Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

Habitat preferences and simulation of physical habitat availability of Perlidae

(Plecoptera) and Corydalidae (Megaloptera) in a neotropical river

Francisco Quesada-Alvarado1; https://orcid.org/0000-0001-9025-3009

Silvia Echeverría-Sáenz1*; https://orcid.org/0000-0001-8214-745X

Anny Chaves-Quirós2; https://orcid.org/0000-0003-4340-9415

1. Central American Institute for Studies on Toxic Substances (IRET), Universidad Nacional, Heredia, Costa Rica. Postal

code 86-3000, Heredia, Costa Rica; FranQAL@gmail.com, silvia.echeverria.saenz@una.ac.cr

2. Instituto Costarricense de Electricidad (ICE), San José, Costa Rica; anny.chaves13@gmail.com

Received 13-XII-2024. Corrected 16-VI-2025. Accepted 10-X-2025.

ABSTRACT

Introduction: Habitat preferences represent the distribution and abundance of species among different habitat

types. These preferences are highly relevant ecological information because they relate to the feeding strategies,

offspring care and predator avoidance refuges of the organisms, therefore potentially influencing their fitness.

Objective: To define the habitat preference of the nymphs of Anacroneuria spp. (Plecoptera) and larvae of

Corydalus spp. (Megaloptera) with respect to current velocity and depth.

Methods: We evaluated the abundance of Anacroneuria and Corydalus with information gathered through 15

field campaigns in three sites of the Savegre River, Costa Rica. Also, we used habitat preferences to create simula-

tions of the physical habitat availability for these species through hydraulic models to determine habitat gain or

loss due to variations in flow.

Results: Anacroneuria (Plecoptera) nymphs preferred velocities of 0.9 m/s and depths between 23-36 cm, while

Corydalus (Megaloptera) larvae preferred velocities between 0.6-0.8 m/s, and depths between 17-29 cm. As a

case study, these preferences were modeled to determine optimal, regular or inadequate habitat availability for

Anacroneuria and Corydalus given hypothetical flow variations in the Savegre River (Costa Rica). A discharge of

< 8 m3/s resulted in a decrease in optimal habitat, mainly because it decreased water velocity below the preferred

ranges. Also, a discharge of > 18 m3/s resulted in a decrease in optimal habitat because of the depth increase.

Conclusions: This type of information is scarce or even absent for neotropical rivers, though necessary for a

description of a healthy habitat. Furthermore, this habitat preference vs. modeled habitat availability approach is

highly useful, -both in tropical and temperate rivers- for understanding the potential effects of any water deriva-

tion or exploitation.

Key words: aquatic insects; stoneflies; dobsonflies; hydrobiological model; river management.

RESUMEN

Preferencias de hábitat y simulación de disponibilidad de hábitat físico de Perlidae (Plecoptera) y

Corydalidae (Megaloptera) en un río neotropical

Introducción: Las preferencias de hábitat representan la distribución y abundancia de las especies entre distintos

tipos de hábitat. Estas preferencias constituyen información ecológica de gran relevancia ya que están relaciona-

das con las estrategias de alimentación, el cuidado de las crías y los refugios para evitar la depredación, por lo

que pueden influir en su supervivencia y capacidad reproductiva.

https://doi.org/10.15517/wykjtt15

AQUATIC ECOLOGY

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

INTRODUCTION

Aquatic macroinvertebrates constitute a

diverse community of organisms that have

been extensively utilized for biomonitoring

purposes over the past several decades (Bonada

et al., 2006, Sumudumali & Jayawardana, 2021).

These organisms are a vital component of

aquatic ecosystems, representing approximately

60 % of the animal biodiversity in continental

freshwater environments (Balian et al., 2008).

Additionally, macroinvertebrates play essential

roles in energy transfer, aquatic trophic dynam-

ics, and nutrient cycling, both within aquat-

ic ecosystems and in the adjacent terrestrial

environments (Dijkstra et al., 2014). Among

aquatic macroinvertebrates, insects form the

largest group and, although they have been

relatively well studied, significant gaps in basic

ecological knowledge persist (Hauer & Resh,

2002; Starr & Wallace, 2021). These knowledge

gaps limit the full potential of aquatic insects

as tools for decision-making processes and

watershed management.

