Comparative energetics and life cycle of the opossum shrimp ( Mysis relicta ) in native and non-native environments

1. Life history, fecundity and energy ingestion were compared for non-native Mysis relicta in Flathead Lake, Montana, and in Waterton Lake, Montana/Alberta, where the mysid is native. 2. Based on estimates from stomach contents, M. relicta in Flathead Lake consumed three to four times more energy per hour while foraging than M. relicta from Waterton Lake. The primary prey of M. relicta in Flathead Lake was Daphnia thorata. Diaptomus sicilis was the primary prey for M. relicta in Waterton Lake. 3. Mysis relicta in Flathead Lake had a 1-year life history. It took 2 years for M. relicta in Waterton Lake to complete a generation. Mysis relicta from Flathead Lake produced significantly (P < 0.05) more eggs per female (19.9 ± 4.7) compared to M. relicta in Waterton Lake (13.7 ± 3.6). The lipid content in eggs from M. relicta in Flathead Lake was also significantly (P < 0.05) higher than eggs from M. relicta in Waterton Lake. 4. In Flathead Lake, M. relicta is able effectively to exploit Daphnia thorata, consuming greater biomass than M. relicta in Waterton Lake. Mysis relicta efficiently assimilates the greater biomass, which reduces generation time, increases fecundity, and increases lipid reserves of offspring. These results provide evidence of energetic mechanisms that enable M. relicta to colonize and affect foodwebs when introduced into lakes.     

Spatial variability in the abundance of pelagic invertebrate predators in relation to depth and turbidity

The abundance of pelagic invertebrate predators in relation to turbidity and depth gradients in Lake Hiidenvesi (southern Finland) were studied. In the shallow (<5 m) and the most turbid (up to 75 NTU) part of the lake, the community of invertebrate predators consisted of cyclopoid copepods (max biomass >500 μg dw l⁻¹) and Leptodora kindtiii (Focke) (17 μg dw l⁻¹), while in the less turbid (10–40 NTU) stratifying area Chaoborus flavicans (Meigen) dominated (max 146 μg dw l⁻¹). In the temporarily stratifying and moderately turbid basin Chaoborus and small-bodied invertebrate predators co-existed. Mysis relicta (Lovén) occurred only in the stratifying area (max 15 μg dw l⁻¹). The results suggested that both water depth and turbidity contributed to the community structure of Chaoborus flavicans. Depth great enough for stratification was of special importance and its effect was amplified by elevated turbidity, while high turbidity alone could not maintain chaoborid populations. Mysis relicta also requires a hypolimnetic refuge but is more sensitive to low oxygen concentrations and may therefore be forced to the epilimnion where it is vulnerable to fish predation. Cyclopoids as rapid swimmers can take advantage at elevated turbidity levels and coexist in high biomass with fish even in shallow water. Leptodora kindtii can form high biomass despite planktivorous fish providing that turbidity exceeds 20 NTU. The results demonstrated that depth and water turbidity can strongly regulate the abundance and species composition of invertebrate predators. These factors must thus be taken into account when applying food web management, which aims to reduce phytoplankton biomass by depressing planktivorous fish.     

Percottus glehni Dybowski — a new colonizer in the ichthyofauna of Lake Glubokoe

Percottus glehni is a new colonizer in the ichthyofauna of Lake Glubokoe. Recently, this species, established in the European part of the USSR nearly 30 years ago, has become common in the lakes and ponds located 3–5 km from Lake Glubokoe. In Lake Glubokoe it keeps mainly to water thyme (Elodea) in shallow water with individuals up to 7 cm in size being predominant. Apparently, these fishes are not abundant due to strong predation pressure from larger predators.     

Distribution,growth, production,and ecological significance of the clam Unio terminalis in Lake Kinneret,Israel

The distribution, body composition, growth rate, and population structure of Unio terminalis were measured at different sites of Lake Kinneret (Israel). Maximum clam density was found on the muddy sand between 0.3–6 m depth. Clams were most abundant in the River Jordan inlet zone, where they showed the highest growth rate. This was probably related to both highest food availability and the highest density of fish hosting Unio glochidia in this area. U. terminalis in Lake Kinneret has a more massive shell and ash content as compared with the European Unio species. The annual P/B ratios of U. terminalis populations at different sites were similar and ranged within 0.17–0.18. The computated filtration capacity and energetic budget permit the assumption that the U. terminalis population plays a substantial role in removal of organic particles from the water in the Kinneret shallow inshore zone (up to 15 m depth), and in nutrient recycling.     

