Quantitative measures of gastrovascular flow in octocorals and hydroids: toward a comparative biology of transport systems in cnidarians

Using microscopy, the gastrovascular systems of four hydroids (Eirene viridula, Cordylophora lacustris, Hydractinia symbiolongicarpus, and Podocoryna carnea) and two distantly related alcyonacean octocorals (Acrossota amboinensis and Sarcothelia sp.) were examined and compared within a phylogenetic framework. Despite a range of stolon widths (means 53–160 μm), the hydroid species exhibited similar patterns of gastrovascular flow: sequentially bidirectional flow in the stolons, driven by myoepithelial contractions emanating from the center of the colony. Unlike the hydroids, the gastrovascular system of A. amboinensis (mean stolon widths for 5 colonies, 0.57–1.21 mm) exhibited simultaneously bidirectional flow with incomplete, medial baffles (width 4–20 μm) separating the flow. Baffles visualized with transmission electron microscopy consisted of endoderm, mesoglea, and occasionally another layer of tissue. Mean flow rates of the gastrovascular fluid for seven stolons ranged from 125 to 275 μm s^?1, with maximum rates of 225–700 μm s^?1. In Sarcothelia sp., stolons were of comparable width (means for 13 colonies 0.55–1.4 mm) to those of A. amboinensis. These stolons, however, were divided by several partitions (width 8–25 μm), both complete and incomplete, which were spaced every 100.5±5.1 μm (mean±SE; range 27.1–283.7 μm) and appeared structurally similar to baffles. In lanes defined by these partitions, ciliary motion was visible in image sequences, and flow was unidirectional. Within a single stolon, flow moved in different directions in different lanes and changed direction by moving from lane to lane via occasional spaces between the partitions. Mean flow rates for 30 stolons ranged from 75 to 475 μm s^?1, with maximum rates of 85–775 μm s^?1. For both octocorals, flow rates of the gastrovascular fluid were not correlated with the width of the stolon lumen. While octocoral gastrovascular systems probably exhibit differences based on phylogenetic affinities, in all species studied thus far, gastrovascular flow is entirely driven by cilia, in contrast to the hydroid taxa.     

Green Fluorescence of Cytaeis Hydroids Living in Association with Nassarius Gastropods in the Red Sea

Green Fluorescent Proteins (GFPs) have been reported from a wide diversity of medusae, but only a few observations of green fluorescence have been reported for hydroid colonies. In this study, we report on fluorescence displayed by hydroid polyps of the genus Cytaeis Eschscholtz, 1829 (Hydrozoa: Anthoathecata: Filifera) found at night time in the southern Red Sea (Saudi Arabia) living on shells of the gastropod Nassarius margaritifer (Dunker, 1847) (Neogastropoda: Buccinoidea: Nassariidae). We examined the fluorescence of these polyps and compare with previously reported data. Intensive green fluorescence with a spectral peak at 518 nm was detected in the hypostome of the Cytaeis polyps, unlike in previous reports that reported fluorescence either in the basal parts of polyps or in other locations on hydroid colonies. These results suggest that fluorescence may be widespread not only in medusae, but also in polyps, and also suggests that the patterns of fluorescence localization can vary in closely related species. The fluorescence of polyps may be potentially useful for field identification of cryptic species and study of geographical distributions of such hydroids and their hosts.     

The Selective Myosin II Inhibitor Blebbistatin Reversibly Eliminates Gastrovascular Flow and Stolon Tip Pulsations in the Colonial Hydroid Podocoryna carnea

Blebbistatin reversibly disrupted both stolon tip pulsations and gastrovascular flow in the colonial hydroid Podocoryna carnea. Epithelial longitudinal muscles of polyps were unaffected by blebbistatin, as polyps contracted when challenged with a pulse of KCl. Latrunculin B, which sequesters G actin preventing F actin assembly, caused stolons to retract, exposing focal adhesions where the tip epithelial cells adhere to the substratum. These results are consistent with earlier suggestions that non-muscle myosin II provides the motive force for stolon tip pulsations and further suggest that tip oscillations are functionally coupled to hydrorhizal axial muscle contraction.     

