Brazil has a significant richness of fish species and plays a fundamental role in discussions about the freshwater ecosystem, as well as its biodiversity and conservation (Azevedo-Santos et al. 2021). Nevertheless, like the rest of the planet, due to the exponential increase in anthropogenic impacts on ecosystems and the high extinction rates of species and populations, leading to a rapid loss of biodiversity, we are facing a serious global phenomenon, known as the “Biodiversity Crisis”, which is rapidly leading our planet towards a mass extinction (Savage 1995; Brooks et al. 2002; Singh 2002; Brook et al. 2006; Pimm et al. 2014; Ceballos et al. 2015; Hogue and Breon 2022).

This global crisis is more serious in freshwater ecosystems, which are more vulnerable to anthropogenic impacts and have higher rates of species and population extinctions, as these freshwater ecosystems are being altered, degraded, or destroyed more quickly and sharply, a phenomenon called the “Freshwater Biodiversity Crisis” (Darwall et al. 2018; Harrison et al. 2018; Reid et al. 2019; Albert et al. 2020; Tickner et al. 2020; Ottoni et al. 2023). Therefore, the mitigation of the effects of human actions on freshwater ecosystems and their conservation needs to be a priority (Albert et al. 2020; Tickner et al. 2020; Ottoni et al. 2023). Brazil is facing all the problems and scenarios mentioned above, including the accelerated rate of degradation, fragmentation, and loss of freshwater ecosystems, and consequently, of the related biodiversity (Pereira-Júnior and Pereira 2017; Azevedo-Santos et al. 2023).

The Cichlidae comprises the greatest species richness among families of Cichliformes, currently comprising 1762 valid species, among which 564 are included within Cichlinae (Fricke et al. 2025a), a subfamily that naturally occurs exclusively in the Neotropical region (Smith et al. 2008; López-Fernández et al. 2010). Cichlinae is divided into seven tribes: Cichlini, Retroculini, Astronotini, Chaetobranchini, Geophagini, Cichlasomatini, and Heroini; the last three being the most diverse ones (Smith et al. 2008; López-Fernández et al. 2010). However, despite the great fish species richness, especially in the tropics, little is known about the diversity and richness of the parasite fauna from these potential hosts (Avise et al. 2008). Tavares-Dias et al. (2022) estimated that only 6% of monopisthocotylan richness is known, incipient knowledge given the richness of fish species that exists, and that each host species hosts at least two monopisthocotylan species.

The proportion of cichlid species worldwide examined for the presence of Monopishtocotyla is low, and the diversity of these helminths parasitizing cichlid fishes in the Neotropical Region remains understudied compared to their counterparts in Africa. However, this imbalance is more likely due to differences in sampling efforts rather than actual variations in species richness among genera (Pariselle et al. 2011).

Sciadicleithrum Kritsky, Thatcher & Boeger, 1989 was proposed to accommodate Sciadicleithrum uncinatum Kritsky,Thatcher & Boeger, 1989 parasite from Cichla ocellaris Bloch & Schneider, 1801. To date, 30 species of Sciadicleithrum have been described from different host species, belonging to the family Cichlidae listed in Table 1.

The Munim River Basin is mainly located at the Cerrado Biome of the Maranhão State; however, its lower portion is located in a region between the Cerrado and Amazon biomes, and its mouth is at the Bay of São José, in a region known as the “Golfão Maranhense” (Vieira et al. 2023). A recent fish inventory, conducted by Vieira et al. (2023), revealed the existence of 123 fish species in this river system, considerably increasing its known fish diversity, which was underestimated until the publication of the inventory.

During studies with Saxatilia brasiliensis (Bloch, 1792), a cichlid endemic to the Northeastern region of Brazil (Fricke et al. 2025b; Froese and Pauly 2024) in the Munim River Basin in Maranhão state, a new species and a previously known species, both Sciadicleithrum, were identified. This new species of Sciadicleithrum is not morphologically similar to those previously known. In the present study, the species are described and redescribed, respectively, based on morphological features. These results expand the knowledge of the biodiversity of the fish parasites of the region.

Specimens of Saxatilia brasiliensis were collected in two tributaries of the Munim River Basin, named Stream Feio (03°51'18.1"S, 043°17'14.0"W) (S1) at the rural zone of the Municipality of Chapadinha and Stream Estrela (03°40'15.6"S, 043°7'9.7"W) (S2) in the Municipality of Anapurus, both located in the Maranhão State, Northeastern region of Brazil (Fig. 1).

Fishes were collected using nets 240 cm long and 100 cm high. A total of sixty-four specimens of S. brasiliensis were obtained, 32 from each locality (sixteen specimens were collected in the dry season and sixteen in the rainy season).

The collections were authorized by the “Sistema de Autorização e Informação em Biodiversidade (SISBIO)” under the number 6441-5, while the laboratory procedures were licensed by the “Comitê de Ética em Experimentação Animal (CEA)” from the Estadual University of Maranhão, under the protocol 11/2021-01200.002200/2015-06.

The fishes were taken to the "Laboratório de Sistematica e Ecologia de Organismos Aquáticos", Federal University of Maranhão (UFMA), where fish euthanasia, preservation and identification were carried out by the laboratory team. The fishes were deposited at the "Coleção Ictiológica do Centro de Ciências Agrárias e Ambientais (CICCAA)" of the Universidade Federal do Maranhão under voucher numbers CICCAA07121 - CICCAA07151.

