Finding a needle in a haystack: larval stages of Didymozoidae (Trematoda: Digenea) parasitizing marine zooplankton.

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 (light microscopy) and molecular (mitochondrial cytochrome c oxidase subunit I gene, cox1) evidence. Didymozoid rediae parasitizing F. desmarestia were observed, just after field collection of the host, containing hundredths of completely developed cystophorous cercariae, releasing them though the birth pore at approximately one cercaria every 12 s. Cercariae lost their tails developing into a 'young metacercaria' in 1 d at 22 °C without need of an intermediate host. 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 cercariae, and the chaetognaths are the second intermediate hosts where non-encysted metacercariae were found. The definitive host is still unknown because cox1 sequences of present study did not genetically match with any available cox1 sequence of adult didymozoid. Our results demonstrate a potential overlap in the distribution of two carnivorous zooplankton taxonomic groups that are intermediate hosts of didymozoids in the pelagic habitat. 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 baseline information for the future morphological and molecular understanding of the Didymozoidae larvae that has been previously based on the recognition of the 12 known morphotypes.

Molecular Identification of Plerocercoids of Clistobothrium montaukensis (Cestoda: Phyllobothriidea) Parasitizing the King of Herrings Regalecus glesne.

PURPOSE: Endo-parasites of the bathypelagic king of herrings Regalecus glesne and oarfish Regalecus russelii are only known from few specimens opportunistically examined. As a consequence, there are few records of parasites from either Regalecus species. We report plerocercoid larvae of phyllobothriidean cestodes parasitizing an adult R. glesne stranded in Bahía de La Paz, Baja California Sur, Mexico. METHODS: Sixty-three plerocercoids were obtained from the intestine of R. glesne and characterized using morphological and molecular methods (nuclear 28S rDNA and mitochondrial cytochrome c oxidase I gene sequences). RESULTS: Following the morphological diagnostic criteria of scolex and muscle bands in the strobila, plerocercoids specimens were preliminary assigned to the genus Clistobothrium. Mitochondrial and nuclear DNA sequences indicate these plerocercoids correspond to Clistobothrium montaukensis Ruhnke, 1993. CONCLUSION: Regalecus glesne is a new host known for C. montaukensis and this report is a new geographical record of C. montaukensis parasitizing species of the genus Regalecus previously known only from California and Florida, USA.

Role of oceanography in shaping the genetic structure in the North Pacific hake Merluccius productus.

Determining the relative influence of biotic and abiotic factors on genetic connectivity among populations remains a major challenge in evolutionary biology and in the management and conservation of species. North Pacific hake (Merluccius productus) inhabits upwelling regions in the California Current ecosystem from the Gulf of California to the Gulf of Alaska. In this study, we examined mitochondrial DNA (mtDNA) and microsatellite variation to estimate levels of genetic differentiation of M. productus in relation to the role of oceanographic features as potential barriers to gene flow. Samples were obtained from nine sites spanning a large part of the geographic range of the species, from Puget Sound, Washington to Costa Rica. The microsatellite results revealed three genetically discrete populations: one spanning the eastern Pacific coast, and two apparently resident populations circumscribed to the Puget Sound and the northern Gulf of California (FST = 0.032, p = 0.036). Cytochrome b sequence data indicated that isolation between the Puget Sound and northern Gulf of California populations from the coastal Pacific were recent phenomena (18.5 kyr for Puget Sound and 40 kyr for the northern Gulf of California). Oceanographic data obtained from the Gulf of California support the hypothesis that permanent fronts within the region, and strong gradients at the entrance to the Gulf of California act as barriers to gene flow. A seascape genetics approach found significant genetic-environment associations, where the daytime sea surface temperature and chlorophyll concentrations were the best predictive variables for the observed genetic differentiation. Considering the potential causes of genetic isolation among the three populations, e.g. spawning areas in different latitudes associated with upwelling processes, oceanographic barriers, asymmetric migration and specialized diet, oceanographic barriers appear to be a likely mechanism restricting gene flow.