Lozano-Cobo, H., A., Oceguera-Figueroa, C.A., Silva-Segundo, C.J., Robinson & J., Gómez-Gutiérrez (2022). Finding a needle in a haystyack: larval stages of Didymozoidae (Trematoda: Digenea) parasitizing marine zooplankton. Enrique Curchitser (Eds.), Progress in Oceanography. 121: 2661-2672. DOI: 10.1007/s00436-022-07593-6.
Horacio Lozano-Cobo 1, Alejandro Oceguera-Figueroa 2, Claudia A. Silva-Segundo 3, Carlos J. Robinson 4 y Jaime Gómez-Gutiérrez 5
Larval didymozoids (Trematoda: Digenea) were discovered parasitizing the hemocoel of the heteropod Firoloida desmares-tia (redia mean intensity = 13) and the chaetognaths Flaccisagitta enflata and Flaccisagitta hexaptera (metacercaria meanintensity = 1) during a 2014–2016 systematic study of parasites of zooplankton collected in the central and southern regionsof the Gulf of California, Mexico. Didymozoid infection route during the early life cycle was inferred combining morpho-logical (light microscopy) and molecular (mitochondrial cytochrome c oxidase subunit I gene, cox1 ) evidence. Didymozoidrediae parasitizing F. desmarestia were observed, just after field collection of the host, containing hundredths of completelydeveloped cystophorous cercariae, releasing them though the birth pore at approximately one cercaria every 12 s. Cercariaelost their tails developing into a ‘young metacercaria’ in 1 d at 22 °C without need of an intermediate host. Molecular analy-sis of cox1 showed that rediae found in F. desmarestia belong to two distinct didymozoid species (Didymozoidae sp. 1 andsp. 2). Metacercariae parasitizing chaetognaths were morphologically identified as Didymozoidae type Monilicaecum andcox1 sequences showed that metacercariae of chaetognaths matched with these two Didymozoidae sp. 1, and sp. 2 speciesfound parasitizing F. desmarestia, plus a third distinct Didymozoidae sp. 3. These are the first DNA sequences of cox1gene from didymozoid larvae for any zooplankton taxonomic group in the world. We concluded that F. desmarestia is thefirst intermediate host of rediae and cercariae, and the chaetognaths are the second intermediate hosts where non-encystedmetacercariae were found. The definitive host is still unknown because cox1 sequences of present study did not geneticallymatch with any available cox1 sequence of adult didymozoid. Our results demonstrate a potential overlap in the distributionof two carnivorous zooplankton taxonomic groups that are intermediate hosts of didymozoids in the pelagic habitat. Thedidymozoid specimens were not identified to species level because any of the cox1 sequences generated here matched withthe sequences of adult didymozoids currently available in GenBank and Bold System databases. This study provides baselineinformation for the future morphological and molecular understanding of the Didymozoidae larvae that has been previouslybased on the recognition of the 12 known morphotypes.
Palabras clave: Rediae; Cystophorous cercariae; Metacercariae; Carnivorous zooplankton; cox1; Gulf of California
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