By David Gardner
This freshwater shrimp (Gammarus pulex) has an unusually orange patch. It turned up in a Riverfly kick sample survey that I was carrying out with Jo Cunningham of the West Wales Rivers Trust on the little River Castell, near Abercastle in north Pembrokeshire.
The orange lump in the shrimp’s interior is a larva of the Acanthocephalan parasitic worm Pomphorhynchus spp. The larval stage known as the cystocanth causes a characteristic orange spot to develop within the body of the intermediate shrimp host, increasing the shrimp’s visibility to fish which prey on them. After the shrimp is ingested the larva burrows into the gut of the new host and develops into the mature worm. There are believed to be two species, P. laevis and P. tereticollis, and there is research currently on going to identify the distribution of these species and their impact on their fish hosts.
Infected Gammarids have been found in other Pembrokeshire rivers, but this is the first time it has been found in the Castell and is particularly interesting because the stream is believed to be fishless after repeated agricultural pollutions in recent years. Does the presence of this infected shrimp suggest that there is an isolated fish population still remaining in the catchment somewhere?