Ecological studies on the community of drifting seaweeds in the south-eastern coastal waters of Izu Peninsula, central Japan. I: Seasonal changes of plants in species composition, appearance, number of species and size

Seasonal patterns of drifting seaweeds in the southeastern coastal waters of Izu Peninsula of central Japan were examined by sampling 966 patches from spring to autumn 1991–1993. In total, 57 plant species appeared, including 10 epiphytic algal species. Monthly totals of the number of species, excluding epiphytic aigae, were highest in May (33) and August (27), though 19–21 species of sargassaceous algae were found from May to August, The number of species, excluding epiphytic algae, in one patch of drifting seaweeds was 1 to 11 (x?= 2.93 ± 2.06) with high richness in May a result of almost entirely sargassaceous species. The wet weight of each patch and maximum stipe length of plants varied from 5 to 6970 g and from 20 to 840 cm (x?= 536.1 ± 782,3 g and 110.6 ± 76.8 cm), respectively, with highs in April and May. Out of 18 species common to all years, 10 species dominated the top or second rank in monthly pooled frequency of appearance. Seasonal changes of these 10 major species were examined, Sargassum horneri (Turner) C. Agardh and Hizikia fusiformis (Harvey) Okamura were abundant in April, but were replaced partly by Sargassum muticum (Yendo) Fensholt in May and largely by Sargassum yamamotoi Yoshida in June. In July, Sargassum nipponicum Yendo and Sargassum piluliferum (Turner) C. Agardh dominated. Subsequently, the major species shifted to Sargassum ringgoldianum Harvey and S. yamamotoi in August, Sargassum micracanthum (Kützing) Endlicher, Sargassum macrocarpum C. Agardh and Zostera marina Linnaeus in September, and S. ringgoldianum and S. micracanthum in October. However, the occurrence of S. yamamotoi, S. nipponicum and S. piluliferum in June or July were particularly heterogeneous compared with other areas of Japan. Dendrogram analysis was done based on frequency of appearance. Pooled monthly samples were divided into three groups characterized from the dominant species, degree of domination, weight, length and number of species of drifting seaweeds as well as the degree of diversity or evenness in appearance. This characterization indicated that the diversity and abundance of drifting seaweeds were higher from April to June than in later months.     

Sargassum boreale sp. nov. (Fucales, Phaeophyceae) from Hokkaido, Japan

A new species, Sargassum boreale Yoshida et Horiguchi is described. It belongs to the subgenus Bactrophycus section Teretia, with cylindrical receptacles and is distinct from Sargassum confusum C. Agardh, S. pallidum (Turner) C. Agardh and Sargassum microceratium (Turner) C. Agardh in having a rather elongated stem with smooth surface and distantly issuing main branches, with narrow leaves. The distinction between S. boreale and these species is also revealed by a difference in internal transcribed spacer 2 (ITS‐2) sequences. In addition to the base substitutions, the existence of a large gap in S. boreale distinguishes this species from others. Sargassum boreale is distributed around Hokkaido and Saghalien to 50°N latitude. A key to the species of section Teretia is provided.     

In situ growth rates of Sargassum horneri (Fucales,Phaeophyta)1

The relative growth rate of the brown alga Sargassum horneri (Turner) C. Agardh in the sea at Maizuru Bay, Japan, was investigated from summer to winter to examine the impact of declining solar radiation and temperature. The observed change in the growth rate between summer and winter was insignificant, in spite of a major decline in solar radiation and water temperature. The mean relative growth rate was 4.6% per day, equivalent to the daily net photosynthetic production previously reported for this species.     

Spatial and seasonal variations in the biofunctional lipid substances (fucoxanthin and fucosterol) of the laboratory-grown edible Japanese seaweed (Sargassum horneri Turner) cultured in the open sea

This work studied the effect of spatial and seasonal differences on the accumulation of functional lipid components in Sargassum horneri (Turner), an edible Japanese seaweed popularly called Akamoku. S. horneri obtained from Samenoura bay area of Japan was laboratory cultured to evaluate the effect of temperature on the accumulation of total lipids (TL), fucoxanthin (Fx) and fucosterol (Fs) by the alga. The laboratory cultured 3 month old S. horneri were cultured in the open sea in two different geographical locations off Usujiri and Matsushima to evaluate the monthly variations, over a year, in their TL, Fx and Fs contents. S. horneri grown off the Usujiri area accumulated the maximum TL close to 193 mg g⁻¹ dry weight during the coldest part of the year. Fx and Fs contributed 5.6% and 16.2% of the TL in S. horneri harvested off Usujiri in February. Further, in spite of being the same species and parent stock, S. horneri grown off the Matsushima area accumulated less TL, Fx and Fs as compared to their Usujiri counterparts. Our study clearly indicates the role of temperature and light apart from nutritional profile and depth of waters where the seaweed was grown on the accumulation of functional lipid components in S. horneri.     

