Seasonal variations in distribution patterns of sympagic meiofauna in Arctic pack ice

During two expeditions of the R.V. Polarstern to the Arctic Ocean, pack ice and under-ice water samples were collected during two different seasons: late summer (September 2002) and late winter (March/April 2003). Physical and biological properties of the ice were investigated to explain seasonal differences in species composition, abundance and distribution patterns of sympagic meiofauna (in this case: heterotrophs >20 µm). In winter, the ice near the surface was characterized by extreme physical conditions (minimum ice temperature: –22°C, maximum brine salinity: 223, brine volume: 5%) and more moderate conditions in summer (minimum ice temperature: –5.6°C, maximum brine salinity: 94, most brine volumes: 5%). Conditions in the lowermost part of the ice did not differ to a high degree between summer and winter. Chlorophyll a concentrations (chl a) showed significant differences between summer and winter: during winter, concentrations were mostly <1.0 µg chl a l^–1, while chl a concentrations of up to 67.4 µmol l^–1 were measured during summer. The median of depth-integrated chl a concentration in summer was significantly higher than in winter. Integrated abundances of sympagic meiofauna were within the same range for both seasons and varied between 0.6 and 34.1×10³ organisms m^–2 in summer and between 3.7 and 24.8×10³ organisms m^–2 in winter. With regard to species composition, a comparison between the two seasons showed distinct differences: while copepods (42.7%) and rotifers (33.4%) were the most abundant sea-ice meiofaunal taxa during summer, copepod nauplii dominated the community, comprising 92.9% of the fauna, in winter. Low species abundances were found in the under-ice water, indicating that overwintering of the other sympagic organisms did not take place there, either. Therefore, their survival strategy over the polar winter remains unclear.Communicated by O. Kinne, Oldendorf/Luhe     

Ciliated epibionts on the Arctic sympagic amphipod<Emphasis Type="Italic"> Gammarus wilkitzkii</Emphasis> as indicators for sympago–benthic coupling

In September 2003 specimens of the sympagic amphipod Gammarus wilkitzkii were sampled in drifting pack ice above 50 m water depth and in the pelagic/benthic environment beneath in the coastal waters off Svalbard (Norway). Abundance values ranged between 1.5 and 8 individuals m^–2 for the pelagic/benthic and the sympagic environments, respectively, and showed corresponding sex ratios of 1:1.4, favoring males. In the ice a significantly higher number of juveniles prevailed. In contrast, ovigerous females were more abundant among pelagic/benthic living specimens. Approximately 25% of the individuals hosted ciliated epibionts of the genera Ephelota, Cryptacineta, Acineta, Podophrya (all suctoria), and Epistylis (peritrichia). Cryptacineta and Ephelota were the most abundant epibionts on this amphipod species. Female specimens of G. wilkitzkii showed the highest degree of infestation (>2,100 individual epibionts per amphipod specimen: indE/A) followed by juveniles (>1,200 indE/A) and males (>220 indE/A). Highest densities of epibionts were found on anterior body parts with the antennae bearing up to 130 individuals. This is the first sighting of epibionts on crustacea from the sympagic environment. We interpret them as biomarkers that indicate the existence of sympago–benthic coupling processes between the ice and the underlying waters and the seafloor. The population structure and the proportion of infested specimens are equal for the amphipods sampled from both the sympagic and the pelagic/benthic environment, indicating the existence of exchange (coupling) processes between the two habitats. Considering the annual ice cycle, local ice drift patterns, and the shallow water depth in the sampling area, we hypothesize that ice amphipods spend ice-free periods near the seafloor where they may serve as basibionts for protozoans and eventually re-colonize the ice with the onset of ice formation. Our observations strongly emphasize that shallow coastal areas serve as both retention (rather than sink) areas for ice fauna during ice-free periods, and as stepping-stones for re-colonizing the ice when the ice is formed in winter.Communicated by M. Kühl, Helsingor     

Spatial and temporal genetic homogeneity in the Arctic surfclam (<Emphasis Type="Italic">Mactromeris polynyma</Emphasis>)

