Mohamed Mahjoub - Academia.edu (original) (raw)
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Papers by Mohamed Mahjoub
Hydrobiologia, 2011
In order to enhance encounters with prey, planktonic predators may display different swimming beh... more In order to enhance encounters with prey, planktonic predators may display different swimming behavior with respect to food availability and distribution. In this study, we used 3D video techniques to record the swimming behavior of malabar grouper (Epinephelus malabaricus) larvae in both the absence and the presence of prey (Artemia sp. nauplii). Swimming properties were investigated in all of the 3D, the two vertical, and the horizontal projections using scale-dependent (mean speed and Net to Gross Displacement Ratio) and scale-independent (fractal dimension) metrics. When prey was added, larvae swam slower and in a less convoluted way as compared to what was observed in the absence of food. The results obtained with scale-dependant metrics were confirmed by those obtained with scale-independent analyses. Both unveiled the anisotropy of the swimming behavior of grouper larvae that tend to swim toward the vertical axis in order to maximize encounters with prey patches. This study shows that malabar grouper larvae can optimize their search volume by switching their behavior and further draws attention to the need to consider both vertical- and horizontal-projections components while addressing the plankter’s swimming trajectories.
Hydrobiologia, 2011
Studies on the behavior of copepods require both an appropriate experimental design and the means... more Studies on the behavior of copepods require both an appropriate experimental design and the means to perform objectively verifiable numerical analysis. Despite the growing number of publications on copepod behavior, it has been difficult to compare these studies. In this study, we studied two species of copepods, Eurytemora affinis and Pseudodiaptomus annandalei, and employed recently developed scaling and non-scaling methodology to investigate the effects of density and volume on the swimming behavior of individual organisms in still water. We also compared the results of two- and three-dimensional projections of the swimming tracks. A combination of scale-dependent and scale-independent analysis was found to characterize a number of behavioral observations very effectively. We discovered that (i) density has no effect except to increase the time spent in the swimming state of “breaking”, (ii) smaller volumes resulted in more complex trajectories, and larger volumes, like density, increased the time spent in the swimming state “breaking”, and (iii) three-dimensional projections gave a more accurate estimation of speed and the time spent cruising. When only a vertical 2D projection was used, “cruising” could be confused with “sinking”. These results indicate that both experimental conditions and the selection of 2D or 3D projection have important implications regarding the study of copepod behavior. The development of standardized procedures with which to compare the observations made in different studies is an issue of particular urgency.
Environmental Biology of Fishes, 2011
Ontogenetic changes in resource use are widespread in many fish species. This study investigated ... more Ontogenetic changes in resource use are widespread in many fish species. This study investigated the feeding habits of whitefish (C. lavaretus L.) larvae in Lake Annecy (France) coupled with experimental behavioral studies in order to identify the underlying mechanisms of the ontogenetic shifts in the diet. The predatory behavior of wild larvae, and the escape responses of their zooplankton prey were both videorecorded in experimental tanks under controlled laboratory conditions. Ontogenetic diet patterns showed that young whitefish larvae have a preference for small cyclops, while older larvae selectively predate cladocerans. Our experimental observations showed that the capture success rate also varied in relation to ontogenetic development in fish. Young larvae were more successful in capturing small copepods, whereas old larvae were more successful in capturing Daphnia. In addition, the larvae were able to adjust their predatory behavior (speed, pursuit) according to the swimming pattern of the prey. These observations suggest that the selective predation on cladocerans observed in old larvae is the outcome of both active and passive choices depending on the escape swimming behavior of the prey, and handling time of the predator.
A wide consensus exists that turbulence, unless reaching a threshold level, enhances ingestion ra... more A wide consensus exists that turbulence, unless reaching a threshold level, enhances ingestion rates in larval fish by increasing encounters with preys. This consensus, mainly derived from modeling exercises, relies on the two following assumptions: i) prey densities should be below saturating level, and ii) maximum intake rates by larval fish are independent of the turbulence level. While fine-scale studies within the feeding areas of larval fish showed that the first assumption is only seldomly met, the second assumption has yet to be validated. In this study, we experimentally investigated the effect of turbulence (Re ranging from 0 to 150000) on the ingestion rates in European Seabass (Dicentrarchus labrax) post-larvae exposed to realistic prey densities consistent with those observed in the natural environment. Contrarly to what was predicted by models in the case of a limiting prey density, our results revealed that increasing turbulence hampers the ingestion rates when food densities are realistic (i.e., high). The dynamics of food intake across three different flow levels (Re=0, Re=60000 and Re=120000) showed that maximum ingestion levels are turbulence-dependent; thus invalidating the above cited second assumption. On the light of these results, we draw the attention to the fact that our knowledge of the factors governing the plankton-turbulence coupling is still incomplete, especially as regards empirical results.
