Enhanced gut fullness and an apparent shift in size selectivity by radiated shanny (Ulvaria subbifurcata) larvae in response to increased turbulence

We studied the relationship between microscale turbulence and feeding success of larval radiated shanny (Ulvaria subbifurcata) in Conception Bay, Newfoundland, during a 3-week period in July-August 1995. Although previous studies had suggested that the relationship between turbulent velocity and larval feeding rates should be dome shaped, we found no evidence of such a functional relationship. Rather, differences in larval feeding success were evident only when days were grouped as either "high turbulence" or "low turbulence" on the basis of Richardson number. Feeding conditions (i.e., prey concentration and composition) were not significantly different on high- versus low-turbulence days. Nonetheless, U. subbifurcata larvae (3-14 mm standard length) contained significantly fewer items in their guts on high-turbulence days. These prey items, however, were (on average) significantly larger than those found in guts on low-turbulence days; the net result was that significantly greater volumes of food were found in larval guts on high-turbulence days. Turbulent velocity did not affect between-day variation in RNA:DNA ratios of the larvae. We suggest that what appears to be a shift in size selectivity by U. subbifurcata larvae under increased turbulence may result from larvae having a higher probability of capturing large prey under increasingly turbulent conditions.