Sperm morphology and morphometry of neotropical catfishes: Pseudoplatystoma reticulatum, Pseudoplatystoma corruscans, Phractocephalus hemioliopterus, and Hemisorubim platyrhynchos

Morfologia e morfometria espermática de bagres neotropicais: Pseudoplatystoma reticulatum, Pseudoplatystoma corruscans, Phractocephalus hemioliopterus e Hemisorubim platyrhynchos

Abstract

The objective of this study was to describe the morphological and morphometric characteristics of spermatozoa from four native fish species using a descriptive approach. Semen samples from adult individuals of Pseudoplatystoma reticulatum, Pseudoplatystoma corruscans, Phractocephalus hemioliopterus, and Hemisorubim platyrhynchos were analyzed. Spermatozoa were fixed and examined by scanning electron microscopy (SEM), and the obtained images were subjected to morphometric analysis using ImageJ software (National Institutes of Health, USA). The sperm cells exhibited typical features of primitive aquasperm, lacking an acrosome and consisting of a head, midpiece, and flagellum, and were classified as complex anacrosomal uniflagellate aquaspermatozoa. Among species, relevant morphometric differences were observed, with greater flagellum length in P. reticulatum; larger head and midpiece volume in P. corruscans; a wider and more compact head in H. platyrhynchos; and greater midpiece and flagellum thickness in P. hemioliopterus. These findings contribute to the understanding of the reproductive biology of these species and provide valuable support for ex situ conservation strategies and the improvement of reproductive protocols.

Keywords:
anacrosomal spermatozoa; electron microscopy; ex situ; siluriformes; species conservation

Resumo

O objetivo deste estudo foi descrever as características morfológicas e morfométricas dos espermatozoides de quatro espécies de peixes nativos, por meio de uma abordagem descritiva. Amostras de sêmen de indivíduos adultos das espécies Pseudoplatystoma reticulatum, Pseudoplatystoma corruscans, Phractocephalus hemioliopterus e Hemisorubim platyrhynchos foram analisadas. Os espermatozoides foram fixados e examinados por microscopia eletrônica de varredura (MEV), e as imagens obtidas foram submetidas à análise morfométrica utilizando o software ImageJ (National Institutes of Health, EUA). As células espermáticas apresentaram características típicas de aquasperma primitivo, sendo desprovidas de acrossoma e compostas por cabeça, peça intermediária e flagelo, sendo classificadas como espermatozoides aquáticos uniflagelados anacrossômicos complexos. Entre as espécies, observaram-se diferenças morfométricas relevantes, com maior comprimento do flagelo em P. reticulatum; maior volume da cabeça e da peça intermediária em P. corruscans; cabeça mais larga e compacta em H. platyrhynchos; e maior espessura da peça intermediária e do flagelo em P. hemioliopterus. Esses achados contribuem para o entendimento da biologia reprodutiva dessas espécies, fornecendo subsídios para estratégias de conservação ex situ e para o aprimoramento de protocolos reprodutivos.

Palavras-chave:
espermatozóides anacrossomais; microscopia eletrônica; ex situ; siluriformes; conservação de espécies

1. Introduction

The order Siluriform represents one of the most diverse groups of freshwater fishes in the world, comprising approximately 3,800 species distributed across 40 families and 490 genera, most of which are endemic to the Neotropical regions (Fricke et al., 2025). Many of these species have major ecological, economic, and aquaculture importance and represent strategic resources for both conservation initiatives and aquaculture production.

Among these species, Pseudoplatystoma reticulatum (Eigenmann & Eigenmann, 1889), Pseudoplatystoma corruscans (Spix & Agassiz, 1829), Phractocephalus hemioliopterus (Bloch & Schneider, 1801), and Hemisorubim platyrhynchos (Valenciennes, 1840) exhibit ecological, biological, and reproductive traits that make them particularly relevant for both aquaculture and conservation. These medium- to large-sized migratory catfish are characterized by a seasonal reproductive cycle, with total spawning and pronounced rheophilic behavior, and reproduction closely associated with fluctuations in river water levels (Resende et al., 1995). In addition to their zootechnical importance, these species play key roles in trophic regulation within aquatic ecosystems (Córdoba et al., 2000; Barthem and Goulding, 1997). From a conservation perspective, their status varies, with P. corruscans currently classified as “Vulnerable” and H. platyrhynchos as “Least Concern” according to the Brazilian Red List (ICMBio, 2025), reinforcing the need for species-specific management and conservation strategies.

