The diet of larval Semotilus atromaculatus (Creek Chubs) was examined in fish collected from an urbanized stream with a limited food base. Chironomids comprised nearly 90% of food items. They appeared in the gut of early larvae and continued to be the main food source as size increased; cladocerans were the second most abundant food item. Both average and maximum prey size were examined. Overall, average prey size increases significantly with standard length (SL). Maximum prey size is gape-limited twice, from the early to mid-mesolarval stage and again in the late mesolarval stage. Significant differences were observed in the maximum size of chironomids ingested among fish of four size ranges, <8.0 mm SL, 8.0–9.6 mm SL, 9.7–14.1 mm SL, and >14.1 mm SL, indicating maximum prey size increases rapidly at 8.0 mm SL, 9.6 mm SL, and again at 14.1 mm SL, yet maximum prey size within each group remains constant. The degree of cranial ossification and fin development at these break points was examined with cleared and double-stained specimens. For fishes <8.0 mm SL, ossification is just commencing, and maximum prey size is gape limited. At 9.6 mm SL, ossification appeared nearly complete in the caudal fin and in key bones involved in food capture and processing. There was no obvious correlation between ossification of the skeleton and the third break point at 14.1 mm SL. The results from this study suggest that ossification of feeding apparatus and caudal fin may play an important role in the ability of Creek Chubs to capture larger prey in the mesolarval stage at about 9.6 mm SL, but other factors likely account for the increase in prey size at about 14 mm SL.
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