Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

Ecomorphological and behavioral differences mediating

resource partitioning among syntopic stream fish species

in the Amazon Rainforest

Gabriel Gazzana-Barros1*: https://orcid.org/0000-0002-6310-9068

Cláudia Pereira-de Deus1, 2: https://orcid.org/0000-0002-5537-7732

Jansen Zuanon1, 2, 3: https://orcid.org/0000-0001-8354-2750

1. Programa de Pós-graduação Biologia de Água Doce e Pesca Interior, Instituto Nacional de Pesquisas da Amazônia,

INPA, Av. André Araújo, 2936, Aleixo, CEP 69060-001 Manaus, AM, Brazil; ggbarros00@gmail.com (*Correspondence)

2. Coordenação de Biodiversidade, Instituto Nacional de Pesquisas da Amazônia, INPA, Av. André Araújo, 2936, Aleixo,

CEP 69080-971 Manaus, AM, Brazil; claudiapereiradedeus@gmail.com, jzuanon3@gmail.com

3. Universidade Santa Cecília (UNISANTA), Rua Oswaldo Cruz, 277, CEP 11045-907, Santos, SP, Brazil (Senior Visiting

Researcher).

Received 09-VIII-2024. Corrected 17-XII-2024. Accepted 09-VI-2025.

ABSTRACT

Introduction: Generalist trophic strategies and opportunistic feeding habits of nektonic fish species inhabiting

oligotrophic streams in the Amazon Rainforest suggest that minor morphological and niche differences can

mediate the occurrence of closely related species in sympatry, alleviating interspecific competition for resources.

Objective: To analyze the ecomorphology, diet composition, vertical and horizontal habitat use, and foraging

behavior of four Characiform species in syntopy, to understand resource partitioning and species coexistence.

Methods: From August to October 2011 (dry season), up to 30 specimens of each species were collected from

each of eight sampled streams in the Adolpho Ducke Forest Reserve, Amazonas, Brazil, for ecomorphological

analyses, with up to 10 of these used for stomach content analysis. Foraging behavior was quantified through

underwater observation of vertical and horizontal space use and foraging frequency in the water column. The

dietary importance of food items was determined using the Feeding Index (FIi), and ecomorphological attributes

were used to characterize body shape and fin morphology.

Results: Differences were detected in foraging behavior and habitat use. Hyphessobrycon. aff. melazonatus pre-

dominantly occupied the stream margins, and the other species utilized the channel. Additionally, only H. aff.

melazonatus exhibited a difference in stomach content composition. Ecomorphological characteristics showed

divergence among species, particularly in body shape, mouth size, and orientation.

Conclusions: The combined analysis demonstrated that differences observed here may mediate syntopic coexis-

tence by alleviating interspecific competition through resource partitioning. The system’s sensitivity to anthropo-

genic impacts and climate change were highlighted on food availability and trophic relations of Amazon stream

fishes and underscore the need for headwater stream conservation.

Key words: Amazonian fishes; oligotrophic streams; habitat use; morphology; co-occurrence.

https://doi.org/10.15517/rev.biol.trop..v73i1.61089

AQUATIC ECOLOGY

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

INTRODUCTION

Understanding the feeding tactics and

habitat use of sympatric fish species that are

phylogenetically close and show similar habits

is crucial for comprehending species coexis-

tence, interspecific ecological interactions and

resource partitioning (Lowe-McConnell, 1999).

Physical environmental factors can influence

habitat use dynamics and feeding behavior

among phylogenetically related species (Leitão

et al., 2015; Peres-Neto, 2004). Resource parti-

tioning in these environments is an important

ecological factor that tends to reduce interspe-

cific competition, thus facilitating coexistence

(Aranha et al., 1998; Baldasso et al., 2019;

Peres-Neto, 2004). However, there is no con-

sensus on the relative importance of these fac-

tors (both physical and intrinsic to the species)

in the dynamics of resource partitioning that

mediate the coexistence of sympatric species in

species-rich Amazonian streams (Baldasso et

al., 2024; Delariva & Neves, 2020).

The coexistence of species through resource

partitioning can be reflected in patterns of

body sizes and/or combinations of morpho-

logical traits within communities (Manna et

al., 2020; Shukla & Bhat, 2022). Subtle morpho-

logical differences can allow the coexistence of

closely related species from a same family. For

instance, the relationship between body size

and shape can indicate how space is utilized by

stream fishes (Brejão et al., 2018; Santos et al.,

2019; Wolff et al., 2023). These studies empha-

size the fine tuning between fish body shape

and functional groups with the characteristics

of the aquatic environment, and how this can

influence swimming performance, particularly

regarding foraging behavior, space use, feeding

behavior and the structure of fish assemblages.

