How do dwarf male barnacles reproduce with larger individuals?

In certain barnacle species where populations are sparsely distributed, a larger individual may develop into a female, while a smaller individual that attaches to it becomes a "dwarf" male.1

The reproductive process for these dwarf males varies based on the species:

• Exposed males: When a dwarf male occupies an exposed position on its partner, it resembles a juvenile and is capable of feeding.1
• Protected males: In some species, the male is protected by the partner and becomes highly reduced, sometimes existing as little more than a sac containing testes.1
• Rhizocephalans: In these parasitic species, a male cyprid larva attaches near a freshly erupted female on the host. It then undergoes metamorphosis into a minute cell mass that migrates into a special "male-cell receptacle" within the female’s brood chamber, where it differentiates into a mass of spermatozoa to fertilize the eggs.1

This adaptation ensures reproductive success for these crustaceans despite the challenges of finding mates in their environment.12