The genus includes more than 80 species, which are all parasitic on the gills of marine fish, especially groupers. Since groupers are mostly abundant in warm waters, species of Pseudorhabdosynochus are found in warm parts of the oceans, especially in coral reefs, but species have also been found in deep-sea groupers. Several species of Pseudorhabdosynochus are pathogens of maricultured groupers.
Female (left) and male (right) sclerotized organs of a species of Pseudorhabdosynochus, P. caledonicus
A species of Pseudorhabdosynochus with captions for main parts and organs
All species of Pseudorhabdosynochus are small animals, ranging 0.3–1 mm in length.
As most monogeneans, they are flat, with an anterior head bearing four oculi and head glands, a main elongate body and a posterior haptor. The digestive system includes an anterior muscular pharynx, and two lateral intestinal branches (or caeca); as in all Platyhelminthes, there is no anus. The haptor, in the posterior part of the body, is a specialized organ used to attach to the host. The haptor includes sclerotized elements, namely a ventral bar, two lateral (dorsal) bars, two ventral hooks and two dorsal hooks, and fourteen hooklets. As in most diplectanids, the haptor bears special, characteristic, structures called squamodiscs. The squamodiscs (one ventral and one dorsal) of species of Pseudorhabdosynochus are made up of numerous rodlets aligned as concentric rows. All species of the genus have two squamodiscs except Pseudorhabdosynochus sinediscus Neifar & Euzet, 2007 in which these organs are completely lacking.
Adults are hermaphroditic. The reproductive organ include a single ovary and a single testis. As in all diplectanids, the ovary (or germarium) is anterior to the testis and loops around the right intestinal caecum.
Species of Pseudorhabdosynochus are characterized by a sclerotized male copulatory organ, or "quadriloculate organ", which has the shape of a bean with four internal chambers. The vagina includes a sclerotized part, which is a complex structure with several chambers and canals, generally used for the diagnosis and differentiation of species.
The origin of the name Pseudorhabdosynochus merits an explanation. ‘Pseudo-’ (from Greek ψευδής, pseudes, "lying, false") is a prefix commonly used in creating a new scientific name for a taxon that superficially appears to be a taxon, but actually is another. The famous Japanese parasitologist Satyu Yamaguti described a ‘membranous plaque’ on the posterior region of Pseudorhabdosynochus epinepheli and created the generic name Pseudorhabdosynochus in reference to the diplectanid genus Rhabdosynochus which, in his interpretation, also had lateral plaques. It has been shown later that ‘plaques’ in P. epinepheli was an erroneous interpretation of damaged specimens. Moreover, the diagnosis of Rhabdosynochus was later modified and it is now considered that species of this genus do not have plaques. Therefore, the genus name Pseudorhabdosynochus is the result of both an erroneous observation and an erroneous comparison by Satyu Yamaguti. However, it is valid according to the ICZN.
Pseudorhabdosynochus is also an interesting nomenclatural case. Yamaguti described the same species twice, the first time in 1938
(as Diplectanum epinepheli Yamaguti, 1938) and the second time in 1958
(as Pseudorhabdosynochus epinepheli Yamaguti, 1958). In both papers, the species was described from parasites collected from the same fish species, the grouper Epinephelus akaara off Japan. Kritsky & Beverley-Burton (1986)
solved this case with reference to the International Code of Zoological Nomenclature. The type-species of the genus is the namePseudorhabdosynochus epinepheli Yamaguti, 1958. Pseudorhabdosynochus epinepheli Yamaguti, 1958 is a junior synonym of Pseudorhabdosynochus epinepheli (Yamaguti, 1938) Kritsky & Beverley-Burton, 1986 and Pseudorhabdosynochus epinepheli (Yamaguti, 1938) Kritsky & Beverley-Burton, 1986 is the modern valid combination for Diplectanum epinepheli Yamaguti, 1938. The latter species (a taxon) and the type-species of the genus (a name) are considered to correspond to the same biological taxon but have different, although valid, names. Recent re-examination of type-specimens have confirmed this interpretation.
As most diplectanids, species of Pseudorhabdosynochus are generally strictly species-specific, i.e. a species is found on a single species of host and no others. However, several exceptions have been reported. Some species of groupers harbour a wide biodiversity of Pseudorhabdosynochus species (and sometimes other monogeneans as well). Examples are Epinephelus maculatus which has 8 distinct Pseudorhabdosynochus species, and Epinephelus malabaricus which has 7 species.
