Sea urchin repelling Tannin– FeIII complex coating for ocean macroalgal afforestation

Abstract Intense seaweed grazing by sea urchins has destroyed kelp forests and accelerated the transformation of these forests into barren areas known as urchin barrens. Once the sea urchins occupy the barren ground, it becomes more challenging to restore the kelp forests. Although phlorotannin, a primary herbivore defense chemical secreted by kelp, has been reported to discourage feeding activities of marine herbivores but the direct application of naturally extracted phlorotannin does not effectively repel sea urchins. In this study, we applied a simple and green Tannin-FeIII (TA-FeIII) coating on substrates as a sea urchin repellent using a cheap, ecofriendly tannin (TA) obtained from biomass as an alternative to phlorotannin. In a model aquarium experiment, most of the sea urchins (Anthocidaris crassispina) in the tank evaded the TA-FeIII-coated substrates. In field tests with 300 sea urchins, the majority of sea urchins could not crawl over the TA-FeIII-coated rope for more than 2 h in contrast to the control group. Hence, the safety, cost-effectiveness, and scalability of the TA-FeIII coating make it a practical candidate to protect the kelp ecosystem from sea urchins.