Role of echinoderms in benthic remineralization in the Chukchi Sea

The role of large, epibenthic organisms in carbon cycling at high latitudes is difficult to assess using standard ship-board collection techniques. We used a remotely operated vehicle equipped with video imaging to examine the distribution and abundance of epibenthic organisms in the northeast Chukchi Sea during June 1998. At each of 11 sites, we collected between 25 and 50 images from a minimum of 20 min of video. We observed 15 different epibenthic taxa, with the echinoderms (Ophiura sarsia, O. maculata, Ophiopholis aculeata, Stegophiura nodosa, and Echinarachinus parma) overwhelmingly dominating the epibenthos. Echinoderm density was highly variable, ranging from 0.2 to 256.6 individuals m^-2 (median=16.3), and echinoderm biomass varied between <0.5 and 4,988 mg C m^-2 (median=737). The highest biomass of ophiuroids recorded (3,388 mg C m^-2) is 30% higher than the highest previously reported from an Arctic shelf. Using a relationship between biomass and respiration developed for deep-sea organisms living at cold temperatures, we estimated respiration rates from <0.1 to 15.0 mg C m^-2 day^-1 (median=1.9). Respiration rates measured on board were several orders of magnitude higher than those obtained from the predictive equation. Further work is needed to assess echinoderm respiration rates accurately under in situ conditions. Even with calculated minimal values for respiration rates, a comparison of epifaunal and infaunal respiration at four stations revealed that echinoderm respiration accounted for as much as 25% of total respiration. High epifaunal respiration rates and biomass values are likely supported by high concentrations of particulate organic carbon carried by Bering Sea water flowing through the eastern Chukchi Sea. Our observations support observations from the Eurasian Arctic that echinoderms dominate the epibenthos of Arctic shelves and that the role of these organisms in carbon remineralization must be considered if we are to generate accurate models of carbon cycling in the Arctic.     

Large-scale oceanic distribution and population structure of Calanus finmarchicus, in relation to physical environment, food and predators

The investigation was carried out from 62°N to 73°N and from 14°E to 11°W in the Norwegian Sea during 19 June-12 July 1997. Regional differences in the phase of the seasonal development of the plankton community were evident, most pronounced across the Arctic front. In the Coastal and eastern Atlantic domains, post-bloom conditions prevailed, characterised by low chlorophyll a (chl a) levels and a phytoplankton assemblage dominated by coccolithophorids and small flagellates. During the study period, egg production rates of Calanus finmarchicus were low (<10 eggs female^-1 day^-1), older copepodite stages dominated, and the seasonal descent to deeper waters had started. In the Arctic domain, bloom conditions were evident by high chl a levels and a high abundance of large diatoms. Egg production rates were higher (a maximum of 29 eggs female^-1 day^-1), but the dominance of stages CI-CIII indicated that considerable spawning had already occurred prior to the spring bloom. The seasonal descent had barely started. Both invertebrate and fish predators were most abundant in the Coastal and eastern Atlantic domains, with abundance strongly decreasing north-westwards. No tight relationship between total abundance of invertebrate or fish predators and that of C. finmarchicus was apparent. However, a weak, but significant, relationship between abundance of young stages of chaetognaths and Euchaeta spp. versus young stages of C. finmarchicus was found, indicating that these invertebrate predators develop parallel to the development of the new cohort of C. finmarchicus. In early summer, C. finmarchicus had reached overwintering stages, and had started to accumulate in deeper waters in areas with the highest abundance of horizontally migratory planktivorous fish.     

