Omar Thiom Thiaw

Reinvestigation of spermatic flagella structure: the teleostean Cyprinodontidae

The study of 7 genera and 15 species of teleostean fishes belonging to the family Cyprinodontidae shows a similar morphology of mobile spermatozoa and a wide diversity of structure of the spermatic flagellum. The flagellar membrane has one, two, or three lateral expansions depending on the species. Peripheral doublets of axonema show only the external arm, except for two species that completely lack them. Intratubular differentiations (ITD) are present in A or B tubules of peripheral doublets as in central tubules of some species, whereas others are totally devoid of them. The ITD can affect all doublets or preferentially doublets 1, 5, and 6. These variations may be due to neutral mutations.

Comparative analysis of reproductive traits in black-chinned tilapia females from various coastal marine, estuarine and freshwater ecosystems.

The black-chinned tilapia Sarotherodon melanotheron is a marine teleost characterised by an extreme euryhalinity. However, beyond a certain threshold at very high salinity, the species exhibits impaired growth and precocious reproduction. In this study, the relationships between reproductive parameters, environmental salinity and condition factor were investigated in wild populations of this species that were sampled in two consecutive years (2003 and 2004) from three locations in Senegal with different salinities: Guiers lake (freshwater, 0 psu), Hann bay (seawater, 37 psu) and Saloum estuary (hypersaline water, 66–127 psu). The highest absolute fecundity and spawning weight were recorded in seawater by comparison to either freshwater or hypersaline water whereas the poorest condition factors were observed in the most saline sampling site. These results reflect higher resource allocation to the reproduction due to the lowest costs of adaptation to salinity in seawater (the natural environment of this species) rather than differences in food resources at sites and/or efficiency at foraging and prey availability. Fecundities, oocyte size as well as spawning weight were consistent from year to year. However, the relative fecundity in the Saloum estuary varied significantly between the dry and rainy raisons with higher values in the wet season, which seems to reflect seasonal variations in environmental salinity. Such a reproductive tactic of producing large amounts of eggs in the rainy season when the salinity in the estuary was lower, would give the fry a better chance at survival and therefore assures a high larval recruitment. An inverse correlation was found between relative fecundity and oocyte size at the two extreme salinity locations, indicating that S. melanotheron has different reproductive strategies in these ecosystems. The adaptive significance of these two reproductive modes is discussed in regard to the heavy osmotic constraint imposed by extreme salinities and high inter-specific competition.

Analysing environmental and fishing effects on a short-lived species stock: the dynamics of the octopus Octopus vulgaris population in Senegalese waters

Short-lived species are extremely dependent on the seasonal and interannual variability of environmental conditions, and determining their stock status is often difficult. This study investigates the effects of environmental variability and fishing pressure on the stock of octopus Octopus vulgaris in Senegalese waters over a 10-year period from 1996 to 2005. Monthly catches-at-age were estimated based on catch-at-weight data and a polymodal decomposition constrained by a given growth curve. Octopus recruitments and fishing mortalities were then estimated using a catch-at-age analysis performed on a monthly basis. Yield and biomass per recruit were simulated using a Thompson and Bell model and used to generate a diagnostic of the fisherys impacts. Results indicate that the high interannual and seasonal variability of the octopus stock biomass is linked to the spring recruitment event, the annual intensity of which was significantly correlated with the coastal upwelling index and sea surface temperature. Yield per recruit varied seasonally but remained almost unchanged from one year to the next. Even when catches vary strongly according to recruitment, the octopus stock appears to be consistently fully exploited, or slightly overexploited in some years. In this context of environmental variability, usual indicators such as the maximum yield per recruit, and the related fishing mortality and spawning potential ratio, remain useful for fisheries management purposes.

Resilience of Key Biological Parameters of the Senegalese Flat Sardinella to Overfishing and Climate Change.

The stock of the Senegalese flat sardinella, Sardinella maderensis, is highly exploited in Senegal, West Africa. Its growth and reproduction parameters are key biological indicators for improving fisheries management. This study reviewed these parameters using landing data from small-scale fisheries in Senegal and literature information dated back more than 25 years. Age was estimated using length-frequency data to calculate growth parameters and assess the growth performance index. With global climate change there has been an increase in the average sea surface temperature along the Senegalese coast but the length-weight parameters, sex ratio, size at first sexual maturity, period of reproduction and condition factor of S. maderensis have not changed significantly. The above parameters of S. maderensis have hardly changed, despite high exploitation and fluctuations in environmental conditions that affect the early development phases of small pelagic fish in West Africa. This lack of plasticity of the species regarding of the biological parameters studied should be considered when planning relevant fishery management plans.

Microtubules and macrotubules in fish meiosis

During meiosis in the male of a cyprinodontid fish, Aphyosemion splendopleure, and during the organization of the spindle of division, the spindle is made of two types of tubules: microtubules (20-25 nm) and macrotubules (30-50 nm). The macrotubules are associated only with the polar region of the meiotic apparatus and are located outside the spindle of microtubules. At the end of meiosis, the spindle microtubules depolymerize whereas the macrotubules remain. One can find them throughout the entire process of spermiogenesis; later, they disappear only at the end of spermatid maturation. We have studied four populations from Cameroon, three of them with macrotubules.