Commercially harvested marine bivalve populations show a broad range of population-genetic patterns that may be driven by planktonic larval dispersal (gene flow) or by historical (genetic drift) and ecological processes (selection). We characterized microsatellite genetic variation among populations and year classes of the commercially harvested Arctic surfclam, Mactromeris polynyma, in order to test the relative significance of gene flow and drift on three spatial scales: within commercially harvested populations in the northwest Atlantic; among Atlantic populations; and between the Atlantic and Pacific oceans. We found small nonsignificant genetic subdivision among eight populations from the northwest Atlantic (F _ST = 0.002). All of these Atlantic populations were highly significantly differentiated from a northeast Pacific population (F _ST = 0.087); all populations showed high inbreeding coefficients (F _IS = 0.432). We tested one likely source of heterozygote deficits by aging individual clams and exploring genetic variation among age classes within populations (a temporal Wahlund effect). Populations showed strikingly different patterns of age structure, but we found little differentiation among age classes. In one case, we were able to analyze genetic diversity between age classes older or younger than the advent of intensive commercial harvesting. The results generally suggest spatially broad and temporally persistent genetic homogeneity of these bivalves. We discuss the implications of the results for the biology and management of surfclam populations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Molecular characterization of the zoanthid genus <Emphasis Type="Italic">Isaurus</Emphasis> (Anthozoa: Hexacorallia) and associated zooxanthellae (<Emphasis Type="Italic">Symbiodinium</Emphasis> spp.) from Japan

The zoanthid genus Isaurus (Anthozoa: Hexacorallia) is known from both the Indo-Pacific and Atlantic Oceans, but phylogenetic studies examining Isaurus using molecular markers have not yet been conducted. Here, two genes of markers [mitochondrial cytochrome oxidase subunit I (COI) and mitochondrial 16S ribosomal DNA (mt 16S rDNA)] from Isaurus specimens collected from southern Japan (n = 19) and western Australia (n = 3) were sequenced in order to investigate the molecular phylogenetic position of Isaurus within the order Zoantharia and the family Zoanthidae. Additionally, obtained sequences and morphological data (polyp size, mesentery numbers, mesogleal thickness) were utilized to examine Isaurus species diversity and morphological variation. By comparing our obtained sequences with the few previously acquired sequences of genera Isaurus as well as with Zoanthus, Acrozoanthus (both family Zoanthidae), and Palythoa spp. (family Spenophidae) sequences, the phylogenetic position of Isaurus as sister to Zoanthus within the Family Zoanthidae was suggested. Based on genetic data, Isaurus is most closely related to the genus Zoanthus. Despite considerable morphological variation (in particular, polyp length, mesentery numbers, external coloration) between collected Isaurus specimens, all specimens examined are apparently conspecific or very closely related based on molecular data and observed morphological variation within colonies. Additionally, obtained internal transcribed spacer of ribosomal DNA (ITS-rDNA) sequences from symbiotic zooxanthellae (Symbiodinium spp.) from all Isaurus specimens were shown to be subclade C1-related Symbiodinium. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Spawning and development in <Emphasis Type="Italic">Osedax</Emphasis> boneworms (Siboglinidae,Annelida)

We report observations on spawning and early development in bone-eating worms of the genus Osedax. Individual females of Osedax rubiplumus were observed at 1820 m depth freely spawning hundreds of oocytes, and females of an undescribed species, Osedax “orange collar”, were observed spawning in laboratory aquaria. Cytological and molecular analysis of the spawned oocytes of two Osedax species revealed no evidence for the bacterial endosymbionts that the female worms require for their nutrition, suggesting that the bacteria must be acquired later from the environment, as they are in other siboglinids. Individual O. “orange collar” females released an average of 335 (±130) eggs per day, but the number of oocytes spawned per day varied greatly, suggesting that not all the females spawned daily. Fertilization rates of the spawned oocytes varied from 0 to 100%, though most females showed nearly 100% fertilization rates. Oocytes spawned in the laboratory at 4–6°C were negatively buoyant. If fertilized, these oocytes extruded polar bodies and then after at least four hours cleaved unequally. Subsequent cleavages occurred in a spiral pattern at roughly 2-h intervals, resulting in free-swimming trochophore larvae after 24 h. These lecithotrophic trochophores swam for 9–16 days before settling with several hooked chaetae, similar to those of dwarf Osedax males. The larval life span of the Osedax species studied in the laboratory appears to be shorter than in closely related Vestimentifera. Osedax rubiplumus, on the other hand, has much larger oocytes and so may have greater dispersal potential than these other Osedax species. The high fecundity and apparently continuous reproduction of Osedax boneworms permits the opportunistic exploitation of sunken vertebrate bones. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Synergistic effects of ultraviolet radiation and conditions at low tide on egg masses of limpets (<Emphasis Type="Italic">Benhamina obliquata</Emphasis> and <Emphasis Type="Italic">Siphonaria australis</Emphasis>) in New Zealand

