Tomohiko Kawamura

Dietary value of benthic diatoms for the growth of post-larval abalone Haliotis discus hannai

The feeding behaviour and the growth of post-larval abalone (1–2 mm in shell length) Haliotis discus hannai fed on nine species of benthic diatom were examined in the laboratory. Digestion efficiencies of abalone fed on the nine diatom species were also measured. All abalone showed active feeding behaviour and diatom cells were observed in their stomachs. However, the growth rates of abalone fed on four diatom species, which were readily digested, were higher than those of abalone fed on the other five species of diatom. The cell walls of the former four diatom species were easily broken open when they were grazed by abalone because of their high adhesive strength (three species), or due to the weakly silicified cell walls (one species with a low adhesive strength). The other five species, whose adhesive strengths are low, were easily ingested by abalone without any deformation of the cell walls and the majority of them were excreted whole and still alive. It is considered that post-larval abalone cannot digest diatom cell contents in their alimentary canal without first rupturing the cell wall with the radula. It is thought that the post-larvae which ingested the latter five diatom species could not readily absorb the diatom cell contents as the cell wall was intact and thus their growth rates were low.

Factors affecting the food value of diatom strains for post-larval abalone Haliotis iris

The growth and survival of post-larval abalone Haliotis iris (mean initial shell length 570 μm) fed on eight strains (seven species) of benthic diatom were examined in the laboratory. Post-larvae showed active feeding behaviour on all diatom strains. Two strains (Achnanthes longipes-1 and Nitzschia sp.) produced significantly faster growth (means 34–35 μm shell length per day) than the remaining six strains (means 11–17 μm/day). The two fast-growth strains were efficiently digested by post-larvae, with 93–94% of live cells ruptured during passage through the gut. Two of the remaining strains could not be ingested during the growth experiment because the cells were too large (A. longipes-2) or too strongly attached (Cocconeis pseudomarginata). For the other four strains (Navicula britannica, Nav. ramosissima, Navicula sp. and Nitzschia ovalis) most cells passed through the gut alive. Survival rate was highest on the two fast-growth strains, and survival was positively correlated with both growth rate (r=0.73, P<0.05) and digestion efficiency (r=0.80, P<0.05) of diatoms. The digestion efficiency of diatom strains appears to be an important factor determining their dietary value. Digestion efficiency can be influenced by diatom morphology, attachment strength, frustule strength and post-larval age/size. Isolation and use of digestible strains may improve hatchery culture of abalone.

Survival and growth rates of post-larval abalone Haliotis discus hannai fed conspecific trail mucus and/or benthic diatom Cocconeis scutellum var. parva

Abstract The settlement rate and growth rate of post-larval Haliotis discus hannai were compared in the laboratory when raised on the following three experimental substrata. (1) Mucus: trail mucus of juvenile H. discus hannai (3 cm in shell length); (2) diatom: monoculture of the benthic diatom Cocconeis scutellum var. parva; (3) diatom and mucus: C. scutellum var. parva coated with trail mucus of juvenile abalone. The mean percentage of larvae that settled were 48.7 on mucus, 98.0 on diatom, and 97.3% on diatom and mucus substrata. All post-larval abalone that settled on the mucus substratum died by the end of the third week, with a mean shell length during the third week of 711.9 μm. Individuals on the diatom substratum died by the end of the second week, with a mean shell length during the second week of 481.6 μm. Post-larvae on the diatom substratum and on the mucus substratum did not reach 800 μm in shell length; the size at which they are able to utilize the cell contents of C. scutellum var. parva. In contrast, on the diatom and mucus substratum, 70.0% of the individuals survived to the end of the experimental period (4 weeks). Individuals on the diatom and mucus substratum grew well over the entire experimental period and reached approximately 1.4 mm in shell length 4 weeks after settlement. Post-larvae reared on the diatom and mucus substratum grew to more than 800 μm by feeding initially on the trail mucus of conspecific juveniles, which was probably supplemented with extracellular diatom mucus and decomposed cells of the diatom. Once they reached about 800 μm, they were able to access diatom cell contents.

Effects of delayed metamorphosis on larval competence, and postlarval survival and growth of abalone Haliotis discus hannai

The effects of delayed metamorphosis on larval competence, and the postlarval survival and growth of Haliotis discus hannai (H. discus hannai) were examined. Competent larvae were induced to metamorphose at 5, 10, 15, and 19 days after fertilization by the addition of 1 AM g-aminobutyric acid (GABA). Larvae in another group were maintained until individuals metamorphosed spontaneously. Metamorphosed individuals (postlarvae) were reared in the laboratory at 20 jC fed on a benthic diatom Cylindrotheca closterium or without any food in equivalent conditions. Metamorphosis, survival, and growth (determined from shell length) rates were measured. The percentage of individuals that had metamorphosed 2 days after the addition of GABA increased with the length of larval swimming period; larvae that were 5 and 19 days old when induced to metamorphose showed 19% (F5.6; S.E.) and 96% (F3.6) metamorphosis, respectively. The percentage of postlarvae that metamorphosed spontaneously increased after 17 days from fertilization and reached 96% (F3.0) at 24 days postfertilization. Survival rates of fed postlarvae depended on larval swimming period. For larvae that swam for V15 days, more than 80% of postlarvae survived throughout the 20-day experimental period. In contrast, the survival rate was only 57% (F2.9) for 19-day-old larvae. Postlarval growth rates did not differ significantly between larval swimming periods V15 days. However, the growth rate of postlarvae from 19-day-old larvae was significantly lower than that of V15-day-old larvae. In the starved treatments, survival rate was lower, and the final shell length of the dead animals was less, as larval period became longer. D 2002 Elsevier Science B.V. All rights reserved.