Probable advanced hydroid from the Early Ordovician of China

A fossil of a colonial organism with pyritized soft tissues of elongated fusiform zooids, found in the middle part of the early Floian Fenxiang Formation in Hubei Province of China, probably represents the oldest reliable record of a hydroid cnidarian. The preservation of the fossil is sufficiently different from that of associated carbonized skeletons of graptolites to exclude affinities with this group. The fossil is unlikely to be a bryozoan because of the mode of budding from proximal, not distal, parts of parent zooids, which is typical rather of hydroids. Although no thecae are preserved, the fossil, named Sinobryon elongatum gen. et sp. nov., is suggested to be a thecate hydroid, possibly related to the Haleciidae. The apparent presence of an advanced member of the thecaphoran Macrocolonia clade in strata 470 Ma old means that much of the hydroid (and cnidarian) diversification preceded the Middle Ordovician.     

Trophic structure of the macrobenthic community of Hornsund,Spitsbergen, based on the determination of stable carbon and nitrogen isotopic signatures

Hornsund is a cold-water fjord in southwestern Spitsbergen, Svalbard Archipelago, with a resident biota that exhibit typical low-temperature Arctic features. Carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic signatures of macrobenthic fauna and its potential food sources were measured in summer 2008 to delineate the trophic structure of the bottom community and to identify its principal carbon sources. The soft-bottom fauna at a water depth of 100 m was found to rely primarily on detritus, which is supplied by sedimentation of suspended organic matter from the water column and horizontal transport of refractory macroalgae from euphotic coastal habitats. Through resuspension by bottom currents, deposited particles also contributed to the diet of benthic filter-feeders. Since benthic organisms were significantly enriched in ¹³C compared to epibenthic zooplankton (mainly amphipods and decapods), the stable carbon signature provides a tool to differentiate benthic and pelagic feeding habits. The benthic food web was characterized by a conventional trophic structure with decreasing species numbers in increasing trophic levels. Primary consumers feeding on a mixture of plant matter, fecal pellets, decaying animal tissue, bacteria, and protists accounted for the greatest biomass share (62 % of the total macrobenthic biomass), followed by secondary consumers (38 %). Based on δ¹⁵N signatures, three trophic levels were detected, corresponding to the following feeding guilds: filter-feeders and feeding generalists (mainly bivalves, crustaceans, polychaetes, and some fish), mixed detritivore–carnivores (polychaetes, priapulids, crustaceans, and ophiuroids) and obligate carnivores (ascidians). The average food chain length (4.5 trophic levels) suggests that high-quality food is readily available in this Arctic fjord ecosystem.     

Grazing effects of a freshwater bivalve (Corbicula leana Prime) and large zooplankton on phytoplankton communities in two Korean lakes

This study examined the effects of a freshwater filter feeding bivalve (Corbicula leana Prime) and large zooplankton (>200 μm, mostly cladocerans and copepods) on the phytoplankton communities in two lakes with contrasting trophic conditions. A controlled experiment was conducted with four treatments (control, zooplankton addition, mussel addition, and both zooplankton and mussel addition), and each established in duplicate 10-l chambers. In both lakes there were significant effects of mussel grazing on phytoplankton density and biomass. The effects were greater in mesotrophic Lake Soyang than in hypertrophic Lake Ilgam. Effects of zooplankton grazing did not differ between these lakes, and zooplankton effects on phytoplankton were much less than the effects of mussels. Although mussels exerted a varying effect on phytoplankton according to their size, mussels reduced densities of almost all phytoplankton taxa. Total mean filtering rate (FR) of mussels in Lake Soyang was significantly greater than that in Lake Ilgam (p=0.002, n=5). Carbon fluxes from phytoplankton to mussels (977–2,379 μgC l^?1d^?1) and to zooplankton (76–264 μgC l^?1 d^?1) were always greater in Lake Ilgam due to the greater phytoplankton biomass (p<0.01, n=6). Based on the C-flux to biomass ratios, the mussels consumed 170–754% (avg. 412%) of phytoplankton standing stock in Lake Soyang, and 38–164% (avg. 106%) in Lake Ilgam per day. The C-flux to biomass ratio for mussels within each lake was much greater than for large zooplankton. Mussels reduced total phosphorus concentration by 5–34%, while increasing phosphate by 30–55% relative to the control. Total nitrogen also was reduced (by 9–25%), but there was no noticeable change in nitrate among treatments. The high consumption rate of phytoplankton by Corbicula leana even in a very eutrophic lake suggests that this mussel could affect planktonic and benthic food web structure and function by preferential feeding on small seston and by nutrient recycling. Control of mussel biomass therefore might be an effective tool for management of water quality in shallow eutrophic lakes and reservoirs in Korea.     

