The polyopisthocotylean ectoparasite Sparicotyle chrysophrii is causing devastating effects on gilthead seabream net-pen farms across the Mediterranean. Adult parasites attach to host gills by the haptor, rigged with dozens of specialized clamps, and feed on fish blood. Severe anemia occurs during high-intensity outbreaks, which provoke direct mortalities, secondary infections and lower fish performance. Fundamental aspects of the parasite’s physiology and morphology remain poorly understood.

Livee adult parasite specimens were retrieved from the gills of experimentally infected gilthead seabream. Parasites were fixed for histology, scanning and transmission electron microscopy (SEM, TEM), as well as confocal microscopy (CM). Histological worm sections, ultrathin sections (TEM), whole specimens (SEM), and fluorescent-labelled actin and tubulin filaments in whole mounts (CM) were examined.

Sensory, reproductive, and secretory/excretory structures were identified. The cephalic region gathers the highest concentration of presumably sensory cilia involved in feeding, whereas microvilli in the remaining parasite body might serve for host encounter and/or orientation with regard to water flow. The parasite's strong ability to lengthen/stretch and shorten/round their body is the result of the contraction of the longitudinal muscle fibers, which most probably contribute to the parasite’s exploratory behavior and/or interaction with other specimens. Further strong muscle fiber arrangements were also observed in the oral apparatus, male reproductive organ, and clamps. Additionally, specialized digestive cells involved in hemoglobin processing were identified.

Many of the observed morphological features have paramount roles in the parasite’s essential biological functions, such as feeding, host encounter, and spreading success. Deeper understanding will contribute to shedding some light on the host–parasite interaction and in turn aid in the development of effective and targeted control strategies against sparicotylosis.