With increasing pressure on aquatic ecosystems, including habitat degradation, unsustainable fishing practices, and hydrological alterations, ex situ conservation strategies are being increasingly recognized as complementary and effective tools for biodiversity preservation (Dagosta et al., 2024; Viveiros and Godinho, 2009; Peixoto et al., 2004; Carolsfeld et al., 2003; Suquet et al., 2000). These pressures, particularly overfishing and river flow obstruction, have contributed considerable to the decline of the natural populations of these species (Brasil, 2022).

The morphological and functional characterization of spermatozoa is thus essential in the current context as such characterization provide fundamental data for the establishment of germplasm banks and assisted reproductive programs (Cabrita et al., 2008; Mylonas et al., 2017). In teleost fishes with external fertilization, spermatozoa generally exhibit conserved morphological characteristics and are described as primitive aquasperm, anacrosomal, and uniflagellate (Jamieson, 1991). Nevertheless, interspecific variation in morphometric parameters may reflect adaptations to reproductive strategies and the fertilization microenvironment (Cosson et al., 2008). Moreover, sperm morphology in externally fertilized species is directly related to sperm motility, fertilization efficiency, and cryopreservation capacity, making it an important indicator of reproductive quality (Costa et al., 2022; Viveiros and Godinho, 2009; Billard and Cosson, 1992).

Scanning electron microscopy (SEM) is an effective tool for analyzing sperm morphology at high resolution and is widely used to characterize reproductive structures in conservation and assisted reproduction studies (Psenicka et al., 2007). Understanding the morphological differences between species enables the optimization of reproductive management techniques and provides deeper insight into the functional diversity of spermatozoa in Neotropical fishes.

In this study, owing to the limited number of individuals sampled per species, we adopted an exclusively descriptive approach, focusing on the morphological and morphometric characterization of spermatozoa as a basis for future research on assisted reproduction and genetic conservation. Accordingly, this study aimed to describe the morphometric characteristics of spermatozoa from four Siluriformes species of interest for conservation and aquaculture, thereby contributing to ex situ conservation strategies and expanding our knowledge of the reproductive biology of these fishes.

2. Materials and Methods

2.1. Location and broodstock maintenance environment

The experiment was conducted at a commercial fish farm located in the municipality of Terenos, Mato Grosso do Sul, Brazil (20°25'57" S and 55°17'11" W), during October and November 2022. The study was approved by the Animal Use Ethics Committee (CEUA) of the Federal University of Mato Grosso do Sul (No. 1083/2019).

Nine adult individuals, including three P. reticulatum, two P. corruscans, two P. hemioliopterus, and two H. platyrhynchos (Figure 1), were used. The fish were housed in an earthen pond of area 1,350 m² at a stocking density of 0.8 individuals/m².

Figure 1
Schematic of the experimental design. The figure illustrates the groups of animals subjected to hormonal reproductive induction and the procedures for semen collection and sample processing for morphological analysis.

Daily water renewal accounted for 10% of the total volume. The mean temperature was 28 ± 0.98 °C, with dissolved oxygen of 8.0 ± 0.26 mg/L and pH of 8.0 ± 0.05. Fish were fed once daily in the morning using extruded feed (32% crude protein, 12% ash, 6% ether extract, and 5.5% crude fiber) at 1% of the total biomass.

2.2. Induced reproduction

Broodstock selection was performed according to Woynarovich and Horváth (1983) based on secondary sexual characteristics, such as the presence of a swollen abdomen and a prominent urogenital opening. Selected animals were then transferred to masonry tanks in the laboratory, where they were maintained undercontinuous water renewal (0.06 L/s) and controlled physicochemical parameters (27.2 ± 1.2 °C; 9.9 ± 0.8 mg/L dissolved oxygen; pH 8.3 ± 0.15).

A single dose of 1.0 mg/kg of carp pituitary extract was administered for hormonal management, according to a protocol adapted from Woynarovich and Horváth (1983). Semen was collected from each individual using sterile syringes, following Sanches et al. (2011). For sample preparation, a 2 µL aliquot of semen from each broodstock was fixed in 2,000 µL of buffered saline formaldehyde (4.6%) and diluted 1:1,000 in a 5 mL microtube.

