Comparative life-history study on sympatric hyperiid amphipods (<Emphasis Type="Italic">Themisto pacifica</Emphasis> and <Emphasis Type="Italic">T</Emphasis>. <Emphasis Type="Italic">japonica</Emphasis>) in the Oyashio region,western North Pacific

Life-history features of the sympatric amphipods Themisto pacifica and T. japonica in the western North Pacific were analyzed based on seasonal field samples collected from July 1996 through July 1998, and data from laboratory rearing experiments. T. pacfica occurred throughout the year, with populations peaking from spring to summer. In contrast, T. japonica were rare from autumn to early winter, but became abundant in late winter to spring. Mature T. pacifica females and juveniles occurred together throughout the year, indicating year-round reproduction. Mature T. japonica females were observed only in spring, and juveniles occurred irregularly in small numbers, suggesting limited, early-spring reproduction in this study area. Size composition analysis of T. pacifica identified a total of eight cohorts over the 2 years of the study. Due to the smaller sample size and rarity of mature females (>9.6 mm) and males (>7.1 mm), cohort analyses of T. japonica were not comparable. Laboratory rearing of specimens at 2°C, 5°C, 8°C and 12°C revealed that a linear equation best expressed body length growth by T. pacifica, while a logistic equation best expressed body length growth by T. japoncia. Combining these laboratory-derived growth patterns with maturity sizes of wild specimens, the minimum and maximum generation times of females at a temperature range of 2–12°C were computed as 32 days (12°C) and 224 days (2°C), respectively, for T. pacifica, and 66 days (12°C) and 358 days (2°C), respectively, for T. japonica. The numbers of eggs or juveniles in females marsupia increased with female body length and ranged from 23 to 64 for T. pacifica and from 152 to 601 for T. japonica. Taking into account the number of mature female instars, lifetime fecundities were estimated as 342 eggs for T. pacifica and 1195 eggs for T. japonica. Possible mechanisms for the coexistence of these two amphipods in the Oyashio region are also discussed.Communicated by O. Kinne, Oldendorf/Luhe     

Salinity favorable for early development and gamete compatibility in two sympatric estuarine species of the genus Hediste (Polychaeta: Nereididae) in the Ariake Sea, Japan

Early development was examined, under various salinities, for two sympatric nereidid polychaetes, Hediste japonica and H. diadroma, which participated in a simultaneous reproductive swarming in an estuary of the Omuta-gawa River in Ariake Sea, Japan. The eggs of both species were isotonic to the medium of 27.5–30 psu salinity. The egg diameter in the isotonic salinity was 180–205 μm in H. japonica, and 130–160 μm in H. diadroma. Successful development of most embryos was observed in a salinity range of 22.5–30 psu in both species, while successful fertilization occurred in wider ranges of salinity, i.e., 10–34 psu in H. japonica and 10 to 30 psu in H. diadroma. In both species, free-swimming larval life started from the indistinct hatching of trochophores out of a jelly layer capsule. The lecithotrophic development appeared to run to the 4-setiger nectochaetes in H. japonica, while to 3-setiger nectochaetes in H. diadroma, resulting in a shorter pelagic larval life in H. japonica. In a comparison of larval morphology among Hediste species, we found a definite negative correlation between the prostomium width, which represents the larval size and depends on egg size, and relative length of chaetae to the prostomium width: the relative length of chaetae was the longest in H. diadroma (with the smallest egg size and long pelagic life), intermediate in H. japonica (intermediate egg size, short pelagic life), and the shortest in H. atoka (largest egg size, no true pelagic life). We also examined the possibility of hybridization between H. japonica and H. diadroma through cross-insemination experiments. The gametes of the two species were reciprocally compatible, and viable hybrid offspring were produced by the laboratory crosses. The hybrid larvae expressed intermediate phenotypes, but with a greater maternal influence in characteristics such as the relative length of chaetae and the lecithotrophic larval duration.     

