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Field and Laboratory Guide to the Seaweeds of Gray's Reef
Prepared by:
Richard B. Searles
Duke University, Department of Botany
Durham, North Carolina 27706
ABSTRACT
This field guide to the seaweeds of Gray's Reef is intended to help visitors and researchers identify the macroscopic or multicellular, benthic (attached) plants living in or near the Sanctuary. These seaweeds belong to four divisions (major groupings) of plants, the Cyanophyta (Cyanobacteria or blue-green algae), the Chlorophyta (green algae), the Phaeophyta (brown algae), and the Rhodophyta (red algae). This flora of the sanctuary was prepared by SCUBA diving to make collections during the winter, spring and summer growing seasons. Sixty-eight species were collected. Dudresnava georgiana and some of the plants described here, but not placed in a particular species are endemic (known only from this limited area). Many, such as Codium isthmocladum. Sargassum filipendula. and Botryocladia occidentalis are common species of the warm-temperate biogeographic region which lies along the southeast coast of the United States. Identification of the species in the sanctuary raises interesting questions about their life histories, ecology, and biogeography and some of these questions are included in the comments following each species description.
STUDY DESCRIPTION
Gray's Reef National Marine Sanctuary is an area of the seafloor off the Georgia coast where the rock of the continental shelf emerges through the overlying sandy sediment in a series of small ledges and flat rock platforms approximately 60 feet below the sea surface. Four different ledges were routinely sampled in preparing this guide. In addition, rock outcrops further offshore, known locally as "The Snapper Banks" were sampled less thoroughly.
This is a precarious habitat for seaweeds. Shifting sediments may cover the rock on which the seaweeds must grow; suspended sediments can obscure much of the light required for their growth and temperatures oscillate with the seasons. In addition to these physical restrictions, there is competition for space with other seaweeds and attached marine invertebrates and grazing by herbivorous animals. In this environment an interesting flora develops in the Sanctuary each year.
During the winter it is a community almost devoid of visible plants. In late winter a few perennial plants such as Sargassum filipendula, Botryocladia occidentalis and Rhodymenia pseudopalmata begin regrowth from the remains of the plants which died back the previous fall, but a March collection yielded only seven species. By June there are scattered annual algae in evidence and over twenty species. In July and early August there is an abundance of seaweeds (more than 60 species) growing along the ledges, emerging though light sand cover on the flat rock surfaces behind the ledges, and growing attached to larger shell and coral fragments. In late August and September most of these plants die back and disappear for the winter. September collections totaled only ten species.
Considering the small size of the area studied, it has a rich flora. Three species were collected only from the Snapper Banks, which in the limited collections made there did not appear to be nearly as rich in seaweeds as the Sanctuary. The remaining sixty-five species all occurred in the Sanctuary. Two species have already been describe as new from the Sanctuary, Dudresnaya georgiana Searles (1985) and Dudresnaya puertoricensis Searles and Ballantine (1986). Eight species are included for which no previously description has been located. Some of these will probably have to be published as new species. Two, Lejolesia sp. and Cladophora sp. are most similar to very distant species in Japan (Lejolesia pacifica Itono) and southern Australia ( Cladophora bainesii van den Hoek), raising interesting questions about the evolution and biogeography of seaweeds.
The plants are part of a warm-temperate flora of about 300 species which occupies the seacoast of the United States from Cape Canaveral in Florida to Cape Hatteras North Carolina, (see Searles, 1984). A few of the species are endemic to this region, Derbesia turbinata and Giffordia onslowensis for example. Many of the species also occur further south in the Caribbean and in many cases still further south to Brasil. Some appear to be cosmopolitan tropical, or warm-temperate species, although as our understanding of the systematics of the seaweeds improves we will probably recognize regional varieties and populations which are distinct from other populations of these widespread species.
There is much we have yet to learn about these plants. The life histories of several of the species in the Sanctuary are incompletely understood; theca tetrasporophyte phases of Dudresnaya georgiana and D. puertoricensis are unknown and the isomorphic tetrasporophyte of D. crassa. which is known from North Carolina and Bermuda, has not been collected here. The gametophyte phases of Derbesia turbinata and Derbesia sp. have never been observed and in Derbesia marina the gametophyte phase has been observed elsewhere, but not south of New England along this coast.
Hopefully, biologists will be stimulated to pursue algal research here through use of this guide and by visits to the Sanctuary. Further research will help us better understand these seaweeds, their relationships to plants in adjacent parts of the Atlantic, their evolutionary history, and their contribution to the animal populations which depend on them as food and habitat.
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