Synchronized release of medusae from three species of hydrozoan fire corals

 The release of medusae from three hydrozoan fire corals, Millepora dichotoma, M. murrayi and M. platyphylla, was investigated at three sites in southern Taiwan from February 1994 to July 1995. All three species were gonochoristic, and developed and released several batches of medusae between April and May. The duration of open ampulla appearing on the surface of coralla was short, about three months, and could be used to infer the reproductive season of the fire corals between April and May. No obvious lunar cycles of medusa release were found in these species. Medusa release started before dark at approximately 17:00 h and continued for several hours. Males began releasing medusae earlier than females. Synchronization of medusa release between colonies, i.e., the probability of occurring on the same nights, was significantly higher within a species than between different species. Hybridization in nature among the three species is, therefore, unlikely due to segregation in the spawning dates. Moreover, the synchronization within each species was often significantly higher within versus between sites. The free-swimming medusae released gametes within approximately one hour, and the spent medusae lived for a few more hours. Medusae may facilitate fertilization rates as a result of an apparently negatively geotactic swimming response that results in medusa aggregation at the surface. No differences in the sizes of medusae, eggs and sperm were detected among the three species; however, some characteristic differences of medusa nematocysts were found. Accepted: 25 September 1997     

Large medusae in surface waters of the Northern California Current: variability in relation to environmental conditions

Blooms of jellyfish around the world have been correlated with climatic variables related to environmental causes. Sizeable populations of large medusae, primarily Chrysaora fuscescens and Aequorea sp., appear annually in shelf waters of the Northeast Pacific Ocean. Previous research has shown that C. fuscescens is abundant seasonally in the inner shelf and exhibits high feeding rates on zooplankton. We examined medusae caught in surface trawls over an 8-year period (2000–2007) using (1) mesoscale surveys sampling 8–10 transects in May, June, and September, and (2) biweekly surveys along two transects from April to August, relating abundance to environmental parameters. C. fuscescens abundances generally peaked in late summer, whereas Aequorea sp. peaked in May or June. General additive models of the mesoscale data indicated that station catches for both species correlated with latitude, temperature, salinity, and distance from shore (and chlorophyll a for Aequorea sp.). Analysis of interannual variability revealed that highest catches of medusae correlated with cool spring–summer conditions, or negative anomalies of the Pacific Decadal Oscillation, and low winter–summer runoff from the Columbia River. Results confirmed our hypothesis of connections between jellyfish populations and regional climate conditions in a region known for strong physical forcing of ecosystem processes.     

Active feeding of downstream migrating juvenile pink salmon (Oncorhynchus gorbuscha) revealed in a large Barents Sea river using diet and stable isotope analysis

The recent, rapid spreading of non-native pink salmon Oncorhynchus gorbuscha in the North Atlantic area has raised concerns about their possible negative impacts on native salmonid species. Potential interactions include competition for food resources during the short freshwater phase of juvenile O. gorbuscha, but little is known about their feeding behavior in the newly occupied North Atlantic rivers. Using stable isotope and stomach content analyses, patterns of freshwater feeding of non-native O. gorbuscha fry were studied in a large Fennoscandian river, the Teno, that discharges to the Barents Sea. Changes in stable isotope values (δ¹³C, δ¹⁵N, δ³⁴S) and stomach contents from the period of emergence (April to mid-May) to estuarine entry (late May/June) were examined and provided both temporally integrated and short-term indicators of freshwater feeding dependency. In addition, the occurrence of juvenile O. gorbuscha and changes in their length and weight during their emergence/migration period were investigated. Juvenile O. gorbuscha were at the spawning grounds from April through to mid-May with abundance peaking in mid-May. Fish moved to the estuary by late May and their abundance decreased toward June, and their body size increased concurrently. Stomach analyses indicated no feeding activity in April–early May in the spawning areas, but the stomach fullness indices increased markedly in fish sampled in the estuary in May and June. The most important prey items in stomachs were Chironomidae and Ephemeroptera larvae. Significant changes in all analysed stable isotopes were detected among sample periods, with a peak in mid-May and June showing significantly lower values than other sample periods. A change from the higher values reflective of parental marine feeding to the lower values reflective of freshwater feeding indicated active in-river feeding by juveniles during the study period. The documented active freshwater feeding of non-native juvenile O. gorbuscha suggests potential resource competition with native fluvial fishes, particularly salmonids.     