Relay-ray way of hydroplasm movement in hydroid colonies

In this study, we continued the investigation of the distribution system of colonial hydroids in the course of its development, starting with its emergence during the planula metamorphosis and ending with the formed colony. The hydroplasmic stream system of two species of colonial hydroids—Perigonimus abyssi G.O. Sars, 1874, and Stauridia producta Wright, 1858—was studied. We found that the main principle by which hydroplasma moves in these colonies, which form no shoots, is the relay-race from one hydranth to another through a stolon fragment connecting them or it moves past the neighbouring hydranth to the next one, etc. We show that the efficiency of the conducting (distribution) system does not depend on the level of complexity of the colonial structure. The results of this study confirm the absence of general colonial processes of integration and self-regulation in hydroid polyps.     

The microbial food web structure of a hypertrophic warm-temperate shallow lake, as affected by contrasting zooplankton assemblages

The composition of zooplankton is known to affect the structure of the microbial trophic web. The zooplankton of the hypertrophic Laguna Chascomús (Argentina) is generally dominated by rotifers and cyclopoids copepods. An unusual dominance by small-cladocerans was observed after a massive winter fish kill in 2007. We hypothesized that small-cladocerans would increase the grazing pressure on heterotrophic flagellates (HF), reducing the degree of coupling between HF and picoplankton. The aim of this study was to investigate the microbial food web structure under two contrasting zooplankton assemblages. The lake was sampled every other week between 2007 and 2009. The abundances of heterotrophic bacteria (HB) and picocyanobacteria (Pcy) laid among the highest values reported for aquatic systems (>10⁸ and 10⁷ cells ml^?1, respectively). Pcy averaged 53% of total picoplanktonic biomass. When small-cladocerans dominated zooplankton HF reached the higher abundance (>10⁵ cells ml^?1) and picoplankton showed the opposite pattern, while the proportion of grazing resistant morphologies (i.e. microaggregates of Pcy) was higher. In contrast, when rotifers dominated, HF abundance decreased and picoplankton increased. Our data suggest that the degree of HF–HB coupling was affected by changes in zooplankton dominance. In contrast to our initial hypothesis, the present results suggest that large numbers of rotifers (>5,000 ind. l^?1) are more efficient than small-cladocerans at controlling HF populations.     

Hydroid assemblages from the Bellingshausen Sea (Antarctica): environmental factors behind their spatial distribution

Although scientific knowledge about the biodiversity of Antarctic benthic hydrozoans has considerably increased in recent years, little is known about their spatial distribution and underpinning factors. Trying to contribute to filling this gap, benthic hydroid spatial distribution in the Bellingshausen Sea (Southern Ocean) was studied. Samples were collected at 32 stations at depths between 86 and 3,304 m during Spanish Antarctic expeditions in 2003 and 2006. Sediments and bottom water properties were analyzed using an USNEL-type box corer and a Neil Brown Instrument System Mark III CTD, respectively. Forty species were reported (Acryptolaria sp., Stegopoma plicatile, Staurotheca dichotoma having the highest percentages of occurrence), representing ca. 19 % of the species richness of the known benthic hydroid fauna of the Southern Ocean. Three well-defined assemblages (shallow, deep and transitional) were established based on significant differences in species occurrence. Benthic hydroid spatial distribution in the Bellingshausen Sea seems to be controlled mainly by depth and substrate (most hydrozoan species are epibiotic), by species dispersal abilities and by species resilience to changing hydrodynamic conditions. The level of species richness found in the present study, compared with other Antarctic areas, gives support to arguments stated by authors against the idea that the Bellingshausen Sea is “a benthos desert” controlled by oligotrophic conditions and intense iceberg traffic.     

