Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

Histomorphometry of the gastrointestinal tract of the fish

Pseudoplatystoma magdaleniatum (Siluriformes: Pimelodidae)

Luisa Zapata-Muñoz1; https://orcid.org/0009-0009-8678-0864

Diana López-Obando1; https://orcid.org/0009-0007-6417-3444

Gersson Vásquez-Machado2; https://orcid.org/0000-0002-4737-7038

Mariana Gutiérrez-Espinosa3; https://orcid.org/0000-0001-6127-9955

Ana Estrada-Posada4; https://orcid.org/0000-0003-3585-3719

Jonny Yepes-Blandón1*; https://orcid.org/0000-0001-6276-5488

1. Research Group on Native and Exotic Aquatic Organisms. Facultad de Ciencias Agrarias, Universidad de

Antioquia, Medellín, Colombia; luisazapata75@gmail.com, diana.lopez11@udea.edu.co, jonny.yepes@udea.edu.co

(*Correspondence)

2. HISTOLAB, Bogotá, Colombia; gmvasquezm@unal.edu.co

3. Centro de estudio e investigación en acuicultura, Villavicencio, Colombia; marianacgutierreze@gmail.com

4. ISAGEN S.A. E.S.P, Medellín, Colombia; aestrada@isagen.com.co

Received 14-II-2024. Corrected 21-IV-2025. Accepted 09-VI-2025.

ABSTRACT

Introduction: Pseudoplatystoma magdaleniatum, commonly known as striped catfish, is an endemic species of

the Magdalena River basin, characterized by its large size and high commercial value. Given its critical endanger-

ment due to overfishing, understanding its gastrointestinal tract morphology is crucial for conservation efforts

and management in fish stocking programs.

Objective: To characterize the morphology, histology, and histochemical qualities of the gastrointestinal tract of

P. magdaleniatum, an endemic fish species in the Magdalena River basin, Colombia.

Methods: Measurements of body height and weight of 22 captured adult individuals were taken, as well as of the

organs comprising the digestive tract (esophagus, stomach, and intestine), and accessory and glandular organs

(liver and gonads). Histological techniques, such as Hematoxylin and Eosin staining, were performed to charac-

terize the organs structurally. Histochemical techniques were employed to describe the dynamics of mucins, and

transmission electron microscopy was used.

Results: The stomach and intestine exhibited four layers: mucosa, submucosa (absent in the esophagus), muscu-

lar, and serosa. The esophagus, with only three layers, was characterized by the presence of stratified squamous

epithelium with goblet cells, club cells, and taste buds. Neutral mucins were detected along the esophagus, while

acidic mucins were observed in the cranial and middle regions. The stomach featured a simple columnar epithe-

lium with abundant gastric glands and exclusively neutral mucins. Finally, the intestine was characterized by a

mucosal tunic of simple cylindrical epithelium composed of enterocytes and goblet cells, abundant folds, and the

presence of sulfated and carboxylated neutral and acidic mucins.

Conclusions: P. magdaleniatum exhibited a relatively short intestine for its size and weight. The histology of

the gastrointestinal tract further supports adaptations for a protein-rich diet. These findings provide valuable

insights for understanding the digestive physiology of this endangered species, which may inform conservation

efforts and management strategies.

Key words: fish; Siluriformes; histology; histochemistry; goblet cells; mucins; digestive physiology; Magdalena

River basin.

https://doi.org/10.15517/rev.biol.trop..v73i1.58796

VERTEBRATE BIOLOGY

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

INTRODUCTION

Pseudoplatystoma magdaleniatum (family

Pimelodidae) described by Buitrago-Suárez &

Burr (2007), commonly known as striped cat-

fish, surubí or pintadillo, is a potadromous fish

endemic of the Magdalena River basin and the

largest within the ichthyofauna of this fluvial

system, reaching up to 1.4 m in length and 70

kg in weight. It inhabits the main rivers of the

basin, as well as its floodplains. It is character-

ized by its elongated dark gray body and white

belly with dark transverse stripes, large head,

and small eyes located in a dorsal position

(Buitrago-Suárez & Burr, 2007), the upper jaw

is longer than the lower jaw and the teeth are

small (Lasso et al., 2011). It is a carnivorous

species that is located among the higher trophic

positions within the ecosystems of the basin,

feeding mainly from fish and shrimp, which

represent approximately 78 % of its stomach

content (Cortés-Millán, 2003).

P. magdaleniatum is a carnivorous cat-

fish, primarily feeding on smaller fish and

crustaceans, which places it at the top of the

river’s trophic chain (Ayala, 2022). This spe-

cies typically inhabits the main branches and

floodplains of the Magdalena River basin, with

seasonal migrations related to reproduction

and feeding patterns (Mojica et al., 2012).

Therefore, understanding the gastrointestinal

morphology of P. magdaleniatum in relation to

its feeding habits and habitat is crucial for com-

prehending its ecological role and for develop-

ing effective conservation strategies.

P. magdaleniatum is one of the species with

the highest commercial value in the Magdalena

River basin; however, this fish is subjected to

high fishing pressure, which has led to a decline

in natural populations and a reduction in catch

RESUMEN

Histomorfometría del tracto gastrointestinal del pez

Pseudoplatystoma magdaleniatum (Siluriformes: Pimelodidae)

Introducción: Pseudoplatystoma magdaleniatum, comúnmente conocido como bagre rayado, es una especie

endémica de la cuenca del río Magdalena, caracterizada por su gran tamaño y alto valor comercial. Dada su

crítica amenaza debido a la sobrepesca, comprender la morfología de su tracto gastrointestinal es crucial para los

esfuerzos de conservación y manejo en programas de repoblación de peces.

