Molecular characterization of muscle-parasitizing didymozoid from a chub mackerel, Scomber japonicus

Morphological and molecular study of Didymodiclinus marginati n. sp. (Trematoda: Didymozoidae) gill parasite of Epinephelus marginatus from the central and western Mediterranean Sea

The current study provides a morphological and molecular characterization of a new species of Didymodiclinus (Trematoda: Didymozoidae) infecting the dusky grouper, Epinephelus marginatus (Teleostei: Serranidae) from the Mediterranean Sea. A total of 279 dusky grouper specimens were examined for didymozoid gill parasites from the Mediterranean Sea between 1998 and 2020. New species differs from the most similar congeneric species by the rudiments of female reproductive organs in functional male specimens, and the seminal receptacle, Mehlis gland and accessory gland cells in functional female specimens, not observed in Didymodiclinus branchialis (Yamaguti, 1970), Didymodiclinus epinepheli (Abdul-Salam, Sreelatha and Farah, 1990) and Didymodiclinus pacificus (Yamaguti, 1938), respectively. These species are also characterized by their different hosts and location within the host tissues, being from other geographical localities. Moreover, this is the first species reported in E. marginatus from the central and western Mediterranean Sea. Genetic analyses were performed on partial 28S and partial internal transcribed spacer-2 ribosomal RNA regions and the mitochondrial cytochrome oxidase 1 (cox1) gene by polymerase chain reaction. Comparison of genetic sequences of Didymodiclinus marginati n. sp. with the available deposited sequences of 28S revealed that the new isolates cluster with several unidentified didymozoids and groups as a sister clade of the Nematobothrinae subfamily. Moreover, 28S and cox1 phylogenetic trees evidenced that Didymodiclinae is well separated from Didymozoinae and other gonochoric didymozoids. Following both morphological and genetic results, a key of identification for the genus Didymodiclinus is proposed.

Didymozoids in Muscle of Atlantic Chub Mackerel (Scomber colias)

BACKGROUND

Digenean trematodes of the family Didymozoidae are tissue parasites that are particularly common in scombrid fish. These parasites can reduce the commercial value of fish, especially the ones occurring in the muscle.

PURPOSE

In the present study and for the first time, we report the occurrence of didymozoids in muscle tissue of Atlantic chub mackerel (Scomber colias) collected in northeast Atlantic (Portuguese coast).

METHODS

The entire musculature of 64 Scomber colias was removed and examined macroscopically and under a stereomicroscope to search for parasites.

RESULTS

The prevalence (3.1%) and abundance (0.03) of infection detected were quite low, but the parasites cause muscle softening decreasing the quality of the infected fish. If the infection levels increase, this may constitute a cause of concern for the fisheries and canning industry.

CONCLUSION

Didymozoids identified in this study are closely related to muscle parasites detected in other fish species of the genus Scomber, most probably the same species. Phylogenetic analysis also corroborates the hypothesis that muscle-parasitizing didymozoids are distinct from the ones parasitizing other fish organs.

Ecology of the Atlantic black skipjack Euthynnus alletteratus (Osteichthyes: Scombridae) in the western Mediterranean Sea inferred by parasitological analysis

Between 2008 and 2011, the head of 150 Euthynnus alletteratus (Osteichthyes: Scombridae) caught inshore off the southeastern Iberian coast (western Mediterranean Sea) were examined for parasites. Two monogeneans, four didymozoid trematodes and four copepods were found. Parasite abundance showed a positive relationship with the annual sea surface temperature, except for Pseudocycnus appendiculatus, but negative with the sea depth (Capsala manteri, Neonematobothrium cf. kawakawa and Caligus bonito). Prevalences and mean abundances differed significantly among sampling areas, except for C. manteri, Oesophagocystis sp. 2 and Ceratocolax euthynni, and sampling years (Melanocystis cf. kawakawa, N.cf. kawakawa, P. appendiculatus and Unicolax collateralis). Results indicate that the parasite abundances of E. alletteratus in the western Mediterranean Sea depend mainly on regional environmental variables, which can show interannual variations. The presence of pelagic parasites, i.e. didymozoids and P. appendiculatus, could indicate that E. alletteratus migrates between inshore and offshore pelagic domains. The different parasite faunas reported in E. alletteratus populations from the western Atlantic Ocean and the Mediterranean Sea appear to point out the geographical host isolation. These results suggest that E. alletteratus inhabiting the western Mediterranean Sea performs inshore-offshore small-scale migrations, and not transoceanic migrations between the western Atlantic Ocean and Mediterranean Sea.