New monopisthocotylan parasite found in Brazil’s Munim River Basin

A field survey of the cichlid Saxatilia brasiliensis in northeastern Brazil identified two species of monopisthocotylan fish parasites, including a new member of the genus Sciadicleithrum named Sciadicleithrum anapurusensis. Researchers examined 64 fish collected from two tributaries of the Munim River Basin in Maranhão state. They counted 274 parasites of the new species and 83 of the previously described Sciadicleithrum araguariensis. The study therefore documents a new taxon, extends the known host and geographic range of an existing parasite, and shows that the same cichlid species can carry two Sciadicleithrum species within one river system. The work was based on field collection and morphological examination; it did not test effects on host health, behavior, survival or reproduction.

The sampling covered Stream Feio at São José Village in the municipality of Chapadinha and Stream Estrela in the balneary district of Anapurus. Thirty-two S. brasiliensis were collected from each locality, with 16 fish taken during the dry season and 16 during the rainy season at each site. The fish were captured using nets 240 centimeters long and 100 centimeters high. Voucher specimens were deposited in the Ichthyological Collection of the Center for Agricultural and Environmental Sciences at the Federal University of Maranhão under numbers CICCAA07121–CICCAA07151. A recent fish inventory cited in the study recorded 123 fish species in the Munim River system, highlighting the substantial biological diversity of the basin and the incomplete characterization of its parasite fauna.

At the Federal University of Maranhão laboratory, investigators removed the branchial arches and placed them in vials containing hot water, approximately 65 °C, to relax and detach the monopisthocotylans. The vials were then shaken vigorously. After cooling, absolute ethanol was added to reach a concentration of 70 percent, and the liquid and branchial arches were examined under a stereomicroscope to recover parasites. For morphological study of the copulatory complex and haptoral sclerites, specimens were mounted in Hoyer’s medium. Characterization and identification took place at the Laboratory of Fish Parasite Helminths at the Oswaldo Cruz Institute in Rio de Janeiro. Holotype and paratype specimens of S. araguariensis were examined for comparison. Measurements were reported in micrometers, with means, ranges and sample sizes, while curved or bent structures were measured as the straight-line distance between their extreme ends.

The new species was assigned to Sciadicleithrum because its male copulatory organ consists of a spiral tube with clockwise rings, its gonads overlap, its testis lies dorsal to the germarium, and its hooks have similar shapes and sizes without dilated shanks. The male copulatory organ of S. anapurusensis contains two clockwise rings. Its haptor bears differentiated dorsal and ventral anchors, a yoke-shaped ventral bar with expanded ends and a delicate postero-medial umbiliform membrane, and a straight dorsal bar whose extremities point posteriorly. The description was based on 27 specimens mounted in Hoyer’s medium. Total body length, including the haptor, averaged 363 micrometers and ranged from 256 to 470 micrometers. Width at the level of the ovary averaged 138 micrometers, with a range of 108–195 micrometers.

Further details distinguish the new taxon from known species. Its elongated, fusiform body has a smooth tegument, three developed cephalic lobes, four pairs of head organs, sparse accessory granules and four equidistant eyespots. The pharynx averaged 32 micrometers in diameter, with a measured range of 28–38 micrometers, and the intestinal caeca joined behind the testis without diverticula. The male copulatory organ was 92–123 micrometers long and 15–22 micrometers wide. Its accessory piece averaged 40 micrometers long and 12 micrometers wide. It was not articulated with the base of the male copulatory organ and divided into unequal distal branches. The longer branch widened distally and ended in a thin subterminal projection and a hook-shaped tip. The vagina was weakly sclerotized, opened ventrally and formed a bulb near the aperture before continuing as a long, thin vaginal canal.

The haptoral structures provided further diagnostic characters. The haptor averaged 97 micrometers wide and ranged from 75 to 133 micrometers. The ventral anchor measured 13–20 micrometers in length and had a short deep root, a developed superficial root and a small hump between the roots. The dorsal anchor was longer, measuring 23–37 micrometers, and likewise had differentiated roots, including a well-developed superficial root and a short, developed deep root. Seven pairs of hooks had ancyrocephaline distribution, with straight shanks, protuberant thumbs and recurved shafts. The functional-hook loop extended for approximately three-quarters of the shaft length. Although S. anapurusensis shares a bifurcated accessory piece with S. satanopercae, S. araguariensis, S. edgari and S. amazoniensis, the new species was distinguished from those taxa by the postero-medial umbiliform membrane on its ventral bar.

