Interactions among juveniles of two freshwater crayfish species and a predatory fish

Two freshwater crayfish species, Astacus astacus L. and Pacifastacus leniusculus Dana, co-occur in some Swedish lakes. Observational studies indicate that the introduced, North American species P. leniusculus may gradually replace the native A. astacus, but the mechanism behind the replacement is not known. This study examined the direct effects of interspecific competition between the crayfish, and indirect effects of competitive interactions and fish (European perch, Perca fluviatilis L.) predation. Three different experiments with young-of-the-year (YOY) crayfish were performed. P. leniusculus was strongly dominant over similar-sized A. astacus in interference competition for shelter in a laboratory experiment. However, in a 35-day experiment in outdoor pools, A. astacus growth and survival were about equally affected by interactions with conspecifics and P. leniusculus. In contrast, P. leniusculus was significantly more affected by intraspecific competition than by competition with A. astacus, suggesting asymmetric competition between the two species. The presence of perch in outdoor ponds with mixed-species groups of the two crayfish species resulted in considerably higher predation rates on A. astacus than on P. leniusculus. Both species showed strong antipredator responses to perch by increasing refuge use. I suggest that higher perch predation rates on A. astacus originate from P. leniusculus being the superior species in interspecific competition for shelter. Because of displacement from refuges, A. astacus individuals become more exposed to the predator. This indirect effect of interactions among the two cray-fish species and the predator may be important in the observed in situ replacement of A. astacus by P. leniusculus.     

Influence of bacteria and salinity on diatom biogenic silica dissolution in estuarine systems

Dissolution of diatom biogenic silica (bSiO₂) in estuaries and its control by water salinity and bacteria were investigated using the river euryhaline species Cyclotella meneghiniana as a model. Laboratory-controlled bioassays conducted at different salinities with an estuarine bacteria inoculum showed a faster dissolution of diatom bSiO₂ at the lowest salinity where bacteria were the most abundant. However in another experiment, salinity increase clearly enhanced the dissolution of cleaned frustules (organic matter free). The presence of active bacteria might therefore predominate on the effect of salinity for freshly lysed diatoms whereas salinity might rather control dissolution of organic-matter-free frustule remains. Incubation of cultivated diatoms at different protease concentrations revealed that high proteolytic activities had little effect on bSiO₂ dissolution at a 1-month scale in spite of an efficient removal of organic matter from the frustules. Altogether it is hypothesized that bacterial colonization increases bSiO₂ dissolution by creating a microenvironment at the diatom surface with high ectoproteolytic activity but also via the release of metabolic byproducts since the presence of organic matter seems generally to facilitate diatom bSiO₂ dissolution.     

The influence of predator presence on utilization of artificial seagrass habitats by juvenile walleye pollock,Theragra chalcogramma

Synopsis Behavioral preference for a structured habitat (artificial seagrass) by juvenile walleye pollock,Theragra chalcogramma, was tested in controlled laboratory experiments. We monitored position of fish in 2000 1 tanks with and without artificial seagrass present in one half of the tank. In addition, we exposed walleye pollock to a predator model, assessing their response when a grass plot was available or unavailable as a potential refuge. In the absence of predators, the fish avoided the artificial seagrass, displaying a preference for the open water side of the experimental tanks. In the presence of a predator model, however, juvenile walleye pollock readily entered the artificial seagrass plots. In addition, they often remained in the grass canopy in proximity to the predator instead of moving out of the grass to avoid the predator (when no grass was present they consistently moved to the opposite side of the tank from the predator). The behavioral choices exhibited in this study suggest that juvenile walleye pollock modify habitat selection in response to perceived predation risk, and recognize the structure provided by artificial seagrass as a potential refuge.     

