First report of a dactylogyrid, Cichlidogyrus sclerosus Paperna & Thurston, 1969 (Monogenoidea) infecting Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) (Cichliformes: Cichlidae) in the United States, with a review of host and locality records in its invasive range and a phylogenetic analysis

Abstract

The parasites of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) are poorly documented in the United States despite the economic importance and global introduction of this African fish. Only one metazoan parasite (Gyrodactylus cichlidarum Paperna, 1968; Gyrodactylidae) reportedly infects Nile tilapia in the United States. Examining Nile tilapia from a flow-through aquaculture system hydrologically linked to Sougahatchee Creek (Tallapoosa River, Auburn, Alabama), we observed a gill infection by Cichlidogyrus sclerosus Paperna & Thurston, 1969 (Dactylogyridae). This monogenoid was originally described from the gill of Mozambique tilapia, Oreochromis mossambicus (Peters, 1852) from Lake Victoria, Uganda. Specimens of C. sclerosus were studied for morphology and phylogenetic analyses using the 28S and ITS1. We identified our specimens as C. sclerosus because they had the following combination of morphological features: marginal hooks shorter than dorsal anchor length; anchor roots reduced; dorsal anchor point bent; dorsal bar pyriform projections approximately half as long as dorsal bar width; penis short (<100 μm), not coiled, tubular, lacking swelling, having irregularly surfaced heel; and accessory piece straight and bifid. Our 28S and ITS1 phylogenies recovered our C. sclerosus sequences in a clade with conspecific sequences and showed no obvious biogeographic pattern. Cichlidogyrus sclerosus reportedly infects 21 fishes of 11 genera and 3 families from 36 countries in Africa, Asia, North America, South America, and Europe. The study of Nile tilapia parasites, especially those exhibiting direct life cycles and low host specificity, is important because they comprise potential invasive species.

Species of Cichlidogyrus Paperna, 1960 (Monogenoidea: Polyonchoinea: Dactylogyridae Bychowsky, 1933) primarily infect African cichlids, have evidently been introduced beyond the natural geographic distribution of their tilapia hosts, have been distributed globally due to production aquaculture (FAO 2022) and the aquarium trade, and have been infrequently reported to spill over into native fish communities (Table 1) (Jiménez-García et al. 2001; Zhang et al. 2019). The introduction of non-native Cichlidogyrus spp. should cause alarm among natural resource managers and aquaculturists because they have direct life cycles, can infect numerous fish hosts, and can cause disease (Kabata 1985; Khalil 1971; Jiménez-García et al. 2001; Shinn et al. 2023). Little is known about the parasites of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) (Cichliformes: Cichlidae) in the United States despite it being an invasive species and aquaculture product. Brule et al. (2024) reported the African gyrodactylid, Gyrodactylus cichlidarum Paperna, 1968 (Monogenoidea: Polyonchoinea: Gyrodactylidae Cobbold, 1864) from the skin and gill of Nile tilapia from the same locality we report herein, Sougahatchee Creek (Tallapoosa River, Mobile-Tensaw Basin, Auburn, Alabama). We herein report the second exotic monogenoid species (Cichlidogyrus sclerosus Paperna & Thurston, 1969) from Nile tilapia in the United States. This also comprises the first record of a dactylogyrid infecting Nile tilapia in the United States.

Table 1. Host and locality records for infection by Cichlidogyrys sclerosus

Materials and methods

During November 2021, hatchery-reared Nile tilapia fingerlings were stocked (200 fish/raceway) into raceways (0.71 m × 0.61 m × 3.00 m; ~ 1 m³ volume) supplied with flow-through water (single pass) at the E.W. Shell Fisheries Center (EWSFC), Auburn, Alabama. This facility comprises a series of earthen aquaculture ponds comprising that segment of Sougahatchee Creek (Tallapoosa River) as well as an indoor flow-through aquaculture facility fed by surface water and having an effluent connected to the creek. Ten (10) Nile tilapia were removed from the raceways one day after stocking, transported to the laboratory alive in buckets containing raceway water, euthanized using tricaine methanesulphonate (MS-222), and necropsied for parasitological examination.

