“Not flying solo”: phylogenetic identification and life-cycle insights of larval cestodes in the European flying squid Todarodes sagittatus (Cephalopoda: Ommastrephidae)

The European flying squid Todarodes sagittatus Lamarck is a widely distributed ommastrephid cephalopod in the Northeast Atlantic and Mediterranean Sea, yet its parasite fauna remains poorly documented in the Mediterranean Sea. In this study, two cestode larvae species infecting T. sagittatus from the Algerian coast (Western Mediterranean) were investigated using an integrative approach combining morphological observations and molecular analyses. A total of 63 squids were examined for parasitic infection. Cestodes in plerocercoid stage were detected in 31 individuals (prevalence of 46%), primarily located within the gastrointestinal tract, including the stomach, intestine, and caecum. Morphological features of the larvae were consistent with members of the order Phyllobothriidea but did not allow identification to the species level. Molecular analysis of the D1-D3 region of the 28S rDNA revealed two distinct larval lineages belonging to the genus Crossobothrium Linton, 1889. One lineage showed 100% sequence identity with the adult cestode Crossobothrium dohrnii (Oerley, 1885), a parasite of hexanchid sharks, confirming the identity of these larvae as plerocercoids of C. dohrnii. The second lineage clustered within the Crossobothrium clade but could not be assigned to a known species and is therefore referred to as Crossobothrium sp. The occurrence of these larvae in T. sagittatus, together with ecological data on squid diet and predator-prey relationships, suggests that this cephalopod acts as an intermediate or paratenic host in the life cycle of hexanchid cestodes. Infection likely occurs through predation on crustaceans acting as first intermediate hosts, while transmission to definitive hosts occurs when infected squids are consumed by sharks. These findings provide the first molecular identification of cestode larvae from T. sagittatus in the Mediterranean and highlight the important role of ommastrephid squids in the trophic transmission of elasmobranch parasites in pelagic ecosystems.