Objetivo: Caracterizar la morfología, histología e histoquímica del tracto gastrointestinal de P. magdaleniatum,

una especie de pez endémica en la cuenca del río Magdalena, Colombia.

Métodos: Se capturaron 22 individuos adultos y se tomaron medidas de altura y peso corporal, así como de los

órganos que componen el tracto digestivo (esófago, estómago e intestino), y órganos accesorios y glandulares

(hígado y gónadas). Se realizaron técnicas histológicas, como tinción de Hematoxilina y Eosina, para caracterizar

estructuralmente los órganos. Se emplearon técnicas histoquímicas para describir la dinámica de las mucinas, y

microscopia electrónica de transmisión.

Resultados: El estómago e intestino exhibieron cuatro capas: mucosa, submucosa (ausente en el esófago), mus-

cular y serosa. El esófago, con solo tres capas, se caracterizó por la presencia de epitelio escamoso estratificado

con células caliciformes, células en copa y botones gustativos. Se detectaron mucinas neutras en el esófago, y las

mucinas ácidas se observaron en las regiones craneal y media. El estómago presentaba un epitelio columnar sim-

ple con abundantes glándulas gástricas y mucinas neutras. Finalmente, el intestino se caracterizó por una túnica

mucosa de epitelio cilíndrico simple compuesto por enterocitos y células caliciformes, pliegues abundantes y la

presencia de mucinas neutras y ácidas sulfatadas y carboxiladas.

Conclusiones: P. magdaleniatum mostró un intestino corto en relación con su tamaño y peso. La histología del

tracto gastrointestinal respalda las adaptaciones para una dieta rica en proteínas. Estos hallazgos proporcionan

valiosas ideas para comprender la fisiología digestiva de esta especie en peligro de extinción, lo que puede infor-

mar los esfuerzos de conservación y las estrategias de manejo.

Palabras clave: pez; siluriformes; histología; histoquímica; células caliciformes; mucinas; fisiología digestiva;

cuenca del río Magdalena.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

by approximately 90 %, which is why this spe-

cies is currently listed as critically endangered

in the red book of Colombian freshwater fish

(Mojica et al., 2012). Different conservation

strategies have been proposed for this species,

such as the establishment of a legal minimum

capture size, which is currently 80 cm (Lasso

et al., 2011), as well as closed seasons and

semen cryopreservation programs (Herrera-

Cruz et al., 2019). In Colombia, other strategies

have been proposed aimed at the recovery and

conservation of fishing resources through fish

stocking programs, taking into account techni-

cal recommendations, such as the stocking time

and site, the guarantee of genetic purity, the

diversity of fish in the body of water and the

survival of the individuals destined for repopu-

lation programs (Atencio, 2001), as well as opti-

mal environmental conditions for adaptation;

however, these processes imply an appropriate

management of the reproducers through fac-

tors such as density and feeding (Atencio, 2001;

Esmaeili et al., 2024).

The study of the structure and organiza-

tion of the gastrointestinal tract in fish is of

great interest because, in general, there is a

correlation between the structure of a fish’s

digestive system and its dietary habits (Hassan,

2013); thus, the basic understanding of this sys-

tem allows optimizing procedures required in

the management of broodstock in fish stocking

programs and consequently, the conservation

of species of great ecological, reproductive,

nutritional, economic and cultural value such

as P. magdaleniatum. Therefore, the objective

of this study was the morphological, histologi-

cal, and histochemical characterization of the

gastrointestinal tract of the P. magdaleniatum,

an endemic fish species of the Magdalena

River basin.

MATERIAL AND METHODS

Animal ethics: All procedures related to

animal handling were performed in accordance

with the Guide for the Care and Use of Labo-

ratory Animals (Albus, 2012), the meeting

minute 134 of the committee, and a permit

granted by the National Aquaculture and Fish-

eries Authority of Colombia-AUNAP, under

the Resolution 0955 (May 27th, 2020).

Animals and husbandry: A total of 22

adult specimens (males and females) of P. m ag-

daleniatum were used, which were captured

from the natural environment in the Magda-

lena River basin, between the years 2018-2020

and kept in the facilities of the Piscícola San

Silvestre Fish Farm (PSS). Prior to the experi-

mental procedures, the individuals were trans-

ferred to land ponds at a density of one fish

per m2 and were subjected to a 24 h quarantine

period in filtered water and two baths with salt

(20 ppm for 30 seconds).

Body indexes: For the morphometric

characterization, from the 22 fish initially col-

lected, 15 specimens were selected for detailed

analysis based on their condition and size range

to ensure a representative sample. The remain-

ing 7 specimens were excluded due to damage

during collection or to avoid bias from extreme

size variations of which total (TL), standard

(SL), intestinal (IL) and gastrointestinal (GIT)

length (cm) measurements were recorded; body

weight (g) (BW), eviscerated (EW), gastrointes-

tinal content (esophagus, stomach, intestine-

GIC) and individual weight of the following

organs: esophagus, stomach, intestine, liver,

gonads. For these measurements, an ichthyom-

eter and an Ohaus® digital scale were used.