Hydrobiologia, 2011
In order to enhance encounters with prey, planktonic predators may display different swimming beh... more In order to enhance encounters with prey, planktonic predators may display different swimming behavior with respect to food availability and distribution. In this study, we used 3D video techniques to record the swimming behavior of malabar grouper (Epinephelus malabaricus) larvae in both the absence and the presence of prey (Artemia sp. nauplii). Swimming properties were investigated in all of the 3D, the two vertical, and the horizontal projections using scale-dependent (mean speed and Net to Gross Displacement Ratio) and scale-independent (fractal dimension) metrics. When prey was added, larvae swam slower and in a less convoluted way as compared to what was observed in the absence of food. The results obtained with scale-dependant metrics were confirmed by those obtained with scale-independent analyses. Both unveiled the anisotropy of the swimming behavior of grouper larvae that tend to swim toward the vertical axis in order to maximize encounters with prey patches. This study shows that malabar grouper larvae can optimize their search volume by switching their behavior and further draws attention to the need to consider both vertical- and horizontal-projections components while addressing the plankter’s swimming trajectories.
Hydrobiologia, 2011
Studies on the behavior of copepods require both an appropriate experimental design and the means... more Studies on the behavior of copepods require both an appropriate experimental design and the means to perform objectively verifiable numerical analysis. Despite the growing number of publications on copepod behavior, it has been difficult to compare these studies. In this study, we studied two species of copepods, Eurytemora affinis and Pseudodiaptomus annandalei, and employed recently developed scaling and non-scaling methodology to investigate the effects of density and volume on the swimming behavior of individual organisms in still water. We also compared the results of two- and three-dimensional projections of the swimming tracks. A combination of scale-dependent and scale-independent analysis was found to characterize a number of behavioral observations very effectively. We discovered that (i) density has no effect except to increase the time spent in the swimming state of “breaking”, (ii) smaller volumes resulted in more complex trajectories, and larger volumes, like density, increased the time spent in the swimming state “breaking”, and (iii) three-dimensional projections gave a more accurate estimation of speed and the time spent cruising. When only a vertical 2D projection was used, “cruising” could be confused with “sinking”. These results indicate that both experimental conditions and the selection of 2D or 3D projection have important implications regarding the study of copepod behavior. The development of standardized procedures with which to compare the observations made in different studies is an issue of particular urgency.
Environmental Biology of Fishes, 2011
Ontogenetic changes in resource use are widespread in many fish species. This study investigated ... more Ontogenetic changes in resource use are widespread in many fish species. This study investigated the feeding habits of whitefish (C. lavaretus L.) larvae in Lake Annecy (France) coupled with experimental behavioral studies in order to identify the underlying mechanisms of the ontogenetic shifts in the diet. The predatory behavior of wild larvae, and the escape responses of their zooplankton prey were both videorecorded in experimental tanks under controlled laboratory conditions. Ontogenetic diet patterns showed that young whitefish larvae have a preference for small cyclops, while older larvae selectively predate cladocerans. Our experimental observations showed that the capture success rate also varied in relation to ontogenetic development in fish. Young larvae were more successful in capturing small copepods, whereas old larvae were more successful in capturing Daphnia. In addition, the larvae were able to adjust their predatory behavior (speed, pursuit) according to the swimming pattern of the prey. These observations suggest that the selective predation on cladocerans observed in old larvae is the outcome of both active and passive choices depending on the escape swimming behavior of the prey, and handling time of the predator.
A wide consensus exists that turbulence, unless reaching a threshold level, enhances ingestion ra... more A wide consensus exists that turbulence, unless reaching a threshold level, enhances ingestion rates in larval fish by increasing encounters with preys. This consensus, mainly derived from modeling exercises, relies on the two following assumptions: i) prey densities should be below saturating level, and ii) maximum intake rates by larval fish are independent of the turbulence level. While fine-scale studies within the feeding areas of larval fish showed that the first assumption is only seldomly met, the second assumption has yet to be validated. In this study, we experimentally investigated the effect of turbulence (Re ranging from 0 to 150000) on the ingestion rates in European Seabass (Dicentrarchus labrax) post-larvae exposed to realistic prey densities consistent with those observed in the natural environment. Contrarly to what was predicted by models in the case of a limiting prey density, our results revealed that increasing turbulence hampers the ingestion rates when food densities are realistic (i.e., high). The dynamics of food intake across three different flow levels (Re=0, Re=60000 and Re=120000) showed that maximum ingestion levels are turbulence-dependent; thus invalidating the above cited second assumption. On the light of these results, we draw the attention to the fact that our knowledge of the factors governing the plankton-turbulence coupling is still incomplete, especially as regards empirical results.