An evaluation of the diet of <Emphasis Type="Italic">Mysis relicta</Emphasis> using gut contents and fatty acid profiles in lakes with and without the invader <Emphasis Type="Italic">Bythotrephes longimanus</Emphasis> (<Emphasis Type="Italic">Onychopoda,Cercopagidae</Emphasis>)

Diets of Mysis relicta from four lakes in central Ontario that had been invaded by Bythotrephes longimanus and three lakes that had not been invaded were investigated using gut content analysis and fatty acid (FA) composition. Gut content analysis of M. relicta revealed a high incidence of cannibalism in all lakes, and consumption of B. longimanus and native zooplanktivorous midges in the genus Chaoborus in lakes where these were present. Cladocera other than B. longimanus were present in the guts of all M. relicta examined except those from Bernard Lake, the lake with the most B. longimanus. In that lake, B. longimanus was the most frequent diet item. Copepod remains were found in 60–100% of M. relicta guts with the lowest frequency occurring in Bernard Lake. Fatty acids (FA) that contributed strongly to the variation in FA composition in M. relicta, as revealed by a principal component analysis, were C16:0 (palmitic acid), C16:1n7 (palmitoleic acid), C18:1n9c (oleic acid), C20:4n6 (arachidonic acid), C20:5n3 (eicosapentaenoic acid), and C22:6n3 (docosahexaenoic acid). Significant differences in FA amount and composition of M. relicta were found between invaded and non-invaded lakes, and among lakes within these groups. Generally, M. relicta in non-invaded lakes had higher concentrations of C16:0, C18:1n9c, C18:2n6c (linoleic acid), C18:3n3 (α-linolenic acid) and C20:4n6, while M. relicta in invaded lakes had higher concentrations of C22:6n3. Two of the non-invaded lakes had lower water transparency, as measured by Secchi depth, which may be the reason why mysids and abundant populations of Chaoborus spp. could be found in the water column during the day. However, differences in FA profiles and gut contents of M. relicta between invaded and non-invaded lakes are consistent with competition for Cladocera in the presence of the invader rather than pre-existing differences among lakes. We conclude that the diet of M. relicta is affected by the invasion of B. longimanus.     

Concurrent evolution of ancient sister lakes and sister species: the freshwater gastropod genus <Emphasis Type="Italic">Radix</Emphasis> in lakes Ohrid and Prespa

Ancient sister lakes are considered to be ancient lakes lying in close geographic proximity, sharing a related origin and significant time of co-existence, usually having hydrological connection as well as a balanced degree of faunal overlap and distinctness. A paradigm for studying sister lake relationships are the ancient lakes Ohrid and Prespa in the Balkans, which are characterized by high degrees of endemicity. Three general patterns of endemic species can be distinguished for these lakes: (1) taxa that are endemic to either lake, with no close relatives in the respective sister lake, (2) closely related but distinct endemic taxa in both lakes (sister species) and (3) shared endemic taxa occurring in both lakes. In the present paper, two endemic freshwater pulmonate gastropod species, Radix relicta (Lake Ohrid) and R. pinteri (Lake Prespa), are used to study the evolution of presumed sister species based on biogeographical and comparative DNA data from world-wide Radix taxa. Phylogenetic, phylogeographical and parametric bootstrap analyses all suggest a sister group relationship of R. relicta and R. pinteri (pattern 2 of endemic diversity). Sister to these two taxa is the widespread R. ampla, which does not occur in the vicinity of lakes Ohrid and Prespa. The southern feeder spring complexes of Lake Ohrid are inhabited by another lineage (Radix sp. 1), which resembles Radix relicta in morphology/anatomy. For Lake Prespa, the widespread R. auricularia was reported in addition to the endemic R. pinteri. Comparative phylogenetic data favour a western Adriatic zoogeographical affinity of lakes Ohrid and Prespa over an Aegean-Anatolian faunal connection. The status of lakes Ohrid and Prespa as sister lakes is evaluated in the light of current knowledge on gastropod speciation and endemism in these hotspots of biodiversity.     