Measurements of all individuals were

made with the same instrument. Photographic

records were made with an EOS Rebel T3i

camera with a Canon EF-S 18-55 mm f/4-5.6 IS

STM lens for the description of the anatomical

characteristics.

From these measurements the following

morphometric relationships were calculated:

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

Histology and histochemistry analysis:

To characterize the microscopic morphostruc-

ture of the gastrointestinal tract, the organs of

the gastrointestinal tract (esophagus, stomach,

and intestine) were removed, and fixed in 10

% buffered formalin solution in a ratio of 1 : 10

for 24 hours. The organs were also injected with

this solution for their subsequent preservation.

Tissues were dehydrated in ethanol at seri-

al concentrations and subsequently cleaned

with toluene or equivalent, followed by imbibi-

tion in hot paraffin and cold molding. Sections

with a thickness of 5 to 6 μm were made with a

rotary microtome.

The sections obtained were stained with

hematoxylin and eosin (H&E), according to

standard procedures, and photographs were

taken with an Olympus CX 23 optical micro-

scope and Basler ACA5472-17UC digital cam-

era for characterization of the structure of the

digestive system (Gosavi et al., 2019).

For the detection of neutral mucins, the

PAS (Periodic Acid-Schiff) technique was used,

and for acidic mucins, AB staining (Alcian

Blue) was performed with pH 1.0 and pH 2.5

(AB pH1.0 and AB pH 2.5, respectively). The

sequential technique AB pH 2.5 + PAS was

carried out to detect the association between

neutral and acidic mucins. For the specific

detection of mucins of epithelial origin and

acidic mucins from the gastrointestinal tract,

staining with mucicarmine was used. Masson’s

trichrome staining was used to detect collagen

fibers in connective tissue.

Transmission Electron Microscopy

(TEM): A tissue fragment is fixed in 2.5 % buff-

ered glutaraldehyde in PBS. Samples were post-

fixed in 1 % osmium tetroxide and 3 % uranyl

acetate, progressively dehydrated, infiltrated

in 1 : 1 plastic resin mixed with acetone, and

embedded in SPURR resin (Electron Micros-

copy Sciences, Fort Washington, PA, USA).

Plastic blocks were cut with a Sorvall MT2-B

ultramicrotome. Semithin sections (1 µm) were

stained with toluidine blue and evaluated to

identify appropriate areas for ultrathin sections.

These areas were diamond cut to a thickness of

80-100 nm (interference color yellow-gold) and

placed on 200 mesh copper grids. Subsequently,

the sections were contrasted with uranyl acetate

and lead citrate and were examined and photo-

graphed with a JEOL 1400 Plus transmission

electron microscope from the Hospital Univer-

sitario Fundación Santa Fe de Bogotá, Depart-

ment of Pathology.

Statistical analysis: A completely ran-

domized experimental classification design

was applied. Descriptive statistics were used to

characterize the sample regarding morphologi-

cal and zootechnical parameters, body parame-

ters and indices were expressed as mean ± STD

(standard deviation). The histochemical results

were interpreted based on the staining intensity

of the tissue for each staining technique. The

level of intensity was determined by qualitative

inspection and the staining intensity results

were described by means of a table of crosses.

RESULTS

The gastrointestinal tract of P. magdale-

niatum was characterized by presenting the

oropharyngeal region followed by a short sac-

shaped esophagus connected to the stomach

followed by a short, slightly coiled intestine that

lacks pyloric caeca (Fig. 1).

The data for body measurements and diges-

tive organs of P. magdaleniatum is presented in

Condition factor (K) = (Body Weight (BW)) / (Total length3(TL3) ×100

Intestinal Coefficient (CO) = (Intestinal Lenght (IL)) / (Total Lenght (TL))

Zihler’ s Index (Z.I) = (IL) / (10× ∛ (TW))

Gonadosomatic Index (GSI) = (Gonads Weight (GW)) / (TW)×100

Hepatosomatic Index (HSI) = (Liver Weight (LW)) / (TW) × 100

Carcass Yield (CY) = (Dressed Carcass Weight (DCW)) / (TW)

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

Table 1. On average, the 15 individuals analyzed

weighed 1690.33 ± 827.6 g. The total and stan-

dard body heights were 65.8 ± 6.1 cm and 55.3

± 5.7 cm, respectively (Table 1).

The body indices for the 15 specimens

analyzed are presented in Table 2. The average

value of the condition factor (K) was 0.56 ±

0.07. Additionally, the intestinal coefficient (IC)

measured 0.49 ± 0.08, while the Zihler index

(ZI) averaged 0.64 ± 0.17. The hepatosomatic

index (HSI) had an average value of 0.51 ± 0.12,

the carcass yield (CY) was 0.94 ± 0.10, and the

gonadosomatic index (GSI) averaged 0.3 ± 0.24.

Histology and histochemistry analysis

Esophagus: Histologically, the esophagus

exhibits a mucosal lining characterized by a

Fig. 1. A. Internal body organization of P. magdaleniatum showing branchial arch, liver, esophagus, stomach, intestine,

gallbladder, swim bladder, and gonads. B. Gastrointestinal tract: esophagus (es), stomach cardial region (ca), stomach fundic

region (fu), stomach pyloric region (pyl), anterior intestine (ai), middle intestine (im), posterior intestine (ip). C. Oral cavity

with structures similar to dental plaques observed in the background.