Habitat preferences describe the distribu-

tion and abundance of species across different

habitat types, enabling the identification of the

most characteristic habitats (Baptista et al.,

2001). This ecological information is highly

relevant, as it is closely linked to feeding strate-

gies, offspring care, predator avoidance, and the

availability of refuges, and environmental toler-

ance, all of which can significantly influence

the fitness of organisms. Furthermore, under-

standing habitat preferences provides valu-

able insights for predicting how species might

respond to habitat loss (Beyer et al., 2010).

In lotic systems, habitat preferences are

typically represented as preference curves or

habitat suitability curves, which are used to

assess the quantity and quality of available habi-

tat in relation to river flow (Kelly et al., 2015;

Shearer et al., 2015; Theodoropoulos et al.,

2015; Vázquez et al., 2020). These preference

curves can also be employed to generate simu-

lations of physical habitats using eco-hydraulic

models, which facilitate the evaluation of habi-

tat gain or loss in response to flow variations

(Im et al., 2011; Im et al., 2018; Kim & Choi,

2018), whether these variations are of natural

origin or result from anthropogenic activi-

ties, such as hydropower dams or irrigation

reservoirs.

Ecological information, habitat preferences

and eco-hydraulic modeling of aquatic species

sensitive to changes in discharge can be used to

calculate environmental flows for rivers where

water extraction or derivation is contemplated.

Objetivo: Definir la preferencia de hábitat de ninfas de Anacroneuria spp. (Plecoptera) y larvas de Corydalus spp.

(Megaloptera) con respecto a la velocidad de la corriente y la profundidad.

Métodos: Con información recopilada a través de 15 campañas de muestreo en tres sitios del río Savegre, Costa

Rica, se evaluó la abundancia de Anacroneuria y Corydalus. Además, las preferencias de hábitat de las especies se

utilizaron para crear simulaciones de la disponibilidad física de hábitat mediante modelos hidráulicos y determi-

nar la ganancia o pérdida de hábitat debido a variaciones en el caudal.

Resultados: Las ninfas de Anacroneuria (Plecoptera) prefirieron velocidades de 0.9 m/s y profundidades entre

23-36 cm, mientras que las larvas de Corydalus (Megaloptera) prefirieron velocidades entre 0.6-0.8 m/s y profun-

didades entre 17-29 cm. Como estudio de caso, se modelaron estas preferencias para determinar la disponibilidad

de hábitat óptimo, regular o inadecuado para Anacroneuria y Corydalus dadas las variaciones hipotéticas de cau-

dal en el río Savegre (Costa Rica). Un caudal < 8 m3/s resultó en una disminución del hábitat óptimo, principal-

mente debido a la disminución de la velocidad del agua por debajo de los rangos preferidos. Asimismo, un caudal

de > 18 m3/s provocó una disminución del hábitat óptimo debido al aumento de la profundidad.

Conclusiones: Este tipo de información es escasa o incluso inexistente para los ríos neotropicales, aunque nece-

saria para la descripción de un hábitat saludable. Además, comparar la preferencia de hábitat frente a la disponi-

bilidad de hábitat modelada es muy útil -tanto en ríos tropicales como templados- para comprender los efectos

potenciales de cualquier derivación o explotación de agua de un cauce.

Palabras clave: insectos acuáticos, moscas de la piedra, megalópteros, modelo hidrobiológico, gestión fluvial.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

Such sensitive species information (bioindica-

tors) is important to assure the biodiversity

and ecological integrity of rivers subjected to

human activities (Chaves et al., 2010). Until

now, almost all flow bioindicator species used

worldwide have been fish (Theodoropoulos et

al., 2018), which render valid information, but

are difficult to sample or quantify, and are also

limited in their altitudinal distribution (e.g. no

native fish species exist in Costa Rica above 1

400 m a.s.l; Angulo et al., 2013). This created the

need for additional biological data derived from

other ecologically relevant groups of aquatic

biota in rivers (Chaves et al., 2010; Vázquez et

al., 2020). Moreover, Kelly et al. (2015) ques-

tioned the applicability of generalized habitat

suitability curves across diverse river systems,

underscoring the need for region-specific data,

particularly for neotropical species and rivers.

