Effects of larval settlement and post-settlement mortality on the distribution pattern and abundance of the spirorbid tube worm Neodexiospira brasiliensis (Grube) (Polychaeta) living an seagrass leaves

The role of larval settlement, post-settlement mortality and competition wi th a red algae in determining the patterns of abundance and distribution of the spirorbid tube worm Neodexiospira brasiliensis (Grube) (Polychaeta: Sp irorbidae) on leaves of three seagrass species: Zostera marina Linnaeus, Zo stera asiatica Miki and Phyllospadix iwatensis Makino were examined in Aini nkap, Akkeshi Bay, Akkeshi, Hokkaido, Japan. Field collections of seagrass shoots were made at about 1-week intervals. The density of newly settled la rvae (< 0.3 mm in tube diameter) increased significantly on Z. asiatica and P. iwatensis, but not on Z. marina during the sampling period. It was high est on Z. asiatica among the three seagrass species, followed by P. iwatens is and Z. marina. Newly settled larvae occurred more on the basal part of y ounger leaves of each seagrass species. Mortality tended to be high on Z. m arina, followed by Z. asiatica and P. iwatensis, although the differences w ere not great. Size-specific mortality showed the existence of high mortali ty in early post-settlement stages on Z. asiatica and P. iwatenisis. Relati vely high mortality was also suffered by individuals with a tube diameter o ver 1.5 mm. Growth in tube diameter of N. brasiliensis was slower on P. iwa tensis than on the other two seagrasses. The effect of a calcareous red alg ae on larval settlement was investigated with removal experiments; however, no effect of red algae was detected. Patterns in the distribution and abun dance of N. brasiliensis on leaves of three seagrass species resulted from the heterogeneity of larval settlement rather than from postsettlement mort ality or competition with red algae. Different densities of larval settleme nt among the three seagrass species or on a leaf are likely to relate to la rval behaviour, such as negative phototaxis.