Sudden appearance of cysts and ellobiopsid parasites on zooplankton in a Michigan lake: a potential explanation of tumor-like anomalies

Cysts on calanoid and cyclopoid copepods, previously reported as tumor-like anomalies (TLAs) in Lake Michigan and Europe, appeared briefly in Patterso n Lake, a small Michigan inland lake. Cysts were rare (4% maximum) in sampl es collected on September 11, 1999, but appeared with high frequency on cal anoid adults (49%) and cyclopoid nauplii (73%) in samples collected on Octo ber 16. By October 30, cysts were again rare (0.4% maximum). Cysts most com monly appeared on the lateral surface of the animal at the articulation of the 1(st) and 2(nd) prosomal segments and often consisted of herniated cope pod tissues. Transparent, pyriform cysts co-occurred in low frequency with other types of cysts and are believed to be the trophomeres and gonomeres o f ellobiopsid parasites. Histologic manifestations of cysts were diverse; h erniations consisted of acellular yolk-like material and apparent host tiss ue, while cysts thought to be Ellobiopsis contained cells with different de grees of nuclear staining and unusual spherical bodies. Hernias were experi mentally induced on live calanoid copepods by piercing the carapace with a fine needle, suggesting that ellobiopsid parasites may cause the hernias by puncturing the carapace of their hosts. Ellobiopsid parasites are common o n marine crustacean zooplankton but have been recorded only once before in freshwater.