Body shape, fin morphology, and mouth orien-

tation reflect the use of different microhabitats

RESUMEN

Diferencias ecomorfológicas y comportamentales que median la repartición de recursos

entre especies de peces de arroyos sintópicos en la selva amazónica

Introducción: Las estrategias tróficas generalistas y los hábitos alimenticios oportunistas de las especies de peces

nectónicos que habitan en arroyos oligotróficos de la selva amazónica sugieren que pequeñas diferencias morfo-

lógicas y de nicho pueden mediar la coexistencia de especies estrechamente relacionadas en simpatría, aliviando

la competencia interespecífica por los recursos.

Objetivo: Analizar la ecomorfología, la composición de la dieta, el uso vertical y horizontal del hábitat, y el

comportamiento de forrajeo de cuatro especies de Characiformes en sintopía, para comprender la partición de

recursos y la coexistencia de especies.

Métodos: Entre agosto y octubre de 2011 (estación seca), se recolectaron hasta 30 especímenes de cada especie en

cada uno de los ocho arroyos muestreados en la Reserva Forestal Adolpho Ducke, Amazonas, Brasil, para análisis

ecomorfológicos. De estos, hasta 10 fueron utilizados para el análisis de contenido estomacal. El comportamiento

de forrajeo se cuantificó mediante observaciones subacuáticas del uso del espacio vertical y horizontal, y la fre-

cuencia de forrajeo en la columna de agua. La importancia dietética de los ítems alimenticios se determinó uti-

lizando el Índice de Alimentación (FIi), y se caracterizaron atributos ecomorfológicos relacionados con la forma

del cuerpo y la morfología de las aletas.

Resultados: Se detectaron diferencias en el comportamiento de forrajeo y el uso del hábitat. Hyphessobrycon aff.

melazonatus ocupó predominantemente las márgenes de los arroyos, mientras que las otras especies utilizaron el

canal. Además, solo H. aff. melazonatus mostró una diferencia en la composición del contenido estomacal. Las

características ecomorfológicas mostraron una divergencia entre las especies, particularmente en la forma del

cuerpo, el tamaño y la orientación de la boca.

Conclusiones: El análisis combinado demostró que las diferencias observadas pueden mediar la coexistencia

sintópica al aliviar la competencia interespecífica mediante la partición de recursos. Se destacó la sensibilidad del

sistema a impactos antropogénicos y al cambio climático en la disponibilidad de alimentos y las relaciones tróficas

de los peces de arroyos amazónicos, subrayando la necesidad de conservar los arroyos de cabecera.

Palabras clave: peces amazónicos; arroyos oligotróficos; uso del hábitat; morfología; co-ocurrencia.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

by fish, indicating adaptations to specific con-

ditions of water velocity, depth, and substrate

type (Casatti & Castro, 2006; Lamouroux et

al., 1999; Langerhans et al., 2003; Teresa et al.,

2021). Additionally, the relationship between

mouth size and orientation can indicate the

type and position of food relative to the fish

and the water column stratum in which the fish

forages (Casatti & Castro, 2006), as well as the

origin (allochthonous or autochthonous) of the

consumed food items (Mazzoni et al., 2010).

However, external morphology alone is not

always a good predictor of trophic niche (Casa-

tti & Castro, 2006; Manna et al., 2017). In such

cases, foraging behavior can provide impor-

tant complementary explanations for these

differences (Ceneviva-Bastos & Casatti, 2007;

Costa-Pereira & Severo-Neto, 2012). Direct

behavioral observation is generally the most

effective method for obtaining such informa-

tion. The use of diving techniques for direct fish

observation is still uncommon in freshwater

aquatic environments (Leite et al., 2023) but has

proven highly effective in studies aiming to elu-

cidate habitat use and foraging characteristics

of species (Brejão et al., 2013; Buck & Sazima,

1995; Casatti, 2002; Nunes et al., 2020; Sabino &

Castro, 1990; Sabino & Zuanon, 1998; Sazima,

1986; Zuanon et al., 2006).

Amazonian streams, regionally known as

igarapés, are small water courses characterized

by the presence of discrete habitats and lim-

ited space availability, making them ideal for

exploring hypotheses about ecological interac-

tions of fish with their environment. Among

the species inhabiting these streams, the most

diverse and abundant group is Characiformes,

especially represented by nektonic fish species

(Dagosta & de Pinna, 2019; Toledo-Piza et

al., 2024), which have very active individuals,

exhibiting pronounced exploratory behavior

and diverse and opportunistic feeding strate-

gies (Barros et al., 2017; Carvalho et al., 2007;

