The Use of Living Mussels as a Substratum for Growing Seedlings of Two Sargassum Species from the Perspective of Coastal Seaweed Bed Restoration in the East China Sea

Abstract

Sargassum vachellianum and Sargassum horneri are the main dominant species and primary producers of seaweed beds in Chinese coastal ecosystems that play an important role in marine blue carbon sinks. However, due to the influence of frequent human activities and global climate change, the seaweed beds formed by Sargassum vachellianum and Sargassum horneri in Chinese coastal waters are gradually declining. In this study, Sargassum zygotes were cultured onto the surface of mussels and then used to repair the declining seaweed bed resources through seaweed transplantation, which was indirectly achieved based on the fast attachment of mussel byssus. The results show that zygotes of Sargassum vachellianum and Sargassum horneri could grow on the surface of mussels and, over time, the force and rate of mussel adhesion gradually increased. The adhesion of Mytilus coruscus was greater than that of Septifer virgatus mussels. After four hours, the rate of adhesion for Mytilus coruscus with a shell length of 30 mm was 100%, and the adhesion force was the highest, at 0.511 ± 0.099 N. Hence, Mytilus coruscus showed better adhesion than Septifer virgatus. After 41 days, the mean length of Sargassum horneri germlings on the surface of Mytilus coruscus grew to 5.554 ± 0.724 mm, the daily growth rate was about 0.154 mm/d, and the mean density declined by 50.59%. After 31 days, the mean length of Sargassum vachellianum germlings increased to 5.510 ± 0.763 mm, the daily growth rate was about 0.191 mm/d, and the mean density declined by 21.21%. After 2 months of development of the mussel–seaweed combinations in coastal waters, the survival rate of Sargassum horneri was 7.6 ± 0.9% and that of Sargassum vachellianum was 25.9 ± 10.5%. Hence, compared with Sargassum horneri, Sargassum vachellianum attached to Mytilus coruscus showed better development, and this system can be used to combat the decline in seaweed bed resources. In this method, mussels were used as an intermediate attachment medium to indirectly achieve the settlement of zygotes for seaweed transplantation, and, therefore, their use as a substratum serves as the basis for a novel technique for seaweed beds restoration.