In situ feeding rates and grazing impact of zooplankton in a South African temporarily open estuary

Recent studies in temporarily open estuaries of South Africa have shown that phytoplankton biomass is at times low, when compared to the high standing stock of the grazers. In situ grazing rates of the dominant zooplankton species were estimated at the Mpenjati Estuary once during the winter closed phase, in August 1999, and once during the summer open phase, in February 2000. The study aimed at determining what proportion of the energetic demands of the dominant grazers of the estuary is met by the available phytoplankton. Results show that the gut of all species exhibited higher pigment concentrations during the night than during the day, both in winter and summer. Gut pigment contents ranged from 0.27 to 5.38 ng pigm individual^-1 in the mysid Gastrosaccus brevifissura, from 0.16 to 1.63 ng pigm individual^-1 in the copepod Pseudodiaptomus hessei, from 0.12 to 0.45 ng pigm individual^-1 in the copepod Acartia natalensis, and from 0.8 to 5.44 ng pigm individual^-1 in the caridean Palaemon sp. [where pigm is the sum of chlorophyll-a (chl-a) and phaeopigments]. During the winter closed phase, gut evacuation rates for G. brevifissura, P. hessei, and A. natalensis were 0.62, 0.42, and 0.46 h^-1, respectively. In summer, gut evacuation rates were 0.68, 0.48, and 0.46 h^-1 for G. brevifissura, P. hessei, and Palaemon sp., respectively. The rate of gut pigment destruction for G. brevifissura was 99.6% of the total ingested, one of the highest values ever recorded for any crustacean. A gut pigment destruction of 79.0% was measured for Palaemon sp., 95.7% for P. hessei, and 93.8% for A. natalensis. During winter the total grazing impact of the dominant zooplankton species ranged from 5.05 to 22.7 mg chl-a m^-2 day^-1 and accounted for 34-69% of the available chl-a in the water column. During summer, the grazing impact ranged between 0.45 and 0.65 mg chl-a m^-2 day^-1, accounting for 17-41% of the available chl-a in the water column. This shows that the dominant zooplankton species of the Mpenjati have a very high grazing impact on algal cells. At times this may exceed 100% of the available phytoplankton production, suggesting that the zooplankton community may often resort to other food sources to meet all its energetic demands.     

Energy budgets and feeding rates of Coryphaenoides acrolepis and C. armatus

This study develops energy budgets and estimates feeding rates for two macrourid fishes, Coryphaenoides acrolepis, dominant in the bathyal eastern North Pacific, and the abyssal cosmopolitan species, Coryphaenoides armatus. Daily energy expenditure by C. acrolepis was nearly twice that of C. armatus. C. acrolepis allocated nearly equal amounts of energy to metabolism and growth. Once sexual maturity was reached reproduction became the dominant energetic cost. Either these costs are necessary to retain adequate numbers of eggs and larvae on the continental slopes, or this fish does not reproduce on an annual basis and the calculated costs are an overestimate. C. armatus allocated relatively more energy to metabolism than growth. It may be semelparous, and this strategy would be of great energetic savings in its food-poor but stable environment. Individual daily ration for C. acrolepis decreased from 0.31% to 0.07% of body weight (BW) and for C. armatus from 0.12% to 0.02% BW with increasing fish length. These rates are substantially lower than those for fishes living in cold waters on the continental shelves. The population feeding rates for C. acrolepis ranged from 0.8 to 15 kg km^-2 day^-1 and for C. armatus from 5 to 2,800 g km^-2 day^-1. The scavenging behaviour of C. acrolepis was used to investigate the role of carrion as a food supply to the deep-sea benthos. It was estimated that the carrion eaten by C. acrolepis is equivalent to 0.04 mg C m^-2 day^-1 or only 0.2-0.4% of the average small particulate flux. Carrion consumption is important for scavengers like C. acrolepis, but it is not an important component of the carbon flux into the deep-sea benthic environment.     

