Many of us have dealt with a clingy boyfriend or girlfriend, but some male deep-sea anglerfish take things to the extreme: In certain species, the male latches onto the much larger female, permanently fusing his body with hers (above). This bizarre mating ritual has long puzzled scientists, as merging two individuals’ blood and tissue should trigger an adverse immune reaction. Now, a new study suggests some males have lost parts of their immune system to compensate, The New York Times reports. Anglerfish that employ the most extreme form of sexual parasitism–where multiple males can conjoin with one female–could not produce functional antibodies and T cells, which typically fight off foreign invaders and differentiate an individual’s own cells from unfamiliar ones. Monogamous anglerfish species had less extreme versions of these dampened immune systems, the team reports this week in Science, as these fish could still produce certain types of antibodies.