Table 1

Records of body data of 15 individuals of striped catfish (P. magdaleniatum).

Measurement Mean SD Minimum Maximum

Weight (gr) 1690.3 827.6 1075.0 4426.0

Eviscerated (gr) 1586.9 805.0 978.0 4220.0

TL (cm) 65.8 6.1 59.0 83.0

SL (cm) 55.3 5.7 50.0 72.0

GIT (cm) 26.6 9.5 13.8 48.7

GIC (gr) 53.4 40.1 26.6 192.6

Esophagus (gr) 8.2 3.1 4.5 14.2

Stomach (gr) 12.9 6.2 6.3 32.2

Intestine (gr) 7.6 2.8 5.5 16.5

Intestine (cm) 32.4 6.2 24.5 53.0

Liver (gr) 8.7 4.5 5.1 22,7

Gonads (gr) 4.7 4.6 1.2 16.3

Abbreviations: TL= Total length, SL = Standard length, GIT = Gastrointestinal length, GIC = Gastrointestinal content, SD

= Standard deviation.

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

non-keratinized stratified squamous epithe-

lium throughout its entire length. Abundant

mucus-secreting cells are present along the

length of the organ, as well as below the epithe-

lium. Additionally, round and oval cells with

eosinophilic cytoplasm were observed, each

containing a centrally located basophilic round

nucleus; some of these cells appeared binucle-

ated (Fig. 2A), potentially corresponding to

club cells. Beneath the basal membrane of the

epithelium, a thick layer of loose connective tis-

sue was observed. This connective tissue layer

Table 2

Body index of 15 individuals of striped catfish (P. magdaleniatum).

Measurement Mean SD (±) Minimum Maximum

K 0,56 0,07 0,47 0,77

CO 0,49 0,08 0,40 0,75

ZI 0,64 0,17 0,23 0,93

HSI 0,51 0,07 0,38 0,66

CY 0,94 0,10 0,63 1,12

GSI 0,30 0,24 0,10 0,94

Abbreviations: K = Condition factor, CO = Intestinal coefficient, ZI = Zihler’ s Index, HIS = Hepatosomatic index, CY =

Carcass yield, GSI = Gonadosomatic index, SD = Standard deviation.

Fig. 2. A. Macroscopic image of the portion of the digestive tract of P. magdaleniatum made up of the esophagus (esf),

stomach (est) and intestine (int). From image. B. histological organization and distribution of esophageal mucins. Stratified

squamous epithelium made up of goblet cells (cc) and club cells (ccl) is observed on the basal lamina (up arrow) and

submucosal lamina propria (lp-s). The presence of neutral (image d – sign +) and acid (images e, f and g – sign *) mucins were

detected in the goblet cells. Staining: b) Hematoxylin and Eosin 100X. C. Masson’s trichrome 40X D. PAS 40X. E. Mucicarmin

40X. F. Alcian Blue (AB) pH 2.5 40X. G. Alcian Blue (AB) pH 1.0. H. PAS/AB pH 2.5.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

is likely the submucosa propria, as no muscular

layer was observed within the mucosa, resulting

in an absence of separation between the lamina

propria and the submucosal layer.

Moving towards the outer layers, we

observed a layer of striated skeletal muscle,

which corresponds to the muscular tunic. In

certain sections, two layers became apparent:

an inner circular layer and an outer longitu-

dinal layer, with the latter appearing thicker.

External to the tunica muscularis, a thin layer

of loose connective tissue was observed, which

corresponds to the tunica adventitia (Fig. 2B).

Within the initial third of the esophageal muco-

sa, the presence of corpuscles or taste buds

was noted. Employing combined staining tech-

niques, we observed numerous goblet cells with

an oval morphology that covered the entire

length of the esophagus.

In the cranial third, we exclusively observed

labeling for sulfated and carboxylated acid

mucins (Fig. 2E, Fig. 2F). In the middle por-

tion, labeling was observed for both sulfated

and carboxylated acidic mucins, along with

neutral mucins, with the labeling of acidic

mucins being predominant (Fig. 2H). Finally, in

the distal third, which is closest to the stomach,

we exclusively observed labeling for neutral

mucins (Fig. 2D).

Stomach: The stomach exhibited a sac-

like structure divided into three regions: the

cardiac, fundic, and pyloric regions, each of

which, presenting mucosa with folds. Histo-

logically, this organ consisted of four layers

(mucosa, submucosa, muscular, and serosa).

The mucosal layer was lined by a simple colum-

nar epithelium composed of tall, slender cells

with oval nuclei primarily located toward the

basal region. Beneath the epithelium was the

lamina propria (LP), and within it, there were

abundant gastric glands that formed clusters.

These glands were either divided or surround-

ed by connective tissue septa and primar-

ily comprised oxynticopeptic cells, which were

most prominently observed in the cardiac and

fundic regions.

In the pyloric region, the glands predomi-

nantly presented cylindrical cells. Beyond the

lamina propria, the mucosal layer presented

a slender layer of smooth muscle known as

the muscularis mucosae. Below the mucosal

layer, there was a substantial layer of connec-

tive tissue, corresponding to the submucosal

layer, comprised of TCL (type connective tis-

sue layer). External to this, the muscularis

layer was observed, consisting of two layers of

smooth muscle: an inner circular layer and an

outer longitudinal layer. Externally, a thin layer

of TCL, termed the tunica serosa, was pres-

ent. The distribution of mucins in all stomach

regions exclusively corresponded to neutral

mucins in both the epithelial and glandular

regions (Fig. 3).

