Factors affecting the distribution of two gobies (Microgobius gulosus, Gobiosoma robustum) in Florida Bay, U. S. A.

Pamela J. Schofield, University of Southern Mississippi
& U. S. Geological Survey, Gainesville, FL

Presented at the Joint Conference on the Science and Restoration of the Greater Everglades and Florida Bay Ecosystem, Palm Harbor, FL April 2003.
 

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Densities of the code goby Gobiosoma robustum Ginsburg and clown goby Microgobius gulosus (Girard) vary across regions of Florida Bay.  Although their distributions overlap to a limited degree, G. robustum is found in great abundance in the seagrass beds of the western and southern portion of the bay, where a strong Gulf of Mexico influence maintains salinities at a relatively constant level (35 – 40 ppt).  Conversely, M. gulosus is most common in the north-eastern section of the bay that is influenced by freshwater inflow and generally is characterized by lower salinities.  In this study, the effects of salinity, habitat complexity, competition, and susceptibility to predation were examined as possible factors underlying the distribution of these two species.

Effects of salinity and competition on growth:

In this study, the relative influences of salinity and competition on growth of both goby species was evaluated in a laboratory experiment.  Both species were grown in the lab over 27 days at two salinity levels (5 and 35 ppt), two food levels (low and high) and both with and without the presence of the other species. Both species exhibited greatest growth at the high food level and the low (5 ppt) salinity.  Neither species was affected by the presence of the other species, and there were no overall differences in growth between the two species.  Thus, the competitive superiority of G. robustum over M. gulosus does not seem to confer an advantage relative to feeding success.  Furthermore, as growth of G. robustum was greater at the lower salinity, it is clear that some factor other than salinity is restricting the bay-wide distribution of this species from north-eastern Florida Bay.

Mean residuals (+ 1 SE) from analysis of variance test showing growth of M. gulosus
 and G. robustum at salinities (5 and 35 ppt). Residuals were used rather than
 raw means to account for differences attributable to initial mass.
 

Acute salinity tolerance:

The premise of this study was to determine whether M. gulosus and G. robustum exhibited differential tolerance to acute salinity shifts, which may be related to their distributions within Florida Bay. Given that M. gulosus inhabits the north-eastern region of Florida Bay that is more likely to be exposed to rapid shifts in salinity than the less variable habitat over which G. robustum is distributed, it was hypothesized that M. gulosus would be more tolerant to salinity shifts. To test this hypothesis, acute (e.g., "plunge-type") salinity tolerance tests were performed with both species. There was no compelling evidence for differences in acute salinity tolerance over the 10-day experimental period, as both species showed significantly reduced survival at similar salinity levels.

Gobiosoma robustum

Density: 0.1 - 9,     > 10 Click graphics and images to enlarge

Microgobius gulosus

Competition and habitat selection:

In Florida Bay, G. robustum is generally found in structurally complex habitats (e.g., seagrass beds), while M. gulosus is more often found in bare mud areas.  To determine whether this habitat partitioning was an effect of interspecific competition, I conducted a series of laboratory experiments, in which each species was presented with structured (artificial seagrass) versus non-structured (bare sand) habitats and their frequency of choosing either habitat type was measured. Use of structured versus non-structured habitats were then examined when the two species were placed together in a mixed group.  Finally, a predator (Opsanus beta) was placed in the experimental aquaria to determine how its presence influenced habitat selection. Both goby species selected grass over sand in allopatry; however, in sympatry, M. gulosus occupied sand more often when paired with G. robustum than when alone.  Gobiosoma robustum appears to directly influence the habitat choice of M. gulosus:  It seems that M. gulosus is pushed out of the structured habitat that is the preferred habitat of G. robustum.  Thus, competition appears to modify habitat selection of these species when they occur in sympatry.  Additionally, the presence of the toadfish was a sufficient stimulus to provoke both M. gulosus and G. robustum to increase their selection for sand.
 

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Gulf toadfish, Opsanus beta,  a goby predator

Habitat effects on susceptibility to predation:

To determine whether the presence of a structurally complex habitat mitigates predation for either species, a laboratory experiment was conducted in which gobies (1 per trial) were placed in aquaria containing either a) bare mud or b) bare mud + artificial seagrass substrate. The predator (Opsanus beta) was more successful preying upon M. gulosus than G. robustum, and there was no habitat (e.g., seagrass) effect.  This was surprising, given that M. gulosus is a burrowing species that was expected to fare better than the non-burrowing G. robustum.
 

CONCLUSIONS:

Based on the results of these experiments, the microhabitat distribution of G. robustum and M. gulosus in the Gulf-influenced (e.g., southern and western) regions of Florida Bay can be explained by the selection of G. robustum for structurally complex habitats (e.g., seagrass) and its competitive superiority over M. gulosus.  However, the mechanisms underlying the distribution of these species in north-eastern Florida Bay is less clear.  G. robustum was as tolerant to acute shifts in salinity as M. gulosus.  Additionally, growth of G. robustum was not negatively affected by low salinities indicative of this region. Finally, G. robustum was less susceptible to predation than M. gulosus by O. beta, even in the absence of seagrass. Restriction of water movements by the anastomosing pattern of mudbanks in north-eastern Florida Bay may negatively affect larval recruitment of G. robustum.  Alternately, it is possible that prey species utilized by G. robustum are not as abundant in the less-vegetated north-eastern Florida Bay as opposed to southern and western regions of Florida Bay.
 

This poster summarizes a multi-year research program that would not have been possible without the support of numerous individuals, including: USM: Stephen T. Ross, Mark Peterson; NOAA: Gordon Thayer, Allyn Powell; USGS: George D. Dennis, Nicholas Funicelli, Mike Robblee, Ken Sulak, Susan Walls, Stephen J. Walsh, George Yeargin and numerous others.  Buck Albert took the lab photographs. Allyn Powell (NOAA) provided the maps of Florida Bay. Clinton Hittle (USGS) and Kevin Kotun (NPS) provided salinity data.