Resource utilization for decorating in three intertidal majid crabs (Brachyura: Majidae)

 For three spider crabs (Tiarinia cornigera, Micippaplatipes and Pugettia quadridens quadridens), patterns of algal utilization for decorating were compared with the dynamics of algae on an intertidal rocky shore reef where the crabs co-occurred. T. cornigera and P. quadridens quadridens were most abundant from autumn to spring when the dominant algae (Boodleacoacta, Sargassum hemiphyllum, S. thunbergii and Corallina pilulifera) occurred in high coverage, while M. platipes was most abundant from spring to autumn. Monthly change of algae used for decorating was not correlated with algae growing in the crab habitat for T. cornigera, but for M. platipes, it was positively correlated for two algal species, and for P. quadridens quadridens, negatively correlated for one algal species. Each species of the spider crabs used some algal species preferentially for decoration. Decorating preference experiments conducted in the laboratory showed that M. platipes and P. quadridens quadridens exhibited similar preference to their algal utilization in the field, whereas for T. cornigera, algal preference in the experiment differed from utilization in the field. Comparisons between materials used for decoration and gut contents revealed that T. cornigera and M. platipes used algal species differently for decorating and feeding, while P. quadridens quadridens used the same algal species for both decorating and feeding. Different tactics for camouflage are discussed in terms of algal utilizations by the three majid species. Received: 25 September 1999 / Accepted: 22 June 2000     

Phenology and demography of a marine specialist herbivore: Placida dendritica (Gastropoda: Opisthobranchia) on the central coast of Oregon

The occurrence of small specialist herbivores in marine rocky intertidal communities has been largely ignored. From 1984 to 1989, the phenology and demography of the common oligophagous ascoglossan opisthobranch Placida dendritica A.& H. was documented at several sites along the central coast of Oregon, USA. P. dendritica was found on the low intertidal green algae Codium setchellii Gardn., C. fragile (Sur.) Har. and Bryopsis corticulans Setch. during spring and summer. Ascoglossan abundance on Codium spp. varied spatially and temporally. Peak density usually occurred in May or June. Maximum ascoglossan live mass was 4 to 6 mg on Codium spp. and 100 mg on B. corticulans. Larval recruitment of P. dendritica to algal hosts was continuous during spring and summer. Rapid recruitment (200 to 400 sea slugs thallus^-1mo^-1) of newly metamorphosed ascoglossans to C. setchellii in wave-protected areas suggests that high densities of competent larvae occurred in nearshore waters and/or that larvae were efficient at locating algal hosts. Ascoglossan growth was rapid, with less than one month elapsing from larval settlement to adult sexual maturity. Therefore, populations of P. dendritica were composed of many overlapping generations. Movement of adult ascoglossans among thalli of C. setchellii was rapid. Continuous larval settlement and adult movement enabled P. dendritica not only to locate and colonize its algal hosts quickly but also to maintain a persistent association with algal hosts during spring and summer.     

The effect of micrograzers on algal community structure in a coral reef microcosm

The effect of amphipod grazing on algal community structure was studied within a 75 l refuge tank connected to a 6500 l closed-system, coral reef microcosm. When amphipods (Ampithoe ramondi) were absent or present in low numbers, a high biomass of mostly filamentous algal species resulted, including Bryopsis hypnoides, Centroceras clavulatum, Ceramium flaccidum, Derbesia vaucheriaeformis, Enteromorpha prolifera, Giffordia rallsiae, and Polysiphonia havanensis. These microalgae disappeared when amphipod density increase beyond approximately 1 individual cm^-2 of tank surface. The macroalga Hypnea spinella germinated in the system in association with amphipod tube sites. H. spinella plants remained rare until filamentous species were eliminated by amphipod grazing. Feeding trials confirmed that H. spinella was protected from grazing by its size rather than a chemical defense strategy. The H. spinella community we observed is similar to the flora described on algal ridges where physical conditions exclude fish grazing. We suggest that amphipods and similar micrograzers are responsible for the algal community structure of these ridges. Caging experiments may be subject to similar effects from increased amphipod grazing on the algae. Introduction of fish that are amphipod predators into the refuge tank caused an increase in algal species diversity but total H. spinella growth rates fell from 25 g dry wt month^-1 to less than 8 g dry wt month^-1. We describe amphipod behavior in relation to changes in population density and food supply, and we stress the potential for increasing the productivity of commercial seaweeds through maintenance of appropriate amphipod species in mariculture facilities.     

