Toshihiro Onitsuka

Community and trophic structures of abalone Haliotis diversicolor habitat in Sagami Bay, Japan

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 (δ13C = 12.4 ± 1.0‰, δ15N = 9.3 ± 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.

Dietary value of benthic diatoms for post-larval abalone Haliotis diversicolor associated with feeding transitions

The feeding behavior and growth of post-larval Haliotis diversicolor with initial shell lengths (SL) of approximately 500 μm (Exp. 1–1 and 1–2), 800 μm (Exp. 2), and 1200 μm (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 μm 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. longipe, Co. sublittoralis, and Cy. closterium, produced faster growth (Exp. 2 mean 29–51 μm/day, Exp. 3 mean 36–44 μm/day) than N. ramosissima (Exp. 2 mean 18 μm/day, Exp. 3 mean 23 μm/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 μm SL, and diatom cell contents were required to produce rapid growth of larger post-larvae (>800 μm 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.

Survival, Growth and Recruitment of Abalone Haliotis diversicolor in Sagami Bay, Japan

Abstract The occurrence of newly settled postlarvae (<500 μm of shell length [SL]) and subsequent growth and survival of the abalone, Haliotis diversicolor, were observed from 2001–2004 at two stations in the rocky shore of Nagai on the coast of Sagami Bay, Japan. Seawater temperature, current velocity, and distance of the weekly stone movement were monitored at both stations to determine factors affecting survival and growth of postlarvae. There were newly settled postlarvae in August and September of 2001, October 2002, August 2003, and September 2004. A total of 6 cohorts were identified between 2001 and 2004. Initial density of each cohort was generally higher at Station 1 than Station 2, but recruits at Station 1 had greater mortality than Station 2. Results from measurements of the physical environmental factors suggested that higher mortality rate of postlarval and juvenile H. diversicolor at Station 1 were produced by greater water turbulence and stone movement caused by storms. Growth rates in the first month after settlement varied between cohorts (35–62 μm SL day−1), and growth rates were slower for cohorts settling later in the spawning season and experiencing lower water temperature. The timing of typhoon-triggered spawning in H. diversicolor population was also suggested to be an important factor affecting growth and survival of postlarvae and early juveniles, along with their subsequent recruitment. The cohorts from 2001–2004 attained 17–40 mm SL in about one year after settlement. To estimate the size and age at the first stage of maturation, the increase in shell length and gonad development of a single cohort from 2001 were measured. This cohort attained 30–55 mm SL and showed high gonad index at 22 mo after settlement, indicating the first stage of maturation of H. diversicolor is achieved at age 2.

Identification of juvenile abalone Haliotis diversicolor based on number of open and sealed respiratory pores

Changes in the number of respiratory pores (open pores) and their imprints (sealed pores) in post-larvae and juveniles were observed and compared for four abalone species Haliotis diversicolor, H. discus discus, H. madaka, and H. gigantea. The first open pore was evident at a shell length (SL) of 1.5 mm in H. diversicolor, 1.9–2.0 mm Sl in H. discus discus, and 2.3–2.4 mm SL in H. madaka and H. gigantea. The number of open pores in H. diversicolor gradually increased with growth, with four to five pores at 2.5–18.0 mm SL and five to six pores at 18.0–27.0 mm SL. The other three species maintained four to five open pores after they reached 3.4–4.5 mm SL. The total number of open and sealed pores (TNP) was greater in H. diversicolor than in the other species at the same SL. Juvenile H. diversicolor were identified among field-caught abalone by the difference in the relationship between SL and TNP (SL-TNP relationship) and also by the monoclonal antibody reaction method. The results of the two methods were in perfect agreement, indicating that our method using the SL-TNP relationship is reliable for the identification of H. diversicolor.

Association between the planktonic larval and benthic stages of Manila clam Ruditapes philippinarum in eastern Hokkaido, Japan

The association between planktonic larval and benthic stages of Manila clam Ruditapes philippinarum and factors determining successful recruitment were studied in Akkeshi-ko estuary and Akkeshi Bay, Hokkaido, northern Japan. The field study showed that planktonic larvae were localized mainly in the bay after dispersal from the estuary and recruitment to the estuarine fishing grounds was not always successful. A numerical analysis suggested that estuarine circulation transported tracers, used to simulate planktonic larva, from the estuary to the bay but the number of returning tracers varied depending on the river discharge and wind direction. The planktonic larvae were affected by unsuitable conditions for survival in the estuary, such as high turbidity. We conclude that most larvae develop within the bay, where conditions are more saline and less turbid, and that although return to the estuary is essential for recruitment, it is not necessarily related to the abundance of larvae.