The life cycle of Pseudorhabdosynochus species is similar to that of other diplectanid monogeneans. It is monoxenous, i.e. there is a single host, the fish. The life-cycle includes a parasitic phase, the adult worm, and two stages which are free in the environment, the egg and the larva. The adult hermaphroditic worm, which lives on the gills of fish, lays eggs in the water. The egg has an operculum. A larva develops in the egg and hatches through the operculum; the swimming larva is ciliated and called an oncomiracidium. The larva finds its host by swimming to it, first attaches to the skin and migrates to the gills, and eventually transforms itself into a young monogenean which is attached to the gill filament by its haptor. During transformation from larva to adult, the monogenean loses its cilia and produces a new tegument, a phenomenon which is considered a general characteristic of the parasitic Platyhelminthes or Neodermata.
Information is available for the life-cycle of Pseudorhabdosynochus lantauensis, a parasite of the orange-spotted grouper Epinephelus coioides, at a temperature of 30 °C. The adult lays 10-22 eggs/day. The eggs hatch within 2–4 days. The oncomiracidium is viable for 4-8 h and should find a host within this delay. After attachment, the oncomiracidium transforms into an adult in 4–7 days. The parasite produces eggs after 7 days. The life-cycle is thus complete in 13–20 days (at 30 °C).
Certain species of Pseudorhabdosynochus parasitize groupers which show spawning aggregations, i.e. the adult fish gather annually at a precise location and meet other fish of their own species only once per year. It has been suggested that transmission of Pseudorhabdosynochus species occurs preferentially during these spawning aggregations, from older infected fish to younger, uninfected, fish.
Pathology and control in mariculture
Species of Pseudorhabdosynochus are attached to the gills of groupers. No information is available on the pathogen effect of these parasites in wild groupers. Since the life-cycle is monoxenous and thus can be completed with a high rate of success in mariculture condition, species of Pseudorhabdosynochus can proliferate on gills of cultured groupers and become dangerous for the fish, inducing mortality.
Lists of species have been provided by Kritsky & Beverley-Burton (1986) and Justine (2007).
The list below (about 80 species) is based on the World Register of Marine Organisms with additions.
^ abcJustine, Jean-Lou (2009). "A redescription of Pseudorhabdosynochus epinepheli (Yamaguti, 1938), the type-species of Pseudorhabdosynochus Yamaguti, 1958 (Monogenea: Diplectanidae), and the description of P. satyui n. sp. from Epinephelus akaara off Japan". Systematic Parasitology. 72 (1): 27–55. doi:10.1007/s11230-008-9171-5. PMID19048406. S2CID9467410.
^Kritsky, D. C., Boeger, W. A. & Robaldo, R. B. 2001: Neotropical Monogenoidea. 38. Revision of Rhabdosynochus Mizelle and Blatz, 1941 (Polyonchoinea: Dactylogyridea: Diplectanidae), with descriptions of two new species from Brazil. Comparative Parasitology, 68, 66-75.
^ abYamaguti, S (1938). "Studies on the helminth fauna of Japan. Part 24. Trematodes of fishes, V". Japanese Journal of Zoology. 8: 15–74.
^ abcdefgMendoza-Franco, Edgar F.; Violante-González, Juan; Rojas Herrera, Agustín A. (2011). "Six new and one previously described species of Pseudorhabdosynochus (Monogenoidea, Diplectanidae) infecting the gills of groupers (Perciformes, Serranidae) from the Pacific coasts of Mexico and Panama". Journal of Parasitology. 97 (1): 20–35. doi:10.1645/GE-2716.1. ISSN0022-3395. PMID21348602. S2CID207251069.
^ abcdefghijJustine, Jean-Lou (2007). "Parasite biodiversity in a coral reef fish: twelve species of monogeneans on the gills of the grouper Epinephelus maculatus (Perciformes: Serranidae) off New Caledonia, with a description of eight new species of Pseudorhabdosynochus (Monogenea: Diplectanidae)". Systematic Parasitology. 66 (2): 81–129. doi:10.1007/s11230-006-9057-3. PMID16972153. S2CID7737882.
