To elucidate the external cues triggering spawning of Haliotis diversicolor, changes in the gonads of adults and the occurrence of newly settled post-larvae were monitored from 2001 to 2004 in Sagami Bay, Japan. The gonad index (GI) and gonad somatic index (GSI) of both sexes began to increase between April and June 2002, peaked in September, and then decreased significantly by early October. In other years, GI and GSI were significantly lower in autumn. Between June and September in each year, almost all females had a mature ovary and most were capable of spawning, but the occurrence of newly settled post-larvae (< 500 µm in shell length) was strictly limited to the periods just after typhoons passed through the study area. The GI and GSI of both sexes also decreased significantly only after typhoon events. Although mature adults experienced various changes in seawater temperature, tidal height and lunar phase during the survey periods, evidence of spawning was observed only after typhoon events. There were no specific trends in seawater temperature and lunar phase before newly settled post-larvae were collected. Our results suggest that spawning of H. diversicolor is strongly associated with typhoon events. Most individuals had an immature ovary just after the passage of a typhoon. However, 1 mo after spawning, most individuals had a mature ovary with a high GI, suggesting that H. diversicolor requires an interval of about 1 mo between successive spawning events.

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The feeding behavior and growth of post-larval Haliotis diversicolor with initial shell lengths (SL) of approximately 500 mm (Exp. 1-1 and 1-2), 800 mm (Exp. 2), and 1200 mm (Exp. 3) were studied in a laboratory setting while they fed on four species of benthic diatom Achnanthes longipes, Cocconeis sublittoralis, Cylindrotheca closterium, and Navicula ramosissima. Exp. 1-1 and 1-2 revealed no marked differences in post-larval growth rates (mean 24-39 mm SL/day) among the diatom species. However, marked differences in growth rates among the species were revealed in Exp. 2 and 3. Three species, A. longipes, Co. sublittoralis, and Cy. closterium, produced faster growth (Exp. 2 mean 29-51 mm/day, Exp. 3 mean 36-44 mm/day) than N. ramosissima (Exp. 2 mean 18 mm/day, Exp. 3 mean 23 mm/day). Post-larvae fed N. ramosissima had lower digestion efficiency (42.8%) than those fed other diatom species (90.7-100%). Diatom extracellular substances appeared to be principally used from post-settlement to 800 mm SL, and diatom cell contents were required to produce rapid growth of larger post-larvae (>800 mm SL). It is likely that the availability of each diatom for post-larvae was affected by diatom morphology, attachment strength, frustule strength, and post-larval size.

The community and trophic structures of the subtidal habitat of the abalone Haliotis diversicolor were examined in Nagai, Sagami Bay, Japan. Conventional community indices showed no significant differences among three sampling stations. The overall continuum of stable isotope ratios was structured into three different trophic linkages: (i) brown algae-dependent benthic food chain; (ii) red algae-dependent benthic food chain; and (iii) planktonic food chain. Brown algae and red algae likely play different roles with respect to carbon sources in the habitat. Conventional fractionation values indicated that the abalone H. diversicolor (d 13 C = -12.4 Ϯ 1.0‰, d 15 N = 9.3 Ϯ 0.5‰) feeds on the lamina of Undaria pinnatifida during juvenile and adult stages (8.0-65.6 mm shell length). Stable isotope signatures suggested that the juveniles of other abalone species as well as some amphipods and a sea cucumber Holothuria decorata are competitors of H. diversicolor, whereas some Muricidae gastropods such as Thais bronni and Ergalatax contractus are predators. The isotopic differences among macroalgal species and the subsequent transfer to consumers indicate that stable isotopic analysis is an effective means of studying food webs in an open rocky shore community with little influence from external primary production such as terrestrial vegetation.KEY WORDS: abalone habitat, food web, Haliotis diversicolor, macroalgae, stable isotope analysis, subtidal rocky shore.

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