Seasonal changes in the local distribution of Yangtze finless porpoises related to fish presence

The Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) is an endangered freshwater porpoise subspecies unique to the Yangtze River basin. Seasonal variations in local distribution of the animal, as well as fish presence, sand dredging, ship navigation, and bridges were examined as potential factors affecting the occurrence of the animals. Passive acoustic surveys were performed regularly from May 2007 to August 2010, near the conjunction of the Yangtze River and Poyang Lake. The distribution of the porpoises was seasonally site‐specific. In May and August, the animals were detected more often at river junctions than in the lake, but vice versa from November to February. The rate of the porpoise detection was significantly higher in areas of fish presence than in areas of absence. The number of porpoises detected did not differ significantly between the sand dredging operation and the prohibition period (in 2008), although the number of vessels obviously declined in 2008. Ship traffic and bridges also did not appear to affect the presence of porpoises. These results showed the relative importance of the various environmental factors, which is important for conservation of not only Yangtze finless porpoise but also endangered isolated cetaceans.     

Global diversity of inland water cnidarians

Global diversity of inland water cnidarians is low, containing <40 species belonging to phylogenetically distinct groups representing independent invasion events: the common and cosmopolitan hydras (12–15 species); the sporadically occurring freshwater medusae (6–16 sp.); the Cordylophorinae (2 sp.); the parasitic Polypodium (1 sp.); the medusae occurring in saline lakes (4 sp.). Freshwater cnidarians inhabit nearly all types of freshwater on all continents (except Antarctica), but only a few species have cosmopolitan distributions. Due to uncertainty in species knowledge, fine scale regions of endemicity are not yet clear. Guest editors: E. V. Balian, C. Lévêque, H. Segers & K. Martens Freshwater Animal Diversity Assessment     

Mesozooplankton of the Kosi Bay lakes,South Africa

The Kosi coastal lake system, a chain of four interconnected basins, is located in the subtropical north-eastern corner of South Africa. Little information is available on zooplankton of the system and the main aim of this study is to report on zooplankton samples collected during 2002 and 2003. The set of samples consists of seasonal, subsurface mesozooplankton samples that were collected during nighttime in each of the lakes. A well-developed salinity gradient was evident along the interconnected lakes in the subsurface water during all seasons, ranging from freshwater in the upper lake Amanzamnyama to a maximum of 22 recorded in Lake Makhawulani. The zooplankton community structures of the lakes reflected the salinity gradient of the system, with some coastal marine taxa recorded in the lakes closer to the mouth and only freshwater taxa recorded in Lake Amanzamnyama. Mesozooplankton diversity and abundance were relatively low compared to other estuarine systems along the eastern coast of South Africa. The dominant taxa were calanoid copepods Acartiella natalensis and Pseudodiaptomus stuhlmanni and the mysid Mesopodopsis africana in the lower lakes, whereas cyclopoids Mesocyclops sp. and Thermocyclops sp. dominated the freshwater lake Amanzamnyama.     

Response of <Emphasis Type="Italic">Chrysaora quinquecirrha</Emphasis> medusae to low temperature