The impact of environmental degradation on reproduction of the black-chinned tilapia Sarotherodon melanotheron from various coastal marine,estuarine and freshwater habitats

There is growing evidence that climate change has greatly altered environmental conditions in many aquatic ecosystems over the last decades, leading to changes in fish distribution and life history traits. Recent works conducted in Senegalese and Gambian coastal marine, estuarine and freshwater ecosystems have shown important changes in the intensity, frequency and breeding timing of wild population of Sarotherodon melanotheron in response to changes in salinity regimes. In addition to salinity, this study investigates the potential influences of other environmental factors that have received less attention on the reproduction of S. melanotheron from three different aquatic ecosystems in Senegal. The results demonstrate that day-length and temperature affect sexual maturity in both males and females from Guiers Lake and Hann Bay, but no such effects were found in upstream of the Saloum Estuary, where the spawning activity seems to be under the synergetic control of rainfall and salinity that apparently predominate over all the other ambient factors. This study demonstrated for the first time that aside from photoperiod and temperature, rainfall also influences fish reproduction in Hann Bay probably through its effects on water quality. Furthermore, our results confirm previous findings that changes in salinity regimes resulting from seasonal variations in precipitation and evaporation are the ultimate causes of temporal changes in the reproductive activity of the species in this estuary.     

Effects of acclimation salinity on the expression of selenoproteins in the tilapia,Oreochromis mossambicus

Selenoproteins are ubiquitously expressed, act on a variety of physiological redox-related processes, and are mostly regulated by selenium levels in animals. To date, the expression of most selenoproteins has not been verified in euryhaline fish models. The Mozambique tilapia, Oreochromis mossambicus, a euryhaline cichlid fish, has a high tolerance for changes in salinity and survives in fresh water (FW) and seawater (SW) environments which differ greatly in selenium availability. In the present study, we searched EST databases for cichlid selenoprotein mRNAs and screened for their differential expression in FW and SW-acclimated tilapia. The expression of mRNAs encoding iodothyronine deiodinases 1, 2 and 3 (Dio1, Dio2, Dio3), Fep15, glutathione peroxidase 2, selenoproteins J, K, L, M, P, S, and W, was measured in the brain, eye, gill, kidney, liver, pituitary, muscle, and intraperitoneal white adipose tissue. Gene expression of selenophosphate synthetase 1, Secp43, and selenocysteine lyase, factors involved in selenoprotein synthesis or in selenium metabolism, were also measured. The highest variation in selenoprotein and synthesis factor mRNA expression between FW- and SW-acclimated fish was found in gill and kidney. While the branchial expression of Dio3 was increased upon transferring tilapia from SW to FW, the inverse effect was observed when fish were transferred from FW to SW. Protein content of Dio3 was higher in fish acclimated to FW than in those acclimated to SW. Together, these results outline tissue distribution of selenoproteins in FW and SW-acclimated tilapia, and indicate that at least Dio3 expression is regulated by environmental salinity.     

The evolution of complex brains and behaviors in African cichlid fishes

In this review, I explore the effects of both social organization and the physical environment, specifically habitat complexity, on the brains and behavior of highly visual African cichlid fishes, drawing on examples from primates and birds where appropriate. In closely related fishes from the monophyletic Ectodinii clade of Lake Tanganyika, both forces influence cichlid brains and behavior. Considering social influences first, visual acuity differs with respect to social organization (monogamy versus polygyny). Both the telencephalon and amygdalar homologue, area Dm, are larger in monogamous species. Monogamous species are found to have more vasotocin-immunoreactive cells in the preoptic area of the brain. Habitat complexity also influences brain and behavior in these fishes. Total brain size, telencephalic and cerebellar size are positively correlated with habitat complexity. Visual acuity and spatial memory are enhanced in cichlids living in more complex environments. However habitat complexity and social forces affect cichlid brains differently. Taken together, our field data and plasticity data suggest that some of the species-specific neural effects of habitat complexity could be the consequence of the corresponding social correlates. Environmental forces, however, exert a broader effect on brain structures than social ones do, suggesting allometric expansion of the brain structures in concert with brain size and/or co-evolution of these structures [Current Zoology 56 (1): 144-156 2010].     