Chemoautotrophic symbiosis in the tropical clamSolemya occidentalis (Bivalvia: Protobranchia): ultrastructural and phylogenetic analysis

Patterns of algal seasonality, and their effect on the diet and feeding preferences of the herbivorous crab Grapsus albolineatus were investigated over an 18-mo period from March 1993 to August 1994 on an exposed tropical rocky shore (Hok Tsui Peninsula at Cape d' Aguilar, Hong Kong). Algal cover was greatest in the winter months, and lowest in the summer. Foliose algae such as Ulva fasciata, Porphyra suborbiculata, and Dermonema frappieri were dominant in the winter, but died off in the summer. During the hot summer months, perennial encrusting algae e.g., Ralfsia expansa, Hildenbrandia rubra, H. occidentalis, coralline crusts and the encrusting cyanobacteria Kyrtuthrix maculans, were the dominant algal species. Seasonal variation in algal abundance influenced the dietary selectivity of the herbivorous crab G. albolineatus. In the winter, the crab fed selectively on filamentous algae (e.g. Hincksia spp., Cladophora spp., Enteromorpha spp., and the cyanobacteria Lyngbya sp.). Foliose algae (e.g. U. fasciata, P. suborbiculata, Pterocladia tenuis) formed a small part of the diet, despite being the dominant species on the shore. Foliose and filamentous algae were virtually absent from the shore in the summer, and the crabs switched to feeding solely on encrusting algae. Electivity indices revealed preferences for green and brown turf species, and avoidance of foliose algae. Faecal analysis revealed that a greater proportion of the food is digested in the winter, suggesting that G. albolineatus is able to digest filamentous algae more efficiently than encrusting algae. Feeding preferences of G. albolineatus appear to be influenced by a number of factors, including the availability, digestibility and morphology of algae. The foraging behaviour and cheliped morphology of the crab also affect food choice. The monsoonal nature of Hong Kong's climate controls the diversity and abundance of intertidal algae and, therefore, indirectly influences the diet and subsequent growth and reproductive success of the herbivorous crab G. albolineatus.     

Dietary preferences of the opisthobranch mollusc Stylocheilus longicauda for secondary metabolites produced by the tropical cyanobacterium Lyngbya majuscula

Pure compounds isolated from the cyanobacterium Lyngbya majuscula Gomont were evaluated in an artificial diet for their influence on the feeding preferences of the sea hare Stylocheilus longicauda (Quoy and Gaimard, 1824), which lives in and feeds on this filamentous cyanobacterium (blue-green alga). Microcolin B, ypaoamide, malyngolide and other natural products acted as feeding deterrents at natural concentrations. At lower concentrations, sea hares were indifferent to ypaoamide and malyngolide in their diets. In contrast, barbamide stimulated sea hare feeding at the concentrations normally found in L. majuscula. Malyngamides and majusculamides, the most common natural products found in samples of L. majuscula from Guam, increased sea hare feeding at low concentrations and inhibited feeding at the higher concentrations that occurred in some collections of L. majuscula. Dietary selection of cyanobacteria by S. longicauda may be regulated by the concentration of specific chemical cues produced by L. majuscula. Received: 28 January 1997 / Accepted: 24 April 1998     

Growth and development of Calanus spp. (Copepoda) during spring phytoplankton succession in the North Sea

Experiments were carried out to determine growth and development rates of the herbivorous copepod Calanus finmarchicus (Gunnerus) under natural conditions during the phytoplankton spring bloom in the northern North Sea. From 28 April to 25 May 1983 copepodite stages I, IV and V were incubated for a 3-d period on board a ship in vessels with naturally occurring phytoplankton or cultured algae as food. Highest rates of growth and development were achieved while the diatom Chaetoceros sp. was the dominant phytoplankton organism. These rates decreased considerably when this chain-forming diatom was succeeded after one week by the small-celled diatom Thalassiosira conferta. Again one week later, during the bloom of the succeeding colonial microflagellate Corymbellus aureus, copepodite stage IV still managed to maintain moderate rates of growth and development, but these rates dropped to almost zero in CV, suggesting the start of a resting stage. Nevertheless, brood collected from this generation and from Calanus helgolandicus (Claus) was raised in the laboratory to the adult stage at high speed. Since temperature and the total phytoplankton concentration in the sea remained almost constant it seems that the retardation and arrestment of growth and development were an immediate response to a qualitative change of the food composition related to the successive blooms of different algal species.     