The gill from each Nile tilapia was removed intact, and each gill arch was separated and examined in a petri dish with raceway water with the aid of a stereo-dissecting microscope (Meiji RZ 3288) with fiber optic light sources as well as light and dark field sub-stage illumination. Live monogenoids intended for morphology were removed from the gill filaments with minutien pins affixed to wooden dowel rods and pipetted onto a glass slide, cover-slipped (no pressure), flame-killed on the slide using a hand lighter, fixed for several days in 10% neutral buffered formalin (n.b.f.), washed in deionized water to remove n.b.f., stained overnight in Van Cleave’s hematoxylin, dehydrated using a graded EtOH series, made basic in 70% EtOH with lithium carbonate and n-butylamine, dehydrated in 100% EtOH, cleared in clove oil, and permanently whole-mounted on glass slides using Canada balsam. Whole mounted specimens were drawn (Figures 1−4) with the aid of an Olympus BX51 compound microscope (Olympus, Tokyo, Japan) equipped with DIC optical components and a drawing tube. Measurements were obtained by using a Jenoptik Gryphax camera (Jenoptik AG, Jena, Germany). Measurements are reported as a range in micrometers (μm) followed by the mean, standard deviation, and sample size. Voucher specimens were deposited in the National Museum of Natural History’s Invertebrate Zoology Collection (Smithsonian Institution, USNM Collection Nos. 1743701–1743707). Hook numbering follows Llewellyn (1963). Sclerite measurements follow Douëllou (1993). Scientific names for fishes, including taxonomic authorities and dates, follow Eschmeyer et al. (2016; online version updated 2023). Common names for fishes follow Froese and Pauly (2023). The hosts were identified as Nile tilapia by having 23 gill rakers and 18 dorsal fin spines as per Boschung and Mayden (2004).

Figure 1. Cichlidogyrus sclerosus Paperna & Thurston, 1969 (Monogenoidea: Polyonchoinea: Dactylogyridae) (USNM No. 1743705) infecting the gill filaments of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) (Cichliformes: Cichlidae) from the E.W. Shell Fisheries Center, Auburn, Alabama (hydrologically linked to Tallapoosa River, Mobile-Tensaw Basin). Scale value aside bar. Abbreviations: accessory piece (ap), anchor (a), distal vagina (dv), eyespot (e), haptor (h), haptoral gland (hg), head organ (ho), intestine (i), marginal hook (mh), mouth (m), ovary (o), peduncle muscle (pm), penis (p), pharynx (ph), proximal vagina (pv), seminal recepticle (sr), seminal vesicle (sv), uterus (u), vitellarium (v).

Two EtOH-preserved specimens intended for DNA extraction were wet-mounted and cover-slipped on a glass slide to confirm the presence of morphological features diagnostic for the species before being digested to extract complete genomic DNA using the Qiagen DNeasy Blood and Tissue Kit (Qiagen, Hilden, Germany) following the manufacturer’s protocol. Primer choice and PCR thermocycling follows Bullard et al. (2021). All PCR reactions were performed with a ProFlex PCR System (Applied Biosystems, Waltham, Massachusetts). Amplification of PCR product was checked in a 5% agarose gel stained with 1% ethidium bromide using 8μl of PCR product. PCR product was purified using the QIAquick PCR Product Purification Kit (Qiagen). Sequencing was performed by Genewiz, Incorporated (South Plainfield, New Jersey). Chromatograms were assembled based on sequence overlap, were proofread by eye, and had low-quality read ends trimmed in Geneious version 2023.2.1 (http://www.geneious.com), resulting in two 28S rDNA segments of 770 base pairs (bp) and ITS1 segments of 541 bp and 542 bp. All sequences were deposited in GenBank under accession numbers PQ728084–7.

Our phylogenetic analysis consisted of 28S and ITS1 phylogenies. Our sequences were aligned with conspecific sequences and appropriate, respective sequences of Cruz-Laufer et al. (2022) with the multiple alignment using fast Fourier transform (MAFFT) tool (Katoh and Standley 2013). Each alignment was trimmed to the length of our included sequence (770 bp, GenBank No. PQ728086 [28S]; 542 bp, GenBank No. PQ728084 [ITS1]). Aligned sequences were exported as a .phy file to run maximum likelihood (ML) tree inference. The ML trees were inferred with IQTREE v.1.16.12 (Nguyen et al. 2015). Substitution model testing was done with ModelFinder (Kalyaanamoorthy et al. 2017) as implemented in IQTREE. After model testing, tree inference was done using best-fitting substitution models (Chernomor et al. 2016). Default tree search parameters were used, except perturbation strength was set to 0.2, and 500 iterations had to be unsuccessful to stop the tree search. Tree inference was preformed 20 times with only the tree with the best log-likelihood score reported. Support for relationships was measured with 1,000 ultrafast bootstrap replicates (UFBoot) (Hoang et al. 2018). The recovered phylogenies were visualized using FigTree v1.4.4 (Rambaut et al. 2014) and further edited for visualization purposes with Adobe Illustrator (Adobe Systems) (Figures 5 and 6).