For this research, we selected nymphs of

the aquatic insects Anacroneuria (Plecoptera:

Perlidae) and larvae of Corydalus (Megaloptera:

Corydalidae) as study organisms. The nymph

and larvae of both orders are typically associ-

ated with riffle areas with fast flowing and

well-oxygenated habitats (Quesada-Alvarado

et al., 2021), as well as to litter and rocky sub-

strates (Baptista et al., 2001; Tamaris-Turizo

et al., 2007). They also have a bigger size than

other aquatic insects, are easy to recognize

in field samples and have a low number of

described genera and species in the Neotropics,

which makes the taxonomic identification pro-

cess more straightforward (Bravo et al., 2019;

Gutiérrez-Fonseca et al., 2015; Stark, 2014).

Both genera are considered sensitive to water

pollution and alteration (Luiza-Andrade et al.,

2022; Rico & Van den Brink, 2015). Due to

their ecological requirements, both can be used

as sentinels for environmental changes, mainly

because they require high concentrations of

dissolved oxygen and, as predators, they rely

on a diverse prey community (Rivera-Gasperín

et al., 2019). Therefore, it is necessary to gain

knowledge about their ecological requirements

in terms of simple hydraulic variables such

as water velocity and depth, which can help

establish baselines for climate change scenarios,

possible hydrological changes in watersheds, as

well as alterations in flow caused by hydroelec-

tric projects or water extraction.

Only recently has research begun to link

hydraulic variables with macroinvertebrate taxa

in this region (Quesada-Alvarado et al., 2020;

Quesada-Alvarado et al., 2021; Vázquez et al.,

2020; Chavarría-Pizarro el at., 2024). Our study

contributes to this effort following the approach

of Quesada-Alvarado (2021), by combining

expert panel input and field measurements to

develop preference curves for two vulnerable

native genera of aquatic insects, Anacroneuria

spp. (Plecoptera: Perlidae) and Corydalus spp.

(Megaloptera: Corydalidae), in a neotropical

Costa Rican river (Savegre). We further dem-

onstrate their application through a case study

modeling optimal environmental flow based on

ecohydraulic data and the habitat preferences

developed in this study. This approach is use-

ful for understanding the potential effects of

water derivation or exploitation in tropical and

temperate rivers.

MATERIALS AND METHODS

Study area and sampling: The study was

executed in the Savegre River watershed, located

in Costa Rica´s Pacific slope (9°34´-9°63´ N &

83°71´-84°07´ W; SMF1). The elevation in this

watershed ranges from 0 to 3 491 m a.s.l., and

the annual precipitation varies from 3 000 to 7

000 mm, with a dry season between December

and April, and a rainy season between May

and November (Espinoza-Cisneros, 2021). This

watershed comprises 600 km2, has an average

flow of 9.7 m3/s in the driest periods, and 98.3

m3/s in the peak of the rainy season (according

to data from this study).

The sampling was executed during the

rainy (May-November 2011) and dry seasons

(January-April 2012) with a total of fifteen field

campaigns at three sites along the Savegre river.

The sites have riverside forests on both margins

and are in a very humid premontane forest,

according to the Holdridge (1964) life zones

classification. Table 1 summarizes some general

characteristics of each site studied.

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At each site during each field campaign, a

50 m section of the river was used for the col-

lection of nymphs of Anacroneuria and larvae

of Corydalus. A total of 12 samples (12 repli-

cates per site) were collected monthly; these

samples were taken in different water current

velocities (assessed by means of a flowmeter

Flow Watch, FW450) to evaluate the distinct

velocity microhabitats offered by the river. At

each one of these microhabitats, depth was also

determined, and then a D-net (500-µm pore)

was introduced to sample macroinvertebrates.

The substrate was removed for 30 seconds to

collect the organisms in the net. The sample

material was deposited in white trays in situ and

the nymphs and larvae were separated and pre-

served in vials (8.0 x 3.0 cm) with ethanol 85 %.