Seasonal changes in feeding, gonad development and lipid stores in Calanusfinmarchicus and C. hyperboreus from Malangen, northern Norway

 Seasonal dynamics of feeding activity, oil sac volume, gonad development, sex ratio and spawning periods in the two sibling species Calanusfinmarchicus and C. hyperboreus, the key zooplankton copepod organisms throughout the northern Atlantic waters, were studied simultaneously in Malangen, northern Norway, during 1992. We were also tracing differences in surface- and deep-dwelling components of these populations in terms of oil sac volume and gonad development during the time period when the G₁ is preparing for a subsequent generation (G₂) or hibernation. The main difference in the life cycle strategies of these species is the earlier maturation and spawning of C. hyperboreus. No feeding activity in either of the two species was found in February, but both commenced feeding in March, prior to the spring phytoplankton peak. The larger copepod, C. hyperboreus, had a more intensive energy deposition than C. finmarchicus. The period of active feeding was much shorter for the former species, only from March through July in copepodite stages CIV and CV, and even less in females – March and April. Basically, a similar pattern of seasonal changes in gonad length and lipids was observed in the two species. In June, oil sacs in the surface- and deep-dwelling specimens were about equal, during the rest of the year, lipids in the deep CVs exceeded those in the surface. We propose that as copepods accumulated sufficient lipid reserves, they started to descend, while others, containing less fat, stayed in the upper layers feeding. The mean length of the gonads in the surface-dwelling copepods was consistently less than in their deep counterparts from October to February, so that gonad development at the expense of accumulated reserves during resting stage was confirmed. C. finmarchicus males were found in considerable numbers only in February and March, and were only occasionally found in the upper layers (0–100 m), while adult male C. hyperboreus were present from October to March, but were never found in the surface layers. The differences in life cycle timing among the two species are discussed in relation to tradeoffs with regard to foraging strategies, generation numbers, bioenergetics and predator avoidance. Received: 31 March 1999 / Accepted: 23 November 1999     

Recruitment and juvenile survival in the sea hare Aplysia juliana (Gastropoda: Opisthobranchia)

The specificity in settling response of larval Aplysia juliana (Quoy and Gaimard, 1832) on species of its food algae, Ulva spp., provided a rare opportunity for the quantitative study of recruitment and early survival rates in a non-sessile marine invertebrate. Post-metamorphic juveniles spend about 3 weeks feeding and growing on the algae before moving to the rocky habitat of the adults. Recruitment and survival were studied by collecting weekly samples of algae and holding them in the laboratory until the recruits were large enough to be seen and reliably counted. Recruitment occurred throughout the year, although the rates were relatively low in late winter and spring. Recruitment was monitored relative to algal weight and bottom area. Variation in larval abundance and algal standing crop influenced recruitment to the study area. Mortality during the juvenile stage was very high in all cases, although the rates and shapes of the survivorship curves varied between algal species and location.     

Feeding preferences,seasonal gut repletion indices,and diel feeding patterns of the sea urchin<Emphasis Type="Italic"> Tripneustes gratilla</Emphasis> (Echinodermata: Echinoidea) on a coastal habitat off Toliara (Madagascar)

The feeding biology of the echinoid Tripneustes gratilla from a seagrass bed off Toliara, on the south-west coast of Madagascar, was investigated during the summer and winter seasons. Repletion indices of the gut of T. gratilla varied seasonally, in such a way that higher amounts of food occurred in the gut in winter than in other seasons. Conversely, a more detailed analysis of one summer and one winter sample of the gut tissue dry weights revealed higher weights in summer (February) than in winter (August). This was interpreted as a varying capability of nutrient storage and closely related to the feeding activity of the echinoid. This inverse relationship between feeding activity and nutrient storage was suggested to be dependent on gonadal growth. Indeed, higher food consumption was observed during the post-spawning period (August), that is, when most of the energy accumulated was used up for gonadal growth. Low feeding activity occurred when a sufficient amount of energy was accumulated in gut tissues (February), the time corresponding to the initiation of gonadal growth. Investigations on the feeding habits, using Ivlev's electivity index, revealed preferential feeding of T. gratilla on Syringodium isoetifolium, the dominant seagrass species found in the gut. This was partly related to the higher availability of that particular seagrass species in the field. However, its soft and terete leaf blade's morphology also makes it easier to manipulate and ingest. Studies on the feeding behaviour indicated the presence of a diel feeding cycle in T. gratilla individuals in both summer and winter seasons. Yet, its pattern varied between the two studied months (one summer and one winter) as the cycle was less contrasted during period of active feeding, i.e. in winter compared to summer. This study provides novel information on how intrinsic factors, such as physiological state, can govern the feeding activity of T. gratilla living in a seagrass habitat.     

