New species of Tereancistrum (Dactylogyridae) monogenean parasites of Schizodon borellii (Characiformes, Anostomidae) from Brazil, and emended diagnosis for T. parvus

Tereancistrum paranaensis sp. n. is described from the gills of Schizodon borellii (Boulenger 1900) (Characiformes) from the upper Paraná river floodplain, Brazil. The new species is mainly characterized by morphology of copulatory complex, dorsal anchor with shaft recurved and pointed and arc-shaped dorsal bar. Tereancistrum parvus was described based on only one specimen and some characteristics were not observed. Now we provide an emendation to the diagnosis of this species.

Tereancistrum was proposed by Kritsky et al. (1980)  In the present study, a new species of Tereancistrum is described from the gills of S. borellii from the upper Parana river floodplain, Brazil, and an emended diagnosis for T. parvus is also presented.

Material and methods
Sixteen specimens of S. borellii were collected from September 2006 to September 2007 using gill nets in the upper Paraná river floodplain (22º50' -22º70'S; 53º15' -53º40'W), South Brazil.Monogenea were removed from the gills under stereomicroscope, killed in a 1:4000 formalin solution and preserved in 5% formalin.Some specimens were mounted in Hoyer's medium to study the sclerotized structures.Other specimens, stained with Gomori's trichrome, were used to observe the internal organs (EIRAS et al., 2006).Measurements are in micrometres, with means followed by the range and number (n) of specimens measured in parentheses.Illustrations were prepared with the aid of a drawing tube connected to a Nikon YS 2 microscope.Ecological terminology is based on Bush et al. (1997).Numbering (distribution) of haptoral hook pairs follows Mizelle (1936) and the description of coiled tube of male copulatory organ follows Kritsky et al. (1985).Vouchers specimens were deposited in the Instituto Oswaldo Cruz Collection (CHIOC), Rio de Janeiro, Rio de Janeiro State, Brazil.Etymology: the specific name refers to the Paraná river, where the parasite was collected.

Remarks:
The new species closely resembles the other members of Tereancistrum by presenting a distinctly spatulate accessory sclerite associated to the ventral anchor.Tereancistrum paranaensis sp.n. is similar to T. kerri by dorsal bar, ventral anchor and vagina but the new species has copulatory complex with coiled and dorsal anchor e a very large superficial root and ventral bar Mshaped, these features not present in T. kerri Tereancistrum ornatus and T. curimba are morphologically closed to T. paranaensis sp.n. by accessory sclerite, copulatory complex and ventral bar, but differs from dorsal bar (Y-shaped) with ends modified.Dorsal anchor has widely divergent roots, short shaft and straight point are significantly different.Tereancistrum curimba and T. toksonum were also described in the floodplain of the upper Paraná river but T. toksonum differs from the new species by the dorsal bar Y-shaped; ventral bar formed by thin sclerotized membrane with thick posterior margin and dorsal anchor with long divergent roots, short shaft and straight point.Tereancistrum arcuatus differs from the new species by the MCO, which is an arcuate tube, while the Tereancistrum paranaensis sp.n. has a coiled copulatory organ.
Remarks: Despite being originally collected from the Amazon Basin, T. parvus was described parasitizing specimens of Leporinus fasciatus from the Steinhart Aquarium, San Francisco, in 1968(KRITSKY et al., 1980).This description was based on only one specimen.Thus, a detailed description is presented with information that was not presented in the original description, such as the position of the vagina, the egg morphology, testis, ovary and seminal vesicle.This study reports a new record of host and locality for the parasite.

Conclusion
The new species Tereancistrum paranaensis is mainly characterized by the morphology of the copulatory complex (male copulatory organ with coils of about 2.5 rings and accessory piece small, non-articulated to male copulatory organ base), the morphology of dorsal anchor and dorsal bar shaped U.For the species Tereancistrum parvus an emended diagnosis was made with information that was not presented in the original description as the position of the vagina, the egg morphology, testis, ovary and seminal vesicle.