Vertical distribution of epifauna on Sargassum horneri, with special reference to the occurrence of bivalve spat

Abstract

The annual macroalga Sargassum horneri often forms a dense canopy at the sea surface. To evaluate the effects of the vertical structure and presence of the sea-surface canopy of S. horneri on the distribution of its epifauna, we collected epifauna from four different vertical portions of S. horneri at Kitsunezaki, west coast of Oshika Peninsula, Miyagi, Japan. The vertical portions of the thalli were defined as follows: sea surface (S), the portion that was always lying on the sea surface; intermediate (I), which was periodically lying on the sea surface with tidal changes; underwater (U), which was always submerged; and bottom (B), the portion around the holdfast. The mean total density of epifauna in the sea-surface portions (S and I) was significantly higher than that in the underwater portions (U and B). Harpacticoid copepods and bivalve spat accounted for more than 90% of the epifauna in all portions. It seems that the canopy of S. horneri collects bivalve larvae and other epifauna from the water column by sweeping the sea surface with the changing of the tide. We proposed the ‘Seasonal Coincidence Hypothesis’ to explain the dense occurrence of bivalve spat on S. horneri in this study.