Hydrobiologia

Monthly observations on the bryozoa Fredericella sultana have been carried out in the outlet of a humic acid lake for more than two years. The growth of the colonies were rapid during algal bloom and in fall the colonies almost covered the free underside of nearly all stones on the bottom. In winter most of the colonies died out, but scattered individuals were also observed alive during this period. Statoblasts produced in summer were ‘hatching’ in March–April next year. F. sultana seems to feed on all particles that can pass through the mouth opening. A special study regarding this was made on the diatoms, by analysis of fecal pellets. It was found that young colonies of F. sultana filtrated most efficiently the smallest particles (<20 µm) while older individuals in addition were able to elevate the filtration of bigger particles (40–100 µm). The feeding takes place both by filtration by the cilia and by an active use of the tentacles themselves on the lophophore.

Naturally dried lichens and mushrooms were collected, stored at various relative humidities and temperatures either under air or argon, and extracted in a 0.2 M sucrose solution to determine the long-term survival of resident bdelloid rotifers. Survivorship of rotifers in samples kept at 21 °C for 8 months declined at both < 1% and 76% humidities, but remained the same as the starting levels at 23% and 43% humidities. Lowering the temperature to 4 °C improved survival at both < 1% and 76% humidities; at -20 °C and <1% humidity, survivorship of rotifers did not decline for up to 18 months. Storage at 21 °C under argon gas improved survival of bdelloids at <1% humidity, but not at 76% humidity. These results suggest that several processes, including oxidation reactions, may be partly responsible for death of anhydrobiotic bdelloids. To facilitate taxonomic work it is recommended that naturally dried samples containing bdelloids be stored over a desiccant at temperatures below 0 °C until they are to be rehydrated.

Many O-group flounder, Platichthys flesus, live in the lower reaches of rivers in North West Europe; while there is extensive information on the niche that they occupy in estuarine and marine environments, little is known of their life in freshwater. The diet of O-group flounder in the Lower Welsh River Dee was studied by stomach contents analysis of diel and seasonal samples. The dominant prey were chironomid larvae and pupae, tubificid oligochaetes and cyclopoid copepods. There was little quantitative variation in diet temporally, although a weak crepuscular pattern of stomach fullness was shown. There was wide variation in length and weight in all months, but the mean values at age I were 60 mm and 2.9 g respectively.

Lake Malaŵi cichlids have evolved rapidly, extensively, and in some cases iteratively to fill an array of ecological niches; however, neither species richness nor trophic diversity is distributed equally across lineages. In the context of evolutionary theory, such differences offer predictions about the magnitudes and patterns of morphological variation within lineages. In this paper, we use geometric morphometrics in three prevalent rock-dwelling genera to address questions related to morphological diversity, disparity, integration, and modularity. In particular, we focus on comparisons between the highly specialized and species-poor genus, Labeotropheus (Ahl in Sitzungsberichte der Berlinische Gesellschaft Naturforschender Freunde zu Berlin 1926:51–62, 1927), and the more ecologically diverse and species-rich genus, Tropheops (Trewavas in Revue Française d’Aquariologie Herpétologie 10:97–106, 1984), as well as between species with lake-wide versus more limited distributions. We find that Labeotropheus exhibits comparable levels of morphological disparity to Tropheops, which suggests that a specialized foraging mode has not constrained cranial variability in this genus. We also find that species with a lake-wide distribution exhibit levels of disparity three times greater than that in a species with a limited distribution. Finally, we show that magnitudes of integration and patterns of modularity are lineage specific, and do not directly correspond to ecology. In sum, these data provide insights into the complex relationship between ecology, morphological variability, and evolvability.

