Parasitic Pachypygus gibber poses a silent threat to reproduction and development in Ciona robusta.

Pachypygus gibber, an ascidicolous copepod of the family Notodelphyidae, is commonly found within the pharyngeal basket of Ciona robusta, a pivotal model species in marine biology. Pachypygus gibber was traditionally viewed as a filter feeder, but its ecological role -whether commensal, kleptoparasitic, or parasitic-remains debated. We investigated, through controlled laboratory experiments, the impact of P. gibber on the reproductive and developmental fitness of C. robusta. We compared infested (C+) and non-infested (C-) parental lines, recording egg production, hatching rates, larval settlement, and juvenile growth and survival. Results revealed no significant differences in egg production per unit body length between C + and C-; however, hatching and settlement rates were significantly reduced in infested individuals. Moreover, offspring of infested parents exhibited marked growth impairment and elevated mortality over four weeks. These findings provide evidence that P. gibber acts as a parasite sensu lato, exerting substantial negative effects on host reproductive output and juvenile viability. Our results partially redefine the ecological role of P. gibber, with implications for C. robusta population dynamics. In addition, we demonstrated that experimental settings where unnoticed infestations are present, might confound biological studies involving this model organism.