1.Introduction

Abalone (Haliotis spp.) is one of the major members in subtidal rocky shore ecosystems and has been heavily harvested as a commercially important fishery resource in Japan. Declines in the abalone natural stocks, mainly due to the long time overfishing, have provoked the fisheries authorities to introduce hatchery-reared juveniles to the wild. This reseeding has been conducted for major abalone species on a commercial scale in Japan since the 1980s. However, the potential impacts of such long-term, large-scale harvesting and reseedings of abalone on subtidal rocky shore ecosystems have not been well studied. Despite the lasting reseedings, the abalone production has been declined since the late 1960s, which is mainly due to very low natural recruitment. Recent studies indicated the low natural recruitment could be caused by ecological problems, such as the decline of adult abalone population and high mortality of post-larvae. Especially, post-larval starvation was suggested to be one of the major causes for the low recruitment, which may be induced by the inter-specific competition for food sources with the other herbivorous gastropods. The previous laboratory studies on feeding and growth of post-larval abalone, H. discus hannai and H. diversicolor, have suggested three feeding transitions during the early growth stages. The inter-specific competition for food sources could be growth-dependent and critical in the abalone habitats. However, studies on ecological niches of abalone species including feeding habits in their natural habitats are very limited and the inter-specific relationships between abalone and other benthic organisms are left unsolved, which are important fundamental information to understand the natural recruitment mechanism of each abalone species.

Stable isotope analyses have increasingly been used to identify sources of dietary organic matter, which can be used for abalone species. Animals that show similar carbon and nitrogen stable isotope ratios are assumed to have similar food sources given that these sources are isotopically distinct. Thus, comparing δ13C and δ15N values may provide clues to analyses of food sources and inter-specific relationships among coexisting species in a habitat.

I examined the abundance of benthic organisms and the species composition of two latitudinally separate habitats, Tomarihama coast, Oshika Peninsula in Miyagi and Nagai coast, Miura Peninsula in Kanagawa as northern and southern habitat of abalone, respectively. Haliotis discus hannai is the only abalone species found at Tomarihama and four species, H. gigantea, H. discus discus, H. madaka and H. diversicolor, inhabit Nagai coast. Stable isotope analyses were carried out to examine the major food sources of the abalone species and to identify the trophic structures of organisms inhabiting the areas.

2.Ontogenetic change in feeding habits of H. discus hannai and H. diversicolor

Feeding experiments at seven growth stages from post-larvae to juveniles of H. discus hannai (1, 1.5, 8, 12, 22, 27 and 35 mm in initial mean shell length, respectively) were performed to test the applicability of stable isotope analyses to study abalone feeding ecology, and find the nearest isotopic fractionation values. Benthic diatoms, macroalgal gametophyte, dried macroalgae and formulated pellets were used as food sources for each growth stage of abalone. The carbon stable isotope ratios of each food source were well differentiated with a little overlaps: dried macroalgae (-12.8 ± 2.0‰), benthic diatoms (-14.2 ± 1.4‰), formulated pellets (-21.8 ± 0.2‰) and macroalgal gametophyte (-31.4 ± 1.3‰) in enriched order. Post-larvae (SL < 2 mm), fed on diatoms or macroalgal gametophyte, showed carbon isotopic values close to their food sources after two weeks of feeding. Juveniles (5-12 mm SL) approached to the carbon isotopic value of formulated pellets after six weeks of feeding. The diet effect and the post-larval feeding transition from diatom to macroalgal gametophyte were clearly revealed by stable isotope analyses, though the fractionation values during the current experiments were different among food types.

From these feeding experiments, the diet-tissue fractionation values for various growth stages of abalone were estimated to be 1.7‰ and 2.9‰ for post-larvae (SL < 2mm) fed on benthic diatoms, 2.4‰ and 2.5‰ for juveniles (5.11-12.27 mm mean SL) fed on formulated pellets, and 3.0‰ and 2.5‰ for juveniles (7.29-17.74 mm mean SL) fed on formulated pellets, for mean δ13C and mean δ15N, respectively. These results were considered to be bases for investigating the following feeding habits of abalone species in natural habitats.