Hydroid epifaunal communities in Arctic coastal waters (Svalbard): effects of substrate characteristics

The knowledge of cryptic epifaunal groups in the Arctic is far from complete mostly due to logistic difficulties. Only recently, advances in sample collection using SCUBA diving techniques have enabled to explore delicate hydroid fauna from shallow waters. This study is the first attempt to examine the relationship between substrate property (such as size of rock, morphological characteristics of algal or bryozoan host) and hydroid community composition and diversity in the Arctic. Samples of substrates for hydroid attachment including rocks, algae, bryozoans and other hydrozoans were collected around the Svalbard. Examination revealed no substrate-specific species. The substrate property did not have a strong influence on hydroid community. Both species composition and richness were not related to colonized rock surface area and to morphological characteristic of algal host. Therefore, results indicate the opportunistic nature of hydroid fauna in terms of substrate preference. However, the presence or absence of hydroids depended on the surface area of rocky substrate. Hydroids were more often present on rocks of larger surface area. Erect hydroids and bryozoans were important attachment surface for stolonal hydroids.     

Near-bottom zooplankton aggregations in Kongsfjorden: implications for pelago–benthic coupling

Near-bottom zooplankton communities have rarely been studied despite numerous reports of high zooplankton concentrations, probably due to methodological constraints. In Kongsfjorden, Svalbard, the near-bottom layer was studied for the first time by combining daytime deployments of a remotely operated vehicle (ROV), the optical zooplankton sensor moored on-sight key species investigation (MOKI), and Tucker trawl sampling. ROV data from the fjord entrance and the inner fjord showed high near-bottom abundances of euphausiids with a mean concentration of 17.3 ± 3.5 n × 100 m^?3. With the MOKI system, we observed varying numbers of euphausiids, amphipods, chaetognaths, and copepods on the seafloor at six stations. Light-induced zooplankton swarms reached densities in the order of 90,000 (euphausiids), 120,000 (amphipods), and 470,000 ind m^?3 (chaetognaths), whereas older copepodids of Calanus hyperboreus and C. glacialis did not respond to light. They were abundant at the seafloor and 5 and 15 m above and showed maximum abundance of 65,000 ind m^?3. Tucker trawl data provided an overview of the seasonal vertical distribution of euphausiids. The most abundant species Thysanoessa inermis reached near-bottom concentrations of 270 ind m^?3. Regional distribution was neither related to depth nor to location in the fjord. The taxa observed were all part of the pelagic community. Our observations suggest the presence of near-bottom macrozooplankton also in other regions and challenge the current view of bentho–pelagic coupling. Neglecting this community may cause severe underestimates of the stock of pelagic zooplankton, especially predatory species, which link secondary production with higher trophic levels.     

Potential antifouling strategies for marine finfish aquaculture: the effects of physical and chemical treatments on the settlement and survival of the hydroid Ectopleura larynx

The hydroid Ectopleura larynx is a common fouling organism on aquaculture nets. To contribute to the development of novel cleaning methods, laboratory and field studies determined the effects of heat (30, 40, 50 and 60°C for immersion times of 1 and 3 s) and acetic acid (0.2 and 2.0% for immersion times of 1, 3 and 10 s, 1 and 5 min) on the settlement of actinulae and the survival of juvenile and adult E. larynx. Laboratory studies showed that, regardless of immersion time, a temperature of 50°C was effective in preventing the settlement of actinulae and the survival of juveniles, while ≤12% of adult hydroids could survive. A temperature of 60°C killed all adult hydroids. For an acetic acid concentration of 0.2%, an immersion time of 1 min substantially reduced the settlement of actinulae and the survival of juvenile and adult hydroids, and none of the juvenile and adult hydroids survived after 5 min. For an acetic acid concentration of 2.0%, all immersion times were effective and reduced the mean settlement of actinulae and the survival of juvenile and adult hydroids to ≤10%. Field studies with fouled net panels exposed to selected heat or acetic acid treatments showed small reductions in mean wet weight and net aperture occlusion of the net panels 2 and 5 days after treatment. Visual inspections of the net panels showed that hydranths of the hydroids were shed, but the dead stolons of the hydroids remained on the treated net panels. Novel cleaning methods and devices may utilise these results to effectively kill E. larynx on aquaculture nets, while further studies are needed to determine the necessity of removing the dead hydroids before further biofouling accumulates on thenets.     