THE LIFE CYCLE OE SARGASSUM HORNERI (PHAEOPHYTA) IN LABORATORY CULTURE1

The life cycle of the large dioecious alga Sargassum horneri (Turner) C. Agardh was completed in unialgal culture by controlling photoperiod in relation to the phase of growth. Embryos isolated from a naturally grown female thallus gave rise to early germlings that rapidly formed blades under both short-day (9 h L) and long-day (15 h L) conditions at 20° C Shoot elongation, which followed early blade formation, occurred under the short-day conditions hut not under the long-day conditions. Functional female and male receptacles developed when thalli 8–14 cm long grown under the short-day conditions were transferred to the long-day conditions; gamete fusion occurred when male and female thalli were grown together. Fertilized oospores gave rise to normal thalli in a manner similar to that for in situ plants. Thus, the life cycle of S. horneri was completed in laboratory culture.     

EVALUATION OF RECOVERABLE FUNCTIONAL LIPID COMPONENTS OF SEVERAL BROWN SEAWEEDS (PHAEOPHYTA) FROM JAPAN WITH SPECIAL REFERENCE TO FUCOXANTHIN AND FUCOSTEROL CONTENTS1

Fucoxanthin (Fx) and fucosterol (Fs) are characteristic lipid components of brown seaweeds that afford several health benefits to humans. This article describes the quantitative evaluation of lipids of 15 species of brown seaweeds with specific reference to Fx, Fs, and functional long‐chain omega‐6/omega‐3 polyunsaturated fatty acids (PUFAs). In addition, fatty‐acid composition of selected species was also accomplished in the study. Major omega‐3 PUFAs in the brown seaweeds analyzed were α‐linolenic acid (18:3n‐3), octadecatetraenoic acid (18:4n‐3), arachidonic acid (20:4n‐6), and eicosapentaenoic acid (20:5n‐3). Both Fx (mg · g^?1 dry weight [dwt]) and Fs (mg · g^?1 dwt) were determined to be relatively abundant in Sargassum horneri (Turner) C. Agardh (Fx, 3.7 ± 1.6; Fs, 13.4 ± 4.4) and Cystoseira hakodatensis (Yendo) Fensholt (Fx, 2.4 ± 0.9; Fs, 8.9 ± 2.0), as compared with other brown seaweed species. Studies related to seasonal variation in Fx, Fs, and total lipids of six brown algae [S. horneri, C. hakodatensis, Sargassum fusiforme (Harv.) Setch., Sargassum thunbergii (Mertens ex Roth) Kuntze, Analipus japonicus (Harv.) M. J. Wynne, and Melanosiphon intestinalis (D. A. Saunders) M. J. Wynne] indicated that these functional lipid components reached maximum during the period between January and March. The functional lipid components present in these seaweeds have the potential for application as nutraceuticals and novel functional ingredients after their recovery.     

Algal communities at Gouqi Island in the Zhoushan archipelago, China

The kelp bed of Gouqi Island lies in the east of the East China Sea, in the Shengsi Archipelago. It is a key component of the island-reef ecosystem. This study evaluated kelp beds on intertidal and subtidal zones from 2004 to 2006. We evaluated seasonal variations in the community structure of macroalgae, and their relationship with water temperature, nitrogen and phytoplankton. The Gouqi Island kelp bed consists mainly of species of Sargassum. Sargassum horneri was the dominant species in subtidal zones, representing 90% of the total biomass. The dominant species in intertidal zones were S. fusiforme, S. horneri, S. thunbergii and Undaria pinnatifida. Fifty-four phytoplankton species were found in the Gouqi Island kelp bed, of which 51 species were found inside the kelp beds, 43 species outside the kelp beds, and 40 species were present both inside and outside the kelp beds. Diatoms were dominant both inside and outside the kelp beds. Except in winter, Skeletonema costatum was dominant, and its abundance in autumn was over 98% of the total abundance. Water temperature was found to be the primary factor influencing the growth of S. horneri in the Gouqi Island kelp bed. S. horneri grew slowly from November to March, and rapidly from March to June. The upper temperature limit for growth of S. horneri was about 18°C. Higher water temperatures can result in a decline in the kelp bed. Nitrogen seemed to be a limiting factor for macroalgal growth. This was particularly true for S. horneri and phytoplankton. Since nitrate is the primary nutrient for S. horneri, its absorption by S. horneri resulted in seasonal changes of nitrate in the ecosystem.     

