more interesting is that these results are the complete opposite of what was concluded after this same study was conducted last summer. Therefore, there must be some factor(s) that changed between these two studies. We’re not exactly sure what these factors are, but nonetheless, this highlights the importance of long term studies, as well as the importance of continuing this study to see how these trends in biodiversity change and pan out in the long run. I think a very interesting takeaway from this project is that invasive species, like G. vermiculohylla, can potentially provide benefits and sustain biodiversity in ecosystems here in Charleston and throughout the world.


Special thanks to Tony Harold and Mary Ann Taylor for their guidance in this research project. This project is funded by the National Science Foundation and is supported by the Fort Johnson REU Program, NSF DBI-1757899.