Diurnal vertical and seasonal horizontal distribution patterns of Mysis relicta in a large Norwegian lake

Mysis relicta was sampled intensively along a depth profilein Lake Snåsavatnet using benthic trawls and verticalnet hauls. Samples were made around 12.00 and 24.00 h in May,June, August and September 1984. The light regime and temperaturewere also recorded. Mysis relicta showed a size-dependent diurnalvertical migration and seasonal benthic horizontal migrationbetween shallow and deep areas. The young individuals left themarsupium in spring while the adults were staying benthic inshallow areas. As they grew in size the young individuals changedhabitat, migrating into the pelagic zone and the deeper benthicareas. Mysis showed a bimodal vertical distribution patternon our deepwater station, with one density peak in the watercolumn and another along the bottom both day and night betweenMay and September. The bimodal distribution pattern is discussedin relation to the light regime and an asynchronic verticalmigration model. The density of Mysis was low in the pelagiczone compared to the shallow benthic area throughout the samplingperiod. Together with the difference in distribution patternin the pelagic and benthic zones, this may influence the distributionpattern of salinonids which seek Mysis as a food item in LakeSnåsavatnet.     

Vulnerability of early life intervals of Coregonus hoyi to predation by a freshwater mysid,Mysis relicta

Synopsis The bloater, Coregonus hoyi, deposits its eggs on deep sediments (70–100 m in Lake Michigan), where its eggs and embryos can be exposed to epibenthic predators. We investigated the vulnerability of early life intervals of bloaters to predation by mysids, Mysis relicta, which are mostly epibenthic by day and planktonic at night. Bloaters were raised from spawn in the laboratory and presented to field-collected mysids in laboratory predation trials. Eggs were not ingested by the mysids. Embryonic bloaters were vulnerable to predation by mysids only during the interval between hatching and swim up, usually 1–24 h under laboratory conditions. The mysids required about a day of exposure to this novel prey before they were able to kill significant numbers of the bloater embryos by making successive attacks with their thoracic legs. In experiments with experienced (2 and 3 days with bloater embryos) mysids, a functional response between embryo density and mysid predation rates was apparent. Temperature and the presence of alternative prey (zooplankton) did not alter the ‘kill rate’ (about 2.5 embryos mysid^-1d^-1) of experienced mysids at high bloater densities (>4 bloaters/mysid). However, more embryos were partially, rather than completely, ingested at 4 versus 9° C and in the presence of zooplankton.     

Foraging behaviors and the interaction of alewife,Alosa pseudoharengus,and bloater,Coregonus hoyi

Synopsis Alewife, Alosa pseudoharengus, and bloater, Coregonus hoyi, are common planktivores in Lake Michigan. Both alewife and bloater use a variety of feeding modes. Alewives can filter, gulp and particulate feed; bloaters can only gulp and particulate feed. We examined handling time per prey and probability of capture for alewife and bloater particulate feeding on Mysis relicta. Using these estimates and available data for filtering alewives, cost curves were derived for alewife and bloater particulate feeding and for alewife using all three modes of feeding. Alewives filter small prey relative to their own body size and particulate feed on larger prey. Feeding mode appears to be dependent on prey size and density and shifts in feeding mode are apparently based on maximizing biomass eaten per time. The ability to filter confers a competitive advantage on alewife when small prey are abundant as they were in the mid 1960s in Lake Michigan. If the zooplankton are large, bloater young-of-year do not suffer this relative disadvantage. In fact, large bloaters can consume prey on the bottom not available to alewife. This shifting competitive balance may explain, in part, the observed dynamics of alewife and bloater.     

Comparison of sampling methods for semipelagic animals in two deep basins of Lake Saimaa

The efficiency of different methods for sampling semipelagic animals was compared in the deep basins of Lake Paasivesi and Lake Puruvesi of the complex Lake Saimaa, Finland, during 1990–92. Certain semipelagic animals, such as Mysis relicta, amphipods and larvae of Chaoborus fiavicans, are important bioindicators of water and benthic quality. However, owing to their high mobility and vertical migrations they are difficult to sample quantitatively with traditional benthic methods. The equipment used for sampling was comprised of:

1. Vertical nets (0.56 and 0.13 m²) placed on the bottom for 5 minutes or 12 hours and hauled up vertically.
2. Plexiglass traps placed on the bottom for 12–14 hours.
3. Mysis-trawl hauled horizontally c. 250–300 metres on the bottom.
4. Ekman-grab.