Taxonomy and distribution of Sargassum (Phaeophyceae) in the Gulf of Thailand

Ten species of Sargassum (Sargassaceae, Phaeophyceae) were found along the Gulf of Thailand. Morphological characteristics of Sargassum baccularia (Mertens) C.A. Agardh, S. binderi Sonder, S. cinereum J.G. Agardh, S.crassifolium J.G. Agardh, S. longifructum Tseng et Lu, S. oligocystum Montagne, S. polycystum C.A. Agardh, S. siliquosum J.G. Agardh, S. swartzii (Turner) C.A. Agardh and one unidentified species were examined and are described in detail. The most common species were S. polycystum distributed widely in almost all the study sites, S. crassifolium restricted to Prachuap Khirikhan Province, S. longifructum restricted to Chumphon Province, S. siliquosum restricted to Surat Thani Province and one unidentified species restricted to Songkhla Province. Three species (S. cinereum, S. longifructum and S. swartzii) are new records for the algal flora of Thailand. Five species (S. baccularia, S. cinereum, S. longifructum, S. polycystum and the unidentified species) belong to the section Zygocarpicae (J.G. Agardh) Setchell.     

Abundance of drifting seaweeds in eastern East China Sea

Drifting seaweeds play important ecological roles in offshore waters. Recently, large amounts of drifting seaweed rafts were found in the eastern East China Sea between the continental shelf and the oceanic front of the Kuroshio Current. However, so far there have been no quantitative reports about this particular area. Two research cruises were organized to survey abundance and standing crop of drifting seaweeds in eastern East China Sea in May 2002 and March 2004, using visual census and net sampling of drifting seaweeds. Visual census data were composed of drifting seaweed raft diameter, perpendicular distance from the transect (navigation course of the research vessel) to the raft, and positions. Using these data, we calculated the “effective stripe width” using the DISTANCE software. Drifting seaweed abundance (composed exclusively of Sargassum horneri) in waters located between the continental shelf peripheral area and the Kuroshio oceanic front was estimated to be higher than in any other area within eastern East China Sea in March and May. Abundance means in May 2002 and March 2004 were 6.14 and 29.05 rafts km⁻², respectively, while standing crop reached 126.81 and 20.35 kg km⁻² (wet weight). Mean diameter and drifting seaweed rafts in May 2002 were significantly greater than in March 2004, reflecting seasonal growth of Sargassum horneri.     

Testing systematic concepts of Sargassum (Fucales, Phaeophyceae) using portions of the rbcLS operon

The taxonomic and phylogenetic concepts within the Sargassum C. Agardh (Sargassaceae) species complex were evaluated through molecular phylogenetic analyses using portions of the chloroplast encoded rbcLS Operon. According to more conservative sequences (rbcL), Turbinaria (Turner) J. Agardh is a close and well‐supported sister lineage to the Sargassum species complex and an appropriate external outgroup for analyses of subgenera and subsections within Sargassum. Both rbcL and more rapidly evolving rbcLS spacer sequences indicated that the East Asiatic genus Myagropsis (Mertens et Turner) Fensholt, along with Sargassum sinicola Setchell et Gardner, represent the closest lineage to Sargassum and form appropriate internal outgroups. The rbcLS spacer region supported three of four subgeneric designations by J. Agardh and sectional levels within the subgenus Sargassum. However, some aspects of Agardh's system were not supported: many of the subsectional ranks or the phyletic concepts; Phyllotrichia was not monophyletic as a subgenus, and its species were also not the most ancestral of Sargassum; and subgenus Sargassum was not the most derived subgenus within the genus. This modern phylogeny suggests a deep evolutionary history for subgenus Sargassum with rapid speciation in closely related subsections and series, and a sister relationship between subgenera Arthrophycus and Bactrophycus.     

