THE IMPACT OF Zn2+ IN WATER ENVIRONMENT ON THE DIURNAL RESPIRATION CYCLICALITY OF PLANORBARIUS CORNEUS S. L. (MOLLUSCA, GASTROPODA, PLANORBIDAE) ALLOSPECIES

Abstract

We investigated the impact of the different concentrations of Zn2+ in the water (in the 0.5 MPL – 3 MPL range) on the chronological features of lung and direct diffusive respiration in representatives of superspecies complex of ramshorns Planorbarius corneus s. l. (“eastern” and “western” allospecies) from the Ukrainian river network. They have a bimodal respiration. Atmospheric oxygen goes into their true lungs through the breathing pore – pneumostome, – during the periodical rises up to the water surface tension film. And the oxygen soluble in the water is acquired by the osmosis through the epithelial body coverings and the surface of adaptive gill. Both ways are almost equally important for the oxygen supply of mollusks’ organisms.
Zn2+ ions are the powerful toxins of local effect for ramshorns. The excess of the MPL may lead to the impairment of stable homeostasis. Both P. corneus allospecies s. l. have clearly expressed diurnal cyclicality of their lung respiration (in the “day-night” rhythm). They perform 1.2–1.3 times more “inhales” during the day with the shorter (1.3–1.4 times) intervals. This apparently is the consequence of physiological activity increasing in the period of their most intensive life-giving metabolic processes occurring.
Under the 0.5 MPL of Zn2+ there were no statistically significant changes in lung and direct diffusive respirations indexes (comparing to the control) in both allospecies during the latent phase of their intoxication process. “Inhales” during the day were 1.1 time longer than during the night in experimental animals. This let mollusks replenish the amount of oxygen they underreceived due to the decreased number of night “inhales”.
Under MPL Zn2+ level the respirational indexes of ramshorns (number, length and volume of “inhales” and intervals between them) increased in average 1.1 times. Their feeding and moving behaviors visibly activated, which promoted the increasing of the daily frequency of their respiration rhythm. Under the 2 MPL concentration of Zn2+ there was further significant increasing (p<0.01) of all aforementioned respiration indexes. Both in day and night the number and length of inhales increased 1.1–1.2 times and their volume 1.3–1.4 times. After Zn2+ concentration increasing from MPL to 2 MPL the mollusks demonstrated the symptoms of stimulatory phase of intoxication.
Under this level of environmental toxicity, the increasing of physiological activity allowed the ramshorns to adapt to new, less favorable conditions. Instead, under the toxicant concentration at 3 MPL they developed fast the symptoms of acute intoxication as last intoxication phases, quickly changing each other: depressive, sublethal and lethal. Mollusks lost their movability quite soon due to the water swelling of their tissues. This process was followed by structural injuries and functional impairments appeared in respirational epithelium, which drove the partial death of the weakest animals; later, all the experimental animals died due to the asphyxia development.
Under the different Zn2+ concentrations lung and diffusive respiration indexes appeared significantly lower (p<0.01) for “eastern” than those for “western” allospecies. This indicates the higher sensibility and lower endurance of the first allospecies regarding the toxicant used.