Biology of fish
The biological approach to populations investigates species acclimatization to salinity and to its variations.
Indeed, according to their nature, duration and extent, the constraints that these variations impose have diverse effects on the biological characteristics of species (growth, production). Acclimatization to salinity and to its variations involves mechanisms of osmoregulation. The principal organs involved in osmoregulation processes are the teguments (permeability), the gills, the digestive tube and the kidney (excretion and absorption). The latter two depend upon the hormonal control of the pituitary gland (secretion of hormones such as prolactine, growth hormone...). These processes engender energy expenditures in function of the intensity and duration of the disturbance. Disturbances will also have direct repercussions on the biological characteristics of individuals and populations (growth and reproduction). The study consists of investigating the physiological and genetic bases of adaptability to stress induced by variations and /or increase in salinity. While such increase constitutes a limiting factor for growth in hypersaline estuaries, its physiological and genetic consequences remain unknown. The interactions and effects on different biological characteristics (for example, reproduction) are also little known. The following questions need answers:
– Which genes are most sollicited in hyperhaline acclimatization?
– Are certain genes involved in osmoregulation subject to pressures of selection?
– Do energy expenditure and hormone synthesis involved in osmoregulation mechanisms vary from one individual to another?
– Do gill structure and hyperstructure differ in function of saline levels?
– What are the repercussions of physiological adaptations to hypersalinity on biological characteristics?
– Are migratory behaviours different for fish species present in the saltiest zones of the estuary?
– Do growth strategies differ from the earliest life stages and onward in milieux of variable salinity?
Using approaches integrating molecular biology, histology, physiology and population genetics will provide some elements of response. Otolithometric approaches provide biological growth and reproduction parameters and link them to environments in which the fishes have resided. This study will bring to light fundamental evolutionary knowledge since the osmoregulation of teleosts has only been studied at an in-depth level for Salmonidae and for the genus Oreochromis of the Cichlidae, but not as yet for Sarotherodon melanotheron. Moreover, in the context of global climatic change, the phenomenon of hypersalinity in certain estuaries may amplify, and the development of biomarkers will be useful for the diagnosis of the impact of hypersaline stress. Sarotherodon melanotheron and Ethmalosa fimbriata are so-called “resistant” species because they are found in a wide range of salinity (from 0 to more than 100 psu – unit of salinity -). Results already obtained show the existence of a significant effect of hypersalinity on growth and reproduction in these two species.
The interpretation of these responses is made difficult by the lack of knowledge of the physiological processes involved in adaptation. IRD will therefore continue to use these species as working models. Indeed, they are characteristic of lagoon and estuarine ecosystems, and present important faculties of adaptation which may be explained by original physiological characteristics. These adaptations concern resistance to deoxygenation, tolerance to important turbidity, resistance to pollution, relatively unselective diet, euryhalinity and eurythermia (Welcomme, 1972). Moreover, these species present continuous reproduction or reproduction with seasonal peaks in function of zones and environments, and sometimes small size at first maturity as well, perhaps the result of adaptive strategies corresponding rather to early maturity. Moreover, the presence in Senegal in the hyperhaline lagoon of Saloum of these two species in milieux of extremely variable salinity brings up a number of questions concerning the causes of this apparent “plasticity”.
A third model of study has been chosen, that of the species Mugil curema (Mugilidae). The family of Mugilidae (mullets) regroups a certain number of ubiquitous species frequenting brackish waters and distributed all over the Atlantic Ocean. The term goal is to compare the biological characteristics and migratory behaviours of West African populations of this species with Brazilian species. Two estuaries with contrasted characteristics were chosen to sample these species and to study the impact of salinity: the inverse hyperhaline estuary of Sine Saloum in Senegal and the normal salinity gradient estuary of Gambia. Five sites were chosen in function of salinity gradient and will be sampled. At a second stage, IRD will prospect new sites for which environmental conditions or comparable species will be found (for example, lagoons and estuaries in Brazil, lagoons in the Gulf of Mexico). The aim will be to test the same indicators (genetic, physiological, behavioural, otolithical…) in order to evaluate specific responses (for example, the same species in different milieux), and/or environmental responses (for example, different species subjected to similar hyperhaline conditions).