Folly Beach Groin

While South Carolina lacks natural rocky intertidal zones, there are artificial rock structures that have developed communities similar to those found on natural zones.

Current Site

The most recent survey was completed on a groin at the north end of Folly Beach in Jan., 2012. Groins on Folly Beach were built in the 1940s. This is a different site from the historical site due to the inaccessibility of the original site. A qualitative survey was completed for an hour during low tide and photos were taken of the zonation pattern. Invertebrate specimens were taken back to Grice Marine Laboratory where they were identified down to species if possible.

Coordinates: 32.684098° N, 79.88657° W

Historical Site

In 1952, Stephenson and Stephenson published a paper on surveys of rocky intertidal zones between Beaufort, NC and northern FL. This paper included the Charleston area, specifically the north and south jetties off Morris Island and Sullivan's Island built in 1882-1895. These jetties were surveyed in Apr. 1947. Area conditions (salinity, air temperature, tidal range, etc.) were noted. One distinguishing feature was the presence of oil on rock surfaces, which was not seen in the current survey. The Stephensons conducted a qualitative survey of the flora and fauna and described the zonation pattern.

Coordinates: 32.749129° N, 79.863138° W

Stephenson and Stephenson 1952

Species Comparisons

The number of species per phylum has not significantly changed between the two surveys. However, the species found in each phylum has changed.

The following organisms were described in both surveys:

- Chthalamus fragilis (barnacle)

- Balanus eburneus (ivory barnacle)

- Bunodosoma cavernata (warty sea anenome)

- Brachiodontes exustus (scorched mussel)

- Crassostrea virginica (Eastern oyster)

- Anguinella palmata (bryozoan)

The following organisms were found in 1947:

- Tetraclita squamosa stalactifera (intertidal barnacle)

- Balanus improvisus (bay barnacle)

- Modiolus demissus (ribbed mussel)

- Asterias forbesi (common sea star)

- Tubularia crocea (hydroid)

- Hymeniacidon heliophila (sponge)

The following organisms were found in 2012:

- Balanus amphitrite (striped barnacle)

- Sphaeroma quadridentata (isopod)

- Echinolittorina placida (interrupted periwinkle)

- Ilyanassa obsoleta (Eastern mud snail)

- Luidia clathrata (gray sea star)

Many of the differences in species can only be attributed to differences in locations, collection methods, and the timing of surveys.  Small deviation in wave action, depending on how sheltered the habitat is, can strongly influence the species that can live there. Despite differences in species, the relative make-up of the communities by phyla was consistent- both were dominated by arthropods and molluscs. 

There were two species collected in 2012 and not in 1947- Balanus amphitrite (striped barnacle) and Echinolittorina placida (interrupted periwinkle)-that can be attributed to invasions.  The shipping industry and man-made structures, such as jetties and docks, have allowed species to expand out of their native range and into new habitat.  The interrupted periwinkle can only crawl and feed on hard substrate.  Humans have built numerous hard structures throughout the otherwise soft-bottom coast of the Gulf of Mexico, allowing the snail to invade from its native along the Yucatan Peninsula into both the Gulf and around Florida into the Atlantic.  As seen in the figure below, this invasion was thought to have occurred after the 1947 survey (Reid, 1956).   

Intertidal zonation patterns are driven by organisms' ability to tolerate pounding waves, exposure to both air and water, and frequently changing conditions.  All of these conditions change based on just how far the habitat is from the water throughout the tidal cycle, making comparisons between two locations difficult.  Nevertheless, we can look at the general patterns from the historical survey and current site to see if anything has changed.

The most distinct feature of both surveys was the presence of the "Chthalamus zone." (a) Abundant at both sites, these barnacles sit highest on the rocks and are exposed to abundant sunshine and desiccation stress.  Below that things get a little more confusing.  Just below or overlapping with Cthamalus, is the acorn barnacle, Balanus improvisus (b).  The central and lower midlittoral zone in the historical survey is characterized by oysters and mussels (c), as well as the red alga, Porphyra, and green alga, Entermorpha (d).  Aside from Cthamalus, every species was noted in all zones.

The present survey had fewer species that were abundant.  The zonation pattern was driven by the "Chthalamus zone" (a), above the oyster zone (c), with the alga Entemorpha only at the lowest part of the rocks.  All other species were found irregularly throughout all three zones.

This study was completed by Leslie Wickes and Robin Garcia for Invertebrate Zoology (BIOL 630 - Spring 2012)

References

Stephenson, T.A. and Stephenson, A. 1952. Life between tide-marks in North America. II: Northern Florida and the Carolinas. J. Ecol. 40(1): 1-49.

Reid, D.G. 2009. The genus Echinolittorina Habe, 1956 (Gastropoda: Littorinidae) in the western Atlantic Ocean. Zootaxa. 2184: 1-103.