Lowe-McConnell, 1999; Sabino & Zuanon,

1998; Sazima, 1986). Phylogenetically related

species may exhibit more significant morpho-

logical similarities and conserved ecological

niches (Casatti & Castro, 2006; Peres-Neto,

2004; Wiens et al., 2010; Winemiller, 1991),

which, in theory, increases niche overlap and

the likelihood of interspecific competition, par-

ticularly in oligotrophic environments such as

small streams in the Amazon Rainforest (Hen-

derson & Walker, 1986). In the Adolpho Ducke

Forest Reserve, previous studies (Barros et al.,

2017; Espírito-Santo et al., 2009; Mendonça

et al., 2005) have documented the syntopic

occurrence (individuals of two or more spe-

cies sharing the same microhabitat) of various

nektonic species in several of these streams,

including Bryconops inpai Knöppel, Junk &

Géry, 1968, Bryconops giacopinii (Fernández-

Yépez, 1950) and Iguanodectes geisleri Géry,

1970 (Iguanodectidae), and Hyphessobrycon

melazonatus Durbin, 1908 (Characidae). These

species exhibit diurnal habits, are considered

trophically opportunistic, and are frequently

observed sharing the same stream stretch-

es, suggesting potential niche overlap among

them. Additionally, the conservation of such

streams and species is relevant due to the high

sensitivity to anthropogenic pressures (where

they can act as bioindicators), the interface of

streams with the forest, the maintenance of the

watershed integrity, and the potential for orna-

mental use of the species.

Considering these ecological aspects of oli-

gotrophic streams, this study aimed to analyze

ecomorphological characteristics, diet, forag-

ing behaviour, and space use of four phyloge-

netically close and syntopic nektonic species,

investigating whether resource partitioning and

morphological dissimilarity act as mediators of

their coexistence in Amazonian streams.

MATERIALS AND METHODS

Study area: The study was conducted at

the Reserva Florestal Adolpho Ducke (RFAD),

a 10 000-ha area of primary lowland tropical

rainforest located North of Manaus, Amazonas

State, Brazil (02º 55’ and 02º 53’S & 59º 58’W).

The study was conducted in the Acará (AC)

and Bolivia (BO) sub-basins, both of which

draining to the Tarumã-Açu River Basin, a trib-

utary on the left bank of the Negro River. Fish

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

were observed and collected from a 50-meter

stretch along the stream banks. Three of these

streams were first-order (AC13, BO12, and

BO13), four were second-order (AC23, BO20,

BO21, and BO22), and one was a third-order

stream (AC30), following the Horton-Strahler

classification system (Petts, 1994), resulting in

a total of 8 sampled streams (SMF 1).

Upland streams (regionally known as

“terra firme”) exhibit a meandering streambed

and a high interface with the riparian forest.

The streambed alternates between riffles, char-

acterized by high water flow, shallow depth, and

a substrate predominantly consisting of sand,

gravel, and small rocks, and pools, which are

deeper and have a substrate mainly composed

of coarse particulate organic matter (leaf litter),

fine particulate organic matter, small branches,

and sand (Fittkau, 1967). The banks are abun-

dant with shrubby plants, leaves, and roots

from the riparian vegetation, and submerged

trunks create natural barriers that contribute

to longitudinal heterogeneity. The water has

a pH of approximately 4.5 due to the pres-

ence of fulvic and humic acids resulting from

the decomposition of plant organic matter.

The average water temperature is 25 °C with

minimal variation throughout the year. These

are oligotrophic streams, where the primary

autotrophic productivity is very low, associated

with the scarcity of inorganic compounds in

the water and dense shading from the riparian

forest canopy (Junk & Furch, 1985; Walker,

1995). The studied streams are deeply insert-

ed in the Reserve and are free from direct

anthropogenic disturbances.

The streams and species Bryconops giacopi-

nii (GIA), Bryconops inpai (INP), Iguanodectes

geisleri (GEI) and Hyphessobrycon aff. mela-

zonatus (MEL) were selected based on records

of species abundance and composition previ-

ously conducted by Espírito-Santo et al. (2009)

and Mendonça et al. (2005), which provided

information on different combinations of spe-

cies co-occurrence in these streams (SMT 1).

First, a search was carried out in the col-

lection database, and the streams in which the

most abundant nektonic species co-occurred

were identified. After this step, we returned

to these streams to double-check for the pres-

ence of the species, to carry out the collec-

tions and to make underwater observations of

their behavior.

Naturalistic observations: Behavioral data

were recorded through direct observation of

fish during snorkeling sessions (Sabino, 1999),

using a visual scanning method (Altmann,

1974). In this method, a single observation

session involved recording the frequency of

occurrence (FO %) of foraging events of an

individual or a group of the same species over 3

min. The observations included the horizontal

space use, distinguishing between the Excava-

tional Margin (EM), identified by bank exca-

vation caused by water flow and velocity; the

Depositional Margin (DM), where sediments

are deposited due to water flow eddies; and the

Channel (C), the central position relative to the

stream banks. Vertical space use was catego-

rized into the Upper Third (UT), Middle (MT),

and Lower Third (LT) of the water column.