Results

Cichlidogyrus sclerosus Paperna & Thurston, 1969

Supplemental description (Figures 1–4)

Based on 10 stained, whole-mounted adult specimens: Body (including haptor) elongate, 444–708 (604 ± 110; 6) long, 123–172 (149 ± 19; 6) wide at maximum width at level immediately posterior to body midpoint, having 4 eyespots; eyespots dorsal; posterior pair of eyespots dense; anterior pair of eyespots diffuse (Figure 1). Mouth, subterminal, at level of anterior eyespot pair (Figure 1). Head organs in 3 pairs, containing string-like substance; gland ducts extending anteriad 78–126 (101 ± 16; 6) before connecting to terminal pores; gland ducts branching immediately anterior to level of mouth (Figure 1). Pharynx circular in outline, 36–43 (39 ± 3; 6) long, 33–41 (37 ± 3; 6) wide, extending dorsally and posteriad before joining with intestine (Figure 1). Intestine bifurcating at level of base of head organs, with cyclocoel, extending posteriad approximately in parallel with respective lateral body margin, looping in hind body (Figure 1).

Haptor irregular in outline, its connection to body indistinct, receiving glandular material from peduncle, associated with 2 muscle cords; cords extending anteriad into peduncle, approximately 44–93 (64 ± 10; 6) long, 74–93 (84 ± 7; 6) wide (Figure 1). Haptoral sclerites comprising 1 pair of ventral anchors, 1 pair of dorsal anchors, a ventral and a dorsal transverse bar, and 7 pairs of marginal hooks (Figures 1 and 2); ventral and dorsal anchors each comprising deep root, superficial root, base, shaft, and point (Figures 2a, b); ventral anchors 24–30 (28 ± 2; 8) long (Figures 1 and 2a); ventral anchor deep root 3–4 (4 ± 1; 8) long (Figure 2a); ventral anchor superficial root 7–9 (8 ± 1; 8) long (Figure 2a); ventral anchor point 11–15 (13 ± 1; 8) long (Figure 2a); ventral anchor length from peak of shaft to notch 23–31 (28 ± 3; 8) long (Figure 2a); dorsal anchors 22–27 (26 ± 2; 4) long (Figures 1 and 2b); deep root of dorsal anchor 3–4 (4 ± 1; 4) long (Figure 2b); superficial root of dorsal anchor 6–7 (6 ± 1; 4) long (Figure 2b); point of dorsal anchor bent, thin, 8–12 (9 ± 2; 4) long (Figure 2b); dorsal anchor length from peak of shaft to notch 23–28 (26 ± 2; 4) long (Figure 2b); ventral transverse bar spanning breath between bases of ventral anchors, lacking associated plate, comprising two projections forming the outline of a V, having a median groove extending along anterior margin, 3–7 (6 ± 2; 6) long at midpoint, 34–36 (35 ± 1; 3) wide at maximum width, 17–21 (20 ± 2; 6) tall (Figure 2c); ventral bar projections 21–27 (25 ± 2; 6) long (Figure 2c); dorsal transverse bar 6–8 (7 ± 1; 2) long at midpoint, 27–37 (32 ± 7; 2) wide, 23–26 (25 ± 2; 2) tall, spanning breath between bases of dorsal anchors, H-shaped in outline, having two pyriform projections ‘auricles’ sensu Pariselle and Euzet (2009) and two peg-like projections (Figure 2d); dorsal bar pyriform projections 10–12 (11 ± 1; 3) long; distance between dorsal bar pyriform projections at base 10–11 (10 ± 1; 3) (Figure 2d); marginal hook pairs: pair I 8–12 (11 ± 2; 5) long; pair II 4–6 (5 ± 1; 3) long; pair III 11–16 (15 ± 2; 6) long; pair IV 11–17 (14 ± 3; 4) long; pair V 13–15 (14 ± 1; 5) long; pair VI 12–16 (13 ± 2; 3) long; pair VII 10–15 (13 ± 2; 4) long (Figures 2e–k).

