Studies on Unionidae populations from the Crapina-Jijila complex of pools (Danube zone liable to inundation)

Summary

This work represents the study of the structure, dynamics and functioning of some Unionidae populations of the Crapina-Jijila complex of pools, integrated into the zone liable to inundation of the Danube.

Of the five species of Unionidae present in the Crapina-Jijila complex of pools, the populations of Unio pictorum, Unio tumidus and Anodonta piscinalis were investigated as these species dominate in the respective ecosystems both in number as well as in biomass.

The following conclusions were arrived at:

• 1. Density, biomass, incidence and abundance are greater in the Crapina populations than those of Jijila, owing to more favourable life conditions (smaller degree of silting, better water circulation, greater variety of the bottom, a.s.o.).

• 2. Spatial distribution is influenced by water level determined by the Danube high floods, nature of the bottom and presence or absence of vegetation.

• 3. Age structure constitutes a chaacteristic trait for each population separately at any moment, while the dynamics of this structure takes places under the action of some environmental factors. Danube high floods exert an important influence upon the Unionidae birth rate, favouring the breeding and development of molluscs in years when these high floods are great, yearly and of long duration. Wind, waves and the exploitation of molluscs are factors which exert a selective action.

• 4. The sex-ratio of the two Unio populations of Crapina fluctuates in the course of the year, and probably indicates consecutive hermaphroditism.

• 5. Observations confirm the amplitude of the active spring and autumn migration of the Unionidae in the area studied, showing at the same time the influence this phenomenon can exert in certain years upon the structure of the respective populations.

• 6. The growth in length in individuals of the three species is continuous, rather fast in the first three years and slowing down afterwards, as exual maturity is attained. Individuals of the same age show a marked individual variability, induced, among other factors, by the variability of the breeding period.

• 7. Comparative biometrical investigation of U. pictorum and U. tumidus individuals of the two pools, showed significant morphological differences among U. tumidus populations. Thus, the relative values of H/L and T/L relationships are greater in individuals of the Crapina population than those of Jijila, due to different local conditions. In Unionidae populations of Crapina a more accentuated polymorphysm was evidenced, expressed by the existence of certain groups of individuals which differ in colour, size and quality of the valves.

• 8. A marked variability is presented by the different phases of the breeding cycle of Unionidae, a phenomenon with adaptive value in the life of the respective populations.

• 9. Ecological investigations and comparative biometrical studies of individuals of the same species, showed that, in spite of the relative isolation between the pools, each of the three species is represented by two distinct populations.

• 10. The energy flow, established for the Crapina populations, shows a low efficiency of net production (Pn/A < 10%). At the same time a high percentage of energy is spent in metabolic processes (R/A > 90%). This energy aspect shows that Unionidae molluscs play a small trophic role in the economy of the respective complex, but are important mineralizers of organic substances.

Résumé

Le présent travail est une étude de la structure, de la dynamique et du fonctionnement de quelques populations appartenant aux espèces U. pictorum, U. tumidus et Anodonta piscinalis, du complexe d'étangs Crapina-Jijila (zone inondable du Danube).

Dans les étangs de la zone inondable du Danube, la distribution spatiale des Mollusques Unionidés est soumise à l'influence des crues du Danube, à celle du fond de l'étang et à la présence ou l'absence de la végétation.

La structure et la dynamique des populations d'Unionidés sont influencées par les facteurs suivants: les grandes crues du Danube, le vent, les vagues et l'exploitation des Mollusques.

Les observations concernant la dynamique saisonière du rapport entre les sexes chez les populations d'Unio de l'étang de Crapina indiquent probablement un hermaphroditisme consécutif.

La croissance en longueur des individus appartenant aux espèces étudiées est continuelle, plus intense durant les premières trois années d'existence et moindre après qu'ils ont atteint la maturité sexuelle.

Les différents phases du cycle reproductif des Unionidés de ces étangs présentent une variabilité prononcée, phénomène d'une valeur adaptative particulière dans la vie de ces espèces.

Les recherches écologiques et l'étude biométrique comparative des individus de la même espèce montrent que malgré l'isolement relatif des deux étangs qui constituent le complexe en question, chacune des trois espèces est représentée par deux populations distinctes.

Du point de vue énergétique, les populations d'Unionidés ont un rôle trophique réduit dans l'économie des écosystèmes aquatiques de la zone inondable du Danube (production nette/assimilation < 10%), mais elles ont par contre un rôle important dans la minéralisation des substances organiques de l'eau (respiration/assimilation > 90%).