INTRAMOLLUSCAN DEVELOPMENT OF THE DIGENEAN ECHINOSTOMA-PARAENSEI - RAPID PRODUCTION OF A UNIQUE MOTHER REDIA THAT ADVERSELY AFFECTS DEVELOPMENT OF CONSPECIFIC PARASITES

Investigations of the early intramolluscan development of Echinostoma paraensei in juvenile M-line snails, Biomphalaria glabrata, have revea led the presence of a previously uncharacterized stage, a morphologica lly and behaviorally specialized mother redia, here termed the precoci ous mother redia (PMR). The PMR, which is produced in a sporocyst atta ched to the inner surface of the host's ventricle, develops before oth er mother rediae and is released as early as 6 days post-exposure (DPE ). Unlike all other mother rediae later produced by the sporocyst, it attaches to the ventricle wall adjacent to the sporocyst and remains t here for at least 31 days. It develops a long, sinuous body and a phar ynx approximately twice as large as that of any other mother or daught er redia present. Developing rapidly, the PMR releases daughter rediae at 10 DPE, about 2 days before other mother rediae. When snails conta ining a single intraventricular sporocyst were challenged by exposure to additional miracidia, fewer challenge sporocysts were observed subs equently in the ventricle than in control snails; this effect was grea ter if the PMR of the original infection had emerged and was present i n the ventricle. The proportion of challenge sporocysts that reached t he ventricle but then subsequently disappeared or failed to develop al so increased significantly in snails in which a PMR was present. These results suggest that the PMR is specialized to protect the sporocyst and that it is capable of adversely affecting the development of chall enge sporocysts. This mode of development was not observed in two othe r echinostome species (Echinostoma liei and E. trivolvis) examined.