Each of these horizontal and vertical segments

was considered a distinct microhabitat due to

their unique characteristics of water velocity,

food availability, and depth, which significantly

influence local fish composition and abun-

dance (Barili et al., 2011; Brejão et al., 2013;

Leitão et al., 2015).

The underwater observations were con-

ducted in the same streams where the speci-

mens were subsequently collected. The diver

surveyed a 50-meter stretch of the stream for

two hours (between 11:00 AM and 1:00 PM), a

time when the species are more active (Sabino &

Castro, 1990; Sabino & Zuanon, 1998; Sazima,

1986). After a 10-minute acclimation period at

each site, observation sessions began, consist-

ing of 3 min of recording foraging events and

habitat use of individuals (or groups) present

at the EM, DM and C, one species at a time, at

five observation sites (evenly spaced along the

50-meter stretch of the stream). A single forag-

ing event was defined as any behavior directed

to a food item, whether or not followed by

manipulation or ingestion of the item. During

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

the direct observation sessions, the following

foraging event types were recorded: i) surface

food picking (Keenleyside, 1979); ii) drift food

picking in the water column (Grant & Noakes,

1987); and iii) bottom substrate food picking

(Sazima, 1986).

Stomach contents: Each 50-meter section

of the stream was sampled in a standardized

manner over two hours by two collectors using

a small hand-held seine net (mesh size 5 mm

between opposite knots) and dip nets. Block

nets were installed at the beginning and end of

the section to prevent fish escape and optimize

capture (Mendonça et al., 2005). After capture,

individuals were immersed in an anesthetic

solution, one liter of water and five drops of

Eugenol (clove oil). When opercular move-

ment ceased, specimens were fixed in 10 %

formalin and subsequently preserved in 70 %

ethanol. INPA’s Institutional Ethics Committee

for Animal Use in Research authorized field

and laboratory protocols (permit # 043 / 2012

granted to GGB).

Stomach content analysis was performed

on ten specimens of each species from each

of the eight streams (10 stomachs/stream/spe-

cies). Following stomach dissection, the degree

of fullness (FD) was determined according to

Goulding et al. (1988).

For each species, the frequency of occur-

rence (Fi) (Hyslop, 1980) of each food item

in the stomachs was calculated relative to the

total number of stomachs containing food.

The relative volume (Vi) of each food item was

estimated visually, as the percentage of the total

volume of each item in the stomach, with the

total volume considered as 100 % (cf. Hynes,

1950, modified by Soares, 1979). The values

of Vi were multiplied by their respective FD to

correct the relative volumes of different food

items present in the stomachs. Food items were

identified to the Taxonomical Order (Merritt &

Cummins, 1996; Passos et al., 2007; Pes et al.,

2005; Salles et al., 2004).

To evaluate the importance of each ingest-

ed item for the species, the Food Index (FIi)

(Kawakami & Vazzoler, 1980) was applied using

the following formula:

FIi = FiVi / [Σ (FiVi)]-1

where FIi = food index, Fi = frequency of occur-

rence of item i, and Vi is the relative volume of

item i.

Food items were also classified according to

their origin (autochthonous or allochthonous).

Data analysis was performed for each species,

and comparisons among them were made using

Pearson’s Chi-square test (Pearson, 1900).

Morphological measurements: Morpho-

metric analyses were conducted on 22 speci-

mens of each species from the eight sampled

streams, with the following size ranges: GIA

(20.6-97.0 mm); MEL (22.7-39.0 mm); INP

(36.0-90.6 mm); and GEI (27.8-50.8 mm).

Seventeen linear morphometric measure-

ments based on Gatz (1979) and Watson &

Balon (1984) were taken point-to-point using

a digital caliper (0.1 mm precision) and related

to standard length (SL). Five body and fin area

measurements, excluding the anal fin (adapted

from Beaumord & Petrere, 1994), were made

for each specimen using the projected image

on graph paper. Fins were extended on the

paper and outlined with a pencil. The fin areas

were then related to body area to maintain pro-

portionality, regardless of size variation among

specimens. The outlines were scanned, and the

pixels representing morphological structures

were converted to cm² using a known area scale

(273 px = 1 cm²) with ImageJ.

The morphometric measurements and

areas were used to calculate 18 ecomorpho-

logical attributes that potentially reflect habi-

tat use and feeding behavior aspects of the

species (Gatz, 1979; Watson & Balon, 1984):

body compression index (BCI), relative body

height (RBH), relative caudal peduncle length

(RCPL), caudal peduncle compression index

(CPCI), ventral flattening index (VFI), relative

dorsal fin area (RDFA), relative pectoral fin

area (RPFA), relative pelvic fin area (RPvA),

relative caudal fin area (RCFA), pectoral fin

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

configuration ratio (PFCR), caudal fin con-

figuration ratio (CFCR), relative head length

(RHL), relative eye position (REP), relative eye

size (RES), relative mouth width (RMW), rela-

tive mouth height (RMH), mouth configuration

ratio (MCR), and mouth orientation (MO).