In 1992, parallel samples were taken with vertically hauled nets (12 h and plexiglass traps baited with fish or liver or provided with a chemical light. Large vertically hauled nets were the most efficient equipment for catching Gammaracanthus lacustris, M. relicta and larvae of Ch. flavicans whilst baited or lighted nets were less effective. Lighted plexiglass traps were most effective in catching Pallasea quadrispinosa and water mites, but baited and control traps sampled inefficiently. The trawl sampled M. relicta and P. quadrispinosa relatively well, but was less efficient in sampling other groups. The Ekman-grab was effective only for sampling Monoporeia affinis. The semipelagic community differed between the studied basins. The dominant semipelagic animals in the humic, deep L. Paasivesi were M. relicta, amphipods and larvae of Ch. flavicans whilst the number of water mites was low in the catches. In the shallower, clear-water L. Puruvesi the densities of crustaceans and Chaoborus-larvae were low whilst some water mites, such as Piona pusilla, occurred in high numbers. Nocturnal sampling was more effective, especially for M. relicta.     

Changes in biomass and spatial distribution of <Emphasis Type="Italic">Elodea nuttallii</Emphasis> (Planch.) St. John,an invasive submerged plant,in oligomesotrophic Lake Kizaki from 1999 to 2002

Distribution of pure Elodea nuttallii vegetation was surveyed from 1999 to 2002, immediately after the most recent expansion of the species in Lake Kizaki, Japan. During 2001 and 2002, areas of E. nuttallii vegetation rapidly diminished and the summer plant height decreased wherever the vegetation remained. The organic matter content, total phosphorus, and extracted P of the sediment from the vegetation bed were measured. A linear relationship was observed between the extracted P in the sediment and the biomass. The extracted P significantly decreased in the shallow littoral vegetation bed, where the biomass clearly diminished. A fertilization experiment using the shallow littoral sediment collected in the vegetation bed was conducted in 2001. In this experiment, apical shoots of E. nuttallii were planted in pots with fertilized sediment (nitrogen, phosphorus, and potassium additions). The growth of E. nuttallii shoots was significantly enhanced by enrichment with phosphorus alone. The ecological implication of sediment phosphorus limitation is discussed in relation to the cause of decline in the E. nuttallii population in Lake Kizaki.     

Patterns of aquatic weed regrowth following mechanical harvesting in New Zealand hydro-lakes

Mechanical harvesting is used to control submerged aquatic weeds in parts of the hydro-lakes in New Zealand's North Island. Problem species are Egeria densa and Lagarosiphon major (Hydrocharitaceae), and Ceratophyllum demersum. Experiments were conducted in two contrasting hydro-lakes. Lake Aratiatia; clear water (K _o 0.2 m^–1) and a low residence time (< 8 h), and Lake Ohakuri; turbid water (K _o = 0.6) and a longer residence time (> 5 days). Growth rates were measured underwater in harvested and control (unharvested) plots. Regrowth of C. demersum was dependent on the prior establishment of the rooted Hydrocharitaceae. Regrowth of the Hydrocharitaceae was inhibited where significant water movement occurred. Regrowth declined after 3 six-monthly harvests allowing the establishment of low growing native Nitella spp. beds in the smaller clear water lake. In Lake Ohakuri there was a change in species dominance from Ceratophyllum to Elodea canadensis in shallow (1–2 m) water. No change in species dominance was observed in deeper (>2 m) water and native species were not able to re-establish. The recommended cutting frequency for management of surface weed growths was only once per year in Lake Aratiatia, but twice per year in Lake Ohakuri.     

