Effects of Morphological Changes Induced by the Rhizocephalan Parasite Polyascus polygenea on Predation Risk of the Asian Shore Crab Hemigrapsus sanguineus.

Parasites can significantly influence ecological communities by altering the traits of their hosts. hizocephalans, 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 Polyascus polygenea 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 Polyascus infection heightens predation risk through non-consumptive effects by inducing host morphological changes.