Evaluation of cryptic invasion in Japanese Undaria populations based on mitochondrial haplotypic analysis

To improve the quality and quantity of seaweed mariculture harvests, seedlings are frequently introduced from regions geographically apart from cultivation sites. The effects of such introduction have been insufficiently studied, despite increasing demands on seaweed cultivation as a measure against global warming. We here evaluated the degree of cryptic invasion occurring through mariculture using a brown algal species, Undaria pinnatifida. Cultivated materials used in the Seto Inland Sea, Japan, were introduced from northern Japan in the 1970s. Based on the clear genetic structure among Japanese Undaria populations, we compared mitochondrial haplotypes between wild populations and cultivated materials in the Seto Inland Sea. All cultivated materials analyzed had haplogroups native to northern Japan. Multiple haplogroups were observed in the wild populations, especially those in the eastern Seto Inland Sea, which is exceptional considering the reported genetic structure. Some northern Japan haplotypes were observed both in cultivated materials and wild individuals. A northern Japan haplogroup was predominant at several sites near Undaria-cultivation sites. These results strongly suggested that cryptic invasion through Undaria mariculture occurred here, although its ecological impact remains unclear. There is a clear risk of invasion by mariculture seedlings from other regions, even when native conspecific populations are present.