Phylogenetic assessment of Megacoelium spinicavum Thatcher & Varella, 1981 (Digenea: Haploporidae) from Pterygoplichthys pardalis (Siluriformes: Loricariidae) in the Peruvian Amazon Basin.

This study provides the first integrative analysis of Megacoelium spinicavum Thatcher & Varella, 1981 (Digenea: Haploporidae) from the Amazon sailfin catfish Pterygoplichthys pardalis Castelnau, 1855 (Siluriformes: Loricariidae) in the Peruvian Amazon. A detailed morphological description is presented, including the first scanning electron microscopy (SEM) images of tegumental structures, which revealed two distinct types of tegumental spines: (1) small, button-like spines and (2) sharply pointed spines. Partial sequences of the 28S rDNA and mitochondrial cox1 genes were generated and analysed to investigate the phylogenetic position of Megacoelium Szidat, 1954, within the Haploporidae Nicoll, 1914. Phylogenetic analyses placed M. spinicavum within the 'robust species' clade of Saccocoelioides Szidat, 1954, clustering with S. bacilliformis Szidat, 1973, although with weak support. These results provide additional evidence that Saccocoelioides is not monophyletic and support restricting the genus to the 'minute species' clade containing the type species. The 'robust species' clade appears to comprise at least three divergent lineages, potentially representing distinct genera, one of which includes M. spinicavum. The absence of molecular data for M. plecostomi Szidat, 1954, the type species of Megacoelium, continues to obscure its phylogenetic placement. We highlight the need for comprehensive morphological and multilocus molecular analyses, including SEM, to clarify the taxonomic status of Megacoelium and to resolve the evolutionary relationships of chalcinotrematine digeneans in Neotropical fishes.