Larger Corydalus larvae (≥ 4 cm) were not pre-

served, only measured in situ and returned to

the river once sampling was completed. In the

laboratory, the identification of Anacroneuria

species was carried out using Gutiérrez-Fon-

seca and Springer (2011), while for Corydalus,

Contreras and Harris (1998) was used. Size

(body length; mm) was also recorded with a

ruler for Corydalus because changes in habitat

selection have been documented according to

larval stage. At each microhabitat evaluated,

dissolved oxygen (mg/l), temperature (°C) and

distance to the river edge (m), were also mea-

sured with multiparameter equipment (Extech

DO700) and a measuring tape, respectively.

The collection of both larvae was carried out

under permit number 121-2011-SINAC grant-

ed by the National System of Conservation

Areas (SINAC) of the Ministry of Environment

and Energy (MINAE) of Costa Rica.

Habitat preferences: To elucidate the habi-

tat preferences, an expert panel was consulted

to determine velocities and depths at which

the nymphs of Anacroneuria and the larvae of

Corydalus are usually registered (Chaves et al.,

2006). With this information, a baseline was

generated for each order to compare later with

data acquired in the field.

The velocity and depth preferences were

classified following the fuzzy logic theory,

which assumes that complex systems are char-

acterized by imprecise transitions between dif-

ferent states of a system. Fuzzy logic allows the

system to have intermediate states since transi-

tions in ecology are gradual, not sharp (Noack

et al., 2013). Therefore, three categories of habi-

tat are generated according to the abundance

distributions of the organisms and following

the fuzzy logic theory to assign the preference:

Optimal, where the species is very likely to be

present as the habitat has the preferred condi-

tions of velocity or depth; Regular, where the

species can be found, but in lower abundances;

and Inadequate, where conditions of water

velocity and depth are not favorable and finding

the species is unlikely.

The fuzzy rules, as mentioned above, gen-

erate overlapping classes (fuzziness). However,

to classify any value of depth and current veloc-

ity into a specific category of habitat preference

(optimal, regular or inadequate), membership

functions are calculated. The membership

functions are usually represented by simple

trapezoidal or triangular functions that are

expressed from 0.0 to 1.0, where 0.0 indicates

that the values of the variable are not part of the

category, and 1.0 indicates that the values are

entirely part of the category (Noack et al., 2013).

Data Analysis: The water velocities mea-

sured in the field were classified in three cat-

egories: low (< 0.4 m/s), moderate (0.4 to 1 m/s)

Table 1

Location and characteristics of the study sites of Savegre River.

Site 1 (S1) Site 2 (S2) Site 3 (S3)

Location N 9º 27’431” - W 83º58’389” N 9º 27’707” - W 83º58’560” N 9°26’ 939” - W 83°59’531”

Elevation 250 m.a.s.l. 294 m.a.s.l. 112 m.a.s.l.

Substrate boulders and cobbles boulders and gravel boulders and gravel

Vegetation of the riverbank trees and shrubs trees, shrubs and grasses trees, shrubs and grasses

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

and fast (> 1.01 m/s) using Extence et al. (1999);

while the depth was categorized in 10 cm

ranges. This was done to ensure replicability,

given the wide range of depths and based on

the premise that similar hydraulic conditions

can be maintained within each 10 cm interval.

We used PERMANOVA tests, using a

Bray-Curtis dissimilarity matrix and 999 per-

mutations, to determine differences in velocity

and depth preferences of Anacroneuria species

(PERMANOVA test did not apply to Corydalus,

as the larvae are difficult to identify at the spe-

cies level and there are no available taxonomic

keys for this purpose). The abundances of Ana-

croneuria and Corydalus were log-transformed

and a Shapiro-Wilk test was applied, due to the

non-normality of the data a Kruskall Wallis test

was performed, we calculated the effect size

(epsilon square) for the tests that were signifi-

cant. With the subsequent post hoc Dunn’s test

we determined if the abundance was different

between velocity categories or depth ranges. To

understand the relationship between velocity

and larval abundance, a Generalized Additive

Model (GAM) and a Linear Model (LM) were

first compared using their AIC values. The AIC

of the GAM was lower than that of the LM,

indicating nonlinearity in the data. However,

when analyzing the quantile plot of the GAM

variance residuals, residual overdispersion was

observed due to the number of zeros present

in the data. Due to the high frequency of zero

counts in the dataset, we fitted a zero-inflated

Poisson (ZIP) model. The model includes a

count component (Poisson) and a zero-inflated

component (binomial). The two models, ZIP

and GAM, were compared using the log-like-

lihood value.