Because of their high abundance in Chesapeake Bay, Chrysaora quinquecirrha medusae may be an important reservoir of organic matter. The timing and location of the decomposition of biomass from medusae may have implications for carbon cycling in the bay. Our objective was to identify the cause of C. quinquecirrha medusa disappearance to better understand when and where decomposition occurs. A time series of visual surface counts and vertical net hauls in the Choptank River, a tributary of Chesapeake Bay, showed that as temperatures approached 15°C, C. quinquecirrha medusae disappeared from the surface, but persisted in net hauls until temperatures reached 10°C. In order to test whether medusae sink upon cooling, we exposed C. quinquecirrha medusae to low temperatures in large static tanks and measured their depth and pulsation rates twice daily for at least 6 days. This procedure was repeated three times through the 2008 jellyfish season. On average, individuals exposed to temperatures below 15°C were found deeper and pulsed slower than those in the warmer control tank. This suggests that low temperatures cause the medusae to sink before cooling to the limit of their physiological tolerance and may have implications for the deposition of organic matter associated with the seasonal disappearance of medusae from Chesapeake Bay.     

A Middle Miocene endemic freshwater mollusc assemblage from an intramontane Alpine lake (Aflenz Basin,Eastern Alps,Austria)

The mollusc fauna of the early Middle Miocene (Langhian) intramontane Alpine Lake Groisenbach is described for the first time. The shells derive from the Feistring Formation in the Aflenz Basin in Austria, which was covered by Lake Groisenbach. The assemblage is moderately diverse with 12 gastropod and 2 bivalve species, suggesting shallow lacustrine and fluvial settings. Among the gastropods, only Theodoxus crenulatus (Klein, 1853) is known from other Miocene localities, whilst all other species are documented so far only from Lake Groisenbach. None of the Early and Middle Miocene lake systems of the Alpine-Carpathian Foredeep and the Balkan Peninsula displays any faunistic resemblance with this new fauna. Even coeval lake faunas from the close-by Graz Basin have no species in common with Lake Groisenbach. This pattern points to a surprising endemicity and biogeographic fragmentation in the Central European freshwater systems during the Early and Middle Miocene. The uniqueness of the newly described fauna is also indicated by the completely erratic occurrence of the otherwise African-Mediterranean genus Bulinus, which is unknown from all other central European Miocene freshwater systems. Emmericia roetzeli Harzhauser and Neubauer nov. sp., Nematurella zuschini Neubauer and Harzhauser nov. sp., Romania fastigata Neubauer and Harzhauser nov. sp., Odontohydrobia groisenbachensis Neubauer and Harzhauser nov. sp., Odontohydrobia pompatica Neubauer and Harzhauser nov. sp., Odontohydrobia styriaca Harzhauser and Neubauer nov. sp., Planorbis austroalpinus Harzhauser and Neubauer nov. sp., Gyraulus sachsenhoferi Harzhauser and Neubauer nov. sp., Bulinus corici Harzhauser and Neubauer nov. sp., Ferrissia crenellata Harzhauser and Neubauer nov. sp. and Stagnicola reinholdkunzi Harzhauser and Neubauer nov. sp. are introduced as new species.     

Population dynamics and population structure ofFerrissia wautieri (Mirolli, 1960) (Gastropoda,Ancylidae) in a pond near Nijmegen (The Netherlands)

Ferrissia wautieri, a freshwater limpet, is a widely distributed species in The Netherlands. In a pond near Nijmegen samples were taken twice a month over the year to study the population size and structure of this species in relation to the water temperature. Only ancyloids were found. Production of juveniles is temperature-dependent; peak numbers occurred in July and August. Just-hatched juveniles (shell length 0.6–1.0 mm) occurred over a lengthy period in the year, but were absent in March and April. During these months the collected numbers of specimens were very low. The largest specimens were collected during March, April and May.     

Prey selection and diel feeding of the freshwater jellyfish, Craspedacusta sowerbyi