^Ehlers, U. (1985). Das Phylogenetische System der Plathelminthes. Stuttgart: G. Fischer.
^ abErazo-Pagador, G.; Cruz-Lacierda, E. R. (2010). "The morphology and life cycle of the gill monogenean (Pseudorhabdosynochus lantauensis) on orange-spotted grouper (Epinephelus coioides) cultured in the Philippines". Bulletin of the European Association of Fish Pathologists. 30: 55–64.
^ abcdeSigura, A.; Justine, J.-L. (2008). "Monogeneans of the speckled blue grouper, Epinephelus cyanopodus (Perciformes, Serranidae), from off New Caledonia, with a description of four new species of Pseudorhabdosynochus and one new species of Laticola (Monogenea: Diplectanidae), and evidence of monogenean faunal changes according to the size of fish". Zootaxa. 1695: 1–44. doi:10.11646/zootaxa.1695.1.1.
^Ruangpan, L. & Tubkaew, R. (1993). Parasites of the cage cultured grouper Epinephelus malabaricus in Thailand. Paper presented at the Proceedings of Grouper Culture, Kaoseng, Songkhla, Thailand.
^Rückert, Sonja; Klimpel, Sven; Al-Quraishy, Saleh; Mehlhorn, Heinz; Palm, Harry W. (2008). "Transmission of fish parasites into grouper mariculture (Serranidae: Epinephelus coioides (Hamilton, 1822)) in Lampung Bay, Indonesia". Parasitology Research. 104 (3): 523–532. doi:10.1007/s00436-008-1226-7. ISSN0932-0113. PMID18855012. S2CID15956362.
^ abVidal-Martínez, V. M., Aguirre-Macedo, L. & Mendoza-Franco, E. F. 1997: Pseudorhabdosynochus yucatanensis sp. n. (Monogenea: Diplectanidae) from the gills of the red grouper Epinephelus morio (Pisces: Serranidae) of the Yucatan Peninsula, Mexico" Folia Parasitologica 44, 274-278. PDF
^ abJustine, Jean-Lou (2007). "Pseudorhabdosynochus argus n. sp. (Monogenea: Diplectanidae) from Cephalopholis argus, P. minutus n. sp. and Diplectanum nanus n. sp. from C. sonnerati and other monogeneans from Cephalopholis spp. (Perciformes: Serranidae) off Australia and New Caledonia". Systematic Parasitology. 68 (3): 195–215. doi:10.1007/s11230-007-9096-4. ISSN0165-5752. PMID17896188. S2CID23506802.
^Sigura, Aude; Chauvet, Claude; Justine, Jean-Lou (2007). "Pseudorhabdosynochus bacchus sp. nov. (Monogenea, Diplectanidae) from Epinephelus coeruleopunctatus (Perciformes, Serranidae) off New Caledonia". Acta Parasitologica. 52 (3). doi:10.2478/s11686-007-0028-x. ISSN1896-1851. S2CID21628540.
^ abOliver, G. & Paperna, I. 1984: Diplectanidae Bychowsky, 1957 (Monogenea, Monopisthocotylea), parasites de Perciformes de Méditerranée orientale, de la mer Rouge et de l'océan Indien. Bulletin du Muséum National d'Histoire Naturelle, Paris, 4° série, 6, section A, 49-65.
^ abHinsinger, Damien D.; Justine, Jean-Lou (2006). "The 'Pseudorhabdosynochus cupatus group' (Monogenea: Diplectanidae) on Epinephelus fasciatus, E. howlandi, E. rivulatus and E. merra (Perciformes: Serranidae) off New Caledonia, with descriptions of Pseudorhabdosynochus cyathus n. sp. and P. calathus n. sp". Systematic Parasitology. 64 (2): 69–90. doi:10.1007/s11230-005-9018-2. ISSN0165-5752. PMID16773473. S2CID26881495.
^Justine, Jean-Lou (2005). "Species of Pseudorhabdosynochus Yamaguti, 1958 (Monogenea: Diplectanidae) from Epinephelus fasciatus and E. merra (Perciformes: Serranidae) off New Caledonia and other parts of the Indo-Pacific Ocean, with a comparison of measurements of specimens prepared using different methods, and a description of P. caledonicus n. sp". Systematic Parasitology. 62 (1): 1–37. doi:10.1007/s11230-005-5480-0. ISSN0165-5752. PMID16132868. S2CID35119181.