We also conducted a log-linear regression

(LLR) to explore the relationship between the

distance from the riverbank and body size of

Corydalus larvae. We used LLR because we

observed an exponential decrease in Corydalus’

body size. This analysis was carried out only for

Corydalus since these larvae have the behav-

ior of burying themselves in the riverbank to

carry out their metamorphosis, so larvae could

change their habitat preference according to

their growth and larval maturation.

These analyses were performed with R

programming environment and language (R

Core Team, 2024), and using the package vegan

(Oksanen et al., 2020), mgcv (Wood, 2017) and

pscl (Jackman, 2020).

We did not use the environmental variables

(DO, pH and others) as predictors of habitat

selection, we used them as controls to deter-

mine whether the presence/absence was due to

water quality conditions or habitat quality. For

this reason, we did not apply statistical methods

to them.

Case Study, habitat availability: The

Costa Rican Institute of Electricity (ICE),

developed a holistic methodology and software

named “RANA” (Chaves et al., 2006; Chaves

et al., 2010; Laporte et al., 2006) to determine

appropriate environmental flows according to

socio-economic, hydrologic and biological data

(fish, macroinvertebrates and amphibian -flow

indicator species), with the aim of reducing the

impacts of dams on ecosystems and communi-

ties (Chaves et al., 2006; Chaves et al., 2010).

This methodology allows processing and data

analysis regarding the habitat preferences of

bioindicator species (in this study Anacroneuria

and Corydalus), to propose an environmental

flow that considers the ecological requirements

of the most sensitive species. RANA also pro-

duces a graphical visualization of the velocities

and depths of a river stretch, making it possible

to see what portions of the river stretch would

be restrictive to the flow indicator species (in

terms of current velocity or depth). We used the

RANA software to create the Anacroneuria and

Corydalus habitat availability models. For more

information about the RANA methodology, the

reader can access the Toolkit for environmental

flows at the United Nations Educational, Sci-

entific and Cultural Organization website (The

United Nations Educational, Scientific and Cul-

tural Organization, 2017).

Within RANA, the ecological require-

ments are expressed using the fuzzy logic

approach because it is impossible to establish

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

precisely the range limits to optimal, regular

or inadequate conditions. For each variable

(current velocity and depth), a fuzzy number

for optimal, regular, or inadequate conditions

is defined. Each fuzzy number represents a set

of values of one variable. Each value in that set

(horizontal axis) has an associated degree of

membership that is expressed by a value in the

interval [0-1] (vertical axis), where 0 indicates

no membership and 1 indicates total member-

ship (Noack et al., 2013).

In the same way, a diffuse rule consists

of arguments combined by logical operators

which conform to a logic expression, and which

have an associated consequence, e.g. given that

A1 and A2 exist, then B is the consequence.

Therefore, in the construction of diffuse rules

some premises need to be established; in this

study, premises about velocity and depth will

generate different responses from optimal to

inadequate. The applicability is a non-binary

function, where the response is a degree of ful-

fillment (DOF) also expressed by a value in the

[0-1] interval. DOF is a value that corresponds

with the conditions given by the rule premises.

RANA methodology uses the minimum

combination to calculate the condition (opti-

mal, regular, or inadequate) for a specific point

in the river based on the associated category of

flow velocity and depth. The result corresponds

to the lower category obtained in evaluating the

value of each premise according to the fuzzy

numbers associated with them. For example,

if we evaluate flow velocity 0.1 m/s and depth

of 1 cm, the result is “inadequate” with DOF

= 1, because 0.1 m/s has a degree of member-

ship of 1 for inadequate, and 1 cm depth has a

degree of membership of 1 for optimal, then

the result condition is inadequate, which is the

lower category.

For this study, stretch S1 (Fig. 1) was used

to evaluate ecological constraints (habitat pref-

erences) for Anacroneuria and Corydalus. We

used the simple hydraulic model generated

by the hydraulic module of RANA software

to evaluate the fulfillment of the ecological

Fig. 1. Scheme of the stretch S1. A. Cross-sectional view. B. three-dimensional view. From Krasovskaia & Rodríguez (2007).