Daphnia exhibit diurnal vertical migration in shallow rock-pools

We report diurnal changes in vertical distribution ofDaphnia longispina andD. magna in shallow (25–60 cm) rock-pools on Baltic islands, southern Finland. During the daytime shallow rock-poolDaphnia reside in water layers close to the sediment while at midnight they either are evenly distributed throughout the water column, or have a modal peak of distribution in the surface water. Contrary to this pattern, no changes were observed in vertical distribution ofD. longispina at noon and at midnight in a 150 cm deep pond. We suggest that the pattern ofDaphina vertical migration in shallow rock-pools is not satisfactorily explained by predation.     

An ecological study of two shallow,equatorial lakes: Lake Mburo and Lake Kachera,Uganda

Lake Mburo and Lake Kachera are shallow, eutrophic lakes in mid‐western Uganda. Lake Mburo recorded higher values of Secchi and eutrophic depths and lower extinction coefficient (k) values. The lakes showed a ‘red shift’ phenomenon in maximum light transmission. The average values of electrical conductivity in Lake Mburo and Lake Kachera were 136 and 244 μS cm^?1, respectively. The pH values indicated high photosynthetic activity. Dissolved oxygen concentration averaged 6.9 and 7.8 mg l^?1 in Lake Mburo and Lake Kachera, respectively. The lakes had high total nitrogen (TN) : total phosphorus (TP) ratios averaging 200 and 280 in Lake Mburo and Lake Kachera, respectively. The lakes are dominated by cyanobacterial blooms that reduce light penetration to less than 1 m. Lake Mburo had a lower algal biomass than Lake Kachera. Chlorophyll a concentrations correlated positively (r = 0.73, P < 0.05) with the extinction coefficient in Lake Mburo but not in Lake Kachera. The correlations between chlorophyll a and TN and TP were also high. Both lakes recorded high primary productivity, Lake Mburo showing higher values. The study highlighted the need to investigate the organism–community interrelationships in the two water bodies.     

Feeding habits ofMysis relicta — an overview

Mysis relicta has been recognized as an important component of many limnetic food webs. The first approach to studying the feeding habits ofM. relicta was the analysis of stomach contents. Assumptions regarding stomach content analysis have been tested and seasonal feeding estimates ofM. relicta have been reported recently. Laboratory grazing and/or predation experiments have recently been completed usingM. relicta primarily from Lakes Michigan and Tahoe.Mysis relicta is concluded to be opportunistic, capable of utilizing a variety of food resources. It may play an important role in structuring limnetic food webs.     

Vertical distribution and movements of brown bullhead (Ameiurus nebulosusLesueur 1819) in Motuoapa Bay,southern Lake Taupo,New Zealand

Ultrasonic telemetry was used to determine the swimming depths of brown bullhead (Ameiurus nebulosus) in Motuoapa Bay at the southern end of Lake Taupo, New Zealand. Sonic transmitters equipped with a pressure sensor were surgically implanted in 12 adult fish (295–345 mm FL). Three automatic receivers recorded a total of 61335 data points on swimming depth between 26 November 1998 and 7 November 1999. Brown bullheads swam in water from 0 to 17 m deep but showed some seasonal variations. They were more active at night when they used shallow water extensively, especially during the winter months. They were most mobile in spring when they left Motuoapa Bay, suggesting migratory behaviour. During winter the fish remained more stationary, only moving around inside the bay. The results of this study provide guidance for establishing a brown bullhead control program should it become necessary.     

The effect of temperature on growth characteristics and competitions of <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis> and <Emphasis Type="Italic">Oscillatoria mougeotii</Emphasis> in a shallow,eutrophic lake simulator system

Blue-green algal blooms formed by Microcystis and Oscillatoria often occur in shallow eutrophic lakes, such as Lake Taihu (China) and Lake Kasumigaura (Japan). Growth characteristics and competitions between Microcystis aeruginosa and Oscillatoria mougeotii were investigated using lake simulator systems (microcosms) at various temperatures. Oscillatoria was the superior competitor, which suppressed Microcystis, when temperature was <20°C, whereas the opposite phenomenon occurred at 30°C. Oscillatoria had a long exponential phase (20 day) and a low growth rate of 0.22 day⁻¹ and 0.20 day⁻¹ at 15°C and 20°C, respectively, whereas Microcystis had a shorter exponential phase (2–3 days) at 30°C and a higher growth rate (0.86 day⁻¹). Interactions between the algae were stronger and more complex in the lake simulator system than flask systems. Algal growth in the lake simulator system was susceptible to light attenuation and pH change, and algae biomasses were lower than those in flasks. The outcome of competition between Microcystis and Oscillatoria at different temperatures agrees with field observations of algal communities in Lake Taihu, indicating that temperature is a significant factor affecting competition between Microcystis and Oscillatoria in shallow, eutrophic lakes.     