The effects of ultraviolet radiation (UVR), desiccation and conditions in tidal pools on embryonic survival were examined for two common pulmonate limpets that lay intertidal benthic egg masses on rocky shores in New Zealand: Benhamina obliquata and Siphonaria australis. Field surveys and manipulative experiments were conducted between December 2006 and September 2007 in the Wellington region of New Zealand (41°17′S, 174°47′E). Egg mass deposition sites in the field were species-specific: B. obliquata deposited eggs primarily in shaded crevices, whereas S. australis predominantly deposited egg masses in the sun and in tidal pools. For both species, however, embryonic mortality was greater in egg masses that had been in full sun compared to shade. For S. australis, there was also high mortality in egg masses in tidal pools or desiccated compared to those that remained submerged in flowing seawater at low tide. In outdoor experiments, embryonic mortality was also always greatest for egg masses exposed to full sun, and lowest for those in shaded treatments. Mortality was also higher if egg masses were in simulated tidal pools, and for S. australis, if desiccated, compared to those submerged in flowing seawater. Periods of particularly sunny conditions with high temperatures also resulted in higher overall mortality. Finally, egg masses of both species that were initially deposited in the shade had greater mortality in response to subsequent UV exposure compared to egg masses initially deposited in full sun. Results from this study suggest that the egg masses of these two species are highly vulnerable to UVR, as well as other intertidal stressors. Embryos of both of these species may be at risk of high mortality particularly during summer when extreme conditions of UV intensity and high temperature coincide with low tide cycles. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Laboratory culture of the aeolid nudibranch <Emphasis Type="Italic">Spurilla neapolitana</Emphasis> (Mollusca,Opisthobranchia): life history aspects

Spurilla neapolitana from the Eastern Mediterranean was cultured in a self-sustained, temperature-controlled laboratory culture system, and its life cycle is described. Adults were collected from three field sites situated 120 km apart, along the Israeli Mediterranean coastline, between March 2006 and August 2007. Cultures of the life-cycle stages were raised at 24°C. S. neapolitana deposited white, coiled, spiral egg masses containing zygotes. Veliger larvae hatched 3.0 ± 0.4 days post oviposition. The veliger larvae are obligatory planktotrophs, with a minimal larval phase of 22 days. In the lab, larvae settled and metamorphosed following exposure to metabolites derived from distinct prey sea anemone species. Reproductive maturity was reached 42 ± 5 days post metamorphosis, resulting in a laboratory generation time of 67 days (egg to egg). The average life span of reproductive specimens in this study was 157 ± 13 days post-oviposition and they reached a length of 7–10 cm. During this period, an average adult deposited ca. 40 × 10⁶ zygotes. This species has several characteristics that suggest it will be a useful model for laboratory-oriented research. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A new source of dimethylsulfide (DMS) for the arctic atmosphere: ice diatoms

We report the first evidence that pennate diatoms growing within the bottom layer of first-year ice in the Arctic produce significant amounts of particulate dimethylsulfoniopropionate (DMSP_p) and dissolved DMSP+DMS. In 1992 in Resolute Passage, a tributary of Barrow Strait, DMSP_p concentrations within the bottom layer of ice reached 1055 mg S m^-3 at the end of the vernal bloom, a value one order of magnitude higher than the maximum value reported in antarctic ice. Bottom-ice concentrations in DMSP_p and DMSP_d+DMS were significantly correlated with the abundance of the dominant pennate diatom Nitzschia frigida. Intracellular concentration in DMSP of ice algae was very low (0.001 pg cell^-1) at the end of April when algae were light-limited and reached 1.17 pg cell^-1 in mid-May following an increase in light and algal growth. We calculate that the rapid release of the dissolved DMSP+DMS from the ice into surface waters following the ice break-up will generate a sea-to-air DMS flux of 0.7 mg S m^-2 d^-1, a pulse ten times higher than the mean arctic summer flux. We estimate that this 1-d pulse represents up to 5% of the annual DMS emission in the Arctic.     