Intestine: The intestine of P. magdale-

niatum was divided into three regions: anterior

(Fig. 4), middle (Fig. 5) and posterior-rectal

(Fig. 6). The three regions presented a mucosal

tunic covered by a simple columnar epithelium

composed mainly of absorptive cells (entero-

cytes) in the anterior and middle portions and

a smaller number of mucus cells (goblet cells).

In the posterior portion, goblet cells are more

abundant, and the number of enterocytes is

reduced. Additionally, the mucosa was char-

acterized by the presence of long folds that are

projected towards the lumen and its branches,

forming an intricate pattern of distribution.

Beneath the epithelium, we observed a well-

visible layer of TCL corresponding to the lam-

ina propria. External to the mucosal layer, we

observed the muscularis layer, consisting of two

layers of smooth muscle: an inner circular layer

and an outer longitudinal layer. Beyond this

layer, there was a thin layer of loose connec-

tive tissue known as the tunica serosa. Histo-

chemical analysis revealed the presence of acid

sulfated, carboxylated, and epithelial mucins,

with a predominant presence. Additionally,

neutral mucins were also detected. The label-

ing was more pronounced in the apical portion

of the folds, as the number of mucus-secreting

cells was higher in this region compared to the

middle and basal sections.

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The presence of neutral mucins was detect-

ed throughout the digestive tract of P. magdale-

niatum, observing a higher intensity of staining

in the stomach. Acidic mucins were detected in

the esophagus, except in the caudal portion and

in the entire stomach. They were also observed

in all three regions of the intestine.

In the organ epithelium of the GIT of

striped catfish, goblet cells are identified in the

tunica mucosa of the esophageal epithelium,

some projecting into the lumen (Fig. 7A). In

the stomach, the epithelium is observed with

a simple organization and with the presence of

some more electron-dense nuclei compared to

the cytoplasm; likewise, the presence of goblet

cells is not detected (Fig. 7B). Concerning the

intestine, we observed the presence of entero-

cytes in the mucosal layer of the epithelium.

These enterocytes featured microvilli project-

ing toward the lumen from the apical region

of the cell, effectively increasing the surface

area for absorption and secretion in the intes-

tine. Additionally, goblet cells were also evi-

dent, characterized by the presence of mucin

granules within them. These granules varied

in electron density, with some being more

electron-dense than others. Furthermore, these

goblet cells projected their contents toward the

lumen of the organ (Fig. 7C, Fig. 7D, Fig.7E).

DISCUSSION

The relationship between body length and

weight is commonly employed in fish stud-

ies as it enables the establishment of poten-

tial growth and nutrition patterns (Bagenal &

Tesch, 1978). The condition factor (K) is used

as a health indicator in studies on fish biology

(Muller-Gomiero et al., 2008). This parameter

is influenced by variables such as the fish’s age,

Fig. 3. A. Macroscopic image of the stomach of P. magdaleniatum, external view of the stomach (+), internal view and folds

inside the stomach (x), opening to the intestine (-). From image. B. Histological organization and distribution of stomach

mucins. The tunica mucosa of the stomach is made up of a simple columnar epithelium (ecs) and the lamina propria (lp)

(images b, c, d and e). The presence of gastric glands (up arrow) is observed in this region. On the tunica mucosa, the tunica

muscularis mucosa (tmm) and the tunica submucosa (tsm) made up of connective tissue (image f) can be seen. The presence

of neutral mucins is detected (image g – sign *); however, acid mucins are not detected (image g – sign *). B.-E. Hematoxylin

and eosin 100X. F. Masson’s trichrome 40X. G. PAS 40X. H. Mucicarmin 40X. I. PAS/AB pH 2.5.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

the sampling season, sex, individual’s stage of

sexual maturity, the amount of stomach and

intestinal content, as well as fat reserves and

muscle mass (Barnham & Baxter, 1998).

This factor allows comparisons to be made

between fish populations subjected to different

conditions of climate, temperature, feeding,

densities, and water quality (de Oliveira-Fel-

izardo et al., 2011). High condition factor val-

ues indicate that animals have experienced

favorable conditions, whereas low values sug-

gest less favorable conditions (Radkhah &

Eagderi, 2015). The catfishes analyzed were

captured in their natural environment in the

Magdalena River basin at different times and

their condition factor K at the time of sampling

was 0.56 ± 0.07.

According to Nandita & Ujjania (2017),

K factor values equal to or greater than 1.0

are adequate since they would indicate a good

level of feeding and appropriate environmental

conditions. On the other side, Bagenal & Tesch

(1978) recommended K value ranges for fresh-

water fish between 2.9 and 4.8. Although the

K factor calculated for the specimens analyzed

in this study represents a single measurement

taken at one point in time, the range of val-

ues obtained (0.44 and 0.77) may be related

Fig. 4. A. Macroscopic image of the anterior region of the intestine (ant) of P. magdaleniatum. Part of the stomach (est) is also

observed, as well as an extension of the intestine (+). From image. B. Histological organization and distribution of mucins

from the anterior region of the intestine of P. magdaleniatum. The simple columnar epithelium made up of enterocytes (e)

and goblet cells (cc) is observed (image b). Followed by the epithelium, the lamina propria (lp) and the muscular tunic

(tm) are observed, as well as the folds of the intestine that branch off (arrow to the left – image). C. The presence of neutral

mucins is detected in the cc (image d), as well as carboxylated and sulfated acidic mucins (images E., F. and G.). Stains: B.