The various growth stages of H. discus hannai and H. diversicolor were sampled from each coastal habitat to investigate the ontogenetic shift of feeding habits. In H. discus hannai, the carbon isotope ratio declined in the early juvenile stage from -16‰ (4 mm SL) to -20‰ (8 mm SL), indicating the diet change from benthic diatoms to juvenile macroalgae. After that, the carbon isotope ratios increased to about -17‰, suggesting the feeding transition from juvenile to adult macroalgae. In H. diversicolor, the first carbon isotopic change from -11.5‰ (4 mm SL) to -14.5‰ (9.6 mm SL) was revealed and then the following increase continued until about -11‰, suggesting the very similar dietary transitions to H. discus hannai. The similarity in feeding transitions among these two taxonomically far abalone species suggests other southern large abalone juveniles would have the similar feeding transition. However, due to a small number of juvenile samples, the ontogenetic shifts in feeding habits could not be detected for the large abalone species found at Nagai coast.

3.Community and trophic structures in the habitat of northern abalone H. discus hannai

The field samplings were conducted in July and December 2006 at Tomarihama coast, Oshika Peninsula. The sampling area was divided into 3 different vegetations within 100 m from land, from the shallowest Sargassum yezoense dominant area (Sy, < 1.5 m depth), Eisenia bicyclis dominant area (Eb, about 3 m depth) and then to the deepest crustose coralline algae dominant area (CCA, about 6m depth). Abalone was mainly collected from both Eb and CCA areas. Turban shells, Omphalius rusticus and Chlorostoma lischkei, and sea urchins, Strongylocentrotus nudus and S. intermedius were recognized as abundant species in Eb and CCA areas.

From stable isotope analyses, adult laminarian macroalgae showed -18‰ to -16‰ of δ13C, while juvenile macroalgae did -23‰ to -21‰. Sargassum has very broad range of δ13C, -21‰ to -16‰. Epilithic microalgae showed similar δ13C values to laminarian macroalgae, but could be distinguished with relatively high δ15N. The values of δ15N indicated that epilithic microalgae were most likely food for juvenile abalone (4-10 mm SL), while laminarian macroalgae for adults (SL > 55 mm). Trophic level indicated by δ15N revealed that starfishes, Asterina pectinifera and Aphelasterias japonica, were dominant predators in all areas. Several gastropods as well as crabs were also recognized as predators: Calliostoma unicum, C. multiliratum, Neptunea arthritica and Thais bronni. Sea cucumber and amphipods were suggested to be competitors for foods, mainly against adult abalone (55-98 mm SL), while chitons and limpets were considered to be much important competitors to juvenile abalone (4 mm-25 mm SL). Several small gastropods including Homalopoma spp. appeared to be more important as competitors for food with post-larval and juvenile abalone (SL < 4 mm) in CCA area.

4.Community and trophic structures in the habitat of southern small abalone H. diversicolor.

The community and trophic structures of the abalone H. diversicolor were examined by collecting various species of animals and plants in H. diversicolor habitat (1-5 m depth) and analyzing and comparing stable isotope ratios of the collected samples. The overall continuum of stable isotope ratios was structured into three different trophic linkages: brown algae- and red algae-dependent benthic food chains and a planktonic food chain. Brown algae and red algae are likely to play different roles with respect to carbon sources in the habitat. Conventional fractionation values indicated that the abalone H. diversicolor (10-65.6 mm SL; mean δ13C = -12.4‰ and mean δ15N = 9.3‰) mainly feeds on the lamina of a brown alga Undaria pinnatifida (mean δ13C = -13.5‰; mean δ15N = 6.7‰), while in the earlier growth stages (SL < 10 mm) they were suggested to have ontogenetic dietary shift from benthic diatoms (SL < 4 mm) to juvenile macroalgae (SL < 10 mm). Stable isotope ratios suggested that the juveniles of other abalone species as well as some amphipods and a sea cucumber species are competitors of H. diversicolor, whereas some Muricidae gastropods such as T. bronni and E. contractus are predators.

