Myxobolus franciscoi sp . nov . ( Myxozoa : Myxosporea : Myxobolidae ) , a parasite of Prochilodus argenteus ( Actinopterygii : Prochilodontidae ) from the Upper São Francisco River , Brazil , with a revision of Myxobolus spp . from South America

Myxobolus franciscoi sp. nov. (Myxozoa: Myxosporea: Myxobolidae) is described from the “curimatá-pacú” fish, Prochilodus argenteus Spix & Agassiz, 1829 (Actinopterygii: Prochilodontidae) from the Upper São Francisco River, Brazil. This parasite forms whitish plasmodia (about 1 x 1 mm) that develop in the connective tissue of fins. The spores are more or less round in frontal view and ellipsoidal in lateral view, measuring 6.4 (6.0-6.9) μm in length, by 6.0 (5.86.4) μm in width and 3.2 μm in thickness. The polar capsules are very small, measuring about 2 μm in length by 1.5 μm in width and ending in a tapered anterior neck. The polar filament makes three turns in a plane at right angles with the longitudinal axis of the spore. Thorough comparisons with the remaining species of Myxobolus Bütschli, 1882 described from South American fish, as well as with almost all species of Myxobolus described so far, are provided. This paper also includes a revision of Myxobolus species from South American fish hosts.

ZOOLOGIA 27 (1): 131-137, February, 2010 checking the presence of an iodinophilous vacuole, a drop of Lugol solution was added to the spores.Infected fins were routinely processed for histology and stained with Haematoxylin and Eosin, and Masson's Trichrome.Syntypes of Myxobolus franciscoi sp.nov.were sent to deposit in the collection of the Museu de Zoologia of the Universidade Estadual de Campinas (UNICAMP), State of São Paulo, Brazil.Voucher specimens of P. argenteus were deposited in the collection of the Museu de Zoologia of the Universidade de São Paulo (MZUSP 95167).Scientific names and their authorship followed the database Fishbase (FROESE & PAULY 2009).

Figs 1-4
Description.This parasite forms whitish plasmodia in the connective tissue of fins.The plasmodia were more or less rectangular or slightly elongated, with rounded ends, measuring about 1 x 1 mm (Fig. 1).The plasmodia occurred singly or in small groups, and were clearly visible to the naked eye.Histologically, the plasmodia were located in the connective tissue, immediately under the dermis, and were surrounded by a dense ± 5 µm thick layer of fibres from the host connective tissue.No membranous wall of parasitic origin was observed.Most of the central part of the plasmodium was occupied by fully mature spores, while in the periphery initial stages of development were observed (Fig. 2).These stages were composed by round, elliptical or slightly irregular cells, with a prominent nucleus, measuring about 8-10 µm in diameter.The spores (Figs 3 and  4) were almost round in frontal view, and ellipsoidal in lateral view, very small in size, measuring 6.4 (6.0-6.9)µm in length by 6.0 (5.8-6.4)µm in width and 3.2 µm in thickness.The spore length/width ratio varied between 1.03 and 1.06, and the spore length/polar capsule length ratio varied between 3.0 and 3.45.The spore walls were smooth, the valves symmetrical, and an iodinophilous vacuole was not present.The polar capsules were very small and equal in size, not reaching the middle part of the spore, more or less rounded, measuring about 2 µm in length by 1.5 µm in width and ending in a tapered anterior neck.The polar filament formed three turns in a plane at right angles to the longitudinal axis of the spore.
In some specimens the gills were infected with an unidentified species of Henneguya Thélohan, 1892.
Etymology: the specific name derives from the São Francisco River.
Remarks.The most striking feature of M. franciscoi in our sampling is the unusually small size of the spores.Our species is morphologically and morphometrically distinct from all other species of Myxobolus described parasitizing fish from South America (Tabs I and II), with two exceptions: M. chondrophilus Nemeczeck, 1926 andM. porofilus Adriano, Arana, Ceccarelli &Cordeiro, 2002.The dimensions of the spores are similar in M. franciscoi and M. chondrophilus, but specimens of the latter are thinner (4.5 µm thick) and have bigger polar capsules (3 µm in length).Also, the host of M. chondrophilus is a marine fish (Sardinella anchovina (sic) (= Sardinella aurita Valenciennes, 1847) and the parasites occur only in the gills (NEMECZEK 1926).These differences are sufficient to justify the separation of these two species.Myxobolus porofilus, infecting a host, Prochilodus lineatus (Valenciennes, 1837), that is closely related to the host of M. franciscoi, has even smaller spores (5.7 by 4.8 µm) and smaller polar capsules than our species.Besides, unlike M. franciscoi, M. porofilus infects the visceral cavity of the host (ADRIANO et al. 2002).Therefore we cannot consider the two forms to be identical.
We also compared the present material with all the 744 nominal species of Myxobolus included in the synopsis by EIRAS et al. (2005a).The first conclusion to be drawn from this comparison is that our specimens can be compared with only a few species due to the small size of the spores.In fact, from the 744 species referred to in that contribution, five have similar spore dimensions: Myxobolus artus Akhmerov, 1960 described infecting the kidneys of Carassius auratus gibelio (= Carassius gibelio (Bloch, 1782)) from the Amur Basin (AKHMEROV 1960), M. calcariferum Basu & Haldar 2003 described from the gills of Lates calcarifer (Bloch, 1790) from India (BASU & HALDAR 2003), M. dermatobius (Ishii, 1915) parasitizing the integument of Anguilla japonica Temminck & Schlegel, 1846 from Japan (ISHII 1915, LANDSBERG & LOM 1991), M. minutus Nemeczek, 1911 located in the gills of Leuciscus leuciscus (Linnaeus, 1758) and L. cephalus (= Squalius cephalus (Linnaeus, 1758) from Germany (NEMECZEK 1911), and M. saranai Landsberg & Lom, 1991 occurring in the gills of Barbus sarana (= Punctius sarana (Hamilton, 1822) from India (TRIPATHI 1952, LANDSBERG & LOM 1991).In general, our specimens are very different from all of these species in several morphometrical and morphological features.Myxobolus artus has considerably wider spores (9 µm) and larger polar capsules (4 x 1.8-2 µm); M. calcariferum has slightly larger spores and considerably larger polar capsules (3.8-4.5 x 2.0-2.7 µm); M. dermatobius has clearly narrower smaller spores in width (4.2-4.9 µm) and larger polar capsules; M. minutus presents smaller spore width values (4.2-5 µm) and larger polar capsules.Finally, M. saranai has spores not so wide (4.5-5.0 µm) and larger polar capsules.Furthermore, these species infect different organs other than the fins, and their hosts are phylogenetically and geographically unrelated to P. argenteus.For these reasons our material cannot be definitely be identified with any of these previously described species.Myxobolus inaequalis Gurley, 1893 was described from the head integument of Pimelodus clarias (Bloch, 1782) from Guiana and Surinam.Only the length (11 µm) and width (7 µm) of the spores were initially described for this species, which has polar capsules of unequal size (GURLEY 1893).Later, spores of 5.  M. franciscoi 6.4 (6.0-6.9)6.0 (5.8-6.4)3.2 2.0 1.5 3 * The measurements indicated relate spores stained in smears; values for fresh spores: SL: 25.2 (25.0-25.3);SW: 15.4 (15.0-15.5).** The parasites were observed in the connective tissue of several organs: gall bladder, urinary bladder, gills, spleen, fins, head surface, liver and heart.*** A small portion of the spores were oval in shape (the majority were elongated ellipsoidal) and measured 105 (10.3-10.9) in length and 8.0 (7.2-8.5) in width.The parasites were found in the connective tissue in gills, heart, serous membranes of the intestine, liver parenchyma and skeletal muscle.**** Different spores in shape (pyriform against oval) and size were described: 8.3 (7.0-9.5) in length and 4.0 (3.5-5.0) in width, with polar capsules being 5.8 (5.0-7.5) in length and 1.5 (1.0-2.0) in width.(1969).Clearly there are significant differences between the different descriptions reported, which are all very poorly detailed, and for these reasons M. inaequalis is considered a species inquirenda.Therefore, we consider that there are currently 29 valid species of Myxobolus parasitic on South American fishes whose diagnostic features are given in tables I and II.