Below-ground nitrogen cycling in relation to net canopy production in mangrove forests of southern Thailand

Rates of accumulation, transformation and availability of sediment nitrogen in four mangrove forests of different age and type in southern Thailand were examined in relation to forest net canopy production. Net ammonification (range: 0.3-2.3 mmol N m^-2 day^-1), nitrification (range: 0-0.7 mmol N m^-2 day^-1) and nitrogen fixation (range: 0-0.6 mmol N m^-2 day^-1) in surface sediments equated to <10% of canopy nitrogen demand (range: 7.5-32 mmol N m^-2 day^-1). By mass balance, we estimated that most of the nitrogen required for tree growth must be derived from root-associated nitrogen fixation and/or mineralisation processes occurring possibly to the maximum depth of live root penetration (75-100 cm). Denitrification, nitrification, rainfall and tidal exchange were comparatively small components of sediment nitrogen flow. Denitrification (range: 0-3.8 mmol N m^-2 day^-1) removed 3-6% of total nitrogen input at three Rhizophora forests, but removed 23% of total nitrogen input in a high-intertidal Ceriops forest. Nitrogen burial ranged from 4% to 12% of total nitrogen input, with the greatest burial rates in two forests receiving the least tidal inundation. Inputs of nitrogen to the forests were rapid (range: 11-37 mmol N m^-2 day^-1), likely originating from upstream sources such as agricultural and industrial lands, sewage and shrimp ponds. Our results indicate that ~70% to 90% of the nitrogen supplied to the forest floor is shunted via the ammonium pool to trees to sustain the rapid rates of net canopy production measured in these forests. Differences in plant-sediment nitrogen relations between the forests appeared to be a function of the interaction between intertidal position and stand age.     

Annual population development and production by Calanus finmarchicus, C. glacialis and C. hyperboreus in Disko Bay, western Greenland

The populations of the copepod species Calanus finmarchicus, C. glacialis and C. hyperboreus were investigated in Disko Bay during a 14-month period in 1996-1997. The three species were predominant in the copepod community. The biomass reached a maximum at the beginning of June (127 mg C m^-3). From the end of July until the end of April the following year, the biomass was <1-6 mg C m^-3. All three species showed seasonal ontogenetic migration. The spring ascent for all three species was just prior to or in association with the break-up of sea ice and the development of the spring bloom, whereas descent occurred over a larger time span during summer. The main overwintering stages were CV for C. finmarchicus, CIV and CV for C. glacialis and C. hyperboreus. Peak abundance of juvenile copepodites, representing the new generation, was in August for C. finmarchicus, in July for C. glacialis and in May/June for C. hyperboreus. From the timing of reproduction and the population development, the life cycles were deduced to be 1 year for C. finmarchicus and at least 2 years for C. glacialis and C. hyperboreus. Secondary production and potential grazing impact of the Calanus community were estimated by two methods based on specific egg-production rates and temperature-dependent production. The Calanus community was not able to control the primary producers during the spring bloom but probably did during post-bloom. The estimates also indicated that grazing on ciliates and heterotrophic dinoflagellates contributes as an essential food source in the post-bloom period.     

Significance of food type for growth of ephyrae Aurelia aurita (Scyphozoa)

We studied growth of newly released Aurelia aurita ephyra larvae fed five different food types, including a large-sized copepod, a phytoflagellate, and suspended POM (particulate organic matter) made from bivalve meat. Experiments were run at saturated food concentration in two different temperatures over 10 days. The effect of small differences in temperature was inconsistent and interacted with the effect of food type, which, in turn, was highly significant. A low average growth rate (4-9% day^-1) was shown when feeding on the large-sized copepod Calanus finmarchicus (80 µg AFDW individual^-1), in spite of an extremely high daily ration of up to 1500% of body AFDW. When feeding on the cryptophyte Rhodomonas baltica (ca. 8 µm cell diameter), the ephyrae showed an average growth rate over the 10 day experiment of 7-11%, but with a considerably higher growth rate during the first days. Suspended POM generated an average growth rate of 7-9% day^-1, whereas fresh bivalve meat, manually placed into the stomach of the ephyra, gave an average growth rate of 12-14% day^-1. Artemia nauplii (ca. 3 µg AFDW individual^-1), used as a general reference, resulted in higher growth rates than any of the other food types (17-31% day^-1). We conclude that A. aurita ephyrae can capture and feed on phytoplankton, large copepods, and POM; that phytoplankton might be of nutritive significance early in development; and that the high quantity of large-sized copepods ingested is inefficiently converted to growth during early development. POM is a potential food source because of the ability of the ephyrae to encounter and ingest it, although concentration, size distribution, and nutritional composition of natural POM probably constrain its effect on growth.     

