Factors Affecting the Distribution of Two Gobies (Microgobius gulosus,
Gobiosoma robustum) in Florida Bay, U. S. A.
Pamela J. Schofield*
University of Southern Mississippi, Hattiesburg, MS
*current address: U. S. Geological Survey, Gainesville, FL
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 characterised 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.
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 northeastern 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-d experimental period, as both species showed
significantly reduced survival at similar salinity levels.
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.
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 d 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.
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.
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 utilised
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.
Pamela J. Schofield, U. S. Geological Survey, 7920 NW 71st Street, Gainesville,
FL 32653, Phone: (352) 378.8181, Pam_Schofield@usgs.gov, Question 5