Brooding of the sub-Antarctic heart urchin, <Emphasis Type="Italic">Abatus cavernosus</Emphasis> (Spatangoida: Schizasteridae), in southern Patagonia

The reproductive cycle of the sub-Antarctic spatangoid sea urchin, Abatus cavernosus, was examined during a 2-year period in southern Patagonia, Argentina. The population studied is the northernmost known coastal population in the austral oceans, and is influenced by a greater annual range of seawater temperature than other Abatus species. The sex ratio of the overall A. cavernosus population did not differ significantly from 1:1, but was not homogeneous across size classes. A clearly defined annual reproductive cycle was found. Spawning occurred from May to July and was synchronous between sexes. Females were observed to be brooding within a 9-month period, from May to February. Juveniles were released during the austral summer, from January to February. The length at which 50% of the females were brooding occurred at a test length of 25.9 mm. A. cavernosus had a large egg size (mean diameter = 1.4 mm) and low fecundity (maximum = 57 eggs per female) compared to closely related species. The number of eggs within each brood pouch was highest in larger anterior brood pouch, which is close to two gonopores, as opposed to the other anterior and two smaller posterior pouches. Significant interannual variation was observed in gonad cycles, fecundity, and embryo development such as: brood size decreased during 2001; adjusted gonad dry weight and fecundity were higher during 2003. Hypotheses concerning the gonadal and brooding cycles and fecundity of sub-Antarctic and Antarctic Schizasteridae are discussed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Population dynamics,reproduction and growth of the Indo-Pacific horned sea star, <Emphasis Type="Italic">Protoreaster nodosus</Emphasis> (Echinodermata; Asteroidea)

The horned sea star (Protoreaster nodosus) is relatively common in the Indo-Pacific region, but there is little information about its biology. This study of the population biology of P. nodosus was carried out in Davao Gulf, The Philippines (7°5′N, 125°45′E) between September 2006 and May 2008. Protoreaster nodosus was found in sand and seagrass dominated habitats at a mean density of 29 specimens per 100 m² and a mean biomass of 7.4 kg per 100 m², whereas a significantly lower density and biomass was found in coral and rock dominated habitats. Adult specimens (mean radius R = 10.0 cm) were found at depths of 0–37 m, whereas juveniles (R < 8 cm) were only found in shallow sandy habitats with abundant seagrass (water depth ≤2 m). Increased gonad weights were found from March to May (spawning period), which coincided with an increasing water temperature and a decreasing salinity. Density and biomass did not change significantly during reproduction, but sea stars avoided intertidal habitats. All specimens with R > 8 cm had well developed gonads and their sex ratio was 1:1. Protoreaster nodosus grew relatively slowly in an enclosure as described by the exponential function G = 7.433 e^{−0.257 × R} . Maturing specimens (R = 6–8 cm) were estimated to have an age of 2–3 years. Specimens with a radius of 10 cm (population mean) were calculated to have an age of 5–6 years, while the maximum age (R = 14 cm) was estimated as 17 years. Potential effects of ornamental collection on the sea star populations are discussed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Thraustochytrids as novel parasitic protists of marine free-living flatworms: <Emphasis Type="Italic">Thraustochytrium caudivorum</Emphasis> sp. nov. parasitizes <Emphasis Type="Italic">Macrostomum lignano</Emphasis>

The Labyrinthulomycota are a relatively poorly studied group of heterotrophic unicellular eukaryotes. They comprise three lineages, labyrinthulids, thraustochytrids, and aplanochytrids, which are all primarily marine organisms and considered to be important components of marine microbial communities. Recently a number of Labyrinthulomycota have been implicated as parasites of marine (but also terrestrial) plants and marine molluscs. Here we describe a new species of thraustochytrid, Thraustochytrium caudivorum sp. nov. that we have isolated from laboratory cultures of Macrostomum lignano (Rhabditophora, Macrostomorpha), a marine free-living flatworm. In these worms T. caudivorum can cause lesions, which start at the tip of the tail plate and which can lead to the dissolution of the posterior part of the animal. Although the worms can frequently cure these lesions and regenerate the lost parts, the lesions can also result in the complete dissolution of the animal. We describe this thraustochytrid based on pure agar cultures and infestations in the worm cultures. Moreover, we describe its pathological effects on the worms and its morphology using both light and electron microscopy. In addition, we report a phylogenetic analysis using a partial 18S rDNA sequence that allows us to place this new species within the thraustochytrids. Finally, we outline a protocol that allows to permanently remove the parasites from infested worm cultures. We conclude that thraustochytrids represent a novel group of parasites of free-living flatworms. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Population structure of <Emphasis Type="Italic">Symbiodinium</Emphasis> sp. associated with the common sea fan, <Emphasis Type="Italic">Gorgonia ventalina</Emphasis>, in the Florida Keys across distance,depth, and time