1. The diet of the invasive freshwater jellyfish, Craspedacusta sowerbyi (Lankester), was assessed by analysing its stomach contents. 2. The medusae ingested various zooplankton and benthic prey in the 0.1–3.0 mm size range. The selectivity indices for prey showed that larger zooplankton (0.4–1.4 mm) and active prey such as copepods were preferred; small potential prey, and the loricate rotifer Keratella cochlearis in particular, were almost never consumed. Measurements suggest that spacing between the tentacle branches could determine prey selection. Another possibility is that some prey are too small or slow to activate nematocysts after collision with the predator. Prey > 1.4 mm can probably escape or are otherwise too large to be handled. 3. The medusae of Craspedacusta medusae stayed in deep water during daytime and migrated upwards at night. The greatest number of prey in freshwater jellyfish stomachs was detected at night. 4. Calculation of consumption rates, based on mean stomach contents and digestion times of 4–5 h, showed that, on average, one medusa takes 190 zooplankton prey day^??1 at a mean prey density of about 100 L^??1. At the observed medusae density of 1 m^??3, the cropping rate was about 0.2 zooplankton prey L^??1 day^??1.     

The benthic macroinvertebrate community in a coastal sand dune lake relative to habitat and changing lake levels

The benthic macroinvertebrate community (BMI) in a freshwater coastal dune lake without a surface outlet was investigated in May and October, 1986. Fifty-three invertebrate taxa were identified from Carter Lake, including three euryhaline crustacean species (Corophium spinicorne, Gnorimosphaeroma oregonensis lutea, and Acanthomysis awatchensis). Corophium spinicorne dominated the BMI communities of the littoral zones and sphaeriid clams dominated the deepwater community.The lake level dropped about 2.5 m between April and October. Based upon this decline, the lake bottom was divided into four major habitats: a sandy temporarily submerged littoral zone (A); a sandy submerged littoral zone (B); and mid-depth zone of mixed mud and sand and the macrophyte, Nitella (C); and a deep zone (D) with soft mud. The average density of BMI was highest in the littoral zones (A and B) in May and in zone B in October (zone A was dry). The lowest density occurred in zone D. In May, BMI biomass was highest in the littoral zones, but the biomass was highest in the mid-depth zone in October. The mid-depth zone in October. The mid-depth zone had the most diverse community.The two most abundant species in the temporarily submerged area, Corophium spinicorne and Juga plicifera, were found in greater numbers deeper in the lake after the water level dropped, suggesting migration by these species in response to changing water levels.     

Papyrus wetlands a lunar-modulated refuge for aquatic fauna

A field study was undertaken of the use of papyrus (Cyperus papyrus) wetlands fringing Rubondo Island, Lake Victoria, Tanzania, as a refuge from predators by freshwater shrimps (Caridina nilotica) and tilapia (Oreochromis niloticus) larvae. During the full moon nights, fish larvae took refuge more than 5 m in the papyrus and did not venture out into the lake, while they ventured into fringing lake waters during moonless nights. The freshwater shrimps and lake flies (Chaoborus sp.) did not migrate with the moon. This fauna took refuge in the papyrus wetlands even though the dissolved oxygen concentration was as low as 0.4 ppm (<5% of saturation).     

Running on empty? Freshwater feeding by spawning anadromous alewife Alosa pseudoharengus

Anadromous alewife Alosa pseudoharengus (n = 202; mean ± s.d . fork length = 231 ± 14 mm) were captured from 10 May to 27 June 2018 in an upper watershed lake on the Isthmus of Chignecto, Canada (45°57′N, 64°14′W). Thirty individuals (mean ± s.d. fork length = 250 ± 12 mm) were captured in an adjacent estuary downstream of a tide gate on 25 April 2018. Comparing estuarine to freshwater specimens, mean gonad mass and gonadosomatic indices in males and females decreased approximately 40% and 60%, and 31% and 50%, respectively. Individuals were characterized as pre-spawners in the estuary and spawners in the lake. Males maintained similar body condition throughout the spawning run whereas female condition decreased 9.4% between the estuary and lake. Stomach fullness decreased comparing estuarine and freshwater specimens, yet 93% of stomachs examined from individuals captured in the lake contained prey. Most males fed throughout all spawning stages (3%–17% empty stomachs), while all females fed during pre- and post-spawning stages and some fasted during spawning (11% empty stomachs). Cumulative prey curve never reached an asymptote, either weekly or for the entire sampling period, so freshwater diet may not have been completely described. Calanoid copepods (79.3%I_A) were a diet staple, with the secondary prey of mayfly nymphs (O. Ephemeroptera) consumed more by females (13.6%I_A) than males (6.2%I_A). PERMANOVA and PERMDISP analyses revealed significant dietary differences in freshwater were weekly and not due to dispersion effects, thus most likely due to feeding on various development stages of insect species. Our results challenge the long-held paradigm that anadromous A. pseudoharengus fast during the spawning migration.     