VARIATION IN MANILA CLAM (RUDITAPES PHILIPPINARUM) FECUNDITY IN EASTERN HOKKAIDO, JAPAN

ABSTRACT Fecundity, condition index, and gametogenic development of the Manila clam Ruditapes philippinarum were quantified at 2 sites on the intertidal fishing grounds of the Akkeshi-ko estuary, Hokkaido, Japan. In this study, an enzyme-linked immunosorbent assay was used to quantify the number of eggs in the clams. Fecundity and condition index were greater at the site with more availability of higher chlorophyll a concentrations and current velocities. However, histological analysis revealed that gametogenic development in the clams was completed at the site with lower food supply. The fecundity and condition index were limited in a part of this site, with high juvenile recruitment (>8,200 individual/m2) and intense food competition between the clams. Moreover, fecundity increased with size. The Akkeshi-ko estuary is located in northern Japan, where low temperatures mean sexual maturation progresses slowly in these clams. These results suggest that Manila clam fecundity increased through weight gain and repeated spawning could not occur due to their slow maturation.

Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future Needs

In this chapter, we estimated carbon sequestration by seagrass and macroalgal beds, defined as the integration of annual plant tissue (organic carbon) production, in Japan. Each of the main four beds, eelgrass beds (Amamo-ba), Sargassum beds (Garamo-ba), warm-temperate kelp beds (Arame-ba), and cold-temperate kelp beds (Kombu-ba), exhibited a distinctive geographic distributional pattern along Japan’s coasts that depended on regional climate and topographic characteristics. The total area of the four beds was approximately 230,000 ha nationwide, based on an analysis of the latest satellite images and information on past distributions of the beds. Carbon sequestration of each seagrass or macroalgal bed was evaluated as integrated annual plant tissue production converted to organic carbon, which was defined by subtracting dissolved organic matter production from net primary production. Plant tissue production of the main constituent macrophyte of the seagrass and macroalgal beds was directly measured in each coastal region, and production values from past reports were also collected and utilized. Annual carbon sequestration by seagrass and macroalgal beds in Japan, expressed in a CO2-converted base, was about 4.7 million tons per year, which is comparable to the CO2 emissions of the industrial sectors of agriculture and fisheries.

Effects of ocean acidification with p CO 2 diurnal fluctuations on survival and larval shell formation of Ezo abalone, Haliotis discus hannai

This study assessed the effects of constant and diurnally fluctuating pCO2 on development and shell formation of larval abalone Haliotis discus hannai. The larvae was exposed to different pCO2 conditions; constant [450, 800, or 1200 μatm in the first experiment (Exp. I), 450 or 780 μatm in the second experiment (Exp. II)] or diurnally fluctuating pCO2 (800 ± 400 or 1200 ± 400 μatm in Exp. I, 450 ± 80, 780 ± 200 or 780 ± 400 μatm in Exp. II). Mortality, malformation rates or shell length of larval abalone were not significantly different among the 450, 800, and 800 ± 400 μatm pCO2 treatments. Meanwhile, significantly higher malformation rates and smaller shells were detected in the 1200 and 1200 ± 400 μatm pCO2 treatments than in the 450 μatm pCO2 treatment. The negative impacts were greater in the 1200 ± 400 μatm than in the 1200 μatm. Shell length and malformation rate of larval abalone were related with aragonite saturation state (Ω-aragonite) in experimental seawater, and greatly changed around 1.1 of Ω-aragonite which corresponded to 1000-1300 μatm pCO2. These results indicate that there is a pCO2 threshold associated with Ω-aragonite in the seawater, and that pCO2 fluctuations produce additional negative impacts on abalone when above the threshold. Clear relationships were detected between abalone fitness and the integrated pCO2 value over the threshold, indicating that the effects of OA on development and shell formation of larval abalone can be determined by intensity and time of exposure to pCO2 over the threshold.

Feeding of the short-spined sea urchin Strongylocentrotus intermedius on macroalgae and benthic animals

To examine the feeding ecology of the sea urchin Strongylocentrotus intermedius, two types of macroalgal diet (kelp Saccharina longissima and red alga Ptilota filicina) and four types of benthic animal diet [barnacle Balanus glandula, limpet Lottia cassis, free-living Perinereis aibuhitensis and P. aibuhitensis attached to a stainless steel wire (wired polychaete)], were separately or simultaneously provided for urchins of 5- to 60-mm test diameter (TD). Sea urchins of even 3-mm TD could graze the kelp, and began to graze the red alga, the wired polychaete and limpet by 5-mm TD, the barnacle by 8-mm TD and the free-living polychaete by 15-mm TD, respectively. These results suggest that the variety of macroalgae and benthic animals which the urchin can graze increases with the ontogeny of their feeding organs, motility and ability to catch animals. Both juvenile and adult sea urchins preferred to eat the polychaete as well as the kelp fronds. Feed conversion efficiency and daily growth rate of the small sea urchins fed benthic animals like the polychaete were higher than those fed macroalgae. The sea urchins are likely to grow better when feeding on animals than on macroalgae, even in habitats where they can access adequate quantities of kelp.