This partnership is probably based on mutual exchange of food, enabling them to survive in extremely nutrient - poor waters. (sciencedaily.com)
The possibilities are limited by the text, by the common mind of the individual community, and by mutual exchange between one community and another. (religion-online.org)
According to one of the most popular Millionaire dating site Casual Dating is a physical or emotional relationship which includes mutual exchange also. (top10millionairedatingsites.com)