Estimating the influence of prawn stocking density and seagrass type on the growth of juvenile tiger prawns (Penaeus semisulcatus): results from field experiments in small enclosures

Fine mesh enclosures (0.9 m² in basal area, 1 m high, with 100 µm mesh) and a jet-net retrieval system were developed to test the influence of juvenile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds in the Embley River estuary of tropical Australia. Small juvenile tiger prawns (3-6 mm in carapace length, CL) were stocked in enclosures at densities of 4-32 prawns per enclosure (4.4-35.5 prawns m^-2) on a high biomass seagrass bed (about 70 g m^-2 of mostly Enhalus acoroides) and one with low biomass (about 10 g m^-2 of mostly Halodule uninervis). After 2-3 weeks in the enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However, not all fish and crustaceans could be excluded from the enclosures. Growth rates were twice as fast on the high biomass Enhalus than on the low biomass Halodule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential for interference competition between prawns, compared with the low biomass Halodule. Growth rates on Enhalus were significantly faster at a stocking density of 4 prawns per enclosure (1.3 mm CL week^-1) than at a stocking density of 16 and 32 prawns per enclosure (both 0.8 mm CL week^-1), and did not differ significantly between April and October/November (temperatures were about 30°C at both times). The mean growth rate at 8 prawns per enclosure (1.1 mm CL week^-1) did not differ significantly from those at 4, 16 and 32 prawns per enclosure. These results from two seagrass beds suggest that the carrying capacity for juvenile tiger prawns was greater in the high biomass Enhalus than the low biomass Halodule bed.     

Arm regeneration in the field in Ophiocoma echinata (Echinodermata: Ophiuroidea): effects on body composition and its potential role in a reef food web

Eighteen percent and 47% of two populations of Ophiocoma echinata in the Florida Keys were undergoing regeneration. An individual would take approximately 720 days to completely regenerate three arms. Regeneration of three arms had a greater effect on gonad production in females than in males, and reduced storage material in the stomach of both. Regeneration in O. echinata was estimated to be 0.07 kJ m^-2 day^-1, which could provide the equivalent of 0.07% of primary production on a reef to higher trophic levels per day.     

Diet composition and quality for <Emphasis Type="Italic">Calanus finmarchicus</Emphasis> egg production and hatching success off south-west Iceland

Egg production and hatching success of the copepod Calanus finmarchicus was measured during spring and summer in the waters south-west of Iceland. Egg-production rates varied greatly, both temporally and spatially, with highest average rates found at a station with low chlorophyll-a concentrations (0.4 mg m^-3). Excluding this high production rate from statistical analysis, the remaining egg-production rates were found to be positively correlated with phytoplankton biomass, as well as with parameters representing healthy phytoplankton condition, food quality and diatom-type fatty acids. Hatching success of eggs was negatively correlated with some saturated and monounsaturated fatty acids related to phytoplankton senescence.     

Mesozooplankton community structure, abundance and biomass in the central Arctic Ocean