Numerous marine invertebrates form endosymbiotic relationships with dinoflagellates in the genus Symbiodinium. However, few studies have examined the fine-scale population structure of these symbionts. Here, we describe the genetic structure of Symbiodinium type “B1/B184” inhabiting the gorgonian Gorgonia ventalina along the Florida Keys. Six polymorphic microsatellite loci were utilized to examine 16 populations along the Upper, Middle, and Lower Keys spanning a range of ~200 km. Multiple statistical tests detected significant differentiation in 54–92% of the 120 possible pairwise comparisons between localities, suggesting low levels of gene flow in these dinoflagellates. In general, populations clustered by geographic region and/or reefs in close proximity. Some of the sharpest population differentiation was detected between Symbiodinium from deep and shallow sites on the same reef. In spite of the high degree of population structure, alleles and genotypes were shared among localities, indicating some connectivity between Symbiodinium populations associated with G. ventalina. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Genetic diversity of Nymphon (Arthropoda: Pycnogonida: Nymphonidae) along the Antarctic Peninsula with a focus on Nymphon australe Hodgson 1902

Sea spiders are conspicuous, and often abundant, members of the Antarctic benthic community. Nymphonidae (Pycnogonida) in Southern Ocean waters comprise over 240 species which are often difficult to assign due to their intraspecific ‘highly variable’ morphology. In particular, Nymphon australe, the numerically dominant species in Antarctic waters is known to have a high level of phenotypic variation in external morphology and is also reported to have a circumpolar distribution. Circumpolarity seems contradictory to the pycnogonid’s brooding lifestyle and presumably limited dispersal. Here we examine the genetic diversity of several Nymphon species collected in the Antarctic Peninsular region. Concomitantly, we assess the genetic structure of N. australe to gain insight into Nymphon dispersal capacity. Cytochrome c oxidase subunit I (COI) and 16S ribosomal gene data suggest a recent common history and/or recent gene-flow of N. australe populations across nearly 800 km of the Antarctic Peninsula. Furthermore, these data support that the Antarctic Peninsula region may hold two previously unrecognized species of Nymphon. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Variation in colony geometry modulates internal light levels in branching corals, <Emphasis Type="Italic">Acropora humilis</Emphasis> and <Emphasis Type="Italic">Stylophora pistillata</Emphasis>

Colonial photosynthetic marine organisms often exhibit morphological phenotypic plasticity. Where such plasticity leads to an improved balance between rates of photosynthesis and maintenance costs, it is likely to have adaptive significance. To explore whether such phenotypic plasticity leads to more favourable within-colony irradiance for reef-building branching corals, this relationship was investigated for two coral species Acropora humilis and Stylophora pistillata, along a depth gradient representing light habitats ranging from 500 to 25 μmol photons m⁻² s⁻¹, during 2006 at Heron Island, Great Barrier Reef (23.44°S, 151.91°E). In the present study changes in flow-modulated mass transfer co-varied with light as a function of depth. In low-light (deep) habitats, branch spacing (colony openness) in A. humilis and S. pistillata was 40–50% greater than for conspecifics in high-light environments. Also, branches of A. humilis in deep water were 40–60% shorter than in shallow water. Phenotypic changes in these two variables lead to steeper within-colony light attenuation resulting in 38% higher mean internal irradiance (at the tissue surface) in deep colonies compared to shallow colonies. The pattern of branch spacing was similar for S. pistillata, but this species displayed an alternate strategy with respect to branch length: shade adapted deep and cave colonies developed longer and thinner branches, allowing access to higher mass transfer and irradiance. Corals in cave habitats allowed 20% more irradiance compared to colonies found in the deep, and had a 47% greater proportion of irradiance compared to colonies in the shallow high-light environment. Such phenotypic regulation of internal light levels on branch surfaces partly explains the broad light niches of many branching coral species. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Microstructures of Antarctic cidaroid spines: diversity of shapes and ectosymbiont attachments

