My students and I combine manipulative field experiments with modern laboratory techniques to address questions related to the ecology of sessile marine invertebrates. Combining a field-based approach with laboratory methods allows us to address problems at multiple scales of organization ranging from physiological to population level processes.

The following represent abstracts of projects that I am currently engaged in or have recently completed:


Dynamics of benthic marine invertebrate communities on temperate offshore reefs of Georgia
Daniel F. Gleason
Principal Investigator

            Subtidal temperate reefs have received substantially less attention from the scientific community than tropical coral reefs. This is somewhat surprising considering that they may be no less important in promoting species diversity and enhancing production. Similar to their tropical counterparts, however, temperate reefs are in peril. Primary threats include commercial trawling, recreational fishing, ocean acidification, and increased sedimentation. Clearly, understanding the basic biology of these systems, including the ecological processes that shape the benthic community is of importance. I propose to investigate the community dynamics of sessile marine invertebrates occupying hard substrata on offshore reefs of Georgia, including those in Gray’s Reef National Marine Sanctuary (GRNMS). The specific objectives are:

A) To determine if succession in sessile benthic invertebrate populations on subtidal hard bottom reefs progresses by a predictable sequence of species replacements and culminates in a single climax community.

B) To determine if established communities of sessile benthic invertebrates on individual subtidal hard bottom reefs vary both spatially and temporally.  

C) To determine if sedimentation is a structuring force in the development and maintenance of subtidal hard bottom reef communities of coastal Georgia.

The first objective will be investigated by following succession on 30x30 cm plots that have been cleared of all sessile benthic invertebrates. The power to separate seasonal patterns of establishment and disappearance from longer term trends will be high because the proposed investigation builds on a study that has accumulated more than 2.5 years of similar data. The second objective will be addressed at two hard bottom sites by quantifying sessile benthic invertebrate populations 3 times each year in 0.25 m2 quadrats placed adjacent to permanent 30 m long transects. Three transect lines will be established at each site and will be deployed to encompass the gradations in community composition that appear to occur across rocky ledges off Georgia. Finally, the role that sediment plays in controlling succession and in contributing to distributional patterns observed along hard bottom ledges will be addressed correlatively by documenting variability in the rates of downward flux of suspended particulate matter. 

Completing the above objectives has practical implications for managing GRNMS and addresses Sea Grant’s high priority research areas for understanding biological processes in key ecosystems and habitats, understanding larval recruitment processes, and understanding the delivery and distribution of sediment in coastal ecosystems. Additionally, accomplishing all three of the proposed goals will provide a baseline for identifying future changes to essential fish habitat and so should be invaluable as a tool for managing offshore reefs.


An Interactive Web-based Guide to the Benthic Invertebrates and Cryptic Fishes of Gray’s Reef

Daniel F. Gleason, Alan W. Harvey, and Stephen P. Vives

Co-Principal Investigators

            Considerable effort has been expended in the study of the benthic invertebrate fauna of Gray’s Reef National Marine Sanctuary (GRNMS) and other southeastern hardbottom communities. However, the resulting information exists primarily in scattered gray literature and is difficult to access. Likewise, although Milton Gray and his associates made extensive collections of invertebrates from the area, much of the supporting data, or in some cases the specimens themselves, are apparently missing or at least not readily available. For all practical purposes, then, the invertebrate fauna of GRNMS is poorly known, and identification of specimens collected from GRNMS and other southeastern hardbottom communities can be a difficult task.  Less effort has been directed toward understanding the benthic cryptic fish community of GRNMS.  Census methods in current use at GRNMS (i.e. video transects, diver transects and point counts) are known to under estimate this component of the fish fauna.  Species composition and temporal dynamics of these fishes is, therefore, poorly known.

We propose to develop new resources that will facilitate the identification of benthic invertebrates and cryptic fishes in GRNMS, consolidate literature associated with those species, and monitor seasonal changes in the invertebrate fauna. Our ultimate goal is to construct comprehensive and illustrated identification keys, usable by both specialists and nonspecialists, for the GRNMS benthic invertebrate and cryptic fish fauna. This information will be made available in a web site for Gray’s Reef invertebrates. Ultimately, this web site will include an annotated, searchable taxonomic list; interactive, illustrated identification keys; and individual species pages, with photographs and basic biological information. In addition, the site will provide researchers working at GRNMS with a data entry form that will allow information, such as taxonomic revisions and new records, to be updated.


Dispersal and recruitment of benthic marine invertebrates at Gray’s Reef National Marine Sanctuary

Daniel F. Gleason

Principal Investigator


Most sessile marine species have a pelagic larval stage that results from spawning of eggs and sperm into the water column followed by fertilization, or release of fully mature brooded larvae (Pechenik 1999). Benthic marine populations are generally considered demographically open with new individuals coming mainly or exclusively from this supply of dispersing planktonic larvae (for review see Caley et al. 1996). Recruitment of these pelagic larvae back to the benthos is dependent on processes that function at multiple spatial scales and that act both prior to and after settlement. In the study outlined here, I propose to investigate questions related to both recruitment and larval dispersal in sessile, benthic marine invertebrates of Gray’s Reef National Marine Sanctuary (GRNMS). The specific goals of this project are two-fold: 1) to follow recruitment of benthic marine invertebrates to newly established patches of bare reef substrata and document changes in patch community structure over time and 2) to determine if recruitment of benthic marine invertebrates to GRNMS is primarily the end product of larval dispersal from adjacent, but physically separated hard bottom communities outside the sanctuary, or the result of recruitment of propagules produced within the sanctuary. The first objective will be addressed by following reestablishment of cleared plots on both flat top ledges and scarps. The second will use allozyme markers to assess population genetic structure and infer dispersal patterns in adult populations of the temperate coral Oculina arbuscula. This project has practical implications for management of the GRNMS because it will provide baseline information that can be used to assess what short and long-term impacts might result from user-induced damage to the benthos.