At the laboratory, the branchial arches were removed and placed in vials containing hot water (~65 °C) for relaxing and detaching of Monopisthocotyla, and then the vials were vigorously shaken. After cooling, absolute ethanol was added to reach the concentration of 70%. The content (liquid and branchial arches) was analyzed under a stereoscopic microscope for collection of the parasites. For morphological studies of sclerotized parts (copulatory complex and haptoral sclerites), parasites were mounted in Hoyer’s medium (Humason, 1979). The material was sent to "Laboratório de Helmintos Parasitos de Peixes, Instituto Oswaldo Cruz, FIOCRUZ", Rio de Janeiro State, Brazil, to be characterized and identified. Specimens of Sciadicleithrum araguariensis Paschoal, Scholz, Tavares-Dias & Luque, 2016 (Holotype CHIOC no. 38091a; paratypes CHIOC no. 38091b, c, d) were examined for comparative purposes.

Drawings were taken using a Zeiss® Axioskop microscope micrographic system with a differential interference contrast (DIC) apparatus and an Olympus BX 41 microscope with phase contrast, equipped with a camera lucida. Measurements are presented in micrometers; means are followed by range and number of structures measured (n) in parenthesis; lengths of curved or bent structures represent the straight-line distances between extreme ends, except for the copulatory complex, measured using IMAGEJ software (Wayne 2010), distributed by the National Institutes of Health (NIH), and available at https://imagej.nih.gov/ij/download.html. Distribution of hooks followed Mizelle and Price (1963). The studied specimens were deposited in the "Coleção Helmintológi­ca do Instituto Oswaldo Cruz (CHIOC)", Rio de Janeiro, Brazil.

Phylum Platyhelminthes Gegenbaur, 1859

Class Monopisthocotyla Brabec, Salomaki, Kolısko, Scholz & Kuchta, 2023

Order Dactylogyridea Bychowsky, 1937

Dactylogyridae Bychowsky, 1933

Sciadicleithrum Kritsky, Thatcher & Boeger, 1989

The parasitofauna associated with the vast majority of Neotropical cichlid hosts remains largely unknown. To date, six genera of dactylogyrids are known to parasitize Neotropical cichlids: Biotodomella Morey, Arimuya & Boeger, 2019 (1 sp.), Gussevia Kohn & Paperna, 1964 (13 spp.), Sciadicleithrum Kritsky, Thatcher & Boeger, 1989 (30 spp.), Trinidactylus Hanek, Molnar & Fernando, 1974, Tucuranella Mendoza-Franco, Scholz & Rozkosná, 2010, and Parasciadicleithrum Mendoza-Palmero, Blasco-Costa, Hernández-Mena & Pérez-Ponce de León, 2017, each with one species described from Brazil, Peru, Venezuela, Mexico, Panama, and Nicaragua. In addition, species of Cichlidogyrus Paperna, 1960, Enterogyrus Paperna, 1963, and Scutogyrus Pariselle & Euzet, 1995, were reported from African introduced hosts, such as Oreochromis spp. (Kritsky et al. 1986, 1989; Bellay et al. 2008, 2009; Carvalho et al. 2008; Yamada et al. 2009; Mendoza-Franco et al. 2010; Melo et al. 2012; Paschoal et al. 2016; Mendoza-Palmero et al. 2017; Morey et al. 2019, 2024; Justo et al. 2020; Seidlová et al. 2022), also in the Neotropical Region.

Monopishtocotylans are known to be host specific, with the majority of species parasitizing a single host species. Nevertheless, members of Sciadicleithrum are recognized to parasitize from one to 15 cichlid species, considering that Sciadicleithrum bravohollisae Kristky, Vidal-Martinez & Rodriguez-Canul, 1994 is reported from 15 different host species (Seidlová et al. 2022). Species of Sciadicleithrum were recorded in 18 genera of fish, all from the family Cichlidae.

Recently, Varella et al. (2023) conducted a taxonomic review and phylogeny of the subtribe Crenicichlina, the largest clade of the tribe Geophagini, and subfamily Cichlinae, including over 100 species, which previously included only two genera: Crenicichla Heckel, 1840 and Teleocichla Kullander, 1988 (Varella et al. 2023). As a result of that work, the genus Crenicichla was split, resulting in the description of four new genera. Thus, the current composition of the subtribe Crenicichlina includes the following genera: Crenicichla, Teleocichla, Lugubria Varella, Kullander, Menezes, Oliveira & López-Fernández, 2023, Saxatilia Varella, Kullander, Menezes, Oliveira & López-Fernández, 2023, Wallaciia Varella, Kullander, Menezes, Oliveira & López-Fernández, 2023, and Hemeraia Varella, Kullander, Menezes, Oliveira & López-Fernández, 2023 (Varella et al. 2023). After this taxonomic review, Crenicichla labrina and Crenicichla brasiliensis were allocated to the genus Saxatilia, currently comprising 23 valid species (Varella et al. 2023; Fricke et al. 2025b).

Considering this, Sciadicleithrum araguariensis is presently known from two congeneric hosts belonging to Saxatilia. In addition, the new species described herein is also a parasite from a member of this genus. Furthermore, Crenicichla niederleinii Holmberg, 1891, the host of Sciadicleithrum joanae Yamada, Takemoto, Bellay & Pavanelli, 2009, became a nomen dubium, and its current taxonomic status is uncertain (Varella et al. 2023; Fricke et al. 2025b).

The finding of two species of Sciadicleithrum on Saxatilia brasiliensis enhances our understanding of parasite biodiversity in the Neotropical Region, specifically within the Munim River Basin located in Maranhão State, and reinforces the host specificity of Sciadicleithrum species to cichlid hosts. The results presented provide data to answer questions of the systematics and taxonomy of Monopisthocotyla in a megadiverse region like the Neotropics.