Excretion of total phosphorus,dissolved reactive phosphorus,ammonia, and urea by Lake MichiganMysis relicta

Rates of nutrient release byMysis relicta in Lake Michigan were measured on five nights at a 45-m station near Milwaukee, Wisconsin, U.S.A., in the summer of 1977. Nocturnal vertical migrations of the mysids were monitored with both echosounder tracings and vertical net tows. Estimates of the total areal dry mass of the mysids ranged from 600 to 1 820 mg m–2. Rates of release of dissolved reactive phosphorus, total phosphorus, ammonia, and urea were measured in dark incubations on shipboard. Excretion experiments were initiated immediately after mysids were collected from each of several vertical net hauls. The depths of maximum mysid densities corresponded approximately with a deep phytoplankton peak located in the vicinity of the thermocline. Semiquantitative demands for N and P by phytoplankton within this peak were obtained from¹⁴C estimates of primary production from a previous study, assuming a constant C:N:P ratio for the algae. These algal nutrient demands were compared to potential N and P release by the mysids to obtain a first approximation of the relative rates of nutrient supply and demand for the field phytoplankton populations. Our analysis indicates that mysids may directly supply about 1–10% of the daily N and P demands of the phytoplankton in the deep peak. However, indirect interactions betweenMysis relicta and other organisms, such as small zooplankton and fishes, could be major factors in nutrient recycling within the metalimnion and subthermocline region of Lake Michigan.Contribution No. 238, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Wisconsin.Contribution No. 238, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Wisconsin.     

Effects of summer drawdown on cyprinid fish larvae in Lake Biwa, Japan

The effects of artificial water-level manipulations in Lake Biwa, initiated in 1992 to prevent flooding, on cyprinid fish larvae were assessed by measuring and estimating the topographic change in the reed zones. The shallow, litter-accumulated water in those zones was inhabited by larvae of two species, Carassius spp. and Cyprinus carpio. More than 70% (by volume) of these shallow areas in Lake Biwa was estimated to be lost when the water level was artificially lowered by 30cm from June to October 1997. During the low water level period, significantly fewer eggs of these cyprinid fishes were collected in a reed zone than in April and May of the same year, a comparison with a previous study suggesting that spawning of the fishes was inhibited during the period of low water level. The artificial reductions in water level probably resulted in a significant decrease in the volume of shallow water in the lake and may be linked to the drastic decline in these cyprinid fishes.     

Seasonal variations of Microcystis populations in sediments of Lake Biwa, Japan

Seasonal variations of colony numbers of Microcystis aeruginosa(Kütz.) Kütz. and M. wesenbergii(Komárek) Komárek in N. V. Kondrat. in sediments of Lake Biwa were investigated over a period of 1 year. At two stations located in the shallow South Basin of Lake Biwa (ca. 4 m water depth), the colony number of Microcystisfluctuated seasonally. The number had a tendency to gradually decrease from winter to early summer, while it increased through mid-summer and autumn. Since the Microcystispopulation in sediment was rather small, intensive growth and accumulation in the water column should be important for the formation of Microcystisblooms in Lake Biwa. Microcystiscolonies in the sediment samples after June were observed to be floating in a counting chamber under a microscope. The observation suggests that the recruitment of Microcystis colonies into the water column mostly occurs in early summer. The number of Microcystiscolonies in the deep North Basin of Lake Biwa (70 – 90 m water depth) was larger than in the South Basin. Because the seasonal variation of colony numbers was not observed in the North Basin, and Microcystiscells do not have gas vesicles, these colonies will not return into the water column. The colonies isolated from the sediment of the North Basin were able to grow in cultured conditions, in the same way as those from the sediment of the South Basin. Therefore, Microcystiscolonies may survive for a long time under stable conditions of low temperature (ca. 8 °C) and darkness, in the sediment of the deep North Basin, accumulating gradually each year.