August Thienemann and Loch Lomond — an approach to the design of a system for monitoring the state of north-temperate standing waters

Limnocnida and Craspedacusta are the two main genera of Cnidaria from continental waters which have a free-swimming medusa in their life cycle. Both originated in tropical-subtropical Asia, but Limnocnida is also found in Africa, with one species (L. tanganjicae) perhaps endemic to lake Tanganyika. Halmomises from Trinidad, and two genera (Mansariella, Keralica) from India have limited ranges. No freshwater medusae evolved in Europe and the Americas (aside from Trinidad), but Craspedacusta sowerbii, by virtue of its drought resistant stages, has managed to colonise all continents, except Antarctica, in the course of the twentieth century. In all, there are about 10–14 freshwater species of medusae. An additional 4–5 species of Moerisia, Australomedusa, and Craspedacusta are sequestered in continental salt-lakes, while a few genera occur in brackish-water seas and estuaries (e.g. Moerisia, Odessia, Ostroumovia). This reduced diversity is probably due to (1) a rarity of drought resistant stages, adjusted to upstream dispersal, in the life cycle, and (2) difficulties to adapt the osmoregulatory system to a hypotonic environment.The feeding ecology of the freshwater medusae is also examined. Like all Cnidaria, they are opportunistic predators. However, fish egg predation might be their major means of subsistence, other types of food being taken only occasionally, or when fish eggs and larvae are scarce. Their impact on the true zooplankton might therefore be limited to short pulses of planktivory. Whether they are themselves limited by predation remains to be studied; many fish in the invasive part of their range avoid them, but some macrocrustaceans readily consume them. It is hypothesized that this might explain their success in Lake Tanganyika, and their absence from lake Baical. Finally, the absence of endemic species from South America could relate to the great diversity bf small-sized predatory aquatic reptiles on this subcontinent.     

Controlled activation of species typical behaviour to low salinity,seawater movement and seawater depth in <Emphasis Type="Italic">Aurelia labiata</Emphasis> (Scyphozoa) Jellyfish in Roscoe Bay,Canada

Controlled observations in Roscoe Bay examined whether Aurelia labiata medusae would respond to three environmentally significant stimuli: low salinity, seawater movement and seawater depth. Significantly more upward-swimming medusae turned and swam down or sideways when they encountered an experimentally created low-salinity plume than did upward-swimming medusae that encountered a seawater plume or a seawater plume containing milk powder. Downward-swimming medusae that encountered a plume containing freshwater continued swimming down. Significantly more upward-swimming medusae exposed to an experimentally created stream of seawater that gently pushed them horizontally turned and swam down than did upward-swimming medusae not exposed to the seawater stream. Downward-swimming medusae continued to swim down when pushed horizontally. A stronger seawater stream that tumbled as well as pushed medusae horizontally was less effective. In a shallow area near the time of a low tide, most medusae were in the top metre of a 2 m water column whilst in an adjacent area where the water was 4–6 m deep, almost all medusae were below 1.5 m. These observations add to a growing body of evidence that the ecological distribution of A. labiata is influenced by their ability to respond adaptively to significant environmental stimuli.     

Fishes and fisheries of Lake Nubia,Sudan

Developing lake conditions in Lake Nubia were found to offer good commercial fisheries potential. This prospect initiated fishery research since 1967.In this study, 26 fish species belonging to 10 families were recorded. Compared with previous studies the total number has decreased and some of the species that were commercially important during the early years of the lake formation, are now negligible in occurrence (Distichodus, Citharinus, Bagrus). The species widely distributed in the lake are Labeo niloticus, Lates niloticus, Sarotherodon niloticus, Hydrocynus forskallii, Alestes baremose and Eutropius niloticus. Their highest production occurs during two periods: February to May and July to September. The dominant fish species feed on different items in a balanced equilibrium with the ecological and biological conditions.     