Figure 2. Haptoral sclerites of Cichlidogyrus sclerosus Paperna & Thurston, 1969 (Monogenoidea: Polyonchoinea: Dactylogyridae) infecting the gill filaments of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) (Cichliformes: Cichlidae) from the E.W. Shell Fisheries Center, Auburn, Alabama (hydrologically linked to Tallapoosa River, Mobile-Tensaw Basin). Scale values aside bars. (a) Ventral anchor of voucher (USNM No. 1743702), (b) Dorsal anchor of voucher (USNM No. 1743702), (c) ventral transverse bar of voucher (USNM No. 1743702), (d) dorsal transverse bar of voucher (USNM No. 1743702), (e) marginal hook I of voucher (USNM No. 1743704), (f) marginal hook II of voucher (USNM No. 1743703), (g) marginal hook III of voucher (USNM No. 1743704), (h) marginal hook IV of voucher (USNM No. 1743706), (i) marginal hook V of voucher (USNM No. 1743704), (j) marginal hook VI of voucher (USNM No. 1743707), (k) marginal hook VII of voucher (USNM No. 1743706).

Testis dorsal to ovary, margin indistinct, approximately 32 long, approximately 11 wide, entirely intercaecal (Figure 3). Vas deferens emerging from anterior margin of testis and extending anteriad (Figure 3). Seminal vesicle comprised of proximal portion and distal tube (Figure 3); proximal portion of seminal vesicle 28–37 (33 ± 4; 4) long, 14–21 (19 ± 3; 4) wide, containing sperm (Figure 3); distal tube of seminal vesicle extending mediad to male copulatory organ (MCO), connecting to MCO dorsally (Figures 1, 3, and 4). MCO medial, comprising penis and accessory piece, 88–136 (119 ± 18; 6) from anterior body end (Figures 1, 3, and 4). Penis ‘copulatory tube’ sensu Douëllou (1993) tubular, tapering distally, lacking swelling, recurved, not coiled, 59–68 (63 ± 4; 4) long, containing irregularly surfaced heel ‘serrated plate’ sensu Douëllou (1993) with elevated rim; heel 8–11 (10 ± 1; 5) long, 12–14 (14 ± 1; 5) wide (Figures 1, 3, and 4). Accessory piece bifid, straight, 50–61 (50 ± 5; 7) long or 74–83% of penis length (Figures 1, 3, and 4).

Figure 3. Male and female reproductive genitalia of Cichlidogyrus sclerosus Paperna & Thurston, 1969 (Monogenoidea: Polyonchoinea: Dactylogyridae) (USNM No. 1743701) infecting the gill filaments of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) (Cichliformes: Cichlidae) from the E.W. Shell Fisheries Center, Auburn, Alabama (hydrologically linked to Tallapoosa River, Mobile-Tensaw Basin). Scale value aside bar. Abbreviations: accessory piece (ap), distal tube (dt), distal vagina (dv), heel (he), ovary (o), penis (p), proximal portion (pp), proximal vagina (pv), seminal receptacle (sr), seminal vesicle (sv), testis (t), transverse vitelline duct (tv), uterus (u), vaginal pore (vp), vas deferens (vd).

Figure 4. Male copulatory organ of Cichlidogyrus sclerosus Paperna & Thurston, 1969 (Monogenoidea: Polyonchoinea: Dactylogyridae) (USNM No. 1743702) infecting the gill filaments of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) (Cichliformes: Cichlidae) from the E.W. Shell Fisheries Center, Auburn, Alabama (hydrologically linked to Tallapoosa River, Mobile-Tensaw Basin). Scale value aside bar. Abbreviations: accessory piece (ap), penis (p), heel (he).

