When someone files for divorce in CT, does the other spouse HAVE to file a counterclaim? Or just if they disagree with the grounds listed (even if no-fault?)

Does it matter which spouse files for divorce first, when one claim will be no-fault and the other for alleged grounds? Does the order in which they are filed have any bearing on how the case works out?

Specifically I am interested in what happens if the grounds are found to be... er, groundless, which there is case law to support, in the situation I am thinking of.