During the "International Arctic Ocean Expedition 1991" (20 August-21 September 1991) mesozooplankton was sampled at six stations in the Nansen, Amundsen and Makarov Basins of the central Arctic Ocean from 1,500 m depth to the surface by multiple opening/closing net hauls. Total mesozooplankton abundance decreased from 268 ind. m^-3 in the surface layer (0-50 m) to <25 ind. m^-3 below 200 m depth. The small copepods Oithona similis and Microcalanus pygmaeus, as well as copepod nauplii, were most abundant close to the surface, while Oncaea borealis and Spinocalanus spp. frequently occurred at greater depth. Mesozooplankton dry mass (DM) integrated over the upper 1,500 m of the water column was surprisingly stable throughout the investigation area and measured 2.0ǂ.3 g DM m^-2. Dry mass in the upper 50 m measured 20.9 mg m^-3 and was dominated by Calanus hyperboreus (57.4%) and C. glacialis (21.1%). C. finmarchicus was very abundant only in the Nansen Basin. Below 200 m the calanoid copepods Metridia longa, Microcalanus pygmaeus and Pareuchaeta spp., the decapod Hymenodora glacialis and chaetognaths of the genus Eukrohnia were the principal contributors to biomass values of <1 mg DM m^-3. Hence, vertical changes in abundance, biomass and species composition were much more pronounced than regional differences between the basins. Three different mesozooplankton communities were differentiated according to their faunistic composition and are discussed in context with the major water masses: Polar Surface Water, Atlantic Layer and Arctic Deep Water.     

Under-ice amphipods in the Greenland Sea and Fram Strait (Arctic): environmental controls and seasonal patterns below the pack ice

During three "Polarstern" cruises to the ice-covered Greenland Sea (spring 1997, summer 1994, autumn 1995) studies on the under-ice habitat (morphology, hydrography, ice-algal biomass) and on the macrofaunal, autochthonous under-ice amphipods (species diversity, abundance) were carried out in order to describe environmental controls and seasonal patterns in this community. In spring, the ice underside was rather smooth and whitish, while in summer melting structures and sloughed-off ice-algal threads were observed, in autumn detritus clumps accumulated in depressions at the ice underside. Only in summer, a thin layer of warm (up to -0.6°C) and less saline (as low as S=6.3) water was found at the ice-water interface above Polar Water. Integrated ice-algal biomass was highest during autumn (2.6 mg chl a m^-2) and lowest during summer (1.2 mg chl a m^-2). Four species of under-ice amphipods occurred in spring and summer (Apherusa glacialis, Onisimus glacialis, O. nanseni, Gammarus wilkitzkii), but only the last species was observed at the ice underside in autumn. A. glacialis and G. wilkitzkii were equally abundant in spring; A. glacialis dominated in summer. The highest total abundance of amphipods occurred during summer (31.9 ind. m^-2), compared to lower abundances in spring and autumn (5.3 and 1.1 ind. m^-2, respectively). A factor analysis revealed seasonal patterns in the data set, which mainly influenced A. glacialis, and species-specific relations between several environmental factors and the distribution of under-ice amphipods. Abundance of A. glacialis was closely related to the under-ice hydrography and ice-algal biomass, whereas the other amphipod species were more influenced by the under-ice morphology. It is therefore stated that the observed thinning of the Arctic sea ice and the resulting increased meltwater input and change in morphology of floes will have a profoundly adverse effect on the under-ice amphipods.     

Blue sprat <Emphasis Type="Italic">Spratelloides robustus</Emphasis> (Clupeidae: Dussumieriinae): a temperate clupeoid with a tropical life history strategy?

This article describes the life-history strategy of the blue sprat Spratelloides robustus in South Australia and compares the demographic traits observed with those of other clupeoids. Validation studies that involved marking the sagittae of captive fish with oxy-tetracycline suggested that growth increments are deposited daily. The oldest fish examined was 82 mm caudal fork length and 241 days old, which suggests S. robustus may live for less than 1 year. Growth rates were high during larval stages (0.34 mm day^у) and remained high throughout juvenile (0.33 mm day^у) and adult stages (0.19 mm day^у). S. robustus reached 50% maturity at approximately 60 mm caudal fork length after approximately 135 days. Spawning occurred from October to February (spring to late summer) and larvae were found mainly in Spencer Gulf, Gulf St Vincent, and Investigator Strait. Females spawned multiple batches of demersal eggs every 1-2 days. Batch fecundities were low (mean=756, SD=341) and increased linearly with length and weight. The life history of S. robustus is dissimilar to other small to medium-sized temperate clupeoids, but similar to those of many small sub-tropical and tropical clupeoids, including other Spratelloides species. Gulf St Vincent and Spencer Gulf may be considered to be "seasonally subtropical systems" in an otherwise temperate region that support a suite of species, including S. robustus, that have life-history strategies similar to those of sub-topical and tropical taxa.     

