Green-gilled oysters — often called “emerald oysters” — present a unique seasonal phenomenon and a fascinating intersection of science, history, and nutrition.
Long cultivated in France and now appearing in Franklin Harbour, South Australia, these oysters have captured the attention of seafood lovers for their vivid colour, distinctive flavour, and potential health benefits.
The Science Behind the Green Gills
The emerald colouration of these oysters comes from Haslea ostrearia, a specific phytoplankton present in certain ocean currents. This blue diatom produces a blue-green pigment known as marennine, first identified in the Marennes-Oléron region of France.
When oysters filter-feed on water containing Haslea ostrearia, marennine stains their gills a vibrant turquoise green. Beyond its visual impact, the pigment also enhances the oysters’ nutty, complex flavour — a hallmark of Franklin Harbour oysters.
Historical and Cultural Significance
Emerald oysters boast a rich culinary history, with records of their consumption dating back to the 1700s. In France, they became associated with refinement and royal taste.
Traditionally, farmers cultivate these oysters in shallow ponds designed to encourage the growth of Haslea ostrearia. As the diatom releases marennine into the water, oysters absorb the pigment through natural filtration — resulting in their distinctive green gills.
This process has historically allowed farmers to command premium prices, often earning up to twice as much for emerald oysters compared to standard stock.
Health Benefits of Emerald Oysters
Beyond their appearance and heritage, emerald oysters offer notable nutritional benefits. Research suggests that marennine possesses antioxidant, antibacterial, and antiviral properties.
These attributes make emerald oysters not only a visual and gastronomic delight, but also a valuable addition to a balanced diet — combining flavour, nutrition, and functional health benefits.
Addressing Consumer Concerns
Despite their benefits, some consumers may initially hesitate when encountering green-gilled oysters, mistaking the colouration for spoilage or toxicity.
In reality, the green hue is entirely natural and harmless — a direct result of the oysters’ diet of Haslea ostrearia. Education is key to dispelling these myths and encouraging people to confidently enjoy these phytoplankton-enriched Pacific oysters.
Current Occurrence in Franklin Harbour
The recent reappearance of emerald oysters in Franklin Harbour is an exciting development for South Australian oyster farmers and seafood enthusiasts alike.
The natural presence of Haslea ostrearia in these waters has allowed local oysters to develop the same vibrant gills and enhanced flavour celebrated in France for centuries — positioning Franklin Harbour as a premium oyster region on the global stage.
Conclusion
With their striking green gills and rich, nutty flavour, emerald oysters represent a compelling fusion of history, science, and culinary tradition.
By understanding the role of Haslea ostrearia and marennine, consumers can fully appreciate the beauty, safety, and health benefits of these unique oysters. As they become more prevalent in South Australia, emerald oysters offer a rare opportunity to celebrate a delicacy with both a storied past and a bright future.
References
Prasetiya, F. S., Safitri, I., Widowati, I., Cognie, B., Decottignies, P., Gastineau, R., Morançais, M., Windarto, E., Tremblay, R., & Mouget, J.-L. (2016). Does allelopathy affect co-culturing Haslea ostrearia with other microalgae relevant to aquaculture? Journal of Applied Phycology, 28(4), 2241–2254. https://doi.org/10.1007/s10811-015-0779-y
Prasetiya, F. S., Decottignies, P., Tremblay, R., Mouget, J.-L., Sunarto, S., Iskandar, I., Dhahiyat, Y., & Cognie, B. (2020). Not only greening: The effects of marennine produced by Haslea ostrearia on physiological traits of three bivalve species. Aquaculture Reports, 18. https://doi.org/10.1016/j.aqrep.2020.100546
Prasetiya, F. S., Comeau, L. A., Gastineau, R., Decottignies, P., Cognie, B., Morançais, M., Turcotte, F., Mouget, J.-L., & Tremblay, R. (2017). Effect of marennine produced by the blue diatom Haslea ostrearia on behavioural, physiological and biochemical traits of juvenile Mytilus edulis and Crassostrea virginica. Aquaculture, 467, 138–148. https://doi.org/10.1016/j.aquaculture.2016.08.029