1,721,012 research outputs found
Survival, growth and selection in a communally reared multifactorial cross of African catfish (<i>Clarias gariepinus</i>)
No full textFormulation of national marine policy: the case of Belgium
No full textThe Third Law of the Sea Conference has put Belgian marine interest in a new perspective. This paper analyses the historical as well as socioeconomic roots of Belgian marine policies, as well as legal and treaty infrastructure. A patchwork of formulated and implicit marine policies characterizes the 'national marine policy'
The implications of heterozygosity in the scallop <i>Placopecten magellanicus</i> (Gmelin)
No full textIn many instances, variation in phenotypic traits has been correlated with electrophoretic heterozygosity of plants, vertebrates and invertebrates. Enhanced growth of selected heterozygous molluscs has been attributed to lower energetic requirements for basal metabolism. I investigated the correlation between single- and multiple-locus heterozygosity and phenotype of the juvenile scallop Placopecten magellanicus (Gmelin). Two questions were being asked: do heterozygous scallops grow faster and which metabolic processes are affected by the degree of heterozygosity? No significant correlation was observed between genotype (scored at 6 polymorphic loci) and growth in six different samples of juvenile scallops, all belonging to the same time series. This result and published records indicate that the allozyme heterozygosity of pectinids does not influence growth to the same degree as in the bivalves Mytilus edulis, Crassostrea virginica and Mulinia lateralis. A decrease in heterozygote deficiency with age suggests selective mortality in scallops between the ages of 2 months and 13 months. The metabolism of 13 month old scallops switched from the catabolite carbohydrate (i.e. glycogen) to protein when they were starved for 4 weeks. Allozyme genotype had no measurable effect on oxygen uptake, excretion rate, O:N ratio and carbohydrate content, either under routine or basal metabolic conditions. Two hours after induction of muscle contractions in 20 month old scallops, the end-product octopine reached a concentration of 4 µmol.(g wet muscle weight)-1. Multiple-locus heterozygosity was positively correlated with octopine accumulation in the phasic part of the adductor muscle. Octopine dehydrogenase activity was on average higher among ODH homozygotes than among heterozygotes. In summary, multiple-locus heterozygosity is correlated with selected traits related to functional anaerobiosis. A model integrates the above mentioned results with the hypotheses of ''associative overdominance" , of a "balanced energy pathway" and of "energetic efficiency". I argue that energy savings due to heterozygosity are used for enhanced "activity" (such as feeding and swimming) in freely moving molluscs and for enhanced "growth" (such as somatic growth in juveniles, gonadal growth in adults and resistance to starvation) in sessile molluscs
Comparing host and parasite phylogenies: <i>Gyrodactylus</i> flatworms jumping from goby to goby
No full textThe combination of exceptionally high species diversity, high host specificity, and a complex reproduction system raises many questions about the underlying mechanisms triggering speciation in the flatworm genus Gyrodactylus. The coevolutionary history with their goby hosts was investigated using both topology- and distance-based approaches; phylogenies were constructed of the V4 region of the 18S rRNA and the complete ITS rDNA region for the parasites, and 12S and 16S mtDNA fragments for the hosts. The overall fit between both trees was significant according to the topology-based programs (TreeMap 1.0, 2.0 beta and TreeFitter), but not according to the timed analysis in TreeMap 2.0 beta and the distance-based method (ParaFit). An absolute timing of speciation events in host and parasite ruled out the possibility of synchronous speciation for the gill parasites, favouring the distance-based result. Based on this information together with the biological background of host and parasite, the following TreeMap solution was selected. The group of gill parasites evolved from a host switch from G. arcuatus, parasitizing the three-spined stickleback onto the gobies, followed by several host-switching events among the respective goby hosts. The timing of these events is estimated to date back to the Late Pleistocene, suggesting a role for refugia-mediated mixing of parasite species. In contrast, it is suggested that co-speciation in the fin-parasites resulted in several host-associated species complexes. This illustrates that phylogenetically conserved host-switching mimics the phylogenetic signature of co-speciation, confounding topology-based programs
Identification of a host-associated species complex using molecular and morphometric analyses, with the description of <i>Gyrodactylus rugiensoides</i> n. sp. (Gyrodactylidae, Monogenae)
