Points in Acrobat vs AI appear different

Hi,

I generated a pdf of various points with specific shapes or symbols.  When I open the pdf in Adobe Acrobat they appear normal:



However, when I open the same file in Illustrator the symbols are shrouded with rectangular black backgrounds:

I am using CS-5 ver 15.1.0 (with a student license).  Any ideas about eliminating the backgrounds without losing the symbols? I am working with thousands of these points and need to used them as a layer in a figure.

Thanks for any help.

Geoldoe (Mike)