Stress effects on fish development and physiology

Team: Patricia Pierson, Guillaume Spennato

Composition with a superposition (red/green) of paired otoliths(sagittae) of Dicentrarchus labrax post-larvae

     Our Laboratory works in particular on the effects of environmental changes (including pollutants flows) on marine species and their habitats. This consists on developing new measurement techniques and new indicators to assess threat impact. The global aim of this work is to allow better monitoring of the biological quality of marine ecosystem, better management of the Mediterranean coastal zone, with an ecotoxicological approach. An important aspect of the study is the characterization of stress effects, at the entire animal organism level, and the validation of new biological indicators of environmental perturbation. Our work is structured around three points:

Part 1 : Stress induction of fluctuating asymmetry or of change in the shape of otoliths in fish

     Studies on various invertebrate or vertebrate models have shown that an environmental stress may result in an increase in the frequency of fluctuating asymmetry acquisition for morphological characteristics during the development of the observed animals. Fluctuating asymmetry is defined as the presence of random differences between left and right sides of an organism for a bilateral character, differences in size, shape or number. From these observations, several authors have proposed fluctuating asymmetry as a biological indicator for stress, while others have concluded the opposite. On the other hand, different forms of otoliths for adult fishes of the same species have been observed by different authors depending on where these fishes were collected.

     In our present project, we measure the impact of the presence of substances in the fish environment that may cause a chemical stress, on fluctuating asymmetry induction or changes in the shape of otoliths. We carry out asymmetry and otolith shape measurements on larvae, post-larvae, or juveniles of fishes, exposed or not to pollutants, in aquaria (phosphate, zinc) or in situ (in three selected Mediterranean coastal areas, each including a type of reference site with the least possible anthropisation, a river mouth, and a recreational harbour). The fishes studied are Mediterranean fishes, Dicentrachus labrax, Oblada melanura, and Diplodus sargus or Diplodus vulgaris. The monitoring of fluctuating asymmetry is carried out by measurements of several morphological parameters of the paired otoliths. The shape analyses of otoliths are based on Fourier descriptors. Observations are made directly on the animal and measurements are made, to increase its accuracy, from photographs of the structures of interest on the right and left sides of the animal, processed with a software for image analysis.

     We have shown, on two types of fishes, that fluctuating asymmetry does not vary under the influence of chemical pollutants, or between a coastal site as protected as possible from anthropisation, a river mouth and a recreational harbour. Hence fluctuating asymmetry cannot be validated as a new indicator of stress in fish.
We are now studying potential changes in the shape of otoliths under stress conditions, under different states of chemical pollution. We have determined the conditions necessary for this study, such as working with a defined fish size class because of change in shape of otoliths during fish growth. We have obtained results demonstrating the influence of environmental conditions on the shape of juvenile fish otoliths. We wish to continue studying the effects of other stressors on the shape of juvenile fish otoliths.

The SPICE project, Fondation Université Côte d'Azur, is included in this part of our research.

Part 2 : Impact of stress on physiological parameters of fish

     We also want to measure the impact of stress on several physiological parameters of fish. The presence of a stressor in the surrounding environment of a fish is susceptible to have a very large impact on this animal, as a fish continuously has to be in contact with any stressor in the water (as long as they do not move far enough away from it). Therefore, it is essential to characterize the impacts of stress on various physiological effects, including fish hormone secretions. Indeed, the response to any stress is of nervous origin and triggers a typical cascade of hormonal secretions (resulting in the secretion of cortisol, the "stress hormone", which can disrupt other hormonal secretions and control changes in the animal's physiology).

     For each tested stress, we will measure its impact on parameters such as fish survival, growth, reproduction (including a possible change of sex), secretions of selected hormones of interest. This study will be completed by characterizing the mechanisms of action of stress.

Part 3 : Study of the reproduction of hake (Merluccius merluccius)

     This work is the result of my collaboration with the Laboratoire Réseau de Surveillance Environnementale of the University of Oran, Algeria. It is based on the study of caught fishes whose size, physical condition parameters (liver and gonad weights), sex and sexual maturity are determined. This leads to the identification of the first size of sexual maturity (for males and for females) and the monthly change in the percentage of egg-laying or pre-laying stages in a population exploited by fishing. The aim is to allow better future management of hake stocks.

Key words: fish, stress, otolith morphometry, endocrinology.

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