Photosynthetic activity by phytoplankton was measured during the ice-free seasons of 1984, 1985 and 1987 using the 14C radioassay in high altitude Emerald Lake (California). Relative quantum yield (αB) and light-saturated chlorophyll-specific carbon uptake (Pm B) were calculated from the relationship of light and photosynthesis fitted to a hyperbolic tangent function. Temporal changes in Pm B showed no regular pattern. Seasonal patterns of αB generally had peaks in the summer and autumn. Phytoplankton biomass (as measured by chlorophyll a) and light-saturated carbon uptake (Pm) had peaks in the summer and autumn which were associated with vertical mixing. Estimates of mean daily carbon production were similar among the three years: 57 mg C m−2 2 d−1 in 1984, 70 mg C m−2 2 d−1 in 1985 and 60 mg C m−2 d−1 in 1987. Primary productivity in Emerald Lake is low compared to other montane lakes of California and similar to high-altitude or high-latitude lakes in other regions.

Only recently have extant Lamprothamnium species been reported from the American continent. L. succinctum (A. Br. in Asch.) R. D. W. was found in Lago Titicaca, Bolivia and L. haesseliae Dont. en Laguna Luro and Laguna La Salada, Argentina. Fossil gyrogonites of L. succinctum and L. haesseliae, however, are here reported to be widely distributed in Quaternary sediments of Argentina, associated with other charophytes, ostracods and foraminiferans. Localities include Laguna del Siasgo, Laguna Salada Grande, Laguna Mar Chiquita, Laguna La Amarga and Salina del Bebedero. Lamprothamnium is a genus that prefers shallow, alkaline, hyposaline to mesosaline environments. Analysis of the characteristics of Laguna La Salada contributes information on the factors that influence the distribution of the genus. The morphology characteristics of the gyrogonites and oospores of L. haesseliae and the gyrogonites of L. succinctum are described. The use of Lamprothamnium as a biomarker and its application in the reconstruction of Quaternary saline environments are also discussed.

The Alkor-Deep (140 m), which forms part of a depression system in the northern Kattegat channel east of the island of Læsø (Denmark), is the location of a self sustaining population of Northern krill, Meganyctiphanes norvegica (Euphausiacea). This population is exposed to one of the most pronounced thermal gradients within the distributional range of this pelagic crustacean. During summer, the temperature of the water column ranges between 4 and 6 in the deep to 16 °C near the surface which results in the krill being exposed to temperature differences of 8–10 °C during diel vertical migration. Oxygen consumption rates were used to investigate the physiological adaptation of the animal to such gradients in temperature. The rates were found to increase exponentially from 31 μmol O2 h-1 gdw -1 at 4 °C to 72 μmol O2 h-1 gdw -1 at 16 °C, giving a Q 10-value of 2.0, and indicating that physiological adaptation to varying thermal conditions does not take place. Behavioural adaptations are discussed which may help the krill to cope with large temperature gradients in their environment.

By means of whole-mount immunohistochemistry, putative inhibitory (GABAergic) neural structures were identified in the central and peripheral nervous system of the tubificid worm, Limnodrilus hoffmeisteri. In the supraoesophageal ganglion (brain) only few strongly labelled cells were observed. However, in its commissural part a high number of stained nerve fibres, arising mainly from the ventral nerve cord and prostomium, occurred. Except for the suboesophageal ganglion the arrangement of γ-amino butyric acid-immunoreactive (GABA-IR) structures proved to be identical in each VNC ganglion. Behind the first segmental nerves three pairs of heavily stained neurones were located. Their processes (both ipsi- and contralateral) form four bundles of fine-fibred polysegmental interneuronal tracts that run close to the dorsal giant axons from the terminal ganglion to the suboesophageal one without interruption. A few small motoneurons and a pair of large ones with contralateral processes were also identified. A bipolar (presumably sensory) neuron was located at the root of each second segmental nerve. GABA-IR neurons were also found in the stomatogastric ganglia and pharyngeal wall; however, the latter structure had a well-developed fibre network, as well. Present results suggest that GABA acts as a common neurotransmitter in sensory, interneuronal and motor system of L. hoffmeisteri. The possible functional role of the identified GABA-IR neural structures in locomotion, escape and withdrawal reflexes in tubificid worms is discussed.