Hematoxylin and eosin 100X. C. Masson’s trichromic 40X. D. PAS 40X. E. Mucicarmin 40X. F. Alcian Blue (AB) pH 2.5 40X.

G. Alcian Blue (AB) pH 1.0. H. PAS/AB pH 2.5.

10 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

to variability in the reproductive cycle of the

specimens, which has been reported in captive

fish populations (Herrera-Cruz et al., 2019).

On the other hand, intestinal length is

a morphometric parameter commonly asso-

ciated with diet and eating habits in verte-

brates (Kramer & Bryant, 1995). In teleost fish,

there is a well-documented close relationship

between intestinal morphology and diet. Her-

bivorous fish tend to exhibit elongated, thin,

loop-organized intestines, whereas fish with

carnivorous diets typically have larger intes-

tines, less folding, and shorter lengths relative

to their body size (Day et al., 2014). Based on

various studies, it has been possible to cat-

egorize fish according to their feeding habits

(Al-Hussaini, 1949; Kapoor et al., 1976; Ward-

Campbell et al., 2005). Taking these references

into account, along with the results of the intes-

tinal coefficient (0.49 ± 0.08) and Zihler index

(0.64 ± 0.17) obtained for the analyzed catfish

in this study, it can be concluded that they fall

within the range associated with carnivorous

fish habits (0.2 to 2.5).

The hepatosomatic (HSI) and gonadoso-

matic (GSI) indices serve as indicators of repro-

ductive activity in fish due to the mobilization

of energy reserves from the hepatopancreas to

the gonads (Revathi et al., 2012). According to

Hisao (1985), teleost fish have an average HSI

between 1-2 with a maximum of 3. The average

HSI of P. magdaleniatum in this study was 0.51,

suggesting that the analyzed catfish had low

liver reserves at the time of sampling, possibly

as a result of the 24 h quarantine period. Addi-

tionally, the average GSI was 0.3, which may

be related to the low HSI. Mitu (2017) suggests

that during the spawning period, the HSI tends

to increase in correlation with the GSI. This,

in turn, would indicate that variations in the

HSI are linked to the storage of energy reserves

for reproduction. However, it is important to

note that this effect could not be conclusively

determined in the catfish analyzed since these

Fig. 5. A. Histological organization and distribution of mucins from the middle region of the intestine of P. magdaleniatum.

The simple columnar epithelium made up of goblet cells (cc) and enterocytes (e) is observed. B. The presence of connective

tissue corresponding to the lamina propria (lp) and the muscular tunic (tm) is observed. C. The presence of neutral

mucins in the goblet cells is detected. D.-G. As well as the presence of carboxylated and sulfated acidic mucins. Staining: A.

Hematoxylin and eosin 100X. B. Masson’s trichrome 40X. C. PAS 40X. D. Mucicarmin 40X. E. AB pH 2.5 40X. F. AB pH

1.0 40X. H. PAS/AB pH 2.5.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

Fig. 6. A. Macroscopic image of the posterior region of the intestine (post) and rectum (+) of P. magdaleniatum. B.

Histological organization and distribution of mucins in the posterior region of the intestine. A.-B.The simple columnar

epithelium of the posterior region of the intestine is observed, made up of goblet cells (cc) and enterocytes. C. Followed by

loose connective tissue that forms the lamina propria (lp). D. The presence of neutral mucins. E.-H. as well as carboxylated

and sulfated acidic mucins is detected. Stains: B.-C. Hematoxylin and eosin 100X. D. Masson’s trichrome 40X. E. PAS 40X.

F. Mucicarmin 40X. G. AB pH 2.5. H. AB pH 1.0. I. PAS/AB pH 2.5.

12 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

indices were assessed at a single time point.

Hence, it is recommended to consider this

aspect in future studies.

Based on the histological analysis of the

esophagus in P. magdaleniatum, it exhibited a

stratified epithelium within the mucosal layer,

featuring goblet cells and taste corpuscles. This

histological observation aligns with descrip-

tions in catfish, such as Glyptosternum macu-

latum, whose esophageal epithelium is also

stratified in nature and characterized by an

abundance of goblet cells and the presence of

taste buds in the anterior region, as reported

by (Xiong et al., 2011). These authors suggest

that the presence of taste buds on the lips and

in the esophagus would indicate that the food

is selected before and during the ingestion

process, so that the presence of striated skeletal

muscle in the esophagus of P. magdaleniatum

would be involved in the generation of volun-

tary movements allowing the fish to regurgitate

unwanted food.

In the esophagus of P. magdaleniatum, the

presence of Club cells or alarm cells was also

observed. Faccioli et al. (2014) report the pres-

ence of Club cells in the esophagus of Hemi-

sorubim platyrhynchos (family Pimelodidae),

with a negative reaction to mucin production,

indicating a possible defense function against

epithelial damage when food is ingested.