It should be noted that hormonal induction, including post-induction timing and the degree of gamete maturation, may influence seminal quality and sperm ultrastructure, potentially resulting in immature or handling-altered cells (Mylonas et al., 2017). However, collections were standardized to minimize such variations.

2.3. SEM

Analyses were performed at the Electron Microscopy and Microanalysis Laboratory of the State University of Londrina (UEL), in Londrina, Paraná, Brazil. For SEM, previously formaldehyde-fixed samples were reprocessed and post-fixed in 2.5% glutaraldehyde solution in 0.1 M sodium cacodylate buffer, pH 7.2, as glutaraldehyde fixation is required for adequate ultrastructural preservation. The samples were then applied onto coverslips pretreated with 0.1% poly-L-lysine and dried in an oven at 40 °C. After drying, the coverslips were transferred to culture plates containing 1 mL of glutaraldehyde solution per well, sealed, and incubated overnight at 4 °C.

After incubation, samples were washed with 0.1 M sodium cacodylate buffer (pH = 7.2) and fixed with osmium tetroxide (OsO₄) for 1 h. They were subsequently dehydrated through an ethanol gradient (30–100%) and dried to the critical point with carbon dioxide (CO₂). The specimens were then mounted on stubs using carbon tape, sputter-coated with gold (Au) under vacuum, and analyzed using SEM (FEI Quanta 200, Oregon, USA).

2.4. Measurement of morphological and morphometric variables

For each species, fifty spermatozoa were analyzed, totaling 200 cells. Sperm structures were measured from SEM images using ImageJ software (National Institutes of Health, USA; available at http://rsbweb.nih.gov/ij/).

Morphometric variables measured for each spermatozoon included head length (HL), head width (HW), head area (HA), midpiece length (ML), total head and midpiece length (THM), midpiece width (MW), flagellum length (FL), flagellum width (FW), midpiece area (MA), total cell length (TCL), and head and midpiece areas (HMA). Figure 2 illustrates the anatomical delineation of these structures.

Figure 2
Schematic representation of a sperm cell of Siluriformes (catfish), highlighting the anatomical boundaries considered for the measurement of morphometric variables in Pseudoplatystoma reticulatum, Pseudoplatystoma corruscans, Phractocephalus hemioliopterus, and Hemisorubim platyrhynchos. Subtitles - (A) Sperm cell showing head length (HL), head width (HW), flagellum length (FL), and flagellum width (FW); (B) sperm head with the midpiece area (MA) outlined in red; (C) sperm head with the head area (HA) outlined in red.

Because of the limited number of individuals analyzed per species, the results are presented descriptively, expressed as mean ± standard deviation, and no inferential statistical tests were performed. Interpretations were based on the direct observation of sperm structures and comparisons with previously published data in scientific literature.

3. Results

All the four species exhibited spermatozoa with a morphology consistent with that of primitive aquasperms, characterized by the absence of an acrosome and the presence of a head, midpiece, and flagellum.

Morphometric analyses revealed notable differences among the studied species (Table 1), potentially reflecting adaptations to the fertilization environment. Pseudoplatystoma reticulatum exhibited spermatozoa with shorter heads and longer flagella (Figure 3a), whereas Pseudoplatystoma corruscans showed the greatest head and midpiece lengths (Figure 3b). Hemisorubim platyrhynchos presented the largest head width (Figure 3c). In Phractocephalus hemioliopterus, the greatest midpiece and flagellum widths were recorded (Figure 3d); however, the absence of intact spermatozoa with fully preserved flagella precluded the measurement of flagellum length and, consequently, total cell length. Therefore, these variables were not included for this species. This limitation did not affect other morphometric parameters, which were measured in adequately preserved cells.

Table 1
Means (± standard deviation) of morphometric parameters (µm or µm²) of sperm cells from P. reticulatum, P. corruscans, H. platyrhynchos, and P. hemioliopterus analyzed using scanning electron microscopy (SEM).

