Peter Christensen

I am researching my Norwegian heritage but I also have some Danish relatives. My great grandfather Peter Christian Christensen was born in Denmark April 7, 1849. His wife Nikolena Petra Christiane Claussen, called Anna, which is written Jane in Danish was also born in Denmark January 21, 1850. They were married on April 7, 1875 in Michigan. Peter may be the Peter C. Christensen listed in the Naturalization Records of Kent County, MI. Is there any way I can track down what ship they may have crossed over on? Or who their parents were? I tried looking in Danish Heritage sites but didn't find a place to post. Thank you Judie