The impact of the rhizocephalan parasitic barnacle on its host crab through non-consumptive effects

Parasites can significantly influence ecological communities by altering host traits. Rhizocephalans, a group closely related to thoracican barnacles within Cirripedia, are common in marine ecosystems and profoundly impact their hosts, most notably by inducing reproductive castration. However, their influence on other host traits, particularly those related to predator defense, remains underexplored. The rhizocephalan Sacculina sp. modifies the length of the cheliped propodus in the host crab Hemigrapsus sanguineus. To determine whether parasitized crabs are more vulnerable to predation, we conducted field tethering experiments comparing parasitized and unparasitized individuals. The results showed that parasitism itself did not directly increase the likelihood of crabs being lost (presumably due to predation) in either sex. However, structural equation modeling revealed that in parasitized males, a reduced cheliped propodus length indirectly increased predation risk. In females, parasitism reduced carapace width, which in turn shortened the cheliped propodus and increased vulnerability to predation. These findings demonstrate that Sacculina infection heightens predation risk through non-consumptive effects by inducing host morphological changes.