Morphology and sinking velocities of fecal pellets of copepod, molluscan, euphausiid, and salp taxa in the northeastern tropical Atlantic

Morphological characteristics and sinking velocities of naturally occurring fecal pellets of copepods, euphausiids, salps, and pelagic mollusks collected in the northeastern tropical Atlantic were investigated during the period of May-June 1992. The fecal pellets of copepods and euphausiids were cylindrical and distinguished only by their size. Those of salps were, in general, rectangular, and slight differences were noted according to the species. The fecal pellets of the molluscan pteropod Clio sp. were conical, while those of the molluscan heteropod Carinaria sp. were spiral. The sinking velocities ranged from 26.5 to 159.5 m day^-1 for copepod fecal pellets, from 16.1 to 341.1 m day^-1 for euphausiid pellets, from 43.5 to 1167.6 m day^-1 for salps' pellets (Cyclosalpa affinis, Salpa fusiformis, Iasis zonaria, and two unidentified species), from 65 to 205.7 m day^-1 for Clio sp. pellets, and from 120.3 to 646.4 m day^-1 for Carinaria sp. fecal pellets. The measured sinking velocities were compared with estimates predicted using the equations of Komar et al. (1981; Limnol Oceanogr 26:172-180), Stokes' law, and Newton's second law, using either a constant density of fecal pellets (1.22 g cm^-3) or densities estimated with the three different equations. When a constant density was used, the three equations overestimated the sinking velocities; Stokes' law resulted in the largest overestimation, and Newton's second law, the smallest. At the taxa level, the overestimation was greatest for euphausiid 1 fecal pellets and smallest for copepod fecal pellets. When the three equations were used to estimate fecal pellet density, the density estimated using the equation of Komar et al. was the greatest, and that using Stokes' law, the smallest, resulting in over- and underestimation of sinking velocities, respectively. Newton's second law resulted in an intermediate density and gave the closest estimate of sinking velocities. We propose that measurement of sinking velocities of a portion of the fecal pellets might guide in choosing an appropriate equation to be used for a reasonable interpretation of vertical mass flux.     

Biomass, photosynthesis and growth of Laminaria saccharina in a high-arctic fjord, NE Greenland

Biomass, photosynthesis and growth of the large, perennial brown alga Laminaria saccharina (L.) Lamour. were examined along a depth gradient in a high-arctic fjord, Young Sound, NE Greenland (74°18'N; 20°14'W), in order to evaluate how well the species is adapted to the extreme climatic conditions. The area is covered by up to 1.6-m-thick ice during 10 months of the year, and bottom water temperature is <0°C all year round. L. saccharina occurred from 2.5 m depth to a lower depth limit of about 20 m receiving 0.7% of surface irradiance. Specimen density and biomass were low, likely, because of heavy ice scouring in shallow water and intensive feeding activity from walruses in deeper areas. The largest specimens were >4 m long and older than 4 years. In contrast to temperate stands of L. saccharina, old leaf blades (2-3 years old) remained attached to the new blades. The old tissues maintained their photosynthetic capacity thereby contributing importantly to algal carbon balance. The photosynthetic characteristics of new tissues reflected a high capacity for adaptation to different light regimes. At low light under ice, or in deep water, the chlorophyll a content and photosynthetic efficiency (!) were high, while light compensation (E_c) and saturation (E_k) points were low. An E_c of 2.0 µmol photons m^-2 s^-1 under ice allowed photosynthesis to almost balance, and sometimes exceed, respiratory costs during the period with thick ice cover but high surface irradiance, from April through July. Rates of respiration were lower than usually found for macroalgae. Annual elongation rates of leaf blades (70-90 cm) were only slightly lower than for temperate L. saccharina, but specific growth rates (0.48-0.58 year^-1) were substantially lower, because the old blades remained attached. L. saccharina comprised between 5% and 10% of total macroalgal biomass in the area, and the annual contribution to primary production was only between 0.1 and 1.6 g C m^-2 year^-1.     

