TPO 26

The passage talks about a kind of freshwater shellfish, zebra mussel, which has been spreading out of it's own habitat and has reached parts of North America. In addition, the passage mentions three reasons to demonstrate that this invasion cannot be stopped. However, the professor refutes these reasons in the lecture. He says that at that time there was not enough information about this matter but now there are possible ways to control this serious thread.

To begin with, the passage states that spread of zebra mussel stems from human transportation. To illustrate, the can spread by fresh water which is used by ships in order to balance their cargo. However, the professor says that if ships use ocean water instead of fresh water this invasion can be stopped. Because salt water can kill zebra mussel.

Second, the passage mentions that zebra mussels are a kind of hardy species which can tolerate hard conditions. In addition, they may have no predators in their new habitats. As a result, they become dominant in the new location. The professor opposes this point. He says that although they may have no predators at the beginning, after a while predators for example birds will find out there is a new source of food for them. Consequently, zebra mussels are not likely dominant there.

Finally, the passage states that zebra mussels compete with other freshwater fish species for food which is a negative effect. However, the professor mentions that they can have a positive influence. Zebra mussels can generate nutrition near the bottom of the ocean which helps bottom eating fish species to have enough source of food.