Effects of temperature and photoperiod on lure display and glochidial release in a freshwater mussel
Freshwater mussels use an array of strategies to transfer their parasitic larvae (glochidia) to fish hosts. We examined the effects of temperature, photoperiod, and female gravidity on mantle lure display and conglutinate release by Ligumia subrostrata (Say, 1831) in 2 laboratory experiments. In the 1st experiment, we examined the use of these strategies in 4 temperature treatments (5, 15, 25, 35°C) and 3 photoperiods (10:14, 12:12, and 14:10 h light: dark). In the 2nd experiment, we observed infection strategies under ambient conditions with flow-through pond water. Water temperature appeared to be the primary cue governing use of these strategies. Lure display occurred over a protracted period, but the highest display frequency occurred between ,11 and 20°C. Lure display declined rapidly above this range and ceased altogether .28°C. Release of conglutinates increased coincident with the decrease in lure display but, at ambient temperatures, occurred over a protracted period similar in duration to lure display. Females that were not gravid at the beginning of the experiment did not display lures, and gravid females whose gills were flushed of glochidia displayed for only a short period during which frequency of display was much lower than gravid individuals. Ligumia subrostrata exhibits a temperature-mediated switch between alternate host strategies. We present evidence that lure display is a primary strategy for host infection and conglutinate release is a secondary, bet-hedging strategy to reduce wastage of glochidia that ultimately must be cleared from the gills at the end of the reproductive season.