Data analysis: The Frequency of Occur-

rence (FO %) data for horizontal and vertical

space use were transformed into proportions

and tested for differences between strata among

species (e.g., 4 species X Channel), and for each

species among strata (e.g., GIA X DM/EM/C)

using a two-way ANOVA with post-hoc Tukey’s

test at a significance level of α = 0.05. FO % data

for foraging events were also tested with a two-

way ANOVA, comparing strata among spe-

cies (e.g., 4 species X mid-water) and for each

species among strata (e.g., GIA X mid-water/

surface). These analyses were conducted using

Statistica 6.0 (Statsoft, 2001).

A non-parametric MANOVA was per-

formed with the Bray-Curtis similarity index

using the Vi values for each food item to test the

significance of differences in stomach content

among species. To compare these differences

between species pairs, a MANOVA with the

same similarity index was performed, including

Holm’s correction (Holm, 1979) (as cited in R

Development Core Team, 2011), with adjusted

significance level (p < 0.006) due to multiple

pairwise comparisons.

To order the species in the ecomorpho-

logical space, a Principal Component Analy-

sis (PCA) was performed using the values of

the 18 attributes. The axes with eigenvalues

greater than one were retained for interpreta-

tion (Motta et al., 1995). This analysis was

conducted using the R statistical software (R

Development Core Team, 2011).

To test the significance of morphological

differences among species, using the eight prin-

cipal components with the highest eigenvalues

from the PCA, a non-parametric MANOVA was

applied with the Bray-Curtis similarity index,

including Holm’s correction (Holm, 1979) for

pairwise comparisons (as cited in R Develop-

ment Core Team, 2011).

RESULTS

Behavior and habitat use: A total of 711

observation sessions of fish behavior and habi-

tat use were conducted, comprising 341 sessions

for MEL (2 356 individuals), 218 for GIA (1 998

individuals), 88 for INP (243 individuals), and

64 for GEI (95 individuals), resulting in a total

observation time of 35 hours and 55 min.

Bryconops giacopinii, INP, and GEI pre-

dominantly used the stream channel, with per-

centages of 80, 87, and 75 % respectively, while

MEL more frequently utilized EM and DM,

with individuals observed in 43 % and 24 % of

the underwater sessions, respectively (ANOVA,

F = 24.5, d.f. = 3 / 707, p < 0.001 (SMF 2A).

A similar pattern of vertical use of the

water column was observed between the spe-

cies (ANOVA, F = 0.617, d.f. = 3 / 768, p =

0.415). However, a higher FO % was observed

for GEI in the lower third (22 %) compared

to INP (14 %), MEL (12 %), and GIA (9 %)

(SMF 2B).

Regarding foraging frequency, similar for-

aging was observed in the water column: sur-

face (52 %) and mid-water (47 %) for GIA

(ANOVA, F = 0.625; d.f. = 2 / 180, p = 0.429),

as well as for MEL (ANOVA, F = 0.341; d.f.

= 2 / 206, p = 0.559) with 51 % and 48 %,

respectively. On the other hand, INP exhibited

a higher FO % at the surface (66 %) compared

to mid-water (34 %) (ANOVA, F = 14.37; d.f.

= 2 / 78, p < 0.001), while GEI showed a more

frequent foraging in mid-water (65 %) than at

the surface (35 %) (ANOVA, F = 5.079; d.f. = 2 /

56, p = 0.026; SMF 2C). Foraging events on the

substrate were rarely observed (less than 1 %

of observations for each species) and thus were

not included in the analyses.

Stomach Content Analysis: A total of 170

stomachs were examined, consisting of 70 from

MEL (18.6-38.4 mm Standard Length, SL), 60

from GIA (18.5-103.3 mm SL), 30 from GEI

(31.1-50.5 mm SL), and 10 from INP (37.7-87.7

mm SL). The degree of stomach fullness indi-

cated that 87.6 % of the examined stomachs (n

= 149) were full.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

The species exhibited a broad dietary spec-

trum, with 31 different food categories identi-

fied from the stomach contents of the four

species. The diet was predominantly composed

of items of animal origin, although there was a

minor but significant presence of plant-origin

items (SMT 2). The analysis revealed that the

food categories Insect Fragments, Hymenop-

tera, Ephemeroptera Larvae, and Plant Frag-

ments were the most important components

of the stomach contents across the four fish

species, though with varying proportions, and

Ephemeroptera Larvae made an important con-

tribution to the difference in the diet between

species (SMF 3).

