Freshwater golden clam surveillance (Corbicula)
Staying ahead of the spread: tracking an invasive clam with eDNA
When the freshwater golden clam (Corbicula fluminea) turned up in the Waikato River in May 2023, Wilderlab's eDNA records showed it had been there for months before anyone saw it. Working with the Ministry for Primary Industries (MPI), we built a new clam assay and screened more than 2,500 samples from around Aotearoa in the second half of 2023.
Golden clams are prolific invaders. Adults reach 2 to 3 cm and can produce hundreds of offspring a day, forming dense beds that clog water infrastructure and compete with native species for food. Overseas, they have proved very hard to control. Once an incursion is found, the most urgent question is how far it has already spread.
Our lab team started with what we already had. A scan of our eDNA records found a sequence matching C. fluminea in a community group's Waikato River sample from April 2023, around a month before the clam was officially discovered.
We then pulled older samples from the freezer and re-tested them with a new clam-specific assay. Corbicula DNA was present in the Waikato River as far back as September 2022. This shows the value of keeping a biobank of historic eDNA samples: they can answer questions nobody knew to ask when the samples were taken.
In 2023, working with MPI's response team, we added the clam assay to our standard testing panel. Every freshwater sample we receive is now tested for Corbicula, so each one doubles as passive surveillance for the clam. If Corbicula is detected in a sample, we are required to report it to MPI. The assay also screens for two other high-risk invasive molluscs, the zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena bugensis).
Because the incursion is a sensitive issue, we want to be 100% sure of any detection before notifying customers, regional authorities or MPI. Not all of New Zealand's native molluscs have had their DNA sequenced and added to public reference databases such as GenBank, so there is a small chance that the C. fluminea DNA barcode is shared by a species that occurs naturally in our waterways. To rule this out, we work closely with the MPI Animal Health Laboratory (AHL) to confirm any positive detection outside the known incursion zone.
Until the AHL confirms C. fluminea through independent qPCR analysis, any sequence that matches its barcode with 100% identity is reported at family level (Cyrenidae). This can mean slightly longer processing times for affected samples. Customers can consent to their samples being made available to the AHL for this testing using the checkbox on our sample submission form, or by letting us know in writing. In line with our terms and conditions, we never share your data or samples with a third party without your permission.
From July to the end of 2023, we screened more than 2,500 eDNA samples from all over the motu. The results suggested Corbicula was confined to the Waikato catchment, and showed no sign that zebra or quagga mussels had reached Aotearoa.
The work is a reminder of how much a national network of eDNA samplers can do. Routine samples collected by councils, community groups and researchers became an early warning system, and a record of the past that helped the response see further back in time.
If you're on the water, remember to Check, Clean, Dry your boat and gear before moving between waterways.
The Venerid clam metabarcoding assay is included in our comprehensive freshwater panel at no extra cost. Wilderlab account holders can also have archived samples re-tested: email info@wilderlab.co with the sample ID numbers and we'll send a quote.
Explore an eDNA sample where Corbicula was found.
Read more about Corbicula from MPI.
*Image credit: Two Asian clams (Corbicula fluminea). Photo by Holger Krisp on Wikimedia Commons