No full textGyrodactylus rugiensis was originally described as a parasite occurring on the marine gobies Pomatoschistus minutus and Pomatoschistus microps. In our preliminary survey this species was also frequently found on Pomatoschistus pictus and Pomatoschistus lozanoi. Subsequent molecular analysis of the internal transcribed spacers rDNA region revealed that this parasite actually represents a complex of two apparently cryptic species, one restricted to P. microps and the other shared by P. minutus, P. lozanoi and P. pictus. Morphometric analyses were conducted on 17 features of the opisthaptoral hard parts of specimens collected from all four host species. Standard discriminant analysis showed a clear separation of both genotypes by significant differences in marginal hook and ventral bar features. Statistical classifiers (linear discriminant analysis and nearest neighbours) resulted in an estimated misclassification rate of 4.7 and 3.1%, respectively. Based on molecular, morphological and statistical analyses a new species, Gyrodactylus rugiensoides is described. This species seems to display a lower host-specificity than generally observed for Gyrodactylus species as it infects three sympatric host species. However, seasonal and host-dependent morphometric variation is shown for G. rugiensoides collected on P. pictus. Host-switching and gene flow might be important factors preventing speciation on closely related and sympatric host species. The presence of host associated species complexes in this Gyrodactylus-Pomatoschistus system is also confirmed by the presence of two host-dependent genotypes within G. micropsi found on P. minutus and P. lozanoi, and P. microps, respectively. By comparing host and parasite phylogeny, phylogenetic and ecological factors influencing host-specificity are discussed
Clinal genetic variation and isolation by distance in the European eel <i>Anguilla anguilla</i> (L.)
No full textThe genetic variability and structure of the European eel (Anguilla anguilla L.) in populations throughout Europe was reassessed using 15 allozymic loci, seven of which were polymorphic. Seven sites were sampled on a latitudinal gradient across the natural continental range, extending from southern France to southern Norway. Heterozygosity (He = 0.05) and level of polymorphism (P = 0.43) were comparable to other marine fish. Populations were poorly differentiated (GST = 0.014, FST = 0.002), which is not surprising considering the high dispersal capability of the European eel. However, a significant geographical cline was detected at two alleles (IDh-1*100 and GPI-1*110), andgenetie distances (DCE) were concordant with geographical coastal distances. Mantel tests, pairwise FST's and multidimensional scaling analyses identify three distinct groups: Northern Europe, Western Europe and the Mediterranean Sea. We propose that the clinal genetic structure in the European eel may be due to (1) isolation by distance (as recently detected with microsatellites), (2) temporal reproductive separation, (3) post-larval selective forces, (4) contact between formerly separated groups or (5) some combination thereof
Is the European eel slipping away towards extinction? A review of research and management challenges
No full textMarine organisms experience a broad range of intrinsic and extrinsic influences during their lives, which impact their population dynamics and genetic structure. Subtle interpopulation differences reflect the continuity of the marine environment, but also pose challenges to those wishing to define management units. The catadromous European eel (Anguilla anguilla) is no exception. Its spawning habitat in the Sargasso Sea and long migration across the North Atlantic qualify it as marine. However, the synergy between hydrographic variability, changing climate, and the impacts of habitat degradation and overfishing in continental waters has negatively affected stock sizes. Its protracted spawning period, variance in age-at-maturity, parental contribution and reproductive success, and the difficulty in sampling the spawning region together may mask a weak geographical genetic differentiation. Recent genetic data report evidence for spatial as well as temporal differences between populations, with the temporal heterogeneity between intra-annual recruitment and annual cohorts exceeding the spatial differences. Despite its common name of ‘fresh-water eel’, the European eel should really be managed on a North Atlantic scale. The fishery may have to be curtailed, migration routes kept open and water quality restored if it is to survive. Eel aquaculture has to focus on efficient rearing in the short term and controlled breeding in the long term. Future research on eel genetics should focus on (i) sampling and analysing spawning populations and recruitment waves to detect spatio-temporally discrete groups, and establishing a biological baseline from pre-decline historical collections for critical long-term monitoring and modelling of its genetic composition; (ii) the analysis of adaptive genetic polymorphism (genes under selection) to detect adaptive divergence between populations, perhaps requiring separate management strategies; and (iii) improving artificial reproduction to protect natural stocks from heavy exploitation, especially now the species has been categorized as endangered (Maes and Volckaert, 2007)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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