A new species of Diaphorocleidus ( Monogenea : Ancyrocephalinae ) from the gills of Argonectes robertsi ( Characiformes ) and new records of dactylogyrids parasitic on fishes from the Xingu River , Amazon Basin , Brazil

Diaphorocleidus altamirensis sp. nov., parasitic on the gills of Argonectes robertsi Langeani, 1999 from the Xingu River, northern Brazil, is described. The new species differs from its six congeners by the morphology of the male copulatory organ (which comprises a coil of six rings), by the midventral vagina, and by the presence of only one pair of eyespots. It is the first species of Diaphorocleidus Jogunoori, Kritsky & Venkatanarasaiah, 2004 described from hemiodontid fishes. In addition, new host and geographical records of seven species of dactylogyrids found on fish from the Xingu River are reported.

Monogeneans represent one of the main components of the parasite fauna of freshwater fishes in the Neotropical Region, and as many as 629 species have been reported by Cohen et al. (2013) from South America, most of which belonging to the Dactylogyridae and parasitizing on the gills of teleost fishes.However, the number of species of these ectoparasites has increased steadily, with descriptions of dozens new species in recent decades, especially from characiform fishes (Cohen et al. 2012, Moreira et al. 2015).
Examination of fishes from the lower reach of the Xingu River near Altamira, a tributary of the Amazon River in Brazil (State of Pará), has revealed dactylogyrid monogeneans on the gills of several fishes, including Argonectes robertsi Langeani, 1999 (Characiformes: Hemiodontidae).This fish occurs in the Tapajós,

TAXONOMY AND NOMENCLATURE
A new species of Diaphorocleidus (Monogenea: Ancyrocephalinae) from the gills of Argonectes robertsi (Characiformes) and new records of dactylogyrids parasitic on fishes from the Xingu River, Amazon Basin, Brazil  Herein, we describe a new species of Diaphorocleidus collected from A. robertsi from the Xingu River in Brazil.In addition, several new host and geographical records of dactylogyrids parasitizing fishes from the Xingu River are provided.

MATERIAL AND METHODS
Fish were collected by local fishermen with nets and hooks in April 2013 from the Xingu River, a tributary of the Amazon River, around Altamira (3°12'S, 52°12'W), state of Pará, Brazil.Gills excised from fish were placed in Petri dishes with tap water and examined for monogeneans using dissecting microscope.Some monogeneans found were fixed in a mixture of glycerin-ammonium picrate (GAP) to study sclerotized structures.After morphological evaluation, the specimens were remounted, dehydrated and mounted in Canada balsam following the procedure of ergens (1969).Other specimens were fixed in 4% hot formalin, stained with Gomori's trichrome and mounted in Canada balsam to study internal organs; some worms were mounted in Gray and Wess medium (huMason 1979) to study sclerotized structures.Drawings were made with the aid of an Olympus BX53 microscope (Olympus Corporation, Tokyo, Japan) equipped with a drawing tube.Measurements, all in micrometers, represent straight-line distances between extreme points and are expressed as the range followed by the mean and number (n) of structures measured in parentheses; body length includes that of the haptor.Numbering (distribution) of hook pairs follows Mizelle (1936) and Mizelle & PriCe (1963).Type and voucher specimens were deposited in the Helminthological Collection of Institute Oswaldo Cruz (CHIOC), Rio de Janeiro, Brazil.

TAXONOMY
Dactylogyridae Bychowsky, 1933 Ancyrocephalinae Bychowsky, 1937 Diaphorocleidus altamirensis sp.nov.New host and geographical records.In addition to the new species described above, another seven species of gill monogeneof the family Dactylogyridae were found on four species of fishes during parasitological surveys carried out in the Xingu River.All fishes represent new hosts of these monogeneans and except for Jainus amazonensis Kritsky, Thatcher & Kayton, 1980, all parasites are reported from the Xingu River, Amazon basin, for the first time (Table 1).

DISCUSSION
The present study provides the first data on the monogeneans parasites of A. robertsi.Currently the family Hemiodontidae comprises five genera and 31 species of fishes distributed in South America (Froese & Pauly 2015).They are poorly studied fishes and only very few species were examined for parasites (azevedo et al. 2009, Cohen et al. 2013, silva Júnior 2014), indicating that many new taxa of parasites can be discovered.
Species of Diaphorocleidus have been recorded from characiform fishes, mainly characids, in a large area which spans from Trinidad through Panama to Brazil, i.e., the Caribbean Region, Central America and South America.Most dactylogyrid monogeneans, including those of Ancyrocephalinae, are considered highly host-specific (Boeger & vianna 2006).Therefore, such large distribution area is uncommon in species-poor Neotropical genera.However no information exists on interrelations of species of Diaphorocleidus to reconstruct possible evolutionary history of this monogenean group in the Neotropical region.
Diaphorocleidus altamirensis sp.nov. is the third species of the genus described in Brazil, but is the first species found on a hemiodontid, which also belongs to the order Characiformes.Diaphorocleidus can be easily distinguished from the other genera of dactylogyrids associated with hemiodontids, i.e., Cleidodiscus Mueller, 1934 andMonocleithrium Price &McMahon, 1966.Monocleithrium is a monospecific genus, with M. lavergneae as its type and only species, which is characterized by having two pairs of eyespots, tandem gonads, coiled MCO with accessory piece directly articulated, and dorsal bar absent.In contrast, Diaphorocleidus exhibits overlapping gonads, a MCO unarticulated to accessory piece and a pair of bars (one ventral and one dorsal).Cleidodiscus is a poorly known genus of ancyrocephaline monogeneans and its taxonomic status remains unclear, especially because inadequate descriptions of its species.Many of the species originally described as members of Based on the study and other recent accounts on the monogenean parasites of fishes from the Xingu River (Moreira et al. 2015, PasChoal et al. 2016), we assume that the actual diversity of monogeneans in the Amazon River basin and South America in general is still poorly known and discovering many new taxa, especially on poorly studied fish hosts, can be anticipated.

Table 1 .
New host and geographical records of dactylogyrids parasitizing fish from the Xingu River, Amazon basin, Brazil.