By Jennies Tran
Efforts on the West Coast to restore native oyster populations have shown promising results; however a particular invasive species has presented challenges. The Japanese Oyster Drill, Ocinebrellus inornatus, has made its way from the waters of Japan to the Pacific Northwest and now threatens the populations of our native Olympia Oysters, Ostrea lurida.
Although the Japanese Oyster drills originate in Japan, the transportation of the Pacific Oysters, Crassostrea gigas, to the United States has brought over these unwelcomed creatures. Specifically during the Gold Rush era, overharvesting of the native Olympia Oysters contributed to the depletion of the native oyster population. The affordable prices meant the demand for oysters was still high, so extensive trade of Pacific oyster seed from Japan to California and Washington began and brought over millions of Pacific Oysters with little regulation. This led to the Japan Oyster drill to populate along the West Coast.
The oyster drills have an adaption which helps them feed on different species of bivalves, called a radula. A radula is an anatomical structure made up of a teeth-like ribbon and enables them to drill through oyster shells. After the oyster drill makes a hole through the oyster shell, the oyster drill is able to secrete a digestive enzyme into the body and excrete its digested tissues. This process can vary in time, depending on the oyster size. Considering the smaller size and growth rate of the Olympia oysters compared to the Pacific oysters, the Japanese Oyster drill will target the Olympias and can consume them at a rate of three per week. Since the Oyster drill can only prey on oysters that have a shell thickness smaller than their radula, Olympia oysters are more susceptible to drilling in comparison to larger oysters. Oyster drills also feed on spat, a term for baby oysters, since spat have yet to develop a shell and are easily accessible.
Like oysters, oyster drills have specific salinity and temperature levels that they need to survive, showing an intolerance to lower salinity and tending to stay further away from fresh water sources. The absence of a planktonic stage in the oyster drill life cycle is vital in limiting the spread of oyster drills, as natural dispersion is limited. With only two life stages to consider, it is more strategic to eradicate the infestation of oyster drill eggs instead of removing adult oyster drills from oyster beds. Furthermore, the rate of adult mortality is high among oyster drills. Other practices include submerging oysters into freshwater encouraging the oyster drills to detach.
Next time you visit Point Pinole Regional Park, make sure to check out our restoration project, the oyster reef balls! Try to spot and eradicate these pesky invasive species!