The specimens came from 27 mackerel bought from local fishermen operating small-scale coastal vessels in Pathio District, Chumphon Province, in August 2024. Five of the fish were Rastrelliger kanagurta and 22 were R. brachysoma, both species of great importance to coastal fisheries across Southeast Asia. The researchers examined the fish immediately after capture at King Mongkut’s Institute of Technology Ladkrabang, Prince of Chumphon Campus, recovering digenean worms from the digestive tract. The worms were washed in saline, heat-killed in near-boiling saline, fixed in 70% ethanol, stained with Semichon’s acetocarmine, dehydrated through a graded ethanol series, cleared in xylene and mounted in Canada balsam for detailed study.

Under the microscope, P. chumphonense is an elongate, cylindrical worm about 968 micrometres long on average — roughly a millimetre — and 186 micrometres wide at the level of the ventral sucker, giving a length-to-width ratio of about 5.2. The body is slightly wider anteriorly and armed with minute spines along the forebody. The oral sucker is subterminal and subglobular, while the ventral sucker is rounded and consistently larger than the oral sucker, with a sucker width ratio of about 1:1.3. The prepharynx is long and distinct, the pharynx oval and muscular, and the esophagus long, with the intestinal bifurcation located in the posterior third of the forebody. The ceca extend to near the posterior extremity of the body.

The reproductive anatomy follows the classic digenean plan: two spherical to oval testes arranged in tandem in the mid to posterior hindbody, a saccular external seminal vesicle, an elongate-claviform cirrus sac about 195 micrometres long occupying the anterior hindbody, and an oval pretesticular ovary. The uterus is preovarian and intercecal, and the eggs are few in number — no more than 15 per specimen — oval, operculate, light brown and unembryonated, measuring about 47 by 30 micrometres. The excretory vesicle is tubular, running forward past the gonads and ventral sucker but terminating below the level of the intestinal bifurcation, a detail that proves crucial for telling this worm apart from its closest relative.

What really sets P. chumphonense apart from most of its congeners is the distribution of the vitellarium — the follicular glands that produce yolk for the eggs. In the new species, the vitelline follicles are confined entirely to the hindbody, extending anteriorly only to the level of the ventral sucker. In many other species of Prodistomum, including P. alaskense, P. angelae, P. gaevskayae, P. hynnodi, P. keyam, P. lichtenfelsi, P. menidiae, P. polonii, P. priedei, P. santoshi, P. siddiqi, P. travassosi, P. vinodae and P. waltairensis, the vitellarium distinctly invades the forebody. Among species where the vitellarium is restricted to the hindbody, P. girellae is distinguished by prominent gland cells associated with the external seminal vesicle, a feature absent in the new worm, while P. mohsini and P. pomatomi have an esophageal region distinctly longer than the prepharynx, unlike the approximately equal lengths seen in P. chumphonense.

The trickiest comparison is with Prodistomum orientale, the species most morphologically similar to the new worm and the one most often reported from scombrid fishes. Historically, P. orientale has been recorded from mackerel across the Pacific, Atlantic and Indian Oceans and the Mediterranean and Red Seas, with a reported host range spanning Scomber japonicus, S. colias, S. scombrus, S. australasicus, Rastrelliger brachysoma, R. kanagurta and even one clupeid. Most descriptions with morphometric data describe the vitelline follicles of P. orientale as well separated from the posterior margin of the ventral sucker, whereas in P. chumphonense they extend anteriorly to that margin. In P. orientale the excretory vesicle also extends forward past the cecal bifurcation, while in the new species it stops short of it.

On morphology alone, separating P. chumphonense from some lineages would be extremely difficult. The new species is morphometrically very similar to a lineage informally labelled Prodistomum “type 4” infecting Scomber japonicus in Japan, sharing both the anterior extent of the vitellarium and the termination level of the excretory vesicle. Yet molecular phylogenetic analyses based on 18S, ITS2 and 28S rDNA sequences, published previously by Suwannarat and colleagues, demonstrate substantial genetic divergence between these lineages, supporting the recognition of P. chumphonense as a distinct species. Interestingly, the new species appears genetically closer to Opechonoides opisthoporus than to some morphologically similar congeners, but O. opisthoporus differs markedly in possessing oblique testes, a uterus overlapping the testes and a long excretory vesicle containing refractile corpuscles, confirming that the new worm clearly belongs within Prodistomum despite its phylogenetic affinities.

The findings highlight a broader problem in trematode taxonomy: several specimens identified or assigned to P. orientale exhibit considerable morphometric variation, complicating differentiation from closely related taxa. As Cribb and colleagues have noted, it is highly improbable that all records attributed to P. orientale represent a single, widely distributed species. The reported host range is exceptionally broad, encompassing scombrid species across extensive geographic regions. Although such a distribution could partly reflect the migratory behaviour and wide dispersal capacity of pelagic hosts such as Rastrelliger and Scomber — highly migratory taxa undertaking extensive regional movements associated with spawning and feeding — host mobility alone does not necessarily imply genetic continuity among parasite populations. Without molecular data, it remains difficult to determine whether the wide distribution attributed to P. orientale represents a single polymorphic species or multiple independently evolving lineages.

The lack of molecular data associated with many historical morphometric descriptions further obscures taxonomic boundaries within this group, with no molecular information available for records from Scomber colias, S. scombrus and Sardinella maderensis. Consequently, it remains unclear whether published records attributed to P. orientale represent a single phenotypically variable species or a complex of undescribed taxa with overlapping morphological traits. Integrative approaches combining detailed morphometrics with multilocus molecular data are therefore essential to clarify species limits within Prodistomum and to resolve potential cryptic diversity, a phenomenon well documented across trematode families.

For the researchers, the combined morphological and molecular evidence unequivocally supports the recognition of P. chumphonense as a distinct species parasitizing Rastrelliger mackerel in the Gulf of Thailand. The holotype and paratypes have been deposited in the Museum of Natural Sciences of Barcelona, and the specific epithet chumphonense honours Chumphon Province, where the type material was collected. The discovery underscores how much remains to be learned about the parasite fauna of commercially important fish in tropical waters — and how molecular tools are transforming our ability to detect species that morphology alone would never reveal.

Subject of Research: Parasitology

Article Title: A new species of Prodistomum ( Digenea , Lepocreadiidae ) from Rastrelliger spp. ( Scombridae ) in the Gulf of Thailand

Article References: Gilardoni, C., Ribas, A., Suwannarat, N., & Poonlaphdecha, S. (2026). A new species of Prodistomum (Digenea, Lepocreadiidae) from Rastrelliger spp. (Scombridae) in the Gulf of Thailand. Zoosystematics and Evolution, 102(3), 845-852. https://doi.org/10.3897/zse.102.190596

Image Credits: AI Generated

DOI: 10.3897/zse.102.190596

Keywords: Prodistomum chumphonense, Digenea, Lepocreadiidae, Trematoda, Rastrelliger, Scombridae, Gulf of Thailand, integrative taxonomy, molecular phylogenetics, cryptic species, species, Prodistomum