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Abstract In this study, we describe a new species of Pseudoparacreptotrema (Allocreadiidae) from the mugilid Dajaus monticola collected in western Puerto Rico, where no allocreadiid has previously been reported, bringing the number of species in this genus to seven (five in D. monticola, two in Profundulus spp.). The new parasite species is distinguished from congeners by its overall size, oral-to-ventral sucker size ratio, pharynx size, cirrus sac, and oral lobe morphology, and by 0.64%–3.45% divergence in a 1019-bp alignment of 28S. We build on prior suggestions that the current concept of P. agonostomi likely includes multiple species and provide the first mitochondrial data (whole mitochondrial genome) as well as the complete nuclear rDNA array from Pseudoparacreptotrema to facilitate future phylogenetic work. Within the Allocreadiidae, phylogenetic analysis of mitochondrial genomes and 28S provides conflicting topologies for the placement of Pseudoparacreptotrema and Allocreadium. The 28S phylogeny of six species of Pseudoparacreptotrema resembles that of four lineages of D. monticola in that in both host and parasite, Pacific coastal lineages branch earliest, and a Caribbean lineage is more recently evolved. 

Abstract The free-living infectious stages of macroparasites, specifically, the cercariae of trematodes (flatworms), are likely to be significant (albeit underappreciated) vectors of nutritionally important polyunsaturated fatty acids (PUFA) to consumers within aquatic food webs, and other macroparasites could serve similar roles. In the context of de novo omega-3 (n-3) PUFA biosynthesis, it was thought that most animals lack the fatty acid (FA) desaturase enzymes that convert stearic acid (18:0) into ɑ-linolenic acid (ALA; 18:3n-3), the main FA precursor for n-3 long-chain PUFA. Recently, novel sequences of these enzymes were recovered from 80 species from six invertebrate phyla, with experimental confirmation of gene function in five phyla. Given this wide distribution, and the unusual attributes of flatworm genomes, we conducted an additional search for genes for de novo n-3 PUFA in the phylum Platyhelminthes. Searches with experimentally confirmed sequences from Rotifera recovered nine relevant FA desaturase sequences from eight species in four genera in the two exclusively endoparasite classes (Trematoda and Cestoda). These results could indicate adaptations of these particular parasite species, or may reflect the uneven taxonomic coverage of sequence databases. Although additional genomic data and, particularly, experimental study of gene functionality are important future validation steps, our results indicate endoparasitic platyhelminths may have enzymes for de novo n-3 PUFA biosynthesis, thereby contributing to global PUFA production, but also representing a potential target for clinical antihelmintic applications.