Biogeography of Aegagropila linnaei (Cladophorophyceae,Chlorophyta): a widespread freshwater alga with low effective dispersal potential shows a glacial imprint in its distribution

Aim Aegagropila linnaei is a freshwater macroalga that is generally regarded as a rare species. It is apparently absent from large but seemingly suitable areas of the Northern Hemisphere, implying a limited dispersal potential and an imprint of Pleistocene glaciations in its biogeography. However, despite the popularity of its enigmatic lake ball‐form, detailed biogeographical studies of A. linnaei have never been conducted. The main means of reproduction of A. linnaei is fragmentation and akinetes are not formed, supporting the assumption of limited dispersal capacity. The aim of this study was to reconstruct the biogeography of A. linnaei, and to identify possible refugia during glaciations, as well as to evaluate dispersal potential by quantitative desiccation experiments. Location Palaearctic. Methods The current distribution of A. linnaei was inferred from herbarium specimens, literature data and recent field observations. All herbarium specimens were morphologically re‐examined. Desiccation experiments were performed with vegetative filaments of three isolates of A. linnaei, as no specialized resistant stages are known. For comparison, the widespread freshwater algae Cladophora glomerata and Rhizoclonium sp. were included. Internal transcribed spacer (ITS) ribosomal DNA sequences were generated and a ribotype network was constructed. Results Aegagropila linnaei was recorded from 283 locations in freshwater and brackish environments. The majority of locations were in central and northern Europe in previously glaciated areas. Desiccation experiments showed that A. linnaei is very susceptible to desiccation. Based on ITS sequences of 34 samples, five different ribotypes were identified. Four of these ribotypes had a restricted distribution. Aegagropila linnaei represents a single species with little genetic variation (0.1–0.5%). Main conclusions This is the most comprehensive study of this species so far, reporting many new locations and tackling several taxonomic problems. Few additional finds were made from North America, and the origin of A. linnaei is inferred to be in Asia. The highest density of its present‐day locations is in previously glaciated areas in Europe, where glacial ice‐dammed lakes might have functioned as refugia. Low effective long‐distance dispersal capacity is inferred, based on high susceptibility to desiccation and its modes of dispersal.     

Submerged macrophytes in the recently freshened lake system Volkerak-Zoom (The Netherlands), 1987–1991

Extensive biological research has been carried out during the first four years after isolation and freshening of Lake Volkerak-Zoom, a former part of the marine-tidal Oosterschelde. Freshening took place in 8 months. Research on submerged macrophytes has been an important part of the investigation because the aim of the water management was to establish a clear and stable freshwater ecosystem, despite high nutrient loading.In the first year after closure the marine Zostera noltii vanished. and the brackish water Ruppia maritima colonized the lake. In subsequent years, several freshwater species colonized the lake (e.g. Potamogeton pusillus, P. pectinatus, Zannichellia palustris, Callitriche sp. and Chara sp.). The total area in the shallow zone (0.5–5.0 m) covered by submerged macrophytes, increased from less than 1% in 1987 to 22.5% in 1991.Characteristics (height, flowering and fruiting) of the main species (P. pusillus, P. pectinatus, Z. palustris, R. maritima) were closely related to water temperature and secchi depth.Waterfowl consumed about half of the production of the submerged macrophytes.     

Seasonal reproductive cycle in the mussel,Lamellidens corrianus

Seasonal reproductive cycle of the freshwater mussel, Lamellidens corrianus has been studied. These mussels are functional or simultaneous hermaphrodites. The spawning was at its peak during the months of September to December. The gonads were in growing stages with reduced gonadal activity during January to April, whereas the maturation of gonads was found to be intense during May to August.