Preference for the presence of substrate in male cichlid fish: Effects of social dominance and context

Many cichlid species dig spawning pits or nests in soft bottoms and exhibit many substrate oriented activities. Despite this fact being of general knowledge, captive cichlids in laboratory and aquaculture set-ups are often kept in the absence of a soft substrate that they can manipulate. This raises a potential welfare issue, depending on how the substrate is valued by cichlid fish. The aim of this study is to assess the importance of substrate for male Mozambique tilapia (Oreochromis mossambicus) in social and non-social contexts. Preferences were established as a measure of time spent in two choice compartments, before and after the presence of a female. Locomotory activity, social interactions and substrate-related behaviours were recorded. Results show that dominant males prefer the area with substrate regardless of social context, and that female's presence strengthens this preference. The same preference is not apparent in the subordinate males, except for foraging. To draw conclusions on the importance of substrate to subordinates, preferences should also be assessed in agonistic contexts, during which substrate may serve to displace aggression. These results, together with related previous studies, show that the lack of substrate is particularly deleterious in a reproductive context, and thus it is likely to decrease the welfare state of breeding males of Mozambique tilapia.     

Reproductive biology of Sarotherodon mossambicus (Cichlidae) in Plover Cove Reservoir,Hong Kong

Synopsis A 16-month study (August 1972 to November 1973) of Sarotherodon mossambicus in the Plover Cove Reservoir, Hong Kong revealed a sex ratio of 1:1 for small fish (below 19 cm standard length), but above this length males predominated in the population. Monthly sex composition also showed this approximately 1:1 ratio except during the months of August and September, when again males predominated. Males and females reached M₅₀ at an average length of 17.8 cm and 16.2 cm respectively. Maturity and gonosomatic index studies showed that spawning occurred from May to October with peaks in June and August, and there were at least two spawnings each year. The relationship between fecundity and standard length of females collected during both spawnings was computed to be F = 118.49L–1150.91.     

Density-habitat relationships of mangrove creek fishes within the southeastern saline Everglades (USA), with reference to managed freshwater releases

The Comprehensive Everglades Restoration Plan aims to make considerable changes to the quantity, quality, and timing of freshwater delivery to the southeastern saline Everglades (SESE), a mangrove ecosystem located between the freshwater Everglades and downstream estuarine embayments. Whereas fishes inhabiting seasonally-inundated areas of the SESE and the shorelines of downstream embayments have been examined, those utilizing the creeks connecting these ecotones have not. To evaluate the functional role of the creek habitat and the possible impact of future hydrologic changes on the fishes inhabiting them, 228 underwater visual surveys were performed at three locations of the SESE over a three-year period. Fish abundance data was related to structural habitat, water level, and salinity over various time periods. The SESE contains taxa from both the freshwater Everglades and downstream embayments, but does not appear to function as a nursery for most fishery taxa. Abiotic variability and fish diversity increased with distance from major freshwater sources. Though there were significant differences in the physical structure of mangrove trees among locations, few meaningful correlations between these parameters and the density of individual fish taxa were found. Small prey-base fishes (<10 cm TL) utilize the expansive ephemeral wetlands (i.e., upper mangle) during wet periods, and were concentrated into deeper creeks when these wetlands became dry. Densities of these fishes increase on the upper mangle with increased flooding (hydroperiod), and we observed greater densities of larger species in the creeks when hydroperiods exceeded 240 days. Based on these results, we recommend that water management create water releases which result in a wet period of ca. 240 days, followed by a gradual dry period lasting ca. 90 days.     

