Distribution and Diversity of Erect Bryozoan Assemblages Along the Pacific Coast of Japan

Abstract

To assess factors involved in the high diversity of benthic fauna in Sagami Bay, we examined the species composition of bryozoans forming rigidly erect colonies, possibly occuring in dense assemblages called bryozoan thickets. We identified erect bryozoans from collections made in the bay in four time intervals over ~125 years. To examine latitudinal effects on diversity, we also identified specimens in collections from near Otsuchi Bay to the north and the Nansei Islands to the south. In addition, we compared the composition and diversity of erect bryozoans in Sagami Bay with those in bryozoan thickets at Otago Shelf, New Zealand. We categorized erect bryozoans into five form categories based on the colony morphologies; detected 17 species in representing the five forms in Sagami Bay, five species representing four forms in the Nansei Islands, and three species representing two forms at Otsuchi Bay. Erect bryozoan diversity thus did not show a latitudinal gradient; it was higher in Sagami Bay than farther north or farther south, though we cannot rule out sampling effects. We speculate that the high diversity in Sagami Bay is due to greater environmental complexity than the other areas, including warming and cooling influences from the Kuroshio and Oyashio Currents, respectively, in different parts of the bay. We detected no clear differences in species composition between eastern and western Sagami Bay, but did detect an apparent loss of diversity of four species overall and nine species in western Sagami Bay between the 1928–1988 and the 2001–2005 intervals, suggesting differential environmental changes in different parts of the bay. Sagami Bay was richer in rigidly erect species than the bryozoan thickets at Otago Shelf, though the same colony morphologies were represented.