Asian clam finds its way to the Wadden Sea
NIOZ Royal Netherlands Institute for Sea Research“It remains to be seen whether this shellfish will become invasive in The Netherlands as well. Potentially, it may become a source of food for shorebirds like the knot as well”, says Livia Brunner of NIOZ.
Pacific shellfish
The Asian clam (Potamocorbula amurensis, Schrenck, 1861) is native to the warmer north-west Pacific biogeographical region. It lives in the waters near Siberia, China, Korea and Japan. It was first introduced to the European continent in 2018 in the Beneden-Zeeschelde and Western Schelde. Since then, it has slowly spread and has been found in Gironde Estuary in France as well.
Intertidal sampling program
During yearly ecological survey 'the Synoptic Intertidal Benthic Survey' (SIBES) in the summer of ’24, the Asian clam was found during field work in three different locations close to the Frisian coast. SIBES is a Wadden Sea-wide, intertidal sampling program in the Dutch Wadden Sea. It has been monitoring the sediment composition and benthic life for the past 17 years now. This means that every field season, between June and October, a research team of eight to twelve people collect samples on roughly 4,500 locations. All benthic species and sediment properties are then analyzed in the lab at NIOZ.
A tooth and an overbite
The Asian clam is a rounded-triangular shaped, thin-shelled, bivalve. It may reach up to 25 millimeters in size. Its distinctive feature is that the right valve is slightly larger than the left, causing a noticeable ‘overbite’: an overlap at the margin, setting it apart from other similar clam species. Furthermore, the right valve has a narrow tooth near the hinge, while the left valve has a long, corresponding socket, the so-called chondrophore.
Potential spreading
Most invasive species in the Wadden Sea have been introduced through fouling of ship hulls, or through natural dispersal. Also, ballast water transport is a known introduction mechanism for bivalves worldwide. Neither route can be ruled out in case of the introduction Asian clam in the Wadden Sea.
Although only three individuals have been found so far, it is expected that more will be identified, as samples from 2024 continue to be processed. The species has a very high spreading potential and is known to be potentially highly invasive. The first invasive introduction of this species was in the San Francisco Estuary. After being introduced in 1986, the Asian clam became the dominant clam species within just two years, significantly altering the benthic community structure and food web dynamics. Also, in the Gironde Estuary, the species reached densities of over 14.000 individuals per square meter in just one year.
It is still uncertain whether and how the species will spread in the Wadden Sea and at what speed. “But spreading is highly likely”, says Brunner. “The clam is tolerant to a wide range of salinities and various substrate types, both intertidal and subtidal. Also, it is known for its short lifespan and high fecundity. It can spawn even at a size of just three to four millimeters. Given this, it seems likely that the individuals that were found during SIBES (2 millimeters, 9.5 millimeters, and 17 millimeters) have already spawned and contributed to the species’ spread.
Food or peril
If the Asian clam manages to establish in the Wadden Sea, it may have severe impact on the ecosystem. One of the potential risks is that it will compete with native species, such as the common cockle, which has more specific requirements for both adult and larval stage. It could have significant ecological impacts in other parts of the food web as well. In the San Francisco Estuary, for example, its introduction led to a drastic decline in zooplankton communities, affecting food availability for other native bivalve species.
On the other hand, in China, clams of the same genus such as Potacorbula laevis are known to be a great food resource for red knots, due to the high flesh-to-shell mass ratio. This suggests that Asian clam could also become a great source of food, influencing bottom-up processes in the ecosystem.
With the long-term impact of the Asian clam in the Wadden Sea still uncertain, this discovery highlights importance of long-term monitoring in programs like SIBES. NIOZ will continue to monitor the spread of the Asian clam and assess its ecological consequences for the UNESCO world heritage site Wadden Sea.
Text and photo's: Koninklijk Nederlands Instituut voor Onderzoek der Zee (NIOZ)