5. Community and trophic structures in the habitat of southern large abalone species

The habitats of southern abalone species, H. gigantea, H. discus discus and H. madaka were investigated in 2005 and 2006 at Nagai coast, Miura Peninsula. H. gigantea were most abundant among three species during the samplings. The competitors for diets of southern large abalones could not be fully investigated during the early growth stages due to the shortage of samples, but several juvenile samples (SL < 20 mm) showed very similar stable isotope values to H. diversicolor juveniles, suggesting diatom-diet and dietary competition with juvenile O. pfeifferi pfeifferi as well as amphipods and sea cucumbers. Several Muricidae gastropods were shown to be possible predator species for abalone post-larvae and juveniles. A Muricidae gastropod Ergalatax contractus was the most abundant species, which occupied more than 30% (17 ind/m2) against total abundance of all samples, suggesting very high predation pressures for post-larval and juvenile abalone.

6. Discussion and Conclusion

The dietary assimilation and the dietary shift during the early growth stages of abalone were revealed for the first time by stable isotope analyses in the present study. The isotopic shift during fast-growing post-larval (SL < 2 mm) to early juvenile stages (SL < 20 mm) were proved from experimental results and were able to be applied to the field samples, showing dietary shift from benthic diatoms to juvenile macroalgae. Between juvenile and adult stages (SL > 25 mm), another significant isotopic change was observed, which indicated the dietary change from juvenile to mature macroalgae. These shifts in major diet during the life cycle of abalone may lead to the habitat change, and subsequent changes in dietary competition and predator-prey relations, indicating different ecological niches depending on growth stages.

In the northern habitat, a large population of small gastropods such as Homalopoma amussitatum and H. sangarense had severe dietary competition pressure on post-larval and juvenile H. discus hannai. In contrast, in the habitat of southern large abalones, both carnivorous gastropods and starfishes were dominant to produce overwhelming predatory pressure on juvenile and adult abalone. In the shallow habitat of H. diversicolor with relatively good reproductive production in natural population, many herbivorous organisms such as abundant turban shells and sea urchins, occupied exclusive quantity of biomass against total community. However, the stable isotope results did not show any clear evidence that these abundant herbivorous organisms could be dietary competitors to H. diversicolor. When the topological complexity and floral diversity of the shallow habitat were considered together, the low level of dietary competition during the life cycle could be one of the reasons to explain the relatively good status of H. diversicolor population in the southern shallow habitat.

アワビ類は、古くから重要な漁業資源として利用されるとともに、人工生産種苗の大量放流が行われてきた。しかし、長年の種苗放流にもかかわらず、その資源量は減少し続けている。これは主に天然発生量の低迷が原因と考えられている。発生量の変動要因を解明するためには、生息地におけるアワビ類各種の生態とともに他の動物との相互関係、すなわち生態系における生態的地位を明らかにする必要があるが、これに係わる既往の研究はほとんどない。そこで本研究では、アワビ類各種の生息地における生物種組成を調べ、安定同位体比を用いて餌生物や競合種、捕食種を成長段階毎に推定することにより、各種の生態的地位の解明を目的とした。調査は、牡鹿半島泊浜と三浦半島長井で実施した。泊浜にはエゾアワビが、長井にはクロアワビ、メガイアワビおよびそれらより小型のトコブシが生息する。長井では、主にトコブシが生息する地点(水深1~5 m)と大型アワビ類2種(暖流系大型アワビ類)が生息する地点(6 ~10 m)の2カ所で調査を行った。

