Morphological features of green and brown hydra in freshwater ecotoxicology

Aluminum (Al) is one of the most abundant chemical elements in the Earth's crust. Yet, the effect of Al on living beings was little known until recently. This was partly because Al becomes soluble in water with increasing acidity only, and it was essentially harmless until acid rains first appeared. Because it is possible for the Al to be a major constituent of surface and subterranean waters, thereby presenting a potential environmental danger for freshwater organisms, the aim of this research was to compare the changes between two closely related hydra species. Specifically, we sought to trace differences and the extent of damage of the effect of Al upon green and brown hydra. Usually found in unpolluted freshwater, hydra is in the the phylum Cnidaria. It has a simple, cylindrical body with an adhesive foot on one end and a mouth surrounded by five to eight tentacles on the other. It comprises two cellular layers separated by mesoglea. Green hydra forms a symbiotic relationship with Chlorella algae. Hydra has a tremendous rate of regeneration. A comparative static toxicity test was performed using the individuals of green (Hydra viridissima Pallas, 1766 ; strain S1J-J1) and brown (Hydra oligactis Pallas, 1766 ; strain S1M-K1) hydra. They were treated with 9 concentrations of aqueous solution of aluminum sulphate (25, 50, 80, 100, 150, 200, 250, 350 and 500 mg/L ; Kemika, Croatia) in laboratory conditions (22.8oC) in subacute exposure for three days and compared to the control groups of organisms. After the exposure, the animals were put in clean aquarium water, and left recovering for 14 days. Al toxicity triggered numerous morphological and behavioral changes, showing the strong deleterious effect on both hydra species. Brown hydra showed higher Al susceptibility in all parameters. Higher concentrations triggered more prominent changes: tentacle reduction, incoherent and sluggish reactions to mechanical stimuli, migration towards water surface and apocrine mucous secretion of ectodermal layer. It strongly induced a hormestic effect in the asexual reproduction of green hydra and even induced sexual reproduction. Mortality was noted only in the highest concentration. An interesting hypothesis was confirmed wherein the brown hydra was able to regenerate itself to its original height by elongating its epithelial cells. This ability in the green hydra is yet to be examined. Control animals did not show any changes. As brown hydra showed higher level of Al susceptibility than green hydra, symbiosis seemed to present an evolutionary advantage. Higher water acidity levels greatly increased the overall toxicity of aluminum to these species. Greater Al concentrations brought harmful impact to both species. However, cooperative ad symbiotic interactions between the green hydra and the green algae give the symbiont stronger and better resistance compared to the brown hydra. This hydra bioassay was shown to be a useful tool for quick corroboration of the expected results.