Variable	P. reticulatum	P. corruscans	H. platyrhynchos	P. hemioliopterus
HL (µm)	1.42 ± 0.72	1.61 ± 0.19	1.60 ± 0.12	1.53 ± 0.17
ML (µm)	0.78 ± 0.17	0.93 ± 0.21	0.76 ± 0.18	0.93 ± 0.16
FL (µm)	34.09 ± 3.41	31.86 ± 4.28	29.47 ± 2.94	–
THM (µm)	2.13 ± 0.18	2.49 ± 0.12	2.39 ± 0.90	2.35 ± 0.27
TCL (µm)	36.93 ± 3.76	35.28 ± 4.60	32.62 ± 3.21	–
HW (µm)	1.60 ± 0.42	1.71 ± 0.12	1.92 ± 0.27	1.91 ± 0.19
MW (µm)	1.34 ± 0.18	1.14 ± 0.58	1.25 ± 0.32	1.46 ± 0.36
FW (µm)	0.32 ± 0.05	0.36 ± 0.05	0.36 ± 0.04	0.40 ± 0.08
HA (µm2)	2.05 ± 0.32	2.38 ± 0.29	2.40 ± 0.37	2.22 ± 0.26
MA (µm2)	0.78 ± 0.16	1.43 ± 0.22	1.20 ± 0.32	1.28 ± 0.32
HMA (µm2)	2.65 ± 0.23	3.27 ± 0.32	3.44 ± 0.35	3.38 ± 0.47

• HL, head length; ML, midpiece length; FL, flagellum length; THM, total head and midpiece length; TCL, total cell length (head, midpiece, and flagellum); HW, head width; MW, midpiece width; FW, flagellum width; HA, head area; MA, midpiece area; HMA, total area (head and midpiece).

Figure 3
Images of sperm head morphology from the four species obtained using SEM at 30,000×. (A) P. reticulatum, (B) P. corruscans, (C) H. platyrhynchos, (D) P. hemioliopterus.

4. Discussion

To the best of our knowledge, this study represents the first comparative morphometric description of the selected four Neotropical catfish species, providing novel data that may support the development of species-specific reproductive management strategies for juvenile production or germplasm conservation.

Morphometric analysis of spermatozoa from the studied species revealed a typical primitive aquasperm structure, commonly observed in externally fertilizing species and potentially associated with selective pressures such as short motility duration, rapid activation, and intense sperm competition (Jamieson, 1991; Fitzpatrick, 2020; Liao et al., 2018).

The morphological diversity of Siluriformes spermatozoa may be associated with variation in spawning environments and mating strategies among species. This relationship has been discussed in comparative analyses of externally fertilizing species (Fitzpatrick, 2020), studies on alternative reproductive tactics in fish (Taborsky et al., 2018), and investigations of the reproductive environment and sperm performance in salmonids (Rosengrave et al., 2024).

In the present study, consistent variations were observed in the mean values of specific sperm structures, such as flagellum length and head area (Table 1). These morphological differences in sperm size may be associated with the fertilization microenvironment, species-specific reproductive strategies, or mechanisms related to the compaction and transport of paternal genetic material (Angrimani et al., 2015; Fitzpatrick and Lüpold, 2014).

The head and midpiece were notably more developed in the spermatozoa of Pseudoplatystoma corruscans than in the other species, suggesting a larger cytoplasmic volume potentially associated with a higher number of mitochondria. This configuration may reflect an adaptation to the high-energy demands of rheophilic environments, where strong currents require greater locomotor effort for spermatozoa to reach the egg. A longer flagellum was observed in Pseudoplatystoma reticulatum, which may provide enhanced propulsion in the aquatic environment. This feature could be advantageous for external fertilization under flowing water, potentially increasing sperm swimming efficiency and fertilization success (Molloy et al., 2007; Liao et al., 2018; Fitzpatrick, 2020).

The compact morphology observed in H. platyrhynchos, with a wider head and shorter midpiece, may be linked to its greater structural stability, which facilitates egg penetration in densely structured substrate environments. This assumption is supported by the observations of Fitzpatrick (2020), Stockley et al. (1997), and Marconato et al. (1996), who suggested that spermatozoa with the configuration found in H. platyrhynchos may represent a functional adaptation for rapid sperm release in environments with lower sperm competition or in spawning areas offering greater physical protection, such as sheltered substrates. In contrast, the spermatozoa of P. hemioliopterus exhibited features indicative of greater structural robustness, such as pronounced width of the midpiece and flagellum. These characteristics may be associated with enhanced stability of motility in turbulent aquatic environments, as discussed in detailed analyses of flagellar morphology and sperm motility (Cosson et al., 2008; Mattei, 1988). To date, there are no records in the scientific literature regarding the morphometric characteristics of P. reticulatum spermatozoa.