Abundance, growth and life cycle of the mesopelagic amphipod Primno abyssalis (Hyperiidea: Phrosinidae) in the Oyashio region, western subarctic Pacific

Abundance and life-cycle features of the mesopelagic hyperiid amphipod Primno abyssalis (formerly P. macropa) in the Oyashio region, western subarctic Pacific, were investigated using samples collected between July 1996 and July 1998. P. abyssalis was collected throughout the entire survey period, with abundance peaks occurring in spring to autumn. While all maturity stages of males and females were observed throughout the study period, the peak reproduction season was in summer. Instar analysis based on the segment number of the pleopod rami indicated that hatched juveniles molted 10 times before becoming adult males and 13 times before becoming adult females. Judging from the dry and ash-free dry weights of each instar, males and females continued to feed throughout the final instar stage. Based on cohort analysis of seasonal samples and laboratory observations on molting frequencies, growth in body length of P. abyssalis was linear with time, and estimated generation lengths were 2.3-3.8 years for females and 1.4-1.9 years for males. Brood size of females ranged from 66 to 337 and increased with increasing female body length. Lifetime fecundity, calculated as the sum of six successive broods, was 1,004. Compared with P. abyssalis in the southern Sea of Japan, those in the Oyashio region have a larger number of adult instars (six versus five for females, three 3 vs one for males), a lower growth rate (0.014 mm day^-1 vs 0.021 mm day^-1), and mature earlier (instar 13 vs instar 15 for females; instar 10 vs instar 11 for males). These characteristics are considered to be advantageous life-history traits to counteract higher niche competition within the mesopelagic community and higher predation pressure by mesopelagic fishes in the Oyashio region than in the Sea of Japan.     

Metabolic rates of juvenile scalloped hammerhead sharks (Sphyrna lewini)

Oxygen consumption of juvenile scalloped hammerhead sharks, Sphyrna lewini, was measured in a Brett-type flume (volume=635 l) to quantify metabolic rates over a range of aerobic swimming speeds and water temperatures. Oxygen consumption (log transformed) increased at a linear rate with increases in tailbeat frequency and swimming speed. Estimates of standard metabolic rate ranged between 161 mg O₂ kg^-1 h^-1 at 21°C and 203 mg O₂ kg^-1 h^-1 at 29°C (mean-SD: 189ᆣ mg O₂ kg^-1 h^-1 at 26°C). Total metabolic rates ranged from 275 mg O₂ kg^-1 h^-1 at swimming speeds of 0.5 body lengths per second (L s^-1) to a maximum aerobic metabolic rate of 501 mg O₂ kg^-1 h^-1 at 1.4 L s^-1. Net cost of transport was highest at slower swimming speeds (0.5-0.6 L s^-1) and was lowest between 0.75 and 0.9 L s^-1. Therefore, these sharks are most energy efficient at swimming speeds between 0.75 and 0.9 L s^-1. These data indicate that tailbeat frequency and swimming speed can be used as predictors of metabolic rate of free-swimming juvenile hammerhead sharks.     

Feeding biology and carbon budget of the sediment-burrowing heart urchin Brissopsis lyrifera (Echinoidea: Spatangoida)