No significant difference was detected

among the stomach contents of the four spe-

cies overall (MANOVA, F = 3.723; r² = 0.06;

p = 0.086). However, pairwise comparisons

revealed significant differences between MEL

and GIA, MEL and INP, as well as MEL and

GEI (SMT 3). In addition to the high impor-

tance of Ephemeroptera Larvae in the diet of

MEL, significantly higher values for Diptera

Larvae, Coleoptera, and Diptera Pupae were

also observed compared to the other species.

The contribution of allochthonous items

(χ² = 4.941; p = 0.176) and autochthonous items

(χ² = 5.396; p = 0.144) to the stomach contents

was not significantly different among the four

species. Of the four species, only INP exhibited

a higher frequency of occurrence (FO %) of

allochthonous items compared to autochtho-

nous items (χ² = 8.100; p = 0.004).

Ecomorphology: Principal Component

Analysis (PCA) revealed a distinct morpho-

logical pattern among the analyzed species.

The first eight PCA axes, with eigenvalues

greater than 1, accounted for 70.5 % of the

cumulative variance.

The first two components (PC1 and PC2)

explained 56.9 % of the total ecomorphological

variation. PC1 (33.2 % variance) was mostly

influenced by attributes such as BCI, RBH,

CPCI, RHL, RES, and RMW. This component

effectively differentiated between GIA, INP,

and MEL, which possess a higher and more

laterally compressed body, a less compressed

caudal peduncle, a longer head, larger eyes,

and a broader, more terminal mouth compared

to GEI. The latter species, in contrast, has a

more terminal mouth position, a lower and

less compressed body, a more fusiform shape,

and relatively smaller eyes and head relative to

its body size, with a more compressed caudal

peduncle (SMF 4).

In the formation of PC2 (23.7 % of vari-

ance), the most important attributes were

RCPL, RDFA, RMH, and MCR, which con-

tributed to the discrimination of INP, GIA, and

GEI, which share shorter caudal peduncles,

smaller dorsal fin areas, narrower mouths with

larger openings, from MEL, which has a rela-

tively longer caudal peduncle, larger relative

dorsal fin area, and a wider mouth with a

smaller opening (SMF 4).

The PC3 of the ordination, representing

13.6 % of the variance, highlighted the impor-

tance of the RPFA and RPvA, contributing to

the morphological divergence between GIA

and its congener INP, as GIA has relatively

smaller pectoral and pelvic fins (SMF 5).

Significant morphological differences were

detected among the studied species (MANOVA,

F = 45.172, r² = 0.61, p < 0.001). For pairwise

comparisons, the smallest significant morpho-

logical difference was found between MEL and

INP, followed by GIA and INP. The pairs with

the most significant differences were INP and

GEI, followed by MEL and GEI (SMT 4).

DISCUSSION

Even though the fact that the nektonic

stream fish species are similar in terms of their

overall ecology and body shape, the results

indicated that there are ecomorphological,

stomach contents, and behavioral differences

that can act as mediators of the coexistence of

individuals of the four syntopic characiform

fish species through the sharing of food and

spatial resources.

Ecomorphology and habitat use:

Despite the overall morphological similarity,

8Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

opportunistic habits, trophic niche overlap, and

phylogenetic proximity among them (Barros

et al., 2017; Brejão et al., 2013; Dagosta & de

Pinna, 2019; Sabino & Zuanon, 1998; Sabino &

Sazima, 1999; Sazima, 1986), interspecific eco-

morphological differences appear to be associ-

ated with the use of specific microhabitats, as

well as the consumption of different combi-

nations of food items and employing slightly

different foraging behaviors, which tend to

alleviate the potential interspecific competi-

tion. The relationship between the observed

morphological divergences and ecological pat-

terns supports the ecomorphological hypoth-

esis (Casatti & Castro, 2006; Winemiller, 1992),

which posits that morphological characteristics

reflect important aspects of the individuals’

ecology and, therefore, indicate modes of life

and adaptations to different habitats and food

resource availability.

The three distinct clusters formed along the

first axis of the ecomorphological ordination

reflect differences in swimming performance

and occupation of distinct microhabitats, as

well as preferences for prey size and type, forag-

ing location, and the position of the food rela-

tive to the fish (Langerhans et al., 2003; Portella

et al., 2017). As a function of movement phys-

ics, body shape strongly influences swimming

performance, microhabitat preference, foraging

frequency, and space sharing (Barros et al.,

2019; Dala-Corte & De Fries, 2018; Souza &

Pompeu, 2020). Therefore, significantly distinct

ecomorphological characteristics are related to

the detected horizontal space segregation, evi-

denced by the higher occurrence of MEL close

to the stream margins. This species has a rela-

tively shorter, higher, and laterally compressed

body, which possibly provides greater swim-

ming performance in low-flow water environ-

ments, such as backwaters or among roots and

holes in the margins (Barros et al., 2019). Sev-

eral species of the same functional group have

shown similar space use (Brejão et al., 2013;

Teresa et al., 2021), even extending beyond

the margins and colonizing temporary pools

along the stream margins (Espírito-Santo et al.,

2017). The other three species, GIA, INP, and

GEI, showed a clear preference for the stream

channel, where the water flow is greater and the

fusiform body shape of these species is better

adapted, being energetically more favorable for

maintaining the body in this microhabitat (Bar-

ros et al., 2019; Brejão et al., 2013; Langerhans,

2008; Neves & Monteiro, 2003).

