What is done to the chromosomes to make them more visible under the microscope?

What do you need with it’s not a question your just stating what it says

The population of unionid mussels decreased after zebra mussels were introduced. zebra mussels competed with the unionid mussels for food. the zebra mussels were able to displace the unionid mussels, allowing the zebra mussel population to increase  over the years. the unionid mussel population decreased because  of lack of food.

Answerchemical;