Annual reproductive cycle of the chiton Acanthopleura japonica

Annual reproductive cycle of the chiton Acanthopleura japonica (Lischke) was studied by observing both gonad index and histological features from October 1980 to October 1981. No significant changes in each factor were observed from October to April. Increased values of gonad index and spermatogenesis or vitellogenesis in gondas were observed from May to September. In the same time, gonad index decreased and sperms or oocytes in gonalds disappeared in the days around spring tide, and they were recovered around neap tide. Suggested spawning around spring tide from May to September is also supported by the occurrence of released eggs in the days around the new and full moons.     

Effect of suspended bottom material on growth and energetics in Mytilus edulis

The influence of suspended, natural silt (0 to 20 mg l^-1) in addition to unicellular algal cells (Phaeodactylum tricornutum) (o to 20.000 cells ml^-1) on clearance, growth and energetics in Mytilus edulis has been studied. Clearance increased by 32 to 43% by the addition of 5 mg silt l^-1 as compared to clearance in a pure algal suspension. Ingestion and growth rate increased with algal concentration, and growth rate was further increased by 30 to 70% by the addition of 5 mg silt l^-1. A growth rate comparable to maximum natural growth rates was reached only at the highest algal concentration in the presence of 5 mg siltl^-1. Assimilation efficiency of P. tricornutum decreased from 77% at 5,000 cells ml^-1 to 52% at 20,000 cells ml^-1. In the experiments with silt added, some 20 to 30% of the assimilated organic matter originated from the suspended bottom material. Net growth efficiency increased with growth rate at a decelerating rate, approaching a maximum of about 70%. It is concluded that suspended bottom material, which is always present in M. edulis' natural habitats, serves as an additional food source, and that M. edulis depends on suspended bottom material to exploit fully its clearance potential, and to reach the maximum growth rates observed in nature.     

Animal community structure and energy budget calculations of a Daphnia magna (straus) population in relation to the rock pool environment

The fauna and environmental conditions in a freshwater rock pool ecosystem were followed by a weekly quantitative sampling program from April to August, 1974. The rock pool, situated on a small island in the northern Baltic Sea, was heavily eutrophicated by droppings from the surrounding colony of sea birds. The intermittent flushing of the pool with rainwater and the input of bird droppings, as well as the biological activity, contributed to the large seasonal variations in nutrients and organic matter that were observed. A dense algal bloom of flagellates occurred in April but vanished when the animal population started to increase in the middle of May. During the rest of the summer, most of the photosynthetic pigments were found in the bottom sediment, mostly as degraded phaeo-pigments due to intensive grazing by the animals in the pool. Very few animal taxa were found and the phyllopod Daphnia magna (Straus) dominated throughout the whole sampling period. D. magna contributed to more than 50% of the total biomass, except in late July when chironomid larvae were most abundant. The total biomass in the pool increased from about 15 mg (dry weight) l^?1 in May to a maximum of about 60 mg l^?1 at the end of June. There were few carnivores in the system, except during the spring when the water bug Deronectes griseostriatus (de Geer) was common.Analyses of size, age and sex structure and calculations of birth and death rates of the D. magna population showed large seasonal variations, correlated with the volume fluctuations and flushings of the pool which stimulated both the algal production and the growth and reproduction of Daphnia. Data from the extensive literature that exists on D. magna and other species of the same genus were used, together with field and experimental data from the rock pool population, in a numerical model describing the energy budget of this species. The model describes variations in weight-specific growth, reproduction, moulting, feeding and respiration rates in relation to temperatures and food concentrations. Energy budget relationships that maximise the chances for survival and utilisation of the available energy for a population exposed to varying food concentrations are predicted by the model. The energy budget model was also used to estimate the secondary production of the rock pool population of D. magna. The total production between April and August was 203 mg (dry weight) l^?1 and the average production per biomass ratio was 0.094 day^?1. The average net production efficiency was 42%, very close to other values reported for D. magna from other biotopes. The relative importance of different factors controlling the production was also analysed with the model. Production per biomass ratio was calculated, assuming a constant temperature and/or food concentration for the whole sampling period. The varying food concentrations in the rock pool had the most pronounced influence, greater than that of temperature and size structure variations in the population.     