This study of the burrowing heart urchin Brissopsis lyrifera includes measurements on feeding and food selection, ingestion rate, absorption efficiency, ventilation and respiration. B. lyrifera regulated feeding depth, ingestion rate and absorption efficiency in relation to food source. When food was added to the top layer of sediment, B. lyrifera burrowed closer to the surface and ingested mainly enriched surface material, whereas it burrowed deeper and ingested deep-seated sediment in the controls. In non-enriched sediment, the feeding rate was 0.04 g sediment DW h^-1 ind.^-1, and, in macro- and microalga-enriched sediment the feeding rate was 0.06 and 0.08 g sediment DW h^-1 ind.^-1, respectively. Absorption efficiency of TOC was about 43% in non-enriched sediment and in microalga-enriched sediment, but was significantly lower (34%) in macroalga-enriched sediment. Absorption efficiency of N varied between 48% and 55%, and was independent of food source. B. lyrifera feed selectively, enriching the gut TOC content about 2-fold and N content about 2.5-fold. The C/N ratio was therefore lower within the gut compared to the surrounding sediment, while the faecal C/N ratio was almost equal to the surrounding sediment. The faeces were, however, slightly richer in TOC and N compared to the surrounding sediment. For 3-5 cm long B. lyrifera, water current rate varied between 4 and 24 ml water h^-1, with a mean of 11 ml h^-1. Mean respiration rate was 205 µl O₂ h^-1 ind.^-1. The water current rate was not sufficient for B. lyrifera to sustain itself by filter feeding only. However, organic-rich particles from the surface are suggested to be an important contribution to the diet. A carbon budget was calculated for B. lyrifera from measured values of consumption, absorption efficiency and respiration, in order to estimate annual production of B. lyrifera. Compared to literature values, growth was overestimated about tenfold in the budget. A large proportion of the absorbed carbon was suggested to leave the animal as dissolved carbon, through mucus production or through anaerobic pathways, either by the heart urchin or by micro-organisms in the gut.     

Growth rate of larval Sardinops sagax from ecosystems with different levels of productivity

Otolith increment data were used to age larval Sardinops sagax from shelf waters between 118°E and 137°E off southern Australia. Maximum and mean growth rates up to an age of 30 days, estimated from a Laird-Gompertz model, were 0.62 and 0.48 mm day^-1, respectively. The growth rate of larval S. sagax from southern Australia was less than, or at most comparable to, those estimated from the substantially more productive upwelling systems of southwestern Africa, the central coasts of North and South America, and inshore of the Kuroshio system east of Japan. The lower productivity of the sardine habitat off southern Australia could be expected to manifest as fewer and/or less extensive patches of suitably dense plankton, or lower levels of sufficient food availability, which results in the lower growth rate of larvae. This probably causes higher mortality and, along with slower growth rates of post-recruit stages, may therefore contribute to the low stock size of sardines in southern Australia. Nonetheless, the similarities in growth rates of larvae within a single species over widely separated regions suggest such rates fall between predetermined bounds, limited by starvation at the lower end and an ontogenetic maximum at the other.     

Early life history of tropical Anguilla leptocephali in the western Pacific Ocean

In order to examine the early life-history characteristics of tropical eels, otolith microstructure and microchemistry were examined in leptocephali of Anguilla bicolor pacifica (27.6-54.1 mm TL, n=20) and A. marmorata (22.0-47.3 mm TL, n=8) collected during a cruise in the western Pacific. A. bicolor pacifica occurred between 10°N and 15°N in the west and between 5°S and 10°N farther to the east. A. marmorata also occurred in two different latitudinal ranges in the Northern (15-16°N) and Southern Hemispheres (3-15°S) of the western Pacific. The increment widths in the otoliths of these leptocephali increased between the hatch check (0 days) and about an age of 30 days in both species, and then gradually decreased toward the otolith edge. Otolith Sr:Ca ratios showed a gradual increase from the otolith center to the edge. The ages of A. bicolor pacifica and A. marmorata leptocephali ranged from 40 to 128 days and from 38 to 99 days, respectively. Growth rates of A. bicolor pacifica and A. marmorata leptocephali ranged from 0.33 to 0.71 mm day^-1 and from 0.45 to 0.63 mm day^-1, respectively. These leptocephali had estimated growth rates that were spread out throughout most of the reported range of growth rates of the leptocephali of the temperate species, the Japanese eel and the Atlantic eels. Differences in the spatial distribution in relation to current systems, and the age and size compositions of the leptocephali of A. bicolor pacifica and A. marmorata suggested different spawning locations for these two species.