You have a Pseudobacciger harengulae (Yamaguti, 1938) – a parasitic digenean fluke
Synonyms: Bacciger harengulae Yamaguti, 1938; ?Pseudobacciger manteri Nahhas & Cable, 1964 (see Rahimian & Thulin, 2003).
Common names: None known
These species are parasitic on the gills of eels.
The adults are less than 0.5 mm long (Dimitrov et al., 1999), possibly up to 1.1 mm (Rahimian & Thulin, 2003), and hence it requires a good microscope to identify them. The body is oval and completely covered with short pointed spines. It has an oral and a ventral sucker, both on the ventral side and also covered with spines. The ventral sucker is located just anterior to the midline of the animal and can change shape from flat to cup-shaped or almost spherical.The oral sucker is smaller than the ventral one and can be retracted. In the posterior end is a small excretory pore connecting to the V-shaped excretory vesicle (Dimitrov et al., 1999; Rahimian & Thulin, 2003). In the Northeast Atlantic two related species occur: Bacciger bacciger (Rudolphi, 1819), which parasitizes sand smelt (Atherina presbyter Cuvier, 1829) and Pronoprymna ventricosa (Rudolphi, 1819), which parasitizes Allis shad, Alosa alosa (Linnaeus, 1758), Twaite shad, A. fallax (Lacepede, 1803) and Sprattus sprattus (Linnaeus, 1758). The latter species is more elongate in body form and has a smooth surface (Bray & Gibson, 1980), whereas the former seems to be very similar. It has a cirrus-sac, an appendage to the copulatory organ?
Unfortunately there are no good illustrations of this species. SEM-pictures can be seen in Rahimian & Thulin (2003) and simple line drawings are found in Dimitrov et al. (1999).
Native distribution: Japan and Korea
Introduced distribution: It was first found in Swedish herring in 1994 and initially identified as P. manteri (Rahimian & Thulin, 2003). It has also been described from Florida, Bimini and Jamaica (as P. manteri), India and Namibia (Rahimian & Thulin, 2003), and Dimitrov et al. (1999) reported it from the Black Sea. Thus it has a very scattered distribution, but mostly in tropical and subtropical waters.
First intermediate host: Tapes philippinarum and Meretrix lusoria (Dimitrov et al., 1999). It has been speculated that Ensis directus could have brought the parasite from Florida/ Caribbean to Sweden as this clam arrived in the Gullmarsfjord at about the same time as the parasite was found in herring (Rahimian, 2007). Second intermediate host Penaeus japonica and Palaemon carinicauda (Dimitrov et al., 1999), but apparently local shrimp species can also be used.
Bray, R.A. 1988. A discussion of the status of the subfamily Baccigerinae Yamaguti, 1958 (Digenea) and the constitution of the family Fellodistomidae Nicoll, 1909. Systematic Parasitology 11: 97-112.
Bray, R.A. and Gibson, D.I. 1980. The Fellodistomidae (Digenea) of fishes from the northeast Atlantic. Bulletin of the British Museum (Natural History) Zoology Series 37(4): 199-293.
Dimitrov, G.I., Bray, R.A. and Gibson, D.I. 1999. A redescription of Pseudobacciger harengulae (Yamaguti, 1938) (Digenea: Faustulidae) from Sprattus sprattus phalericus (Risso) and Engraulis encrasicholus ponticus Alexandrov off the Bulgarian Black Sea coast, with a review of the genus Pseudobacciger Nahhas & Cable, 1964. Systematic Parasitology 43: 133-146.
Rahimian, H. 2007. Parasites of fingerling herring Clupea harengus L.: ecology and fine morphology. Journal of Helminthology 81: 199-217.
Rahimian, H. and Thulin, J. 2003. Pseudobacciger harengulae from the Atlantic herring Clupea harengus: a new host and locality record. Journal of Helminthology 77: 69-75.