Hidden burrow associates: macrosymbiotic assemblages of subtidal deep-burrowing invertebrates in the northern part of the Sea of Japan

The activity of deep-burrowing macrofauna strongly influences all biogeochemical processes in sublittoral soft sediments. Despite this key role, these organisms are difficult to sample and, thus, often remain ignored in environmental studies. This study is the first in comprehensively exploring the diversity of the macrosymbiotic communities associated with the dominant subtidal deep-burrowing invertebrates from the southern part of the Russian coast of the Sea of Japan, represented by the species of the genera Upogebia Leach, 1814 (Arthropoda: Crustacea: Decapoda) and Urechis Seitz, 1907 (Annelida: Polychaeta: Echiura). The associated symbiotic communities mostly consist of obligate, host-specific species, while those species found in burrows of both hosts are probably using them just as refuges. Most symbionts occurred solitary or in heterosexual pairs, likely due to aggressive and strictly territorial behavior. This is certainly a hidden biodiversity, as more than half of the species reported here were not previously known from these “relatively simple and well-studied” boreal marine ecosystems. Our findings also allowed us to describe a new species belonging to the symbiotic genus Hesperonoe Chamberlin, 1919 (Annelida: Polychaeta: Polynoidae), based on morphological and molecular evidences, the latter being here presented for this genus for the first time.