Hooked on fish blood: the reliance of a gill parasite on haematophagy.

Parasitism involves diverse evolutionary strategies, including adaptations for blood feeding, which provides essential nutrients for growth and reproduction. (Polyopisthocotyla: Microcotylidae), an ectoparasitic flatworm, infects the gills of gilthead seabream (), significantly affecting fish health, welfare and Mediterranean cage farm profitability. Despite its impact, limited information exists on its feeding behaviour. This study demonstrates the presence of blood and exogenous haem groups in and explores its digestive tract using light and electron microscopy, elucidating its internal morphology and spatial arrangement. Elemental analysis of the digestive haematin cells shows residual oxidized haem depots as haematin crystals. Additionally, we studied the impact of the blood feeding on the host by estimating the average volume of blood intake for an adult parasite (2.84 ± 2.12µl·24h) and we described the significant drop of the plasmatic free iron levels in infected hosts. Overall, we demonstrate the parasite's reliance on its host blood, the parasite's buccal and digestive morphological adaptations for blood feeding and the provoked effect on the fish host's health.