Structure and signaling in polyps of a colonial hydroid

Abstract. After feeding, polyps of colonial hydroids contract regularly, dispersing food throughout the colony via the gastrovascular fluid. Such contractions may trigger signaling pathways that allow colonies to grow in an adaptive manner, i.e., to initiate development of more polyps in food‐rich areas and to suppress polyp development in food‐poor areas. In this context, we investigated the structure and potential signaling of the junction between polyps and stolons in colonies of the hydroid Podocoryna carnea. Using transmission electron microscopy, we found that the density of mitochondrion‐rich epitheliomuscular cells was low in polyp and stolon tissues except at or near the polyp‐stolon junction, where many of these mitochondrion‐rich cells occur in ectodermal tissue. In vivo fluorescence microscopy suggests that these mitochondria are a principal source of the metabolic signals of the colony. Both native fluorescence of NAD(P)H and fluorescence from peroxides (visualized with H₂DCFDA) co‐localize to this region of the polyp. Rhodamine 123 fluorescence suggests that both these metabolic signals emanate from mitochondria. To test whether such metabolic signals may be involved in colony pattern formation, inbred lines of P. carnea were used. Colonies of a runner‐like inbred line grow with widely spaced polyps and long stolonal connections, much like wild‐type colonies in a food‐poor environment. Colonies of a sheet‐like inbred line grow with closely spaced polyps and short stolonal connections, similar to wild‐type colonies in a food‐rich environment. Polyp‐stolon junctions in runner‐like and sheet‐like colonies were imaged for the fluorescence of H₂DCFDA. Densitometric analysis of this signal indicates that the mitochondria in epitheliomuscular cells of runner‐like polyps emit greater amounts of peroxides. Because peroxides and other reactive oxygen species are frequently intermediaries in metabolic signaling pathways, we suspect that such signaling may indeed occur at polyp‐stolon junctions, affecting colony pattern formation in these inbred lines and possibly in hydroid colonies in general.     

Ontogenetic and spatial variability in trophic biomarkers of juvenile saffron cod (<Emphasis Type="Italic">Eleginus gracilis</Emphasis>) from the Beaufort,Chukchi and Bering Seas

Climate models indicate the Arctic will undergo dramatic environmental change with forecasted increases in temperature and river runoff. Saffron cod (Eleginus gracilis) is abundant in nearshore waters and appears in the diet of many Arctic sea birds and marine mammals; however, little is known about its early ecology and consequently how they might be affected by environmental changes. We aimed to characterize the mechanisms of spatial and ontogenetic variation in trophic biomarkers (lipid classes, fatty acids and bulk C and N stable isotopes) of saffron cod from the Western Arctic, Chukchi and Bering Seas. Size-standardized analyses showed a significant difference in lipid condition metrics and trophic biomarkers as a function of survey location. Both ontogeny and sampling location played an important role in determining lipid stores with elevated levels in both small offshore juveniles (<55 mm) and larger inshore juveniles (>75 mm). Higher lipid storage in Arctic juveniles was associated with elevated levels of diatom fatty acid markers, but not with nearshore carbon input. Increased lipids were found in age-1 juveniles from Prudhoe Bay in the Western Beaufort that were feeding at a lower trophic level than similarly sized age-0 juveniles from surface trawls in the Bering Sea. The use of otolith annuli revealed two discrete patterns of growth that help explain the trade-offs between energy storage and rapid growth that diverge between the Arctic and Bering Sea. Laboratory temperature-growth experiments confirmed that saffron cod have a eurythermal growth response and are able to store excess lipids at temperatures as high as 20 °C.     