SEASONAL CHANGES IN BENTHIC DIATOM COMMUNITIES COLONIZING GLASS SLIDES IN ABURATSUBO BAY, JAPAN

The seasonal changes in communities of benthic diatoms colonizing glass slides were observed in Aburatsubo Bay, Kanagawa Prefecture, Japan. Once every month during the study periods June 1984 to July 1985 and February to October 1987, glass slides were immersed at 1 m depth for 10 days. Thirty diatom species belonging to 14 genera were identified. Each of these species constituted more than 1% of the total cells in at least one sample. Direct sunlight duration, which is the total time exposed to direct sunlight during the immersion period, and grazing pressure seem to be major factors controlling the seasonal changes in diatom density. Generally, the total cell density of diatoms increased as the direct sunlight duration increased. However, diatom densities were comparatively low under heavy grazing pressure of amphipods in spite of long direct sunlight durations, especially in summer. Physical factors of the seawater, such as water temperature, salinity and nutrients were measured and determined to be mi...

Contribution of diatoms as food sources for post-larval abalone Haliotis discus hannai on a crustose coralline alga

Abstract Survival and growth rates of post-larval abalone Haliotis discus hannai reared on a crustose (non-geniculate) coralline alga (CCA) Lithophyllum yessoense with diatoms (CCA + diatoms) and without diatoms (CCA − diatoms) were compared in the laboratory in order to determine the contribution of diatoms on CCA as food sources for post-larvae and the dietary value of CCA themselves. Experiments were performed for 5 weeks with two groups of different developmental stage, newly metamorphosed stage (younger postlarvae) and over 1 mm stage (older post-larvae). Both stages of post-larvae reared on CCA + diatoms grew well (53.4 ± 2.4 μm/day; mean ± SE and 85.0 ± 4.0 μm/day; mean ± SE, respectively). Younger post-larvae grew to over 2 mm in 5 weeks, and older ones reached over 4 mm in 5 weeks. There was no difference in shell length of younger post-larvae reared on CCA + diatom and CCA − diatom at the week 1. However, the mean growth rate of abalone on CCA − diatoms from the second to fifth week (24.4 ± 1.9 ...

FACTORS AFFECTING SETTLEMENT OF ABALONE (HALIOTIS IRIS) LARVAE ON BENTHIC DIATOM FILMS

Abstract Diatoms are widely used in abalone hatcheries to induce larval settlement (=attachment and metamorphosis) but there are few data on the factors that influence their effectiveness as settlement cues. Of 16 diatom strains tested in Experiment 1, half induced ≥80% attachment and >50% metamorphosis within 4 days. Settlement success did not correlate significantly with diatom abundance or adhesive strength (P > 0.05). Several diatom strains interfered with settlement. Examples included (1) smothering by highly mobile diatoms (Nitzschia longissima); (2) shells becoming stuck to sticky secretions (Navicula britannica, Achnanthes longipes); and (3) unstable diatom cells preventing pedal attachment (Licmophora sp.). In Experiment 2, only 1 of 15 diatom strains induced over 70% metamorphosis, and three others induced 22–36% metamorphosis, within 4 days. Older cultures of a strain induced higher attachment and metamorphosis than younger cultures of the same strain (P < 0.001), but some old cultures still gave poor settlement. For the young diatom cultures, larval attachment correlated positively with diatom percent cover (r = 0.89, P < 0.05), and metamorphosis with the growth phase of the diatom film (r = 0.91, P < 0.05). Experiments 3–5 examined the role of bacteria in settlement induction by diatom films. In Experiment 3, films of 8 diatom species from Experiment 1 were regrown from ∼6 individually isolated and rinsed cells, likely altering the associated bacterial flora. Metamorphosis was 179 fold lower on average than in Experiment 1 (P < 0.0001), but attachment and diatom density were not significantly different (P = 0.87 and P = 0.75 respectively). In Experiment 4, Nitzschia ovalis grown and assayed with antibiotics had lower metamorphosis after 2 days than the same strain grown and assayed without antibiotics (6 vs 64%, P = 0.004), whereas attachment after 2 days did not differ (94 versus 92%, P = 0.49). In Experiment 5 bacteria from a Nitzschia ovalis culture induced as much attachment and metamorphosis as the intact diatom film, whereas cell-free supernatant was much less effective. The presence of antibiotics in settlement assays reduced attachment and metamorphosis by biofilms, but not by coralline algae (Phymatolithon repandum) or GABA. In Experiment 5 metamorphosis on diatoms and bacteria occurred gradually over 2 wk, whereas on coralline algae and GABA it occurred within a few days. This study suggests that many diatoms cue rapid larval attachment, but few induce consistently strong metamorphosis within 4 days in laboratory conditions. The bacteria present in diatom films affect the settlement-inducing activity of the film in at least some cases. The effectiveness of diatom films was generally higher for more mature films but overall, physical factors explained little of the variation in the activity of diatoms.

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.

Introduction: the coastal ecosystem complex as a unit of structure and function of biological productivity in coastal areas

Coastal seas are composed of relatively independent ecosystems, such as estuaries, rocky shores, mangroves, and coral reefs. Such individual ecosystems (IEs) are linked closely to each other by the dispersal and circulation of nutrients and organic matter and the movement of organisms, and form a coastal ecosystem complex (CEC). The CEC is understood as a unit of structure and function of coastal seas. It is characterized by a high degree of species diversity and it plays a large role in fishery and aquaculture production, as it provides important marine services for humans. This special volume compiles recent ecological studies of target species and IEs, to facilitate the conservation of coastal seas and the sustainability and production of their fisheries and aquaculture based on our understanding of the structure and function of the CEC in these environments.