We studied small-scale variation in morphology and growth rate of the seagrass Halophila ovalis in an intertidal flat of Thailand coast where the seagrass bed is subjected to grazing by dugongs. Our objectives were to examine whether morphology and growth rate differ between rhizomes in diiferent positions within a patch (center, edge and dugong trails), and to test whether H. ovalis shows plastic growth response to disturbance by dugong feeding. Seagrass biomass and leaf density were 3 to 4 times greater at the patch center than the patch edge and the dugong trails. In contrast, branching rate and internode length of rhizomes were significantly greater at the patch edge and dugong trails than at the patch center. The results of marking experiments of H. ovalis revealed that net production, leaf production and rhizome elongation rates were 2 to 3 times greater at the patch edge than at the center. Growth and production rates at the edge of the dugong trails and artificial trails mimicking dugong trails were not significantly higher than at the patch center. Nevertheless, significant differences in some morphological parameters such as branching frequency and internode length between the dugong trails and the patch center suggest that morphological changes in H. ovalis can be induced over short time scales. Recovery of H. ovalisafter disturbance by dugong herbivory was estimated to be quite rapid (<20 d). High growth rates of H. ovalis were probably responsible for its persistence under grazing by dugongs. KEY WORDS: Halophila ovalis. Seagrass. Within-patch variation. Growth. Production. Dugong herbivory. Morphological plasticity. Thailand. Small-scale heterogeneity 0 Inter-Research 1999 Resale of full article not permitted

Seagrasses are composed of four families belonging to angiosperms and they are thought to become adaptive to aquatic life independently. Zosteraceae is one such family and because of the relatively high species diversity around Japan and Korea coast areas, the family might have arisen therefrom. To elucidate the origin and evolution of Zosteraceae which consists of three genera, Phyllospadix, Zostera , and Heterozostera, 2.8 kb nucleotide sequences of rbcL and matK genes in the chloroplast genome were examined for various species, including cosmopolitan Z. marina and endemic Z. caulescens . The phylogenetic analysis reveals the following three features. First, based on the synonymous nucleotide substitution rate of the rice chloroplast genome, we estimated the divergence times between Zosteraceae and its closest relative, Potamogetonaceae, and between different genera, Zostera and Phyllospadix, as approximately 100 million years (myr) and 36 myr, respectively, suggesting that Zosteraceae emerged somewhere in the period from 36 myr ago to 100 myr ago. Second, two subgenera of Zostera , Zostera and Zosterella , exhibit their reciprocal monophyly and appear to have differentiated from each other approximately 33 myr ago. However, the third genus Heterozostera branched off only 5 myr ago from the stem lineage leading to Zosterella and this seems too recent in comparison with the ancient divergence of the two subgenera. Third, we estimated the most recent common ancestor of subgenus Zostera as 6 myr. In Z. marina four haplotypes were found in the sample and have diversified in the past 1.5 myr. One haplotype is shared by both sides of the Japan Archipelago and its closely related haplotypes occur also in eastern Pacific Ocean. Based on these phylogeographic analyses, we propose a provisional age related classification of Zosteraceae to argue the origin and evolution.

Abstract. We studied the population dynamics and reproductive traits of two phytal gastropods, Lirularia iridescens and Hiloa tristis, inhabiting a seagrass bed consisting of two seagrass species, Zostera marina and Zostera caulescens, in Otsuchi Bay, north‐eastern Japan. The main objectives of the study were to (1) determine seasonal and between‐substrata variations in gastropod populations and (2) examine two major factors potentially responsible for the variations, namely space (seagrass) and food (epiphytes). The biomass of Z. marina reached a peak in summer and decreased through autumn, whereas the biomass of Z. caulescens showed little seasonal fluctuation. Epiphyte biomass was maximum in April and remained low from summer to winter on both seagrass substrata. The population densities of both gastropod species increased rapidly due to increases in their new recruits from May to July for L. iridescens, from June to October for H. tristis. Eggs of L. iridescens were found in almost all months of the study period, with a breeding peak in April, whereas oviposition of H. tristis occurred only from June to October. The new recruits of both gastropods grew rapidly, with their abundance decreasing from summer to autumn. The peak densities of new recruits of the two species coincide with that of seagrass biomass rather than epiphyte biomass, and the juveniles grew when the epiphyte biomass was fairly low. Hence, large postrecruitment declines in gastropods may be caused by food shortage. Egg density, size structure and growth rate of both gastropod species were not significantly different between seagrass substrata. Total abundance was not significantly different for L. iridescens. For H. tristis, however, abundance was lower on Z. caulescens than Z. marina during most of the year, mainly due to higher post‐recruitment decline on the former. The ability of L. iridescens to breed continuously may enable them to successfully inhabit two seagrass species with different phenology, whereas the much shorter breeding season of H. tristis, which coincides with the periods of higher Z. marina biomass, may be more adaptive for inhabiting Z. marina than Z. caulescens.

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