The Sea Cucumber Holobiont and Probiotics: Recent Progress on Apostichopus japonicus.

After the first definition of the term "Holobiont" by Margulis in the introduction of symbiosis as "Association throughout a significant portion of the life history" in 1991 [1], the understanding of holobiont has become an important goal in modern biology today [2]. Recent advances in microbial collection, genome/metagenome/transcriptome sequencings, and bioassays for host-microbes interactions push us towards a fuller understanding of holobiont in various aspects of life on Earth. Historically, holobiont and related hologenome concepts have been tested and expanded through research on marine organisms such as coral, fish, sea cucumber, sponge, and squid. In particular, the sea cucumber Apostichopus japonicus is a physiologically and ecologically unique marine invertebrate in which the holobiont can be studied with its significant capability of organ regeneration, presence of microbes in coelomic fluid, their mysterious nutrition connected to slow growth, and improvements in seed production for the bio-conservation of endangered and essential fisheries resources. The animals are also important in evolutionary terms on a branch of the Deuterostomia clade sharing ancestry with humans, so we can also compare to and learn from knowledge on the human-microbes interactions. In this review, recent progress in the sea cucumber A. japonicus holobiont studies, and the discovery of probiotics candidates among its pioneer microbiomes are described. By understanding this recent progress, we expect to stimulate new and further perspectives on basic biology, bio-conservation, and sustainable aquaculture of sea cucumber.