The symposium of the Society of Zoological Systematics was held on September 7, 2023 (18:00–20:30) at Yamagata University as a session of the related topics in the 94th Annual Meeting of Zoological Society of Japan. The title of the symposium was “The World of Small Creatures: Interesting Recent Topics on Polychaetes and Amphipods”. Six exciting topics were presented by younger and promising researchers.

The family Spionidae Grube, 1850, one of the most abundant polychaete groups in Annelida in terms of species numbers and biomass, is globally found in a wide variety of marine environments. Among this family, members belonging to the Polydora-complex (or referred to as polydorids, Tribe Polydorini, or polydorins) are well known for their ability to bore into various hard calcareous substrates. This group comprises boring and non-boring species, both of which include symbiotic species and therefore utilize a variety of substrates, including other invertebrate species, as habitat. In the course of research on spionid polychaetes in Japan, the author recently discovered and described two new species, Polydora lingulicola Abe & Sato-Okoshi, 2020 and Polydora tunicola Abe, Hoshino & Yamada in Abe, Hoshino, Yamada, Ogino, Kawaida & Sato-Okoshi, 2022, that show novel and unusual obligate symbiotic relationships with lingulid brachiopods and ascidians, respectively. Here I review the known symbiotic relationships in spionid polychaetes, including above two newly discovered symbiotic relationships.

Spionid polychaete Polydora websteri is known to inhabit commercially important mollusc shells and is reported globally. This review introduces the ecology and larval development of Polydora websteri inhabiting wild oyster shells (Crassostrea gigas) in Gamo Lagoon, Miyagi, Japan. Notably, over 100 worms were found in a single oyster valve and many mud blisters were observed on the inner surface of oyster shells. Females were observed to deposit a string of egg capsules from May to October. Culturing of these egg capsules revealed unique larval development of P. websteri, with adelphophagic and planktotrophic larvae coexisting within a capsule at the time of hatching. Those different larval developmental modes, also observed in other Polydora species, may be a key factor contributing to their widespread global distribution.

Although the marine environment, especially deep-sea bottom, provides a vast habitat for organisms, the diversity of deep-sea invertebrates is still poorly understood. In this article, we introduce a review article of the records of deep-sea polychaetes that have been collected from depths below 2,000 m and recent progress in the survey of deep-sea polychaetes around northern Japan. These topics highlight the potential problem of many deep-sea polychaetes identified as the same species that have been described in shallow zones. Also, we present examples of our publications and preliminary results on the deep-sea polychaetes off the Pacific coast of northeastern Japan by molecular phylogenetic assessment.

The gammaridean amphipod is a large taxon with over 7000 species described to date. They occupy important positions in the ecosystem as decomposers and as food for fish, birds, and mammals. Gammaridean amphipods occur in a wide variety of environments, from marine to brackish and freshwater to terrestrial. Recently, however, species of this group have been found in the mouth of a whale shark and in hot spring, previously unexpected and unique environments. This paper introduces gammaridean amphipods that occur in these special environments, as studied by the author.

Ceinina japonica, an amphipod of the family Eophliantidae, bores tunnels into laminarian brown algae to construct nests. As for the populations that utilize Undaria pinnatifida, individuals of C. japonica reproduce within the nest, and adults and juveniles coexist for months. The individuals which live inside of the seaweed are protected from predators and utilize resources supplied with the growth of the seaweed. However, U. pinnatifida, an annual seaweed, disappears during the summer. It has been reported that the infestation rates of C. japonica increased in Saccharina japonica var. ochotensis during the period when U. pinnatifida was lost along the coast of Hokkaido. This suggests the seasonal migration of C. japonica and its preferences for laminarian kelps. On the other hand, C. japonica has also been reported as an epifauna on some Sargassum species. Further investigation is needed to determine the identity of the populations occurring in laminarian kelps and on Sargassum seaweeds. In order to elucidate the population maintenance mechanisms of C. japonica, it is necessary to investigate its habitat preferences, adaptation to multiple seaweed species and dispersal patterns.

The behavioral features of clinging to a self-constructed mast-like structure have been described in eight species belonging to three genera of gammarid amphipods in the family Dulichiidae. The mast is usually composed of mud and amphipod feces and constructed with the aid of ‘amphipod silk’ spun out from the dactyli of pereopods 3 and 4. The major function of masts is believed to be the provision of conditions to optimize suspension-feeding. However, there are many questions about mast-building that remain to be determined, such as the reason for the presence of diatoms within the muddy material of masts and the way by which these amphipods avoid predation. Masts appear to be constructed mainly by adult females and guarded as territory for their reproduction and care of young. Dulichia species may culture benthic diatoms on the mast and provide as the food for adults and juveniles. Examination of the generating process and fiber structure of the amphipod silk threads will be useful for the development of new water-proof fiber materials.