Marine management affects the invasion success of a non‐native species in a temperate reef system in California,USA

Despite promises that ‘healthy’ marine systems show increased resilience, the effects of ecosystem management strategies on invasion success in marine systems is still unclear. We show that resistance to the invasive alga, Sargassum horneri, in a temperate reef system occurs through alternate mechanisms in different ecosystem states. In an old marine protected area (MPA), invasion of S. horneri was suppressed, likely due to competitive pressure from native algae, resulting from protection of urchin predators. In a nearby fished urchin barren, invasion of S. horneri was also suppressed, due to herbivory by urchins whose predators are fished. Within newer MPAs with intermediate levels of interacting species, S. horneri was abundant. Here, neither competition from native algae nor herbivory was sufficient to prevent invasion. We confirm that invasion in marine systems is complex and show that multiple mechanisms in single systems must be considered when investigating biotic resistance hypotheses.     

Floating period of Sargassacean thalli estimated by the change in density

The density (fresh weight/volume) of the detached thalli of four sargassacean species (Myagropsis myagroides, Sargassum horneri, S. patens, S. siliquastrum) was measured weekly to determine their floating period. Thalli of the four species were collected with respect to the difference in their reproductive stage, and their floating abilities at different stages were determined. Floating thallus was ballasted with pre-weighed leads until the final addition of a weight caused sinking. The density and floating period of sargassacean thalli were dependent on the species and the specimens’ reproductive stage. The floating period of thalli detached before maturation was estimated to be between 4 and 14 weeks. However, thalli detached during maturation had a floating period of between 1 and 8 weeks, and those detached after maturation sank within 2 weeks. Sargassum horneri had the lowest density and the longest floating period among the four species investigated and this may explain the high abundance of S. horneri in floating seaweeds observed in the Sea of Japan.     

Haplotypic differentiation between seasonal populations of Sargassum horneri (Fucales,Phaeophyceae) in Japan

Conspecific populations with different life‐cycle seasonality, particularly of their reproductive season, have been reported for many seaweed species. However, the number of genetic analyses of such seasonal populations is limited. Herein, based on the mitochondrial cytochrome oxidase subunit 3 gene (cox3) haplotype, we evaluated the genetic diversity of winter (January–March) and spring (April–June) fertile populations of the brown alga Sargassum horneri in Sado Island, Japan. Obviously, dominant cox3 haplotypes were different between seasons (even at a single site), and the N _ST values indicated the presence of a strong genetic differentiation between the two seasonal populations. AMOVA analyses confirmed that most of the genetic diversity was between seasons rather than between regions. Herein, the possibility of allochronic isolation is discussed. Considering the phylogeny of the mitochondrial cox3 DNA sequence, autumn or winter seasonal populations such as those in this investigation have evolved independently at least twice in the species. The present study shows that a shift in breeding season could be expected to conserve the genetic diversity of S. horneri in a certain region.     

Growth and maturation of the 'autumn-fruiting type' of Sargassum horneri (Fucales, Phaeophyta) and comparisons with the 'spring-fruiting type'

The growth and maturation period of the autumn-fruiting population ot Sargassum horneri C. Agardh (Phaeophyta) was investigated in Hiroshima Bay, Seto Inland Sea. Surveyed traits were compared to those of the spring-fruiting type and ecological features of this species were discussed. The annual lifetime of the autumn-fruiting population could be divided into four phases according to the daily increase in thalius length:phase I from December to May (increase in length < 0.1 mm/ day), phase II from May to September (= 0.3–1.0 mm/ day), phase Ml (from September to December > 10 mm/day) and phase IV which was the senescence phase from December to March. Receptacle formation‘as observed in November and gamete release from November to February. Conversely, the spring-fruiting type germinated in April and exhibited swifter growth in its early stage of development than the autumn-fruiting type. Rapid increase in thalius iength in autumn was common in both fruiting types, although the spring-fruiting type continued to grow during winter. Receptacle formation of the spring-fruiting type started in February but gamete release was not observed until April and May. The difference in life-history patterns of both types of S. horneri was in the overwintering period. The autumn-fruiting type spent that season as germi-ings or as young plants exhibiting slow-paced growth, while the spring-fruiting type overvwintered as adult thal-li preparing for gamete release in spring.     

Egg release and germling development in Sargassum horneri (Fucales, Phaeophyceae)

The mature female conceptacle of Sargassum horneri (Turner) C. Agardh has an ostiole filled with a gelatinous plug. The oogonium in the conceptacle has cell walls that can be differentiated into a dense outer and a less dense inner microfibrillar layer. Just prior to egg release, stalk material is produced inside the outer layer and the inner layer disappears. At this stage the gelatinous plug is extruded and mucilage is released through the ostiole. The released eggs are retained on the receptacle by the stalk and are surrounded by a large amount of the mucilage. Three-celled germlings form a primary wall with a polylamellated structure of microfibril layers. In multicellular germlings that have differentiated into thallus and rhizoids, the peripheral thallus cells have an outer cell wall consisting of a microfibril layer under the primary wall, while the cell wall of the rhizoid tip has an amorphous structure. The germlings are released from the stalk and become attached to the substratum by an adhesive substance secreted from rhizoidal cells.     