ACKNOWLEDGMENTS
Thanks to Yoshimi Sato (leader of the EPT/CODEVASF, Três Marias, Minas Gerais) CEMIG-GT/CODEVASF working arrangement and UFRRJ/IBAMA, Minas Gerais, technical-scientific agreement co-operation for providing logistical and material support.Also to Osvaldo T. Oyakawa for his kindness in Therefore we believe that the present material represents an undescribed species and propose to name it Myxobolus franciscoi sp.nov., a name relating to the São Francisco River.There are 30 nominal species of Myxobolus described for South American hosts.With the exception of two species, Myxobolus magellanicus Szidat, 1893 and Myxobolus paranensis Bonetto & Pignalberi, 1965, described from Argentinean hosts -Galaxias maculatus (Jennyns, 1842) and Salminus maxillosus (= Salminus brasiliensis (Cuvier, 1816) -, widespread in hosts distributed in other South American countries, all species were described from Brazilian fishes.While most species have freshwater hosts,, two species, -M.chondrophilus (NEMECZEK 1911) and M. platanus (EIRAS et al. 2007) -were described from the saltwater fishes S. anchovina (sic) and Mugil platanus (Günther, 1880), respectively.A species misidentified as Myxobolus cerebralis Hofer, 1903 which is parasite of salmonids, was reported from Mugil brasiliensis (= Mugil liza Valenciennes, 1836) by MENDES (1980).
Figures 1-4.Myxobolus franciscoi sp.nov. of Prochilodus argenteus: (1) plasmodia in the fin; (2) histological section of the plasmodium, parcial view; the centre of the plasmodium is filled with mature spores while at the periphery there are developmental stages.Insert: total view of the plasmodium.Masson's Trichrome; (3) spores; (4) schematic drawing of the spores in frontal view.Bar = 3 µm.

Table I .
Features (measurements in µm) of the valid Myxobolus species described from South American fishes.(SL) Body spore length, (SW) body spore width, (ST) body spore thickness, (LPC) length of the polar capsules, (WPC) width of the polar capsules, (NC) number of coils of the polar filament.

Table II .
References for features listed in table I.