Interrelationships Among Monorchiid Trematodes with Special Emphasis on Some Northwestern Atlantic Genera

The Monorchiidae Odhner, 1911 is a cosmopolitan family of flukes (Trematoda: Digenea) comprising species that parasitize the digestive tract of estuarine and marine fishes as adults. Compared with other oceans, recent morphological or molecular taxonomic work conducted on monorchiid species from the northwestern Atlantic Ocean has been sparse (Manter, 1931; Overstreet, 1969; Andres et al., 2018; Wee et al., 2018, 2019, 2020). Therefore, the present work investigated the interrelationships of some monorchiids from the northwestern Atlantic Ocean with emphasis on several genera and investigated if Lasiotocus minutus (Manter, 1931) Thomas, 1959 constitutes a complex of cryptic species. New morphological and molecular data are provided for 3 species; new molecular data are provided for 5 species; 6 new monorchiid species are described and illustrated. Phylogenetic analysis of the 28S rDNA fragment revealed Genolopa Linton, 1910 represents a natural lineage, supporting that presence of spines in the genital atrium and a bipartite, anteriorly spined terminal organ are key diagnostic features for the genus, and provided further evidence that Lasiotocus Looss in Odhner, 1911 is polyphyletic. Phylogenetic analysis of the 28S rDNA fragment and morphological analysis of L. minutus did not support a complex of cryptic species because all isolates of the 28S rDNA region were identical across locations and definitive hosts. However, more data are needed to come to a well-supported conclusion, such as molecular data from additional DNA regions (ITS2 rDNA, mtDNA) and data from more geographic locations and intermediate hosts