Factors affecting the distribution and densities of two gobies (Microgobius gulosus and Gobiosoma robustum) in a lagoonal estuarine system (Florida Bay, United States)
Densities of the code goby Gobiosoma robustum and clown goby Microgobius gulosus 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. Conversely, M. gulosus is most common in the northeastern section of the bay that is influenced by freshwater inflow and generally is characterized by low salinities. I examined the effects of salinity, habitat complexity, competition, and susceptibility to predation as possible factors affecting the distribution of these two species. The two species are strikingly similar: Both had similar abilities to withstand acute shifts in salinity, and both exhibited greater growth rates at low salinities. Gobiosoma robustum is competitively superior and can displace M. gulosus from a preferred habitat (seagrass). However, although competition modifies habitat selection in syntopy, it does not affect either growth or predation rates. 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 fresh-water influenced northeastern Florida Bay are less clear. Both species were equally tolerant of the chronic low salinity and rapid salinity variability characteristic of northeastern Florida Bay. Additionally, G. robustum was less susceptible to predation than M. gulosus by Gulf toadfish (Opsanus beta ), even in the absence of seagrass. Restriction of water movements by the anastomosing pattern of mudbanks in northeastern Florida Bay may negatively affect larval recruitment of G. robustum . Alternately, prey species utilized by G. robustum may not be as abundant in the less-vegetated northeastern Florida Bay as opposed to southern and western regions of Florida Bay. Further research on the larval ecology and physiology of these species may lend insight into patterns structuring their abundances.