Detection of an ontogenetic diet shift in the naked goby, Gobiosoma bosc

Background/Question/Methods

Ontogenetic shifts in diet allow organisms to maximize energy conservation, presumably by reducing the time spent foraging or increasing net energy intake. Diet shifts are especially significant when individuals switch between prey types of ecologically distinct size classes, such as when gobies and flatfishes switch from consumption of meiofauna to consumption of macrofauna. The goals of this study were to describe the diet of Gobiosoma bosc, examine if there is a threshold body size at which the diet shifts from dominance of meiofauna to dominance of macrofauna, and examine if the diet shift is gradual or rapid. Gobiosoma bosc were collected from oyster reefs in the Charleston Harbor estuary by examination of removable oyster shell and seine. To investigate diet composition, the digestive tract was removed from G. bosc individuals and prey types were identified to the lowest taxonomic level possible, measured, and enumerated. A comparison between predator standard length and portion of macrofauna in the diet was made to determine the threshold body size and rate of a potential diet shift.

Results/Conclusions

Stomach content analysis reveals G. bosc are consuming primarily harpacticoid copepods and ostracods from the meiofauna, and amphipods and polychaetes from the macrofauna. Consumption of macrofauna begins at a predator standard length of 20 mm, and significant consumption of meiofauna is not present in individuals above 30 mm standard length. These results suggest a meiofauna to macrofauna diet shift is occurring at an intermediate rate, and proportion of macrofauna in the diet increases non-linearly as predator standard length increases. Many described ontogenetic diet shifts cite changes in habitat or external environmental conditions as a driving mechanism. As juvenile and adult G. bosc frequent the same habitat, our results are ecologically intriguing because they suggest that significant changes in morphology or behavior may be driving a dietary transition. Specifically, small individuals (<20 mm) are most likely feeding on meiofauna in the interstices of soft sediments through a substrate sieving behavior. Large individuals (>30 mm) are consuming more epifaunal organisms, presumably by visual predation.