Research @ Otago - Sorrel O'Connell-Milne

Sorrel is studying how parasite infection in clams varies in a commercial harvested environment.

Sorrel's research looks at the number of parasitic infections in the clam Austrovenus stutchburyi in  areas where commercial harvesting takes place and compares these with areas where there has been no harvesting.  She also examines the effect of these parasites on juvenile clam mortality, growth, body condition and foot length.

Mud snails were collected from Otago Harbour and their parasites were used to experimentally infect the juvenile clams and observe changes in their growth, body condition and foot length.

The parasites she studies are Curtutaria australis and Acanthoparyphium spp., two trematodes that play very similar roles within the ecosystem. These trematodes have a complex life cycle and require three different host species before they can reach maturity. The first intermediate host is a gastropod (whelk or mud snail) where the parasite infects the gonadal material, castrating the host and effectively turning the host into a 'parasite factory'.  The parasite then infects the second intermediate host, the clam, where the inactive juvenile stage is encapsulated as a cyst in the clam's foot.  The oystercatcher is the definitive host for the parasite and the trematode’s life cycle is completed when the oystercatcher preys upon the clam and ingests the trematode.

The trematodes encyst within the foot of the clam where they reduced foot muscle contractions and hinder burrowing behaviour.

As these parasites infect the foot of the clam they reduce the ability of the clam to bury into the sediment, increasing the chance of predation by birds seven-fold.  Effectively, the trematode is changing the clams burrowing behaviour and actively enhancing its own transmission to the definitive host, the oystercatcher.  However, not all is doom and gloom, parasites also have effects on the surrounding infauna; the shells of clams held above the sediment provide attachment surfaces and shelter for other organisms, which can result in increased biodiversity of the area.

Sorrel and her field volunteers, Chris and Steph, collecting clams in Blueskin Bay, Otago.

Areas with a reduced density of shellfish due to commercial harvesting were found to have 36% more parasites on average than equivalent unharvested areas. Results indicate that although mortality was unaffected by parasites infection directly, increased infestations negatively affect the growth, body condition and foot length of clams.  Therefore, in harvested areas increasednumbers of infections may result in the clam reaching maturity later and their ability to filter feed and bury into the sediment may be reduced.  This study portrays the complex impacts that human activity may have on the intertidal environment and highlights that commercial clam harvesting not only removes shellfish biomass but also has more inconspicuous effects on the surrounding ecosystem.