Finding a Needle in a Haystack: larval Stages of Didymozoidae (Trematoda: Digenea) Parasitizing Marine Zooplankton

Abstract

Abstract Larval didymozoids (Trematoda, Digenea) were discovered parasitizing the hemocoel of the heteropod Firoloida desmarestia (redia mean intensity = 13) and the chaetognaths Flaccisagitta enflata and Flaccisagitta hexaptera (metacercaria mean intensity = 1) during a 2014–2016 systematic study of parasites of zooplankton collected in the central and southern regions of the Gulf of California, Mexico. Didymozoid infection route during the early life cycle was inferred combining morphological and molecular evidence (light and scanning electron microscopy and mitochondrial cytochrome c oxidase subunit I gene, cox1, respectively). Didymozoid rediae parasitizing F. desmarestia were observed alive releasing hundredths of cystophorous cercariae at a mean rate of one cercariae every 12 s. Cercariae developed into young metacercariae in 1 d at 22°C. Molecular analysis of cox1 showed that rediae found in F. desmarestia belong to two distinct didymozoid species (Didymozoidae sp. 1 and sp. 2). Metacercariae parasitizing chaetognaths were morphologically identified as Didymozoidae type Monilicaecum and cox1 sequences showed that metacercariae of chaetognaths matched with these two Didymozoidae sp.1, and sp. 2 species found parasitizing F. desmarestia, plus a third distinct Didymozoidae sp. 3. These are the first DNA sequences of cox1 gene from didymozoid larvae for any zooplankton taxonomic group in the world. We concluded that F. desmarestia is the first intermediate host of rediae and second intermediate host cercariae and chaetognaths are the third intermediate hosts of didymozoid metacercariae demonstrating a potential overlap distribution of these two carnivorous zooplankton taxonomic groups. The didymozoid specimens were not identified to species level because any of the cox1 sequences generated here matched with the sequences of adult didymozoids currently available in GenBank and Bold System databases. This study provides valuable information for the morphological and molecular understanding of the Didymozoidae larvae that has been previously based on the recognition of the 12 known morphotypes.</jats:p