Rhizocephalan sacculinid barnacles such as Loxothylacus panopaei
start life as a nauplius larva, then a cyprid as do many other crustaceans. Instead of settling on rocks as other barnacles do, females settle at the base of the seta of a crab, usually recently molted. They attach by the base of their first antennae. Then they metamorphose, lose their eyes and legs, and inject the rest of their body into the host. They begin to grow inside, sending projections through the crab body until most of the weight of the crab may actually be barnacle tissue. When sexually mature the barnacle erupts in a sack-like or muchroom-like brood chamber from the abdomen, as seen above. Male sacculinid barnacles (in the cyprid stage) settle within this sack. The male extrudes cells which migrate into the female and form into a testis, which fertilizes the eggs the female lays. The barnacle is called a parasitic castrator because they usually destroy the gonads of the host crab, causing them to display intermediate sexual characteristics. They also inhibit the crab from molting. Some crab hosts of this parasite may find refuge in low-salinity water. Rhizocephalan barnacles have no appendages, digestive tract, segments, or plates; and the part which normally develops into a peduncle (as in goose barnacles) becomes a footlike absorptive process called "nutrient rhizoids" that ramify through the host.