Feeding interactions between the introduced ctenophore Mnemiopsis leidyi and juvenile herring Clupea harengus in the Wadden Sea

We analysed feeding interactions between Mnemiopsis leidyi and juvenile Clupea harengus in the Wadden Sea. Biomass, diet overlap, prey selectivity, predation impact and stable isotope composition (C, N) of both species were assessed from June to September 2010. High biomass of C. harengus was found in June and July (wet weight 3.0 ± 1.8 g m^?3) followed by a steep decline from August to September (wet weight 0.01 ± 0.01 g m^?3), coinciding with a dramatic increase in M. leidyi biomass (wet weight 18.3 ± 16.1 g m^?3 during August). These two species showed a high overlap in their respective diets (copepods, meroplankton) during the study period. Predation impact of C. harengus on calanoid copepods was highest in June and July where 84 and 41 % of the standing stock were eaten per day in June and July, respectively. Predation impact of M. leidyi on calanoid copepods was highest in September (16 %). Based on stable isotope analysis C. harengus and M. leidyi were assigned to a trophic level of 3.08 and 2.47, respectively. Furthermore, we assessed the potential of competition between M. leidyi and C. harengus in a mesocosm experiment. Results indicated that at present zooplankton densities intraspecific competition in C. harengus seemed to be greater than interspecies competition with M. leidyi. Due to the low predation impact of M. leidyi and the reduced temporal overlap, competition between M. leidyi and C. harengus during the study period was estimated as low. Nevertheless, considering the high dietary overlap and the inter-annual variation in biomass and occurrence of both species and their zooplankton prey, competition in the Wadden Sea area cannot be excluded.     

Polar night ecology of a pelagic predator,the chaetognath <Emphasis Type="Italic">Parasagitta elegans</Emphasis>

The annual routines and seasonal ecology of herbivorous zooplankton species are relatively well known due to their tight coupling with their pulsed food source, the primary production. For higher trophic levels of plankton, these seasonal interactions are less well understood. Here, we study the mid-winter feeding of chaetognaths in high-Arctic fjord ecosystems. Chaetognaths are planktivorous predators which comprise high biomass in high-latitude seas. We investigated the common species Parasagitta elegans around the Svalbard archipelago (78–81°N) during the winters of 2012 and 2013. Our samples consisted of individuals (body lengths 9–55 mm) from three fjords, which were examined for gut contents (n = 903), stable isotopes, fatty acid composition, and maturity status (n = 352). About a quarter of the individuals contained gut contents, mainly lipid droplets and chitinous debris, whilst only 4 % contained identifiable prey, chiefly the copepods Calanus spp. and Metridia longa. The δ¹⁵N content of P. elegans, and its average trophic level of 2.9, confirmed its carnivorous position and its fatty acid profile [in particular its high levels of 20:1(n-9) and 22:1(n-11)] confirmed carnivory on Calanus. Observations of undeveloped gonads in many of the larger P. elegans, and the absence of small individuals <10 mm, suggested that reproduction had not started this early in the year. Its average feeding rate across fjords and years was 0.12 prey ind.^?1 day^?1, which is low compared to estimates of spring and summer feeding in high-latitude environments. Our findings suggest reduced feeding activity during winter and that predation by P. elegans had little impact on the mortality of copepods.     

Differences in the contributions of dietary water to the hydrogen stable isotope ratios of cultured Atlantic salmon and Arctic charr tissues

Hydrogen stable isotopes of animal tissues are well established tracers of migration ecology in terrestrial ecosystems. Recent research has highlighted δ²H as a potential tool in studies of aquatic ecosystems, particularly as a robust tracer for quantifying the importance of allochthonous subsidies. Although the use of δ²H has clear potential, some uncertainties remain, in particular with regard to the contribution of dietary water to consumer δ²H. Here, we quantify the contribution of dietary water to δ²H in two salmonid fishes, Atlantic salmon (Salmo salar L.) and Arctic charr (Salvelinus alpinus L.), reared on diets of known isotopic composition. Furthermore, we examined the capacity of fins (adipose and caudal) to provide a non-lethal means of estimating consumer δ²H. The proportion of deuterium derived from environmental water of all tissue was substantial in both Atlantic salmon (mean = 0.43 ± 0.1 SD) and Arctic charr (mean = 0.48 ± 0.15 SD) but varied considerably between both individuals and tissue type. White muscle proved to be the least variable of the tissues analysed. Although fins proved to be a possible non-destructive substitute, a degree of caution is recommended with their use, as the proportion of dietary water contributing to the deuterium of fins was considerable more variable.     

