Gracilaria: A Weedy Invader

Nick Partington, St. Olaf College

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The problem: Invasive species come in many different shapes and sizes, with a great variety of effects on the environment. For example, some invasive species infiltrate and destroy native trees, while some are introduced into lakes where they may displace native species and absorb nutrients. Whether aquatic or terrestrial (or perhaps even extraterrestrial), invasive species have been shown to have considerable effects on the environments they invade, and have been proven to play a major role in affecting global change (Vitousek, 1996). Gracilaria vermiculophylla is an invasive seaweed that has been introduced to many regions throughout the world, including the east coast of the United States (Thomsen, 2006, 2007, 2009). It takes the form of a thin, brownish red algae and originated off the coasts of Japan, from which it has dispersed throughout the world by hitching a ride in the ballasts of commercial ships (Krueger-Hadfield, 2017). In South Carolina, it can be observed in dark patches on beaches when the tide recedes.


A patch of Gracilaria vermiculophylla on Grice Beach, where we will be collecting samples this summer.

Many small fishes use G. vermiculophylla as habitat; it provides them with food, as well as shelter from predators (Byers, 2012). Many of these fishes serve as prey to larger fishes, and eventually the energy they contain travels up the food web to commercially and recreationally important fishes across the world, including within the Charleston harbor area. That is, G. vermiculophylla provides habitat to fishes, which in turn serve as food for larger fishes that are consumed by humans. Having a good understanding of how these fishes use G. vermiculophylla as habitat can aid the conservation and fishing industries in understanding this low-level component of the food web.

My research project this summer is aimed at improving this understanding. We will be replicating the design of a study implemented in the summer of 2017 by studying fish communities occurring in patches of Gracilaria vermiculophylla. Particularly, we will be exploring differences in the abundance and diversity of fishes utilizing dense patches of G. vermiculophylla as compared to sparse patches. We are also interested in any differences that might exist between dense and sparse patches concerning habitation patterns among different developmental stages of these fish species. Our findings may support that which was discovered last summer, or they might reveal a completely new piece of information. I am excited to see what we will find!

Special thanks to Dr. Tony Harold for his 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.


Byers, J. E., P. E. Gribben, C. Yeager, and E. E. Sotka. 2012. Impacts of an abundant introduced ecosystem engineer within mudflats of the southeastern US coast. Biological Invasions 14:2587-2600.

Krueger-Hadfield, S. A., N. M. Kollars, A. E. Strand, J. E. Byers, S. J. Shainker, R. Terada, T. W. Greig, M. Hammann, D. C. Murray, F. Weinberger, and E. E. Sotka. 2017. Genetic identification of source and likely vector of a widespread marine invader. Ecology and Evolution 7:4432-4447.

Thomsen, M. S., K. J. McGlathery, and A. C. Tyler. 2006. Macroalgal distribution patterns in a shallow, soft-bottom lagoon, with emphasis on the nonnative Gracilaria vermiculophylla and Coldium fragile. Estuaries and Coasts 29:465-473.

Thomsen, M. S., K. J. McGlathery, A. Schwarzschild, and B. R. Silliman. 2009. Distribution and ecological role of the non-native macroalga Gracilaria vermiculophylla in Virginia salt marshes. Biological Invasions 11:2303-2316.

Thomsen, M. S., T. Wernberg, P. Staehr, D. Krause-Jensen, N. Risgaard-Petersen, and B. R. Silliman. 2007. Alien macroalgae in Denmark – a broad-scale national perspective. Marine Biology Research 3:61-72.

Vitousek, P. M., C. M. D Antonio, L. L. Loope, and R. Westbrooks. 1996. Biological invasions as global environmental change. American Scientist 84:218-228.



One Fish, Two Fish, Red Fish, Killifish

Melanie Herrera, U. of Maryland, College Park

After 9 sampling days, 18 collections, and over 3000 fish, we’ve discovered fishes’ habitat preferences are much more complex than we thought. To recap, our hypothesis predicted fish would prefer dense sites of the invasive seaweed, Gracilaria vermiculophylla, over sites with more open water (thus, less Gracilaria).  We also predicted that dense site would have greater diversity by attracting various types of fish due to its branches that conceal fish from predators.

Our belief that Gracilaria would fulfill the refuge effect, attracting more fish and more diverse species, was supported through the copious amounts of fish found in Gracilaria. Despite more abundance in the dense sites of Gracilaria, more diversity was shown in sparse sites (Figure 1). Among both the dense and sparse sites Atlantic Silversides and Bay Anchovies, Pipefish, and Striped Killifish were the most abundant and common species. While similar species occurred in both habitats, the sparse site had more occurrences of species that were considered rare in dense sites. For example, sparse sites had more occurrences of Spade fish and Florida Pompanos than dense sites. Additionally, sparse sites had species of fish such as leatherjackets and lizardfish that never occurred in dense sites.

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Figure 1: Rank abundance patterns of fish in dense sites (represented by triangles) and sparse sites (represented by circles) of G. vermiculophylla at Grice Cove. The number of fishes were calculated as a logarithm as a measure of relative abundance of fish at each site. Species are ranked from most abundant (1) to least abundant (8-10). Slopes show differences in species evenness amongst sites. Steeper slopes exhibit less species evenness.


Supporting our hypothesis, dense sites did demonstrate more abundance. In total, 2944 fish were collected from the dense sites while 361 fish were caught in the sparse sites. It is predicted that smaller-bodied fish used Gracilaria more as a refuge because of their increased vulnerability to threats as small animals. Lack of abundance in sparse sites could be explained by increased exposure to predators and environmental threats.

Increased use of the dense sites shows Gracilaria does contribute towards housing all types of fish, most importantly economically important fishes. According to the National Marine Fisheries Service’s report on fisheries economic in 2011, the seafood industry alone brings in a minimum of $88 million dollars annually. In order to support this important industry, commercial fisheries can use our research to establish sustainable fisheries by understanding the various habitats that help rear economically important fishes. Our identification of the invasive seaweed’s role on housing fish can be used as a protective measure for these fish in future sustainable management.


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Figure 2: Two of the top three most abundant species collected from dense sites of Gracilaria. (Left: Striped Kilifish; Right: Atlantic Silversides).


Thank you so much to my mentors Dr. Tony Harold and Mary Ann McBrayer for their advice and guidance. This research is funded through the National Science Foundation and College of Charleston’s Grice Marine Lab.