Parasitisation by Bathycreadium elongatum (Digenea, Opecoelidae) in pyloric caeca of Trachyrincus scabrus (Teleostei, Macrouridae).

A novel process of transmural passive displacement of a digenean parasite was studied in the digestive tract of the roughsnout grenadier Trachyrincus scabrus, which is found in the northwestern Mediterranean Sea. This mechanism seems to facilitate the elimination of a significant portion of intestinal parasites. The digenean parasite Bathycreadium elongatum was found in the intestine, mainly within pyloric caeca, in 74.4% of T. scabrus, with a mean abundance of 44 individuals per fish. Nodule-like lesions were also found in the mesentery of pyloric caeca of infected T. scabrus. Histological sections of the nodules revealed granulomatous inflammatory responses surrounding degraded digeneans. Partial nucleotide sequences of the 28S rRNA gene obtained from intracaecal B. elongatum and from the core of the nodules of the mesentery of pyloric caeca showed 100% mutual identity with an overlap of 971 bp. The greatest abundance of both intracaecal B. elongatum and nodules occurred in spring. During summer, and especially autumn, the abundance of intracaecal B. elongatum decreased. Prevalence and abundance of nodules increased in autumn. In winter intracaecal parasite abundance and prevalence began to increase, but decreased again in nodules. During spring and summer, parasites pass into the visceral cavity, hypothetically owing to the fragility of the wall of pyloric caeca in their apical zone, and become degraded through a granulomatous inflammatory response. This process seems to have a detrimental effect on the B. elongatum cycle since some of parasites are trapped and degrade in the connective tissue in which they are unable to complete their life cycle.