Trophic variability of Arctic fishes in the Canadian Beaufort Sea: a fatty acids and stable isotopes approach

Trophic ecology of most demersal Arctic fishes remains one of the major knowledge gaps for understanding food web dynamics and connectivity among ecosystems. In this study, fatty acids (FA) and stable isotopes (SI) were used to study the feeding ecology of seven species (n = 106) of the most abundant benthic fishes (eelpouts, sculpins and agonids) in the Canadian Beaufort Sea from shallow (20–75 m), slope (200–350 m) and deep (500–1000 m) habitats. Both FA and SI results revealed among- and within-species variability in diet composition. Correspondence analysis of FA signatures identified high within-species variability in diet, resulting in high overlap among species. Calanus-derived FA were present in all species (Calanus markers up to 13 % of total FA) and were particularly important in Ribbed Sculpin, Adolf’s and Longear Eelpout collected in deep habitats, suggesting a strong contribution of pelagic-derived FA to benthic fish communities. Incorporation of this signal in the benthos may result from either direct consumption of deep overwintering copepods (i.e., off-bottom feeding) or through detrital accumulation in benthic invertebrate prey. Mean SI values differed among species and indicated that a large range of trophic positions (δ¹⁵N varied from 14.09 to 17.71 ‰ for Canadian Eelpout and Adolf’s Eelpout, respectively) and carbon dietary sources are preyed upon (δ¹³C range from ?21.13 to ?23.85 ‰ for Longear Eelpout and Ribbed Sculpin, respectively). SI analyses suggested that most species examined were low- to mid-trophic generalist benthic carnivores, with the exception of Ribbed Sculpin, which was a low-trophic pelagic predator.     

Vertical distribution of hydroids in wharf fouling in the Northwestern Sea of Japan

The features of the vertical distribution of hydroids on various types of wharves in the northwest part of the Sea of Japan were studied. It was shown that the highest values of biomass of the common species of hydroids Obelia longissima and Bougainvillia ramosa were at wharf sites below the zone of varying submergence up to a depth of 1 m. The spatial variability of hydroid abundance is discussed in relation to the local specifics of their environment and the character of formation and the structure of the fouling community.     

Water transparency affects the survival of the medusa stage of the invasive freshwater jellyfish <Emphasis Type="Italic">Craspedacusta sowerbii</Emphasis>

Craspedacusta sowerbii is a global invader of inland waters, but little is known on what environmental factors control its colonization success. We hypothesized that water transparency and thus, exposure to UV radiation are relevant for the colonization success of this relatively transparent invasive hydroid. Here we used field and laboratory experiments to assess the effect of natural solar radiation and artificial UV radiation on the medusa stage of C. sowerbii across natural and simulated water transparency gradients in three Chilean Patagonian lakes. Short-term exposure of the jellyfish to artificial UV radiation under low Dissolved Organic Carbon (DOC) treatments induced mortality and caused sublethal effects such as swimming anomalies and production of reactive oxygen species. Outdoor exposure of jellyfish to full solar radiation in the most transparent water (Lake Ranco; DOC = 0.6 mg l^?1) resulted in almost complete mortality. However, higher DOC contents provided partial or almost complete protection against the adverse impact of UV radiation. Overall, our findings provide evidence of the role of underwater light conditions in the underlying mechanisms that may favor the invasion of this gelatinous alien species in inland waters. The results imply that under the current “brownification” of lakes, an increase in water color might provide more favorable conditions for the invasion process of this hydroid.