The echinoderm endoskeleton, located in the connective layer of the tegument, is organized into a three-dimensional mesh, the stereom. Among echinoids, the cidaroids depart from this pattern, and the shaft of the spine lacks an epidermis. Thus, the spines lack antifouling protection, allowing ectosymbionts such as bryozoans and foraminiferans to attach. This raises a question about the adaptive role of the cortical layer of the stereom. This study examined the micro- and mesostructure of the spines of 11 cidaroid species collected in the Weddell Sea and Drake Passage, and the nature of their ectosymbiont attachments. Scanning electron microscopy was used to characterize the cortex surface and X-ray micro computed tomography (μCT) to describe the symbiont attachments. Spine microstructure features provide a useful taxonomic character for distinguishing among three species in the genus Ctenocidaris, and challenge a previous parasitic interpretation of cortical filaments on the spines of Rhynchocidaris triplopora. Ectosymbiont attachments were classified as Anchoring, Molding, Cementing, or Corroding. The study suggests that some microstructure features may be protective, keeping the ectosymbionts away from the cortex and loosely attached at intervals along the shaft of the spine, while other micro-structures facilitate attachment over considerable areas of the shaft. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Influence of environmental factors and female size on reproductive output in an invasive temperate marine gastropod <Emphasis Type="Italic">Rapana</Emphasis><Emphasis Type="Italic">venosa</Emphasis> (Muricidae)

Life history and reproductive strategies influence population dynamics at the inter- and intra-specific level. Environmental conditions suitable for gonad development and spawning, the reproductive range, may be a smaller portion of the broader species distribution. The only known breeding population of veined rapa whelks (Rapana venosa) in North America is in Chesapeake Bay, USA. There is considerable interest in the potential reproductive range of this non-indigenous species given the rapa whelk’s negative impacts on commercial shellfish species in both its native and introduced ranges. Weight-specific reproductive output is described for wild caught Chesapeake Bay rapa whelks maintained in flow-through mesocosms for 2 years. Measured reproductive output within and between egg capsule deposition seasons (years) in relation to water temperature, salinity, daylength, and female size is used to describe the rapa whelk’s reproductive range. Egg capsule production is influenced by seasonal and absolute water temperatures as well as seasonal daylength cycles. Egg capsule deposition by Chesapeake Bay rapa whelks begins at water temperatures of approximately 18°C and continues for 11–15 weeks. Forty to 70% of female whelks deposited egg capsules in most weeks during this season, producing 150–200 egg capsules female⁻¹ week⁻¹. Water temperatures >28°C caused reduced egg capsule production relative to temperatures of 20–25°C. Egg capsule production was positively related to seasonal changes in daylength, and two peaks of egg capsule deposition were observed in the 2001 and 2002 deposition seasons. The combination of declining daylength and higher water temperatures in late summer was associated with the cessation of egg capsule deposition. A lower average weight specific reproductive output in 130–145 mm SL rapa whelks (average 12 ± 1%) than in 90–106 mm SL rapa whelks (average 22 ± 1% of body weight) may reflect a life history that balances the physiological costs of maintaining a large body mass with the production of many planktonic larvae from multiple clutches of egg capsules per breeding season over a 10–15-year lifespan. Estimates of the cumulative day-degree requirements corresponding to the annual initiation of egg capsule deposition were 238 and 236 for 2001 and 2002, respectively. Reproductive output and day-degree requirements for Chesapeake Bay rapa whelks were similar to values calculated from previous studies of native muricids (Eupleura caudata and Urosalpinx cinerea). A latitudinal range of 30–41° (N and S) is predicted as the realized reproductive range for rapa whelk populations on the basis of the day-degree requirements for native whelks and reproductively active invasive rapa whelk populations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The foraging ecology of Arctic cod (Boreogadus saida) during open water (July–August) in Allen Bay, Arctic Canada

Arctic cod (Boreogadus saida) is a schooling fish providing a critical link between lower and upper trophic levels in the Arctic. This study examined foraging of Arctic cod collected from Allen Bay, Cornwallis Island, Canada (~75 N 95 W), during summer 2010 using temporal indicators of diet including stomach content, and carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes of liver and muscle. Foraging at the time of capture reflected sympagic and epi-benthic habitats indicated by the prevalence of cyclopoid and harpacticoid copepods in stomachs, whereas stable isotope data, which provide an estimate of feeding over a longer period, indicated pelagic prey as important. Prey selection of juveniles differed from adults based on stable isotopes, while large adults showed the most separation based on stomach contents, suggesting size-related diet shifts. Compared to studies near Resolute in the 1970s, 1980s, and 1990s, growth and pre-spawning gonadal conditions of Arctic cod have not changed.