Ovary irregular in shape, thin-walled, containing many fully developed ova, intercaecal, 47–64 (52 ± 7; 5) long, 24–43 (36 ± 8; 5) wide, 217–271 (251 ± 24; 4) or 37–43% of total body length (including haptor) from anterior body end, 280–368 (358 ± 38; 4) or 48–55% of total body length from posterior body end (including haptor) (Figures 1 and 3). Oviduct extending anteriad, dorsal to seminal receptacle (Figure 4). Transverse vitelline duct intersecting oviduct between seminal vesicle and seminal receptacle (Figure 4). Oötype not observed. Vitellarium extending posteriad from pharynx and slightly surpassing cyclocoel, dorsal and ventral to intestine, spanning the maximum breadth of body (Figure 1). Vagina funnel-shaped, originating ventral to seminal receptacle, comprised of proximal and distal portions (Figures 1 and 4); proximal vagina thin-walled, extending anterodextrad from seminal receptacle, 12–17 (14 ± 3; 4) in maximum width (Figures 1 and 3); distal vagina sclerotized, looped, surrounded by muscle distally, 33–37 (35 ± 2; 4) long (Figure 3). Vaginal pore slightly dextral, 57–59 (58 ± 1; 3) or 34–45% of body width from dextral body margin, 79–104 (92 ± 13; 3) or 56–79% of body width from sinistral body margin (Figure 1). Uterus thick-walled, dorsal to seminal vesicle, curving ventrally around dorsal portion of seminal vesicle, having crenulated luminal surface, tapering distally, extending sinistrad towards MCO, 34–64 (54 ± 17; 3) long, 2–3 (3 ± 1; 3) thick (Figure 3).

Taxonomic summary

Host: Oreochromis niloticus (Linnaeus, 1758) (Cichliformes: Cichlidae), Nile tilapia.

Locality: E. W. Shell Fisheries Center, Auburn, Alabama, USA (Sougahatchee Creek, Tallapoosa River, Mobile-Tensaw Basin).

Specimens and sequences deposited: Voucher specimens (USNM Nos. 1743701–7);

28S rDNA (GenBank Nos. PQ728086–7); ITS1 (GenBank Nos. PQ728084–5)

Site on host: Gill filaments.

Taxonomic remarks

Cichlidogyrus sclerosus is most easily differentiated from its congeners by having the combination of marginal hooks that are shorter than the dorsal anchor length, reduced anchor roots, a bent dorsal anchor point, dorsal bar pyriform projections that are approximately half as long as the dorsal bar width, a short penis (<100 μm) that is tubular (not coiled, lacking swelling) and having an irregularly surfaced heel, and an accessory piece that is straight (not curved) and bifid. Our specimens matched the original (Paperna and Thurston 1969) and supplemental descriptions (Douëllou 1993; Vidal-Martínez et al. 2001) of C. sclerosus by having that combination of features. This monogenoid was originally described from the gill of Mozambique tilapia, Oreochromis mossambicus (Peters, 1852) (as Tilapia mossambica [Peters, 1852], Cichlidae, type host) from Lake Victoria, Kajjansi, Uganda (type locality); Nile tilapia (as Tilapia nilotica [Linnaeus, 1758]) and Haplochromis sp. (Cichlidae) from Lake George, Uganda; blue spotted tilapia, Oreochromis leucostictus (Trewavas, 1933) (as Tilapia leucosticta Trewavas, 1933) from Lake Victoria, Jinja, Uganda; and redbelly tilapia, Coptodon zillii (Gervais, 1848) (as Tilapia zillii [Gervais, 1848]; Cichlidae; experimental infection, locality unspecified) (Table 1).

Herein, we collected specimens of C. sclerosus and a congener. These congeneric specimens generally resemble C. thurstonae (type-host: Nile tilapia; type-locality: Nile River, Cairo, Egypt) by having marginal hook pairs III–VII that are approximately twice as long as the marginal hook pair I; an s-shaped accessory piece with a proximal finger-like process; and a straight vaginal sclerite having a jagged margin (Ergens 1981; Pariselle et al. 2003). However, these specimens were poorly fixed, and we cannot formally detail them further herein.

Phylogenetic results

The two 28S sequences of C. sclerosus (GenBank Nos. PQ728086–7) produced herein each comprised 770 bp and were identical. Our included 28S sequence was recovered in a clade of other sequences of C. sclerosus. Our sequences differ from the other 12 extant 28S sequences ascribed to C. sclerosus (GenBank Nos. DQ157660, MH767401, MK524728, MK524729 MN078060, MT994744, MW580351, MW580352, OL415084, OR338287, OR528026, PP191180) by 0 (0%) to 8 bp (1%) (Figure 5). The 28S sequences of C. sclerosus used herein were recovered sister to that of Cichlidogyrus amphoratus Pariselle & Euzet, 1996 (GenBank No. HE792772) infecting Guinean tilapia, Coptodon guineensis (Gunther, 1862) from Senegal (Mendlová et al. 2012), and our sequence differed from it by 37 bp (4.5%) (Figure 5).