Growth and size distribution of some large brown algae in Ohori,east coast of Korea

About fifty individuals of four dominant large brown algae were collected by random sampling at monthly intervals using SCUBA. Size frequencies of Sargassum confusum and S. horneri were concentrated mainly in the small size classes during the initial growth phase, but were more or less evenly distributed in most size classes during the fast growing phase. Undaria pinnatifida and Costaria costata showed the same tendency as Sargassum spp., that frequency distribution became even when the plants reached their maximum growth. The kelp species recorded their maximum lengths in March, whereas Sargassum spp. showed their peaks in early summer. The relationships between the length and weight in different growth phases appeared to be dependent on the phenology of each species.     

Finding of regeneration in a large number of wounded receptacles in Sargassum ringgoldianum (Phaeophyceae,Fucales)

Sargassum ringgoldianm is a canopy‐forming perennial macroalga dominant in a shallow waters along the central Pacific coast of Honshu, Japan, where macroalgal forests have been drastically declining by browsing of herbivorous fish since the 1990s. Accordingly, we tried to reveal seasonal fluctuation of browsing damage on S. ringgoldianum. In the course of the investigation, we found signs of receptacle regeneration in S. rinngoldianum, which has been reported in a few species of the order Fucales. Regenerations occurred most frequently from 3.0 mm to 3.9 mm of the basal part of original receptacles and were also observed at the tips of remaining stalks as well as branchlets that seemed to lose original receptacles. The regenerated receptacles formed antheridia and oogonia, and released eggs from them, suggesting that regenerated receptacles are functional. In addition, no regeneration was observed in only three individuals among 51 plants observed. We show herein a high regenerative ability of wounded receptacles in S. ringgoldianum. This frequent regeneration of functional receptacles is important for the persistence of the population.     

基于5.8S rDNA-ITS序列的我国浙江沿海铜藻群体遗传多样性分析

为了调查我国浙江沿海铜藻(Sargassum horneri)群体遗传多样性,对浙江省南麂岛火焜岙、洞头岛、枸杞岛的野生群体和南麂岛马祖岙养殖群体的铜藻44个样品的5.8S rDNA-ITS序列进行了PCR扩增和序列分析,获得DNA片段长度为1485 bp,包括部分ITS1、完整的5.8S rDNA和部分ITS2序列。序列分析表明仅有1个变异位点,说明群体间没有明显的遗传分化,遗传多样性较低。另外,将包含这一变异位点的基因型与GenBank公共数据库中另外2株铜藻的5.8S rDNA-ITS区序列进行比对,共有31个变异位点,其中插入/缺失位点10个。因此,建议加快人工铜藻场的建设以及野生铜藻场的恢复,保持铜藻场的生态平衡。     

Cultivation of the brown alga Sargassum horneri: sexual reproduction and seedling production in tank culture under reduced solar irradiance in ambient temperature

As a large conspicuous intertidal brown alga, individuals of Sargassum horneri can reach a length of more than 7 m with a fresh weight of 3 kg along the coasts of the Eastern China Sea. The biomass of this alga as a vital component in coastal water ecology has been well documented. In recent years, a steady disappearance of the algal biomass along the once densely populated coastal areas of the Eastern China Sea has drawn attention in China. Efforts have been made to reconstruct the subtidal algal flora or even to grow the alga by use of long-lines. As part of the efforts to establish an efficient technique for producing seedlings of S. horneri, in this investigation a series of culture experiments were carried out in indoor raceway and rectangular tanks under reduced solar irradiance at ambient temperature in 2007–2008. The investigation demonstrated that: (1) sexual reproduction of S. horneri could be accelerated in elevated temperature and light climates, at least 3 months earlier than in the wild; (2) eggs of S. horneri had the potential to be fertilized up to 48 h, much longer than that of known related species; (3) suspension and fixed culture methods were both effective in growing the seedlings to the long-line cultivation stage; and (4) the life cycle of S. horneri in culture could be shortened to 4.5 months, thus establishing this alga as an appropriate model for investigating sexual reproduction in dieocious species of this genus.