Different aspects of tubulin polymerization in spermatids of cyprinodontidae (fish, teleost)

Abstract Our study concerns 10 genera and 26 species of cyprinodontid fish. In the cytoplasm of spermatids tubulin polymerizes in various forms according to species. We have demonstrated the presence of classic microtubules with a diameter of 24 nm and also of tubules of smaller diameter (15 run) and greater diameter (30 to 50 nm). Microtubules are very numerous in the cytoplasm of certain species. They may be arranged without any order or form bundles which may contain several hundred parallel elements. They never form a manchette. In two species (Aplocheilichthys normani and Epiplatys fasciolatus) only spermatids that degenerate show this peculiarity. The microtubules present two kinds of decorations. The first type are small elements composed of MAP which enable two or more microtubules to link up. The second type are curved tubulin elements that give the microtubule that bear them the appearance of an incomplete doublet. Doublets and triplets may also be formed. Cyprinodontidae spermatocytes and spermatids probably synthesize a very large quantity of tubulin which polymerizes in certain species.

Le spermatozoide de Ophidion sp. (Poisson, téléostéen): Particularités ultrastructurales du flagelle

Abstract The spermatozoon of Ophidion sp. possesses an elongated nucleus 8 μm long, a short midpiece (0,6 μm), and a long flagellum (100 μm). The flagellar membrane extends in the form of two diametrically opposed sidefins. Evolving spermatids and spermatozoa are found in the lumen of the seminiferous tubes. The sections of flagella show filamentary and tubular elements disposed parallel to the axoneme microtubules. We have divided the flagella into three types. In type 1 the tip of the sidefins contains 20 to 30 filaments 5 run in diameter and between these and the axoneme 20 to 30 tubular elements 15 to 20 nm in diameter. Type 2 possesses a dense cytoplasm and a few tubular elements 10 nm in diameter disposed at the tip of the sidefins. Type 3 contains a cytoplasm which is not dense and in which we found polysaccharides and 1 to 8 tubular elements forming a palisade which lines the plasma membrane at the tip of the sidefins. We interpret these three types as three successive stages in the organization of the flagellum during spermiogenesis. Type 3 corresponds to the spermatic flagellum. These 10-nm-diameter tubules do not have the same chemical composition as the microtubules. Elements of the cytoskeleton serve as a support for the sidefins.

MATURATION OF MOSQUITO SPERMATOZOA DURING THEIR TRANSIT THROUGHOUT THE MALE AND FEMALE REPRODUCTIVE SYSTEMS

The testes, seminal vesicles and spermathecae of 22 species of mosquitoes belonging to the genera Aedes, Anopheles, Culex, Mansonia and Toxorhynchites are investigated under the electron microscope. Modifications of the acrosome and sperm wall occur during the transit of the spermatozoon from the lower region of the testes to the spermathecae throughout the seminal vesicles. The origin and fate of the cell coat and the possible roles of somatic cell layers both in the testes and the seminal vesicles are discussed.

Changes in population structure of the white grouper Epinephelus aeneus as a result of long-term overexploitation in Senegalese waters

In Senegal, a significant decrease in catches indicates that many demersal fish stocks are being overexploited. The white grouper Epinephelus aeneus, locally known as the ‘thiof’, is exploited by both small-scale and industrial fisheries. A 28-year database of E. aeneus catches along the Senegalese coast provided by the Centre for Oceanographic Research of Dakar-Thiaroye, and size at maturity measured in Dakar (Senegal) from monthly samples in 2010, were used to analyse changes in population structure in the area over the past 37 years. Catches from the northern fishing areas were lower than those from the southern fishing areas, and decreased steadily during the period (Kolmogorov–Smirnov test, D = 0.243, p = 0.0002). The individual mean weight of catches decreased from 1974 to 2010 (linear regression, r 2 = 0.40, n = 37) and only 60% of the individuals were mature. The calculated sizes at maturity were 49 cm total length (TL) for females and 55 cm for males, and the optimal length of capture for a sustainable fishery was 96 cm, but only 0.03% of E. aeneus caught reached this length. Most of the catch consisted of juveniles; the larger reproductive individuals had disappeared. The number of individuals caught decreased significantly between 1974 and 2010 (1974–1983, r 2 = 0.98, n = 74 674; 1984–1993, r 2 = 0.95, n = 96 696; 1994–2003, r 2 = 0.93, n = 12 619; 2004–2010, r 2 = 0.91, n = 12 887), whereas the length range remained the same (10–110 cm TL). Biological indicators clearly showed that E. aeneus stocks in Senegal are overexploited and the species is now endangered. Immediate active management of fishing pressure is needed, therefore, to maintain E. aeneus populations in the area. Our results suggest a minimum size of <50 cm should be introduced and that fishing effort should be reduced.

Process of cytoplasmic elimination during spermiogenesis in two cyprinodontidae (Teleostean fishes)

The mechanism of excess cytoplasm elimination during spermiogenesis was studied in two Cyprinodontidae species Aphysemion riggenbachi and Aphyosemion splendopleure. The cytoplasm is progressively filled with numerous vesicles resulting mainly from the phenomenon of pinocytosis. These vesicles coalesce and give birth to concentric cisternae in the aged spermatid. The residual body is formed by this membranous system.