Ecomorphological attributes associated

with PC2 mainly contributed to the differen-

tiation of MEL. This species had a dorsal fin

with a larger relative area, typical of species

adapted to lentic or low-flow water environ-

ments, where a larger fin can function more

effectively (Gosline, 1971; Langerhans et al.,

2003), aiding the fish in making short move-

ments and maneuverability.

Bryconops giacopinii had relatively larger

pectoral and pelvic fin areas compared to INP.

Although GIA showed body shape characteris-

tics that confer greater adaptability to the main

channel environment, fish with large pectoral

fins are potentially more efficient at maneuver-

ing (Howe et al., 2021). Large pelvic fins are

associated with demersal habits (Gatz, 1979),

and these characteristics may also make GIA

efficient in other microhabitats, such as back-

waters and margins. These characteristics help

explain the broad distribution of this species in

small streams of various hydrographic basins in

the Amazon (Dagosta & de Pinna, 2019), sug-

gesting high environmental adaptability.

Foraging: Mouth orientation data are quite

revealing regarding the water column stratum

in which the fish forages, the origin of the

ingested food, and the fish’s position relative

to the food (Motta et al., 1995; Portella et al.,

2017). For example, Bryconamericus stramin-

eus Eigenmann, 1908 was observed capturing

items at the water surface, explained by the

predominance of terrestrial insects in its diet

(Casatti & Castro, 1998), as was Astyanax altip-

aranae Garutti & Britski, 2000 (Casatti et al.,

2003). Individuals of Hemigrammus marginatus

Ellis, 1911 are insectivorous but prefer aquatic

insects, evidenced by foraging in the mid-water

column (Casatti et al., 2003). In the present

study, GEI had a terminal mouth position and

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

a higher frequency of observed foraging events

at mid-water, collecting items carried down-

stream. Bryconops inpai had a slightly upward

positioned mouth compared to the others and

foraged mostly at the water surface, which was

associated with a higher proportion of alloch-

thonous items found in the stomach. Regarding

GIA and MEL, which had intermediate mouth

positions compared to GEI and INP, there was

no preference for foraging location (surface

or mid-water), as well as a similar proportion

of allochthonous and autochthonous items in

the stomach. The lack of preference for one

of the foraging strata can be attributed to

opportunistic life history strategies and greater

feeding behavioral plasticity in individuals of

these species (Aranha et al., 1998; Barros et al.,

2017; Brejão et al., 2013; Sabino, 1999; Sabino

& Zuanon, 1998; Sazima, 1986). Although less

specialization in foraging stratum in the water

column may result in lower competitive ability

of these species compared to more specialized

fish, the use of a greater variety of microhabi-

tats for feeding, on the other hand, increases

their versatility and competitive advantage for

available trophic resources, allowing them to

explore other foraging locations and alternative

food items of lower energetic value (but more

abundant). These conditions are typical of oli-

gotrophic streams with low carrying capacity,

like those studied here, where fish heavily rely

on allochthonous resources and are adapted to

opportunistically handle high unpredictability

in food availability (Barili et al., 2011; Hender-

son & Walker, 1986; Walker, 1995).

Stomach contents: Greater similarity in

ingested food items was observed between

GEI, GIA, and INP compared to MEL, as well

as similar horizontal space use (GEI, GIA, and

INP in the Channel and MEL in the margins).

The lower number of analyzed stomachs for

INP may have blurred the true diversity of

food items ingested by this species because of

the lower probability of detecting some rare

items in its diet; however, the use of somewhat

broad food categories in this study probably

attenuated such problem. Moreover, the main

differences in the diet of the four species were

mainly related to MEL, which also reduces the

risk of misinterpretation of the results pre-

sented herein.

Dietary differences among ecologically and

morphologically similar species are commonly

associated with differences in body and mouth

shape, size, and feeding behavior among spe-

cies (Aranha et al., 1998; Barros et al., 2017;

Brejão et al., 2013; Esteves et al., 2021; Portella

et al., 2017). Gorman & Karr (1978) proposed

that food item selection among related species

is primarily a consequence of the habitat in

which individuals are found, and where they

select among available food items. Dala-Corte

& De Fries (2018) and Montaña & Winemiller

(2010) observed that body size, and to a lesser

extent habitat use, are the main factors causing

dietary segregation among syntopic congeneric

species. In MEL, a higher proportion of Cole-

optera, Diptera, and especially Ephemeroptera

larvae were detected in the stomach contents,

which significantly contributed to the trophic

divergence between species. The selection of

different food items by MEL compared to other

species relates to different patterns of use of

margin and channel microhabitats, but also

to its smaller body size relative to the others.