Ecological aspects of ophiuroids from the phytal of S. W. Atlantic Ocean warm waters

Nine species of ophiuroids — Amphipholis januarii, Axiognathus squamatus, Hemipholis clongata, Ophiocnida scabriuscula, Ophiactis lymani, Ophiactis savignyi, Ophionereis reticulata, Ophioderma appressum and Ophiothrix angulata —were collected in various habitats from the mid-littoral and upper infralittoral on the northern coast of the State of São Paulo, Brazil. The abundance of 6 species occurring in 28 species of algae was investigated. A total of 220 samples was collected at monthly intervals from April, 1969 to May, 1970. The distributional pattern of the populations, as determined by Mobisita's index, shows aggregation of the ophiuroids. No ophiuroid species was limited to one algal species. Each species prefers certain algal species and/or a different level on the shore. The most abundant species on algae was Ophiactis lymani, with a maximum density of 1,510 individuals/100 ml of algae. Four species of ophiuroids occurred inside sponges. The most abundant species in sponges was O. savignyi, with a maximum density of 1,892 individuals/100 g of dry sponge. Two species of ophiuroids occurred inside the bryozoan Schizoporella unicornis. Feeding observations have shown Axiognathus squamatus to capture its food by mucous nets. Decapod crustaceans fed on ophiuroids in aquaria. The copepod Thespesiopsyllus sp. was found in the body cavity of the 2 Ophiactis species. Simultaneous sexual and asexual reproduction were abserved in O. savignyi.     

The ecology of temperate salt-marsh fucoids. II. In situ growth of transplanted Ascophyllum nodosum ecads

Growth, in terms of length, weight, and number of branches and/or dichotomies, in transplanted specimens of Ascophyllum nodosum ecad scorpioides in a temperate salt marsh is described. The ecad scorpioides, when transplanted from its characteristic habitat on the mid-intertidal, Spartina alterniflora-dominated, marsh flats to a location near mean low-water developed characteristics normally associated with A. nodosum ecad mackaii. The growth of these plants was more rapid than those in the mid-intertidal region and was not affected by the shading of algal fronds by S. alterniflora. Unusually high temperatures and light intensities during the winter and spring months were major factors affecting growth in plants that were subjected to relatively long periods of tidal exposure. The presence of S. alterniflora during the summer months may act in a protective capacity for mid-intertidal ecad populations.This research was supported by research grants AG-375 and BO 38018 from the National Science Foundation and, in part, by the State University of New York Research Foundation and the Energy Research and Development Administration (ERDA).     

Annual cycle of biomass of a threatened population of the intertidal seagrass Zostera japonica in Hong Kong

The phenology and primary productivity of a population of Zostera japonica (Aschers. & Graebn.) threatened by the construction of Hong Kong's new international airport were studied over a 12-month period. The need to conserve the population, and the small leaf size of Z. japonica rendered traditional destructive or marking techniques inapplicable for percentage cover and biomass estimation. A nondestructive method based on image analysis techniques was therefore devised for repeated estimation of percentage cover, biomass and leaf area index. This technique, which involved random quadrat sampling, photographic recording and image analysis, was able to provide data on the three parameters with acceptable precision and was cost-effective in the field. Z. japonica demonstrated a strongly seasonal cycle of vegetative growth, with different patterns for leaf density (peak in March) and overall bed area (peak in June). Total (above- and below-ground) net primary productivity was estimated at between 344 and 688 g AFDW m⁻² yr⁻¹. Percentage cover of Z. japonica was negatively correlated with total suspended solids (TSS) in the water column while total bed area was negatively correlated with water salinity. Increased sedimentation associated with the new airport project was identified as one important factor affecting the growth of the seagrass, as TSS reached the high level of ≈1 g DW l⁻¹ during the first half of the study period. Sediment traps set in the beds also recorded potential sedimentation rates at between 2.89 and 14.5 mg cm⁻² d⁻¹. This high turbidity resulted in a sharp decrease in the density of Clithon spp., the dominant grazers of epiphytic algae on Z. japonica. Effects of sedimentation and shading on growth of Z. japonica were investigated by field manipulative experiments. Experimental increase of sedimentation rate and shade both resulted in larger decreases in percentage cover and above-ground AFDW compared with the control. Received: 3 March 1997 / Accepted: 14 March 1997     

Population biology of hyperbenthic crustaceans in a cold water environment (Conception Bay,Newfoundland). I.<Emphasis Type="Italic"> Mysis mixta</Emphasis> (Mysidacea)

