Infection, infestation, and disease: differential impacts of Labyrinthula sp. on the seagrass Thalassia testudinum (Banks ex König) in Florida Bay, USA
Barbara A. Blakesley , Donna M. Berns, and Margaret O. Hall
Florida Marine Research Institute, Florida Fish and Wildlife Conservation
Commission, St. Petersburg, FL
In the spring of 1995, concern over past acute seagrass die-offs of Thalassia testudinum in Florida Bay and continuing chronic seagrass losses there prompted the initiation of a large-scale monitoring effort of seagrass health. This study was done in conjunction with the Fisheries Habitat and Assessment Program (FHAP).
Our work in Florida Bay from 1995 to 1999 on Labyrinthula sp.
represents the first studies of the occurrence and distribution of this pathogenic slime net in the subtropical seagrass T. testudinum and its role in chronic and acute disease.
The field data focused our attention on the need to establish the role of
Labyrinthula in both acute and chronic seagrass disease and mortality. Evaluation of that data, together with the results from concurrent laboratory studies of infection in both wild and cultured Thalassia , by Labyrinthula sp. provided insights on the impacts of the slime net in this seagrass in Florida Bay. Results from these studies first established the relationship between infection of this seagrass by the pathogen and the presence of necrotic lesions associated with disease. Another acute Thalassia die-off that occurred during the course of our study allowed us to determine that Labyrinthula sp. was not the primary pathogen involved in this acute event.
Historical parallels to this study include the impacts of infection by L. zosterae in the temperate eelgrass Zostera marina that were documented after massive seagrass die-offs along the coasts of northern Europe and North America in the
1930’s and 1940’s. A recurrence of this “wasting disease” in New England in the
1980’s renewed interest in the pathogen and it’s affects on seagrass. More recently, studies of L. zosterae in Z. marina in France raised questions as to what constitutes an infestation and what is truly a “disease” (Hily et al. 2002). The authors of that study found infection at all of their study sites and “infestation” during discrete time periods, but never detected adverse effects on seagrass beds such as changes in occurrence or distribution. It has been suggested that “disease” only occurs when the youngest blades on a short shoot become lesioned (Vergeer and den Hartog 1994, Hily et al. 2002).
Results from our study showed that increases in the occurrence and distribution of
Labyrinthula sp. infection and associated lesions in Thalassia correspond with

decreases in abundance and changes in the distribution of this seagrass. Baywide, the percent of microscopically verified infected Thalassia shoots increased 10fold over the first 4 years of the study, while lesion prevalence and severity doubled. During this same time period, Thalassia cover abundance and short shoot density increased slightly baywide, but substantial localized Thalassia losses occurred in western and central Florida Bay.
We attributed localized chronic Thalassia losses to differential impacts of
Labyrinthula sp. on its host. We propose that there may be a “threshold of disease” below which infected seagrass beds are not adversely affected (i.e. significant seagrass losses do not occur). Generally, we defined this “threshold of disease” in Florida Bay
Thalassia as infection of approximately one quarter of the examined short shoots at a particular site.
When a small percentage of shoots show Labyrinthula presence, they are merely “infected” as in Eagle, Calusa,
Crane, and Madeira Basins (Table 1). “Infested” seagrass beds with slightly higher infection rates may be thinned, as has been observed in Blackwater, Twin, and Whipray Basins (Table 1). If the threshold is exceeded, the seagrass beds cannot outgrow the infection and chronic losses occur as in Rabbit, Johnson and
Rankin Lake Basins (Table 1) and a true disease condition is present. We have also generally found a positive linear relationship between blade age and infection, with older blades being more severely lesioned than younger ones.
However, when “infestation” and “disease” occur, the proportion of severely lesioned younger blades increases, with significantly greater lesion severity in
“diseased” plants.
The “threshold of disease” can theoretically be shifted up or down depending on the ambient environmental conditions at the infection site. Some factors likely to influence the threshold are salinity, temperature, pathogen virulence, and host resistance. In basins where severe seagrass losses have already occurred from acute die-off (Rabbit, Johnson, Rankin Lake), Labyrinthula infection affects many of the shoots in the basin (Table 1) and causes seagrass mortality either directly or in conjunction with other stressors such as elevated sediment sulfide levels. We propose that, at these high infection levels, Labyrinthula can cause dramatic seagrass losses and may lead to the succession of Thalassia by Halodule . In
Calusa, Crane, and Eagle Key Basins, infection levels remain low (Table 1), most likely due to relatively low-density seagrass beds. In basins where the average salinity periodically drops below 15‰ (Eagle and Madeira), infection levels remain low with occasional spikes in infection and lesion presence coinciding with, or following, elevated salinities. Usually, lesion severity in these cases remains low; this may be due to differences in virulence in different Labyrinthula species or strains. In Madeira Basin, seagrass densities are high enough to sustain

Labyrinthula infection and transmission, but periodic low salinities control infection.
Basin
Eagle
Calusa
Crane
Madeira
Blackwater
Twin
Whipray
Rabbit
Johnson
Rankin
Range of %
Shoots Infected
0-3
0-13
0-18
0-22
0.9-22
0.7-28
0-29
4-42
4-61
1-59
Average %
Shoots Infected
“Infected” 1
4
4
7
“Infested” 9
11
14
“Diseased” 24
27
35
Average %
Lesion Severity
0.1
0.3
0.2
0.5
0.9
0.8
1.1
1.7
2.0
2.5
Table 1 . The range and average percent of shoots microscopically verified as infected, and the average percent lesion severity of all blades in the 10 study basins (1995-1999).
Several interacting factors, both biotic and abiotic, were identified which influenced the occurrence and distribution of Labyrinthula sp. in Thalassia within the Bay. These same factors, along with other as yet unidentified factors, influence the degree of impact of Labyrinthula sp. on its host T. testudinum .
These impacts, both “beneficial” and detrimental, affect not only the spatial and temporal occurrence and distribution of Thalassia in Florida Bay, but also the overall health of these vitally important seagrass beds in one of the most extensive submarine seagrass meadows in the world.
Literature Cited
Hily C, Raffin C, Brun A, den Hartog C, 2002. Spatio-temporal variability of wasting disease symptoms in eelgrass meadows of Brittany (France).
Aqat. Bot. 72: 37-53.
Vergeer LHT, den Hartog C, 1994. Omnipresence of Labyrinthulaceae in seagrasses. Aquat Bot 48: 1-20
Blakesley, Barbara A., Florida Marine Research Institute, Florida Fish and
Wildlife Conservation Commission, 100 8 th
Ave. S.E., St. Petersburg, FL
33701, Phone: 727-896-8626 X4221, Fax: 727-823-0166, Email: barb.blakesley@fwc.state.fl.us