Toluene toxicity: Effects of sublethal levels on enzyme activities in seawater adapted tilapia(Sarotherodon mossambicus Peters)

The effects of exposing seawater adapted cichlid fish, tilapia(Sarotherodon mossambicus Peters) to sublethal concentrations of toluene on the activities of lactate dehydrogenase, succinate dehydrogenase and acetylcholinesterase were studied. The activity of lactate dehydrogenase increased while those of succinate dehydrogenase and acetylcholinesterase were inhibited in most tissues of the exposed fish. The alterations in the dehydrogenases suggested that some changes in carbohydrate metabolism may have occurred. Acetylcholinesterase inhibition in brain and other tissues indicated impairment of nervous function in toluene-intoxicated tilapia.     

Responses of the PAS-positive pars intermedia cells in the cichlid fish Sarotherodon mossambicus to ambient calcium and background adaptation

Summary The function of the PAS-positive pars intermedia cells in fish has been connected with control of background adaptation and of plasma calcium levels. Since background reflectivity and illumination influence calcium metabolism in S. mossambicus, we have tested the hypothesis that the effects of background reflectivity on the PAS-positive cells are mediated by changes in plasma calcium. However, total plasma calcium does not correlate with the activity of the PAS-positive cells as estimated by morphometrical criteria. Transfer of S. mossambicus to a white background leads to a drop in plasma calcium, and a marked reduction of the activity of the PAS-positive cells. Adaptation to low-calcium freshwater, on a neutral background, has the same effect on plasma calcium but has no effect on the PAS-positive cells. The characteristic structural features of highly active PAS-positive cells in fish from a black background are not due to the slight hypocalcemia that occurs in these fish, since addition of calcium to the water in concentrations that fully prevent the drop in plasma calcium does not suppress the PAS-positive cells. These findings make it very unlikely that these cells produce a hypercalcemie factor in S. mossambicus.     

Effects of 1,25- and 24,25-dihydroxyvitamin D3 on bone formation in the cichlid teleost Sarotherodon mossambicus

1,25-Dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 have antagonistic effects on the acellular bone of the tilapia Sarotherodon mossambicus. 1,25-Dihydroxyvitamin D3 inhibits the activity of the lining osteoblasts. Prolonged administration leads to demineralization of the bone matrix. Injection of 24,25-dihydroxyvitamin D3 induces structural signs of greatly enhanced appositional bone growth within three days. No effects were observed on the mineral content of pre-existing bone. Both 1,25- and 24,25-dihydroxyvitamin D3 may have distinct, but different physiological functions in fish.     

Impact of predation on early stages of the armoured catfish Hoplosternum thoracatum (Siluriformes-Callichthyidae) and implications for the syntopic occurrence with other related catfishes in a neotropical multi-predator swamp

This study investigated the role of predators in preventing competitive exclusion among three closely related armoured catfishes (Callichthys callichthys, Hoplosternum littorale and H. thoracatum) that occur synthopically in multi-predator freshwater swamps of Suriname, South America. The potential impact of predation on armoured catfish was determined by combining laboratory measurements of predation rates on five early developmental stages of the armoured catfish H. thoracatum for 24 aquatic predators with field studies of the density of the predators in the swamps. The contribution of a particular predator to the total predation pressure on its prey was determined to a large extent by the density of the predator in the swamp. Seemingly innocuous predators with low or moderate predation rates in the laboratory may be extremely important in the swamps due to their high abundance. Small-sized omnivorous fishes and aquatic invertebrates were major predators of early developmental stages of armoured catfish. Both qualitative and quantitative ontogenetic changes in the predation pressure on armoured catfish were observed. Major predation on eggs, larvae and juveniles of H. thoracatum resulted from a different set of predators in each developmental stage of the prey. In all developmental stages of H. thoracatum the predation pressure involved several predator species and not a single, dominant predator. The potential predation pressure of the 24 predators taken together and the number of predators that were able to prey on H. thoracatum decreased sharply with increasing age (size) of the prey. Even if egg (nest) predation is prevented by the guarding male, the potential impact of the 24 predators on the populations of armoured catfish is large. Predation may account for the high mortality of H. thoracatum observed in the swamps. The high predation pressure on callichthyid catfishes may help to explain the coexistence of three closely related and morphologically quite similar armoured catfishes in Surinamese swamps.     