Seasonal population dynamics of Mysis mixta Lilljeborg were studied from December 1998 to November 2000 at a 240 m deep site in Conception Bay, Newfoundland. At this depth, temperature was <0°C and salinity between 32.0 and 34.0 psu year-round. The spring phytoplankton bloom began in early or late March and reached a maximum in late April to mid-May. M. mixta exhibited a highly synchronised life cycle, with spawning and mating occurring in October to November, embryos brooded for ~5 months, and juveniles released during spring bloom sedimentation in April and May. Females were semelparous and died at age 2.5 years, following release of juveniles in spring, whereas the majority of mature males died at age 2 years, following mating in November. The biennial life cycle of this population resulted in the presence of two cohorts in the hyperbenthos at any given time. Variation in density and biomass was low among cohorts but high within cohorts, the latter probably due to the high motility of mysids. Densities in 1999 and 2000 were 242±379 and 544±987 ind. per 100 m³ (mean±SD), respectively. Although growth rates were similar between years, rates measured from changes in dry mass differed both seasonally and among life-history stages (range from –4 to 7 mg month^–1). Annual secondary production was estimated at 29–73 mg C m^–2 in 1999 and 53–205 mg C m^–2 in 2000. The annual P/B ratios were 1.62 and 1.19 in 1999 and 2000, respectively.Communicated by J.P. Grassle, New Brunswick     

Stable isotope analysis of two species of anguilliform leptocephali (<Emphasis Type="Italic">Anguilla japonica</Emphasis> and <Emphasis Type="Italic">Ariosoma major</Emphasis>) relative to their feeding depth in the North Equatorial Current region

Anguilla japonica leptocephali are transported from their offshore spawning area to their recruitment areas in East Asia, but their depth distributions, food sources and feeding are still poorly known. This study analyzed carbon and nitrogen stable isotope ratios of leptocephali of A. japonica, Ariosoma major and Ariosoma spp., and of particulate organic matter (POM), their likely food source, at five different depths in 2004–2009. We used mixing models to show that A. japonica appeared to be feeding at depths between 5 and 50 m, but sometimes deeper. A. major appeared to have a tendency of mostly feeding at depths of 50 m or shallower. Although the A. japonica and Ariosoma spp. collected in the same area during the leptocephalus stage appeared to have different feeding ecologies possibly related to different types of POM, their different depth distributions, sizes and transport histories may also help explain these differences.     

Habitats and tidal accessibility of the marine foods of dabbling ducks and brant in Boundary Bay,British Columbia

The role of birds in intertidal food webs is mediated by effects of tides on the habitat, distribution, and accessibility of food organisms. We studied effects of elevation, sediment characteristics, vegetation, and tidal cycles on waterfowl feeding and food organisms in Boundary Bay, British Columbia, Canada from September to March 1990–1992. Percent dry mass of esophageal contents was mainly Zostera japonica leaves (84%) for American wigeon (Anas americana Gmelin), and leaves of Z. japonica (57%) and Z. marina (41%) for brant (Branta bernicla L.). Northern pintails (A. acuta L.) and mallards (A. platyrhynchos L.) ate mostly the leaves, seeds, and rhizomes of Z. japonica (48 and 72%, respectively) and gastropods (12 and 28%). Green-winged teal (A. crecca L.) ate mainly amphipods (68%) and saltmarsh seeds (19%). The well-sorted fine sand (median 0.125 mm) to medium sand (0.25 mm), and the low organic carbon content (1.31±0.12% dry mass), did not appear to vary sufficiently along 2.1- and 2.6-km transects to affect macroinvertebrate distributions. Three habitat zones were recognized: (1) upper intertidal (sandflats); (2) mid-intertidal (largely monotypic Z. japonica); and (3) lower intertidal (mixed Z. japonica and Z. marina). These zones, although related to elevation, had greater influence on amphipod distributions than did elevation alone or detrital biomass. Biomass and numbers of most invertebrate foods of waterfowl were greatest in the eelgrass zones, with the upper elevational limit of Z. japonica occurring at about Mean Water Level (MWL). Accordingly, wigeon, pintails, and mallards fed mostly below MWL. Because of seasonal tidal cycles, daily accessibility of areas below MWL but within reach of surfacefeeding waterfowl decreased from a mean of 10.8 h in October to 9.6 h in December. In October only 10 d and <12 h available feeding time, whereas in December and January every day had <12 h available feeding time. Seasonal effects of tides on accessibility might limit sufficient feeding by some waterfowl species in intertidal areas, increasing their reliance on alternative feeding sites.