The skin of the earthworm secretes mucous. This mucous keeps the skin moist always, which is vital for the survival of earthworms. Because earthworms have no respiratory organ and hence the respiration takes place through the moist skin.
The oxygen from the environment diffuses passively across the moist skin and it is carried by the circulatory system (blood) to the cells. Similarly, carbon dioxide from the cells will also be carried by the circulatory system and diffuses out through the moist skin. Hence, the earthworms will die if their skin dries out.
Salinity (saltiness) of the soil in which an earthworm lives is an important factor that decides its survival because earthworms are highly sensitive to salt stress. The salinity may reduce their growth at low salt concentrations or cause mortality at high salt concentrations.
They are unable to tolerate high ionic strength, because high salt concentrations destroy their sensitive skin, and the earthworms cannot have control over the osmotic regulation. In addition, the neurosecretory cells in earthworms play a vital role in water balance as well as ionic and osmotic regulation.
The salt stress and desiccation significantly interfere with the functions of these neurosecretory cells. Hence, salts like sodium chloride are extremely toxic to most of the earthworm species.
Sodium chloride could cause mortality at a concentration of 0.5 per cent itself, whereas other salts such as potassium chloride are toxic at a concentration of more than 1 per cent. However, the salts at lower (sublethal) concentrations may not cause mortality in earthworms, although they would substantially reduce the reproduction, especially production of cocoons (egg cases).
Source: The Hindu