The composition,nutritional status and digestibility of the diets of Sarotherodon mossambicus from nine man-made lakes in Sri Lanka

Synopsis The dietary composition and the nutritional status and the digestibility of the diets of Sarotherodon mossambicus from nine reservoirs in Sri Lanka were evaluated. The feeding habits of S. mossambicus were variable from reservoir to reservoir; they ranged from herbivory to total carnivory. The protein, total lipid, carbohydrate and total organic matter content of the ingested material were related to the dietary composition and ranged from 18.53% to 35.15% (x−24.18%), 5.94% to 9.84% (x−7.91%), 11.6% to 34.7% (x−22.34%) and 34.4% to 64.4% (x−45.71%), respectively. Irrespective of the feeding habits, the diet contained a significant proportion of organic material which cannot be accounted for by protein, total lipid and carbohydrate. As much as the ingested material was related to the feeding habit, the digestibility of the nutrient components was related to the food material devoured. For example, the mean digestibility of the total organic matter in S. mossambicus feeding on detritus, plants and animal were 36.85, 33.5 and 29.5 respectively, and compared well with observations from elsewhere. It is hypothesised that the favourable nutrient quality of the available dietary material in the reservoirs of Sri Lanka, which could be and is effectively utilized by S. mossambicus, may have been, at least partially, responsible for its almost unprecedented success in Sri Lanka.     

Structural changes in the pars intermedia of the cichlid teleost Sarotherodon mossambicus as a result of background adaptation and illumination

Summary The pars intermedia of Sarotherodon mossambicus (Tilapia mossambica) contains two cell types which can be differentiated at both the light and electron microscopic level. The predominant cell type is lead haematoxyline positive, and has been shown to be the MSH producing cell type by means of immunocytochemical staining at the ultrastructural level. The changes in cellular and nuclear volume, as well as the results of stereological measurements on the cytoplasmic organelles, show that the activity of MSH cells is high on a black background and low on a white background or in total darkness. In blinded fish under a normal day-night regime the activity of the MSH cell is as high as that in black adapted fish, whereas the activity is low when the blinded fish are kept in total darkness. From the observed differences in activity of the MSH cells between the experimental groups, it is concluded that the MSH cells are not activated by the absence of reflected light, but by a high ratio between direct and reflected light. A second light-sensitive organ, supposedly the pineal gland, is also involved in the background response of the MSH producing cells.     

Structural changes in the pars intermedia of the cichlid teleost Sarotherodon mossambicus as a result of background adaptation and illumination

Summary The MSH producing cells in the pars intermedia of Sarotherodon mossambicus have been shown to be involved in background adaptation processes. Reflected light received by the eyes affects the activity of these cells. In the present study the hypothesis has been tested that also the pineal organ, as a second photoreceptor, is involved in regulation of the metabolic activity of the MSH cells. The pineal organ appears to contain photoreceptor cells and is considered to be capable of transferring information about light conditions to the animal. Removal of the pineal organ of fish kept on a black background has no effect on activity of MSH cells, whereas the activity of these cells in fish kept in darkness is increased. Thus it seems that the pineal organ exercises its influence on MSH cells only in darkness and that this influence results in a reduced activity of these cells. It is therefore concluded that the metabolic activity of MSH cells is inhibited not only by reflected light received by the eyes, but also by the action of the pineal organ as a result of the absence of illumination.No structural signs of secretory activity can be observed in the pineal, which might indicate synthesis or release of substances like melatonin. However, administration of melatonin reduces the activity of MSH cells. Neither pinealectomy nor treatment with melatonin has any influence on the second cell type of the pars intermedia, the PAS positive cells.     