Researchers also redescribed S. araguariensis, which had originally been described from Saxatilia labrina, formerly classified as Crenicichla labrina, collected in the Araguari River in Amapá state. The Maranhão specimens resembled the original material. Re-examination of the holotype and paratypes also revealed a distal cap on the accessory piece, a structure not mentioned in the original description. The redescription was based on 20 mounted specimens. Body length, including the haptor, ranged from 250 to 397 micrometers, and the haptor measured 81–128 micrometers across. Its male copulatory organ was tubular and thin, with 1.5 to two clockwise rings. The accessory piece measured 21–38 micrometers long and 8–15 micrometers wide, was bifurcated through a tube at the middle and carried a slightly sclerotized distal cap. The new distribution record makes S. araguariensis known from two congeneric hosts in the genus Saxatilia.

Among the 64 fish examined, S. anapurusensis had a prevalence of 64.1 percent. Infected fish carried one to 27 parasites, corresponding to a mean intensity of 6.7 parasites per infected fish, while mean abundance across all analyzed fish was 4.3 parasites. S. araguariensis occurred in 50 percent of the examined hosts. Its intensity ranged from one to six parasites per infected fish, its mean intensity was 2.6 and its mean abundance was 1.3 parasites per analyzed fish. These measurements apply to the sampled hosts from two Munim River tributaries and should not be generalized to every population of S. brasiliensis or to other Brazilian rivers. The study did not assess the clinical or ecological consequences of either parasite, including possible effects on host condition, transmission or population persistence.

The records add to a broader pattern of incomplete knowledge about monopisthocotylan diversity. The researchers cited an estimate that only 6 percent of monopisthocotylan richness is known. They noted that the parasite fauna of Neotropical cichlids has been studied less extensively than that of African cichlids, a difference they considered more likely to reflect unequal sampling effort than actual differences in species richness. Six dactylogyrid genera had been reported from Neotropical cichlids, and Sciadicleithrum contained 30 described species at the time of the study. Members of the genus parasitize cichlids and had been recorded in 18 fish genera, although individual species may use between one and 15 cichlid hosts. The findings from S. brasiliensis therefore contribute another host association and another described species to this incompletely sampled assemblage.

Taxonomic revisions are also relevant to the host records. A review of the crenicichline cichlids transferred Crenicichla brasiliensis and Crenicichla labrina to Saxatilia, a genus comprising 23 valid species. Both cichlid hosts discussed in the study consequently belong to the same genus. The host previously reported for Sciadicleithrum joanae, Crenicichla niederleinii, was designated a nomen dubium, leaving its current taxonomic status uncertain. By describing S. anapurusensis and reporting S. araguariensis from a new host and region, the study expands the documented parasite fauna associated with Saxatilia and the Munim River Basin. Its contribution is taxonomic and ecological: it provides formal morphological descriptions, prevalence and intensity measurements, and verified host-parasite records for further systematics, while leaving the biological significance of infection unassessed.

Subject of Research: Morphological description and prevalence of two Sciadicleithrum parasites infecting Saxatilia brasiliensis in Brazil’s Munim River Basin

Article Title:  Sciadicleithrum spp. ( Dactylogyridae , Monopisthocotyla ) parasitizing Saxatilia brasiliensis ( Cichlidae , Cichliformes ) from the Munim River Basin, northeastern Brazil, with a description of a new species

Article References: Silva, M. L., Ottoni, F. P., Viana, D. C., Saraiva, D. J., Justo, M. C. N., & Cohen, S. C. (2025). Sciadicleithrum spp. (Dactylogyridae, Monopisthocotyla) parasitizing Saxatilia brasiliensis (Cichlidae, Cichliformes) from the Munim River Basin, northeastern Brazil, with a description of a new species. Zoosystematics and Evolution, 101(4), 2161-2169. https://doi.org/10.3897/zse.101.147382

Image Credits: AI Generated

DOI: 10.3897/zse.101.147382

Keywords: Sciadicleithrum anapurusensis, Sciadicleithrum araguariensis, Saxatilia brasiliensis, monopisthocotylans, fish parasites, Munim River Basin, Maranhão, parasite morphology, cichlid fish, Neotropical biodiversity, Sciadicleithrum, Dactylogyridae