Factors influencing cercarial emergence and settlement in the digenean trematode Parorchis acanthus (Philophthalmidae)

• Published in: Journal of the Marine Biological Association of the United Kingdom Vol. 91; no. 8; pp. 1673 - 1679
• Main Authors: Prinz, K., Kelly, T.C., O'Riordan, R.M., Culloty, S.C.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 01.12.2011
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Aquatic birds; Biological and medical sciences; Biotic factors; Bivalvia; Cysts; Experiments; Fundamental and applied biological sciences. Psychology; Gastropoda; Influence; Inhalation; Invertebrates; Laboratories; Marine; Mollusks; Mytilus edulis; Nemathelminthia. Plathelmintha; Nucella lapillus; Parorchis acanthus; Philophthalmidae; Sea water ecosystems; Seawater; Semibalanus balanoides; Shellfish; Synecology; Trematoda; Valves; Ireland; metacercariae; transmission; encystment; Mytilus edulis; Nucella lapillus; cercariae; filtration current; Filtration; Transmission; Plathelmintha; Gastropoda; Trematoda; Marine environment; Metacercaria; Bivalvia; Emergence; Population recruitment; Helmintha; Invertebrata; Mollusca
• ISSN: 0025-3154; 1469-7769
• DOI: 10.1017/S0025315410000718

The trematode Parorchis acanthus is widely distributed and exhibits a broad host spectrum, with larval stages occurring in marine gastropods and adult flukes parasitizing on various seabird species. Unlike the majority of digeneans, its cercariae do not invade a second intermediate host but encyst on a substrate in the open. In a series of laboratory experiments, we investigated factors influencing cercarial emergence and settlement of P. acanthus. Emergence of cercariae from the gastropod intermediate host Nucella lapillus increased with increasing temperature, whereas cercarial encystment success decreased. Numbers of emerged cercariae were highest in the presence of light and water. However, although in much lower numbers, cercariae were also released from N. lapillus and successfully encysted under relatively dry conditions. Cercariae were found to settle predominantly on filtering blue mussels Mytilus edulis, suggesting that cercarial concentration on the valves is largely facilitated by the bivalve's inhalation current. After being inhaled, cercariae actively emerged from the mussels using their suckers and subsequently encysted on the shells, preferably along the rim of the valves. Barnacle epibionts Semibalanus balanoides were observed to impair cercarial settlement on mussels, by disturbing or preying on cercariae. Whilst settlement success of cercariae on M. edulis increased with increasing mussel density, intensity of encysted metacercariae decreased. Furthermore, cercariae settled more efficiently on small than on large mussels. Our findings suggest that the transmission of P. acanthus cercariae is regulated by a range of abiotic and biotic factors, with filtering activity of mussels playing a key role by enhancing accumulation of cercariae on a substrate suitable for transmission to the bird final host.