Structural changes in the pars intermedia of the cichlid teleost Sarotherodon mossambicus as a result of background adaptation and illumination

Summary The pars intermedia of Sarotherodon mossambicus contains two structurally different endocrine cell types. The predominant cell type is assumed to synthesize MSH and related peptides. The second cell type is PAS positive; its function and products are unknown. In this second cell type changes occur in relation to background colour and illumination. Thus, PAS positive cells of fish adapted to a white background are less numerous and metabolically less active than those of fish adapted to a black background, and are most active in fish kept in total darkness. In blinded fish, whether in light or in darkness, the activity of the PAS positive cells is similar to that of the black background-adapted animals. The significance of these responses in relation to the control of background adaptation is discussed.     

Morphological changes of integumental chloride cells to ambient cadmium during the early development of the teleost, Oreochromis mossambicus

Synopsis Newly-hatched embryos of Oreochromis mossambicus were reared in freshwater and treated with 0 (control), 50 (low level) or 200 (high level) ppb cadmium for 4 days. Changes in the numbers and dimensions of chloride cell apical crypts on the skin of the free embryos were examined daily using scanning electron microscopy. The apical crypts of the chloride cells were rarely observed on the skin of the embryo trunk, and unevenly distributed on the surface of the yolksac. Two days after hatching, the chloride cells of the free embryos exposed to 50 ppb Cd were more active than those of the other two groups. Compared with the control group, the maximum dimensions of the developing apical crypts were stimulated by 50 ppb Cd and inhibited by 200 ppb Cd. The results indicated that the development of chloride cells in tilapia free embryos was provoked by low level Cd exposure and stunted by high level Cd exposure, suggesting the existence of structure/function relationships in which the activation of chloride cells may be related to the ionoregulatory mechanism in adaptation to Cd exposure.     

Habitat use of arctic charrSalvelinus alpinus in Thingvallavatn,Iceland

Synopsis Habitat use by four morphs of arctic charr,Salvelinus alpinus, was investigated in Thingvallavatn, Iceland, by sampling with pelagic and benthic gill nets. Sampling was done in May/June and August/September. Greatest abundance of fish was recorded in the littoral and epipelagic zone in early autumn. Catches were low in early summer. The four morphs are partly segregated in habitat. Small (SB-) and large benthivorous (LB-) charr have a more restricted spatial distribution than piscivorous (PI-), and especially planktivorous (PL-) charr. Both benthivorous morphs are mainly found in the littoral zone, and occur in largest numbers in stony shallows at depths between 0 and 10 m. PL-charr usually dominates in numbers in all habitats. PI-charr is most abundant in epibenthic habitats, although numbers are always low. All morphs are caught in higher numbers at night than during the day, but the diurnal activity difference is highest among SB-charr. The habitat use by different morphs is as may be expected from their morphology and diets. Within the population of PL-charr, young and small fish are more abundant on the bottom than in the pelagic zone, and there is a surplus of females in the pelagic zone. Along the benthic profile, young, small and immature PL-charr are more abundant in deep than in shallow waters. The results are discussed in relation to food supply, competition and predation. Possible reasons for the occurrence of four arctic charr morphs are also discussed.Contribution from the Thingvallavatn project.     

The response of freshwater plants to salinity pulses

To test whether a pulse of saline water affects the growth and survival of aquatic plants, the response of four common freshwater macrophytes; Triglochin procerum, Myriophyllum simulans, Cotula coronopifolia and Baumea arthrophylla was examined. Plants established in freshwater were exposed to a pulse of diluted sea water of <1000, 4000, or 8000 mg L⁻¹ for either three or six weeks followed by four weeks in water of <1000 mg L⁻¹. All four species survived exposure to salinity pulses. At the end of the recovery period all species had a positive relative growth rate of between 0.01 and 0.06 g g⁻¹ day⁻¹. It can be concluded that all four species can tolerate short periods of exposure to saline conditions and this gives some flexibility to managers when deciding the best way to utilise and dispose of saline groundwater.