Grazing and predation of the grass shrimp Palaemonetes pugio

The grass shrimp Palaemonetes pugio did not significantly consume live sea grass (Halodule wrightii), but assimilation of the attached epiphytes was quite significant. Salinity changes from 11-33%. had no effect on the amount of epiphytes assimilated but increasing the temperature from$14^\circ-30^\circC$exponentially increased the epiphyte consumption rate. The assimilation rate per unit body weight declined significantly for shrimp ranging from 6 to 54 mg dry weight (11-24-mm total length). Total assimilation of epiphytes increased with shrimp size up to 25-30 mg, after which it leveled off or declined. Assimilation efficiencies averaged about 83% and were not affected by changes in temperature and salinity. Small shrimp (11-15 mm) did not prey on mysids but larger ones (19-22 mm) did. The maximum consumption rate was six mysids per shrimp per day and was attained at a mysid density of about 400· m-2. Halodule without epiphytes did not provide a refuge for this mysids, but the addition of epiphytes as an alternative food significantly reduced the predation pressure.