Glochidia of the freshwater mussel Lampsilis overwintering on fish hosts

Largemouth Bass were infected with glochidia of the freshwater mussel Lamps ilis cardium. Three fishes each were held at 4.5, 10, and 15.5 degrees C; f ive fish were held at 21 degrees C. By 64 days, metamorphosed juveniles wer e found in the 15.5 and 21 degrees C trials but not in the 5.5 and 10 degre es C trials, indicating that the lower threshold temperature for metamorpho sis was between 10 and 15.5 degrees C for the duration. In a second experim ent, Largemouth Bass were infected with glochidia of L. cardium and held at 10 degrees C. A sample of fishes was removed monthly and brought to 21 deg rees C. Numbers of glochidia that metamorphosed after being warmed were com pared to the number that metamorphosed without warming. The percentage that metamorphosed after warming decreased linearly with time. At one month, 10 0% of the glochidia metamorphosed after warming. This decreased to 80% by t wo months, to 30% by four months and 3% by six months. Although this post-w arming percentage decreased with time, the total percentage of metamorphose d juveniles (at all temperatures) was not correlated with time. Controls ke pt at 21 degrees C required three weeks to reach peak metamorphosis, but te st subjects subjected to 10 degrees C required less than nine days to metam orphose once warmed. Many overwintering glochidia therefore complete a port ion of their development on the host at winter temperatures, but stop short of excystment. Some glochidia metamorphosed without being warmed, but this phenomenon is not understood. This study confirms that glochidia may overw inter on hosts, with some glochidia persisting for more than six months bef ore metamorphosing when warmer conditions return.