According to (Chivers et al., 2007), club cells

exhibit holocrine secretion within epithelial

Fig. 7. TEM micrographs of the epithelia of the gastrointestinal tract of P. magdaleniatum. A. Esophagus with the presence

of goblet cells (cc). B. Stomach, with simple columnar epithelium (ESC). The presence of goblet cells is not detected. C.-E.

Anterior, mid and posterior intestines, respectively. In all three regions, the presence of goblet cells, enterocytes. E. And

microvilli (asterisk *) projecting towards the lumen of the intestine is observed.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

wounds of the esophagus, suggesting their role

as a defense mechanism for the underlying tis-

sues during prey ingestion. The organization of

the stratified epithelium is associated with the

esophagus’s protective function against abra-

sion. Additionally, the presence of mucosal

cells aids in lubrication during swallowing,

facilitating the ingestion process (Wilson &

Castro, 2010).

On the other hand, while in P. magdale-

niatum there is a separation between the epi-

thelium and the lamina propia-submucosa by

means of the basal lamina and absence of the

muscularis mucosae, in G. maculatum the mus-

cularis tunica extends directly into the lamina

propia and submucosa (Xiong et al., 2011). In

the case of the esophagus of Trachelyopterus

striatulus (Siluriformes, Auchenipteriadae), the

submucosa tunica presented abundant connec-

tive tissue, similar to P. magdaleniatum. The

absence of the muscularis mucosa might be

countervailed by the presence of abundant con-

nective tissue, which plays an important role in

the protection and strengthening of the esopha-

gous in carnivorous fish, similar to the walking

catfish, Claris batrachus (Raji & Norouzi, 2010).

The muscular tunica presented circular and

longitudinal muscle fibers, like T. striatulus (dos

Santos et al., 2015).

In fish, the stomach is involved in food

storing and hydrochloric acid production

(Purushothaman et al., 2016). In a study con-

ducted on the morphology and histology of the

gastrointestinal tract of the catfish Pimelodus

maculatus (Pimelodidae), similar findings to

those observed in P. magdaleniatum were docu-

mented. These similarities included the histo-

logical organization of the three regions of the

stomach and the presence of a simple columnar

epithelium in the mucosa. Additionally, the

study identified glands composed of oxyntico-

peptic cells within the lamina propria, followed

by the muscular layer of the mucosa, which

consists of smooth muscle, and, finally, by the

serosa (Santos et al., 2007). In H. platyrhynchos,

gastric glands made up of oxynticopeptic cells

related to the production of hydrochloric acid

were observed (Faccioli et al., 2014), so these

cells are associated with a high energy demand

due to ionic exchange (Naguib et al., 2011).

The presence of abundant gastric glands is a

characteristic of carnivorous fish, since they

are essential in the digestion of protein-rich

foods (Purushothaman et al., 2016). In catfish

such as Pachypterus khavalchor, a reduction

in the number of gastric glands is observed

towards the pyloric region of the stomach,

which could indicate that this part is more

involved with food storage than with digestion

(Gosavi et al., 2019).

Similar results were found in P. khavalchor

(Siluriformes: Horabagridae), where the exami-

nation revealed the presence of four layers

(mucosa, submucosa, muscularis, and serosa)

in the gastrointestinal tract. The transition from

a stratified epithelium in the esophagus to a

simple columnar epithelium in the stomach was

observed, as well as the absence of mucous cells.

Notably, the lamina propria and gastric glands

were observed in detail. Like in P. magdalenia-

tum, the separation between the tunica mucosa

and submucosa was evident due to a thin layer

of smooth muscle known as the lamina mus-

cularis mucosae. Additionally, both circular

and longitudinal muscle layers were recognized

within the muscularis layer, and the outermost

layer, known as the serosa, was also identified

(Gosavi et al., 2019). The presence of thick

muscular and serous layers in the stomach is

associated with the fragmentation and unwind-

ing of the food bolus, which favors the secretion

of digestive enzymes (Faccioli et al., 2014).

H. platyrhynchos is a pimelodid with dis-

tribution in the Amazon and Orinoco basins

(Lasso et al., 2011). As in striped catfish, the

presence of a simple columnar epithelium made

up of enterocytes and goblet cells is reported for

the intestine of this species, with greater abun-

dance of the latter in the posterior region of the

intestine (Faccioli et al., 2014). The enterocytes

and goblet cells present in the epithelium could

fulfill a lubrication and absorption function

in the intestine, facilitating the transit of food

(Purushothaman et al., 2016). Other studies

reported that the microvilli present in the

apical zone of enterocytes could amplify the

14 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

area of intestinal absorption. The intestine

of the striped catfish presented a high degree

of folding and an absence of pyloric caeca,

which is consistent with that reported for G.

maculatum, whose intestinal mucosa presented

a great abundance of folds and goblet cells

(Xiong et al., 2011).

In terms of histochemical characteriza-

tion, neutral mucins were observed throughout

the gastrointestinal tract of the striped catfish.

However, acidic mucins were detected in the

cranial and middle sections of the esophagus

and throughout the intestine, but they were not

observed in the caudal part of the esophagus or

in the stomach. These findings align with those

reported for the catfish P. khavalchor (Gosavi

et al., 2019), T. striatulus (dos Santos et al.,

2015), H. platyrhynchos (Faccioli et al., 2014),

Pelteobagrus fulvidraco (Cao & Wang, 2009)

and Rhamdia quelen (Hernández et al., 2009).