第1章の緒言に続き第2章では、安定同位体比を用いて、天然生息地におけるエゾアワビとトコブシの成長に伴う食性の変化を推定した。一般に、栄養段階が1上がると炭素および窒素安定同位体比がそれぞれ約1 ‰、3.4 ‰上がるとされ、この値を用いて餌の推定や食物連鎖の解析が行われているが、これらの濃縮係数は生物種や成長段階によって異なる可能性が示唆されている。そこで、殻長1~35 mmの7成長段階のエゾアワビに、付着珪藻5種、ワカメ幼体、マコンブ成体、人工餌料を与えて飼育し、安定同位体比の変化から濃縮係数を推定した。その結果、炭素および窒素の濃縮係数は1.6 ‰および2.2 ‰と推定された。これらの濃縮係数と実際に調査海域で採集された各種植物およびアワビ類の安定同位体比から、エゾアワビとトコブシの成長段階毎に天然生息域での餌料を推定した。エゾアワビの炭素安定同位体比は、殻長5~8 mmで顕著に減少し、20~30 mmで増加に転じた。この結果は、主餌料が付着珪藻から海藻幼体、海藻成体の順に変化したことを示すと考えられた。調査時における稚貝(殻長1.8~6.9 mm)と成貝(38.2~117.5 mm)の主餌料として、付着珪藻と紅藻マクサ、褐藻ワカメとアラメの成体がそれぞれ推定された。トコブシでもエゾアワビとほぼ同様に炭素安定同位体比が変化し、成長に伴う主餌料の変化は類似すると考えられた。稚貝(4.2~10.4 mm)と成貝(34.8~77.1 mm)の主餌料として、褐藻アラメの幼体と紅藻マクサ、アラメとワカメの成体および幼体が推定された。

第3章から第5章では、それぞれエゾアワビ、トコブシ、暖流系大型アワビ類生息地における食物網構造を解析し、成長段階毎に競合種と捕食種を推定した。エゾアワビ生息地では、異なる海藻群落が水深に伴い帯状に発達しており、成貝は水深2~5 mの褐藻アラメ群落内に、稚貝は水深6 m以上の紅藻無節サンゴモ群落内に多く生息していた。安定同位体比からエゾアワビ成貝と餌料を巡り競合する可能性があると考えられたのはマナマコ、ワレカラ類であり、稚貝と競合する可能性がある種は見つからなかった。濃縮係数1および3.4を使い、捕食種の可能性があるものを推定すると、成貝に対してはイトマキヒトデ、ショウジンガニなどが考えられたが、稚貝に対しては見つからなかった。トコブシ生息地では、トコブシ稚貝と成貝は同所的に生息していたが、安定同位体比は異なり、生態的地位が異なることがわかった。稚貝の競合種として可能性があるものはカサガイ類など非常に多かったが、成貝の競合種と考えられたのは大型アワビ類稚貝、フジナマコおよびヨコエビ類のみであった。稚貝と成貝の捕食種としては、イトマキヒトデやヒメヨウラクなど小型巻貝類が考えられた。暖流系大型アワビ類の稚貝の競合種としてはサザエとウスヒザラガイ、捕食種としてはアカヒトデとエビスガイが考えられた。成貝に対して競合種および捕食種の可能性があるものは認められなかった。

第6章の総合考察では、天然生息地におけるアワビ類の食性変化を論議し、アワビ類各種の生態的地位を比較した。アワビの種によって分布様式と生息地の環境は異なるが、食性や競合種、捕食種は非常によく似ていることが明らかとなった。しかし、稚貝と成貝では生態的地位が異なることがわかった。

以上、本研究は、安定同位体比を用いた野外調査と室内実験の組み合わせにより、アワビ類各種の天然生息地における成長に伴う食性の変化を解明し、成長段階毎に生態的地位の概要を明らかにした。本研究の結果は、アワビ類の生態的特性を考慮した資源管理や放流事業の確立に必要不可欠な基礎的情報を提供するだけではなく、今後の岩礁生態系における新しい研究手法を提示した点でも重要である。よって審査委員一同は本論文が学位(農学)に値するものと判断した。