Images obtained using SEM supported these observations. The analysis allowed the characterization of external sperm morphology, consistent with the anacrosomal pattern described for teleost fish. In addition, the definition of sperm structures and their visual proportions may provide insights into functional interpretations, such as the relationship between cell shape and displacement capacity in aquatic environments. However, precise delineation between structures, particularly between the head and midpiece, is not possible using SEM alone, requiring complementary analysis by transmission electron microscopy (TEM) for more accurate structural identification.

On comparing the results obtained in our study with data available in the literature, the dimensions of the head, midpiece, and flagellum of the spermatozoa were observed to be smaller than those reported by Velarde et al. (2023) for P. corruscans. The authors described an HL of 2 µm, an HW of 2.32 µm, an ML of 1.5 µm, and an FL of 32.6 µm. For H. platyrhynchos, an HW of 6.7 ± 0.3 µm, an FL of 84.1 ± 0.56 µm, and a total length of 93.2 ± 5.5 µm were reported; for P. hemioliopterus, an HW of 6.3 ± 0.3 µm, an FL of 82.8 ± 8.2 µm, and a total length of 91.4 ± 7.9 µm were observed.

The discrepancies reported in the literature may be associated with variation in sperm quality among the individuals analyzed, which is influenced by a range of extrinsic and intrinsic factors (Costa et al., 2022; Gallo et al., 2022; Żarski et al., 2016; Kowalski and Cejko, 2019; Araujo et al., 2014). These discrepancies can be largely attributed to the methodological differences, particularly regarding the type of microscopy, adopted in the studies mentioned. Although light microscopy tends to overestimate cell dimensions owing to resolution limitations and distortion caused by optical artifacts, SEM, which was employed in this study, may slightly underestimate the values because of sample preparation (fixation, dehydration, and sputter coating) and surface visualization of structures.

From an applied perspective, the findings from this study reinforce the importance of accurate morphological characterization for the development of assisted reproduction protocols and ex situ conservation. Although Fitzpatrick (2020) highlighted sperm morphology as a predictor of fertilization success, the caveat noted by Kommisrud et al. (2020) must be considered; despite its valuable insights on sperm quality, sperm morphology does not constitute a definitive predictor of fertilization success in artificial insemination protocols.

The differences observed in the morphometric and structural parameters of spermatozoa among the species may have direct implications for cryopreservation efficiency and fertilization rates and should be considered when selecting species-specific protocols for the production of Neotropical fish juveniles.

5. Conclusions

The spermatozoa of P. reticulatum, P. corruscans, P. hemioliopterus, and H. platyrhynchos exhibited characteristics consistent with those of primitive aquasperms. Pseudoplatystoma reticulatum had the largest flagellum, P. corruscans had the largest head and midpiece volume, H. platyrhynchos had the widest and most compact head, and P. hemioliopterus had highest midpiece and flagellum thickness.

Acknowledgments

This study was partially funded by the Coordination for the Improvement of Higher Education Personnel (CAPES) - Financial Code 001. This study was partially funded by the Federal University of Mato Grosso do Sul – UFMS/MEC – Brazil. We thank the State University of Londrina (UEL) and Piscicultura Piraí for their support. We also thank the Conselho Nacional de Desenvolvimento Científico e Tecnolgico (CNPq) for financial support through projects: 444572/2024-4 and 403157/2024-2. Researchers D.P. Streit Jr (CNPq grant 305387/2022-7), D.H. Siqueira-Silva (CNPq grant 313053/2022-7) and J.A. Povh (CNPq-grant 312072/2021-0) are CNPq research fellows.

Data Availability Statement

All the data supporting the results of this study were published in the article itself;

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Edited by

• Editor:
  Takako Matsumura Tundisi

Publication Dates

• Publication in this collection
  10 July 2026
• Date of issue
  2026

History

• Received
  10 Nov 2025
• Accepted
  25 Mar 2026