Figure 5. Phylogenetic relationships of species within Cichlidogyrys Paperna, 1960 reconstructed using maximum likelihood tree inference using the large ribosomal subunit (28S) gene. Numbers aside nodes indicate bootstrap values. New sequence of Cichlidogyrus sclerosus Paperna & Thurston, 1969 is shown in bold. GenBank numbers are in parentheses following each taxon.

Figure 6. Phylogenetic relationships of species within Cichlidogyrys Paperna, 1960 reconstructed using maximum likelihood tree inference using the internal transcribed spacer 1 (ITS1). Numbers aside nodes indicate bootstrap values. New sequence of Cichlidogyrus sclerosus Paperna & Thurston, 1969 is shown in bold. GenBank numbers are in parentheses following each taxon.

The two ITS1 sequences of C. sclerosus (GenBank Nos. PQ728084–5) produced herein comprised 541 bp and 542 bp and were identical. Our included ITS1 sequence was recovered in a clade of other sequences of C. sclerosus (Figure 5). Our sequences differed from the other 9 extant ITS1 sequences ascribed to C. sclerosus (GenBank Nos. DQ537359, KX869722, MH767390, MN905938, ON819193, ON819218, ON819279, ON819301, OR335571) by 0 (0%), 1 (0.3%), or 3 bp (0.6%) except for GenBank No. ON819218, which differed by 29 bp (4.7%). The ITS1 sequences of C. sclerosus used herein were recovered sister to C. amphoratus (GenBank No. HE792782) infecting Guinean tilapia from Senegal (Figure 5) (Mendlová et al. 2012), and our sequence differed from it by 48 bp (10.4%).

The ITS1 and 28S phylogenetic analyses failed to recover clades of C. sclerosus from the same geographic locality (i.e., sequences of C. sclerosus from the same continent are not monophyletic).

Discussion

Cichlidogyrus sclerosus exhibits characteristics of a potential invasive pathogen, especially to endemic cichlids. It has a direct life cycle, a geographically broad (worldwide) introduced/invasive range, and a relatively low degree of host specificity, infecting cichlids of various genera as well as a carp (Cyprinidae) and a splitfin (Goodeidae) (Table 1). It has spread globally probably because of African cichlid aquaculture (FAO 2022; Table 1). Since its original description from cichlids in Uganda, C. sclerosus has been reported from 35 other countries in Africa, Asia, North America, South America, and Europe. Our phylogenetic analysis showed that isolates of C. sclerosus do not clade by geographic locality, which could indicate that this parasite has been introduced and reintroduced repeatedly. Cichlidogyrus sclerosus has been reported from 21 host species belonging to 11 genera and 3 families (Cichlidae, Cyprinidae, Goodeidae), including non-African hosts (Table 1). Many of these records need confirmation and are not accompanied by a voucher specimen or published morphological diagnosis. Cichlidogyrus sclerosus has been identified as a primary pathogen in fish culture facilities by Kabata (1986) and Paperna (1996). A congener, Cichlidogyrus philander Douëllou, 1993, causes gill hyperplasia, lamellar fusion, ablation of epithelium, and filament malformation in high intensity infections among wild southern mouthbrooders, Pseudocrenilabrus philander (Weber, 1897) (Cichlidae) from a manmade South African reservoir (Igeh and Avenant-Oldewage 2020). More experimental work that documents intensity-dependent pathological change and studies that demonstrate Koch’s postulates for C. sclerosus would help determine its threat level as an invading, primary pathogen. Rather alarming from the perspective of fisheries and conservation management, infections of C. sclerosus have been reported from local, non-African, non-cichlid fishes in Mexico (blackfin goodea, Goodea atripinnis Jordan, 1880) and Iraq (common carp, Cyprinus carpio Linnaeus, 1758) (Table 1). Considering all host records and phylogenetic host specificity (Table 1), it seems relatively unlikely that non-cichlid endemic fishes would be at high risk for infection and disease caused by Cichlidogyrus spp. However, the fact that non-cichlids can be infected (Abd Al-Khenifsawy and Al-Mayali 2022; Salgado-Maldonado and Rubio-Godoy 2014) is cause for concern. We also acknowledge that probably few workers are examining the gill of native stream fishes for infections by these minute ectoparasites, and perhaps infections among non-cichlid fishes are underreported.