Some authors suggest that the abundance of

goblet cells and the presence of acidic mucins

in the esophageal epithelium are associated

with the ability to direct food more easily due

to the lubricating action provided by these pro-

teins, as well as protection against pathogens

and mechanical damage (de Oliveira-Ribeiro

& Fanta, 2000; Xiong et al., 2011) which, along

with taste buds, would allow a high degree of

food selection by P. magdaleniatum. Converse-

ly, the presence of neutral mucins would be

associated with the transformation of the bolus

into chyme, which would indicate pre-gastric

digestion (Cao & Wang, 2009).

Similar to the yellow catfish P. fulvidraco

(Cao & Wang, 2009), the exclusive presence of

neutral mucins in the stomach of striped catfish

would be associated with enzymatic coopera-

tion during digestion, as well as the transfor-

mation of food into chyme, the absorption and

transport of molecules across cell membranes.

They also have a buffering effect on the acid-

ity level in the stomach (Petrinec et al., 2005).

According to Cao & Wang (2009) the presence

of gastric glands, as well as the secretion of

neutral mucins in the stomach of yellow catfish

provide a great absorption and digestion capac-

ity, as well as a high concentration of collagen

fibers that make up the stomach lamina pro-

pria-submucosa, which increases stomach elas-

ticity, and favors food storage capacity.

The presence of neutral mucins in the

intestine could be related to enzymatic activity

in food degradation (Anderson, 1986), while

acidic mucins in the intestine would be linked

to the protection of the epithelium against

degradation by the action of glycosidases (Car-

rassón et al., 2006). In the intestinal epithelium

of striped catfish, the presence of sulfated and

carboxylated acid mucins was observed, which

are related to the absorption of proteins, ions,

among other particles and, in turn, are involved

in the protection of the epithelium against the

action of pathogens, which facilitates the transit

of food (Díaz et al., 2008).

The abundance and distribution of goblet

cells observed in the intestine of P. magdaleni-

atum provide important insights into its diges-

tive adaptations. Similar to other carnivorous

fish species, such as the sea bass Dicentrarchus

labrax (Carrassón et al., 2006), P. magdale-

niatum exhibits a high concentration of goblet

cells, particularly in the posterior region of

the intestine. This pattern is consistent with

the need for increased mucus production to

facilitate the transit of protein-rich food and

protect the intestinal epithelium from mechani-

cal damage (Sukkhee et al., 2024).

However, the distribution pattern of gob-

let cells in P. magdaleniatum differs markedly

from that observed in omnivorous species like

Oreochromis niloticus, which shows a more

uniform distribution along the entire length of

the intestine (Díaz et al., 2008). This difference

likely reflects the distinct digestive require-

ments of carnivorous and herbivorous diets.

In herbivorous fish, the uniform distribution

of goblet cells may aid in the continuous lubri-

cation necessary for processing plant mate-

rial, which often requires longer transit times

and more extensive mechanical breakdown (de

Matos et al., 2021).

Furthermore, the types of mucins pro-

duced by these goblet cells in P. magdaleni-

atum, with a predominance of acidic mucins in

the posterior intestine, align with observations

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e58796, enero-diciembre 2025 (Publicado Jun. 30, 2025)

in other carnivorous fish species such as the

European catfish Silurus glanis (Kozarić et al.,

2008). This prevalence of acidic mucins may

serve multiple functions, including protection

against bacterial colonization, facilitation of ion

transportation, and possibly helping in the final

stages of protein digestion (Matheus et al., 2021;

Yun-Chieng et al., 2020).

The observed goblet cell distribution and

mucin composition in P. magdaleniatum, when

compared to fish with different feeding habits,

underscore the species’ adaptation to a carnivo-

rous diet (Gonçalves et al., 2024; Nunes et al.,

2020). These features likely contribute to effi-

cient protein digestion and absorption, while

also providing necessary protective functions

in the gastrointestinal tract (Kotzé & Huys-

seune, 2020). Such adaptations are crucial for

P. magdaleniatum’s role as a top predator in its

native ecosystem and highlight the intricate

relationship between diet, digestive physiology,

and ecological niche in teleost fishes.

This study revealed that P. magdaleniatum

presents a gastrointestinal tract with histo-

logical and ultrastructural features adapted for

carnivorous feeding. Our findings include a

relatively short intestine lacking a submucosal

layer, distinct mucin distribution patterns in the

esophagus, stomach, and intestine, abundant

gastric glands in the stomach, and the pres-

ence of specialized cells such as club cells in

the esophagus and enterocytes with prominent

microvilli in the intestine. These adaptations

provide insights into the digestive physiology of

this endangered catfish species, contributing to

our understanding of its biology and potentially

informing conservation strategies. However,

these findings are based on a limited sample size

from a single location and period of time, using

only histological methods. Further research

incorporating larger sample sizes, multiple

populations, and advanced techniques such as

immunohistochemistry is needed to fully char-

acterize the digestive physiology of this species

and its implications for conservation.

Ethical statement: The authors declare

that they all agree with this publication and

made significant contributions; that there is no

conflict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are fully

and clearly stated in the acknowledgments sec-

tion. A signed document has been filed in the

journal archives.

ACKNOWLEDGMENTS

We thank the company ISAGEN S.A.

(Colombia), for financing the study, the com-

pany Piscícola San Silvestre S.A. (Barrancaber-

meja, Colombia) for the logistical support.

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