Evolution of sex determination and male-male competition in parasitic barnacles

Rhizocephalan barnacles are parasitic living in crabs and hermit crabs. There are two reproductive systems. In the GSD (genetic sex determination), larger size larvae become males and smaller size larvae become females. Sex of larvae is determined by the mother. In the ESD (environmental sex determination), no sexual size difference in larvae is observed. We develop a theory to explain this pattern.