^Vidal-Martínez, V. M. & Mendoza-Franco, E. F. 1998: Pseudorhabdosynochus capurroi sp. n. (Monogenea: Diplectanidae) from the gills of Mycteroperca bonaci (Pisces: Serranidae) of the Yucatan Peninsula, Mexico" Folia Parasitologica 45, 221-224. Article PDF
^Zhang, J. Y., Yang, T. B. & Liu, L. (2001). [Monogeneans of Chinese marine fishes] (in Chinese). Beijing: Agriculture Press.
^Bu, S. S. H.; Leong, T. S.; Wong, S. Y.; Woo, Y. S. N.; Foo, R. W. T. (1999). "Three diplectanid monogeneans from marine finfish (Epinephelus spp.) in the Far East". Journal of Helminthology. 73 (4): 301–312. doi:10.1017/S0022149X99000505.
^Zeng, Bijian; Yang, Tingbao (2006). "Description of Pseudorhabdosynochus justinei n. sp. (Monogenea: Diplectanidae) and redescription of P. vagampullum (Young, 1969) Kritsky & Beverley-Burton, 1986 from the gills of the longfin grouper Epinephelus quoyanus (Valenciennes) (Perciformes: Serranidae) in Dapeng Bay, South China Sea". Systematic Parasitology. 66 (3): 223–235. doi:10.1007/s11230-006-9067-1. ISSN0165-5752. PMID17143574. S2CID29694108.
^Beverley-Burton, Mary; Suriano, Delia Mabel (1981). "A revision of Cycloplectanum Oliver, 1968 (Monogenea: Diplectanidae) and descriptions of C.hongkongensis n.sp. and C.lantauensis n.sp. from Epinephelus spp. (Serranidae) in the South China Sea". Canadian Journal of Zoology. 59 (7): 1276–1285. doi:10.1139/z81-180. ISSN0008-4301.
^Aljoshkina, L. D. (1984). "[New species of the family Diplectanidae (Monogenea) in the South-West Atlantic]". Zoologicheskii Zhurnal. 63: 1253–1256.
^Laird, Marshall (1958). "Parasites of South Pacific fishes II. Diplectanum melanesiensis n. sp., a monogenetic trematode from Fiji and the New Hebrides". Canadian Journal of Zoology. 36 (2): 167–173. doi:10.1139/z58-017. ISSN0008-4301.
^Justine, Jean-Lou; Vignon, Matthias (2008). "Monogeneans of the grouper Epinephelus tauvina (Perciformes, Serranidae) off Moorea, French Polynesia, with a description of Pseudorhabdosynochus pai n. sp. (Monogenea: Diplectanidae)". Systematic Parasitology. 72 (2): 113–125. doi:10.1007/s11230-008-9159-1. ISSN0165-5752. PMID19115085. S2CID7679605.
^Oliver, Guy (1986). "Cycloplectanum riouxi n. sp., une nouvelle espèce de Diplectanidae (Monogenea, Monopisthocotylea) parasite d'Epinephelus guaza (Linnaeus, 1758) (Pisces, Serranidae)". Systematic Parasitology. 8 (4): 317–322. doi:10.1007/BF00009739. ISSN0165-5752. S2CID39530925.
^Yang, Tingbao; Zeng, Bijian; Gibson, David I. (2005). "Description of Pseudorhabdosynochus shenzhenensis n. sp. (Monogenea: Diplectanidae) and redescription of P. serrani Yamaguti, 1953 from Epinephelus coioides off Dapeng Bay, Shenzhen, China". Journal of Parasitology. 91 (4): 808–813. doi:10.1645/GE-518R.1. ISSN0022-3395. PMID17089747. S2CID19512689.
^Yang, Tingbao; Gibson, David I.; Zeng, Bijian (2005). "Pseudorhabdosynochus summanoides n. sp. (Monogenea: Diplectanidae) from Epinephelus coioides in Dapeng Bay, South China Sea, with observations on several similar species of Pseudorhabdosynochus Yamaguti, 1958". Systematic Parasitology. 62 (3): 221–239. doi:10.1007/s11230-005-5497-4. ISSN0165-5752. PMID16315082. S2CID7907556.