Barros et al. (2017) attributed the proportional

use of different feeding tactics and space use

to the narrowing of the feeding niche in MEL

when syntopically co-occurring with the same

species studied here. The authors suggested

this occurred i) due to changes or differences in

items collected at the surface or mid-water; ii)

through the ingestion of food items of different

sizes and origins, mediated by mouth morphol-

ogy (e.g., mouth size, mouth orientation) and

specific behaviors (e.g., ability to locate small

food items); iii) and through the use of different

microhabitats for foraging (e.g., central channel

vs. margins). The results found here support

and complement such hypotheses, adequately

explaining the margin use segregation by MEL,

correlated with functions of specific morpho-

logical structures (mouth configuration and

size), different feeding tactics employed, and

10 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

local conditions of microhabitats (low water

speed and shelter among roots).

Resource partitioning and competition:

Ecological interactions associated with micro-

habitat heterogeneity can mediate variations in

feeding tactics and space use by opportunistic

species (Brejão et al., 2013; Casatti & Castro,

1998; Ceneviva-Bastos et al., 2010), and verti-

cal and horizontal partitioning of the water

column plays an important role in microhabitat

occupation patterns (Leitão et al., 2015; Peres-

Neto, 2004; Portella et al., 2017). The high over-

lap in the use of the middle third of the water

column by the four species suggests a reflection

of the evolutionary history of this phyloge-

netically related group, which has retained a

generalized body morphology, allowing it to

perform similar ecological functions, resulting

in similar patterns of vertical water column use.

On the other hand, the pressure for channel

microhabitat uses by GIA, INP and GEI may be

alleviated by the apparent segregation of MEL

in the margins, potentially resulting in reduced

interspecific competition. Individuals of MEL

were only observed continuously occupying

the channel in the absence of the other syntopic

species in the BO13 stream (channel 56 %, ME

21 %, and MD 23 %). This pattern of space use

by MEL and other species suggests the evi-

dence of interference competition (Schoener,

1974), associated with a dynamic mechanism

of resource partitioning, where ecomorphologi-

cal and dietary differences, foraging behavior

and space use segregation plays a mediating

role, facilitating coexistence (Baldasso et al.,

2019; Dala-Corte & De Fries, 2018; da Silva et

al., 2017; Delariva & Neves, 2020; Leitão et al.,

2015; Manna et al., 2017; Portella et al., 2017;

Souza & Pompeu, 2020).

These amazonian nektonic stream fish-

es here studied are phylogenetically related,

morphologically similar, trophic opportunist,

with high niche overlap. Several studies have

reported minimal niche differentiation in tropi-

cal stream fish assemblages (Aranha et al.,

1998; Barros et al., 2017; Esteves et al., 2021;

Goulding et al., 1988; Herder & Freyhof, 2006;

Kliemann et al., 2021; Peres-Neto, 2004; Sabino

& Zuanon, 1998), suggesting that the composi-

tion of these tropical stream fish assemblages

may reflect stochastic processes. However, the

combined analysis of direct observation of

behavior (foraging and habitat use), stomach

contents, and ecomorphological characteristics

demonstrated that, despite being seemingly

subtle, the significant differences observed here

suggest they act as a mechanism for reduc-

ing interspecific competition, emphasizing the

mediating role of resource sharing and spatial

partitioning in the coexistence of these syntopic

species in small, oligotrophic upland streams in

the Amazon Rainforest.

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.

See supplementary material

a38v73n1-suppl1

ACKNOWLEDGMENTS

We thank the Igarapés Project for provid-

ing support to field and laboratory activities

related to our research with stream fishes,

especially during the development of the Mas-

ters Thesis of GGB. This study was financed in

part by the Coordenação de Aperfeiçoamento

de Pessoal de Nível Superior-Brasil (CAPES)-

Finance Code 001, and Fundação de Amparo

à Pesquisa do Estado do Amazonas (FAPEAM)

through the Post-Graduate Program Biologia

de Água Doce e Pesca Interior at INPA. We are

also grateful to CNPq, FAPEAM and CAPES

for long-term financial support to the Igarapés

Project; and for scholarship grants to GGB.

INPA provided logistic support to field and

laboratory activities, as well as office space and

other facilities, for which we sincerely thank.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61089, enero-diciembre 2025 (Publicado Jun. 30, 2025)

Eurizângela Dary with analysis of fish stomach

contents, and José da Silva Lopes (“Seu Zé”)

provided invaluable help during the field trips.

Fish specimen collections were authorized by

SISBIO (permit # 63419-1 to GGB).

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