Spiders observed at sea: family-level composition and seasonal occurrence patterns in a coastal environment

The sea has traditionally been considered an unsuitable habitat for spiders, so there have been few systematic surveys of their presence there. We conducted a kayak-based visual survey of spiders in the coastal waters of Tokyo Bay, Japan, from December 2020 to May 2021, and documented the observed individuals photographically. A total of 58 individuals representing eight families were observed and photographed. The most common family was Linyphiidae (n = 30), followed by Phrurolithidae (n = 9) and Agelenidae (n = 5). The number of spiders observed on the sea surface varied seasonally; a peak of 20 individuals was recorded in late March, but none was observed from April through the end of the survey in May. Spider behaviour included sailing on the sea surface by a juvenile of the genus Tetragnatha. The relatively high sea-surface temperatures in the inner bay during winter, together with the presence of prey insects on the sea surface, suggest that conditions at the sea surface may allow spiders to survive there, at least temporarily. We also discussed the potential ecological significance of spiders moving onto the sea surface in the context of resource subsidies from terrestrial to marine ecosystems.