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

requirements at this point. The hydraulic model

allows us to obtain simulations for different

water discharges. The simulations provide a set

of points in the stretch for which we have the

x, y, and z arbitrary coordinates (to generate

visualizations) and the flow velocity and depth

(to evaluate the habitat preferences).

RANA software evaluates the ecological

constraint for each simulated point in the site of

interest and produces a graphical visualization

of resulting conditions evaluation, using green

for each point that has optimal habitat for the

flow-indicator species, yellow for regular habi-

tat and red for inadequate habitats. The most

restrictive value (lowest category) is assigned

when flow velocity and depth evaluation are

combined for each point. For example, in each

case where flow has an inadequate condition,

but velocity has a regular or optimal condi-

tion, the simulated point in that site would be

assigned inadequate (red). Furthermore, RANA

calculates the amount of optimal, regular and

inadequate habitats for any user-given flow

included in the simulations.

RESULTS

Habitat preferences: The expert panel

(composed of six experienced aquatic entomol-

ogists) indicated that stoneflies and dobsonflies

are rheophiles with a preference for moderate

and high current velocities. They also consid-

ered Perlidae to be the family with the greatest

demand for high velocities. In terms of depth,

they indicated that Perlidae and Corydalidae

could live in shallow waters and up to a depth

of 1.8 m (Table 2).

For the Savegre river watershed, registered

variables in each study site (15 sampling cam-

paigns) are described in Table 3. The maximum

velocity recorded was obtained in a microhabi-

tat composed of rock and at a shallow depth,

allowing an increase in water velocity. The

maximum flow was recorded during the month

of October, which is the month with the highest

rainfall in Costa Rica´s Pacific slope, while the

minimum flow was recorded during the month

of March, which is the peak of the dry season.

In total, 194 macroinvertebrate samples

were processed and 179 individuals of four dif-

ferent species of the genus Anacroneuria were

collected (A. marca, A. benedettoi, A. holzenthali

and A. perplexa; Plecoptera: Perlidae). There

was no difference in the water velocity prefer-

ence between these four species (Permanova =

1.53; p = 0.77). All Anacroneuria nymphs were

found in habitats with velocities between 0.5

to 1.8 m/s. Moreover, as velocity increased, the

abundance of organisms increased, reaching a

peak between 0.6 and 1.2 m/s, and after that,

Table 2

Velocity and depth preferences of Perlidae and Corydalidae for Costa Rican rivers (Theoretical rule: according to the opinion

of the experts panel).

Classification

of habitat

Velocity (m/s); theoretical rule Depth (cm); theoretical rule

Anacroneuria Corydalus Anacroneuria Corydalus

Optimal 1.0-5.0 0.4-0.9 0.5-1.7 0.2-1.8

Regular 0.3-1.2 0.2-0.4; 0.8-1.2 0.1-0.05; 1.4-3.1 0.01-0.2; 1.4-3.2

Inadequate 0-0.5 0-0.5; 1.1-2.0 2.7-6 0.1-0.01; 2.7-6.0

Table 3

Summary information of the three study sites, throughout the 15 field campaigns. Savegre river, 2011-2012.

Site DO (mg/l) Temperature (°C) Flow (m3/s) Velocity (m/s) Depth (m)

Max Min Max Min Max Min Max Min Max Min

S1 10.3 8.3 24.3 20.6 92.4 9.7 2.13 0 0.93 0.07

S2 8.9 7.5 26.7 21.7 92.4 9.7 1.67 0 0.81 0.13

S3 9.3 7.7 27.9 18.7 98.3 9.9 1.3 0 0.51 0.17

8Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

the abundance began to decrease at extreme

velocities (> 2 m/s; Fig. 2). A greater abundance

of stoneflies was recorded within the moderate

and fast velocity categories, in comparison with

low velocity (Kruskal-Walis = 15.12; p < 0.05;

effect size: 0.150). However, there was no dif-

ference between moderate and fast velocities

(Dunn’s test p > 0.05). According to the count

component of the zero-inflated Poisson model,

the expected abundance of Plecoptera increases

by approximately 4 9 % for each 1 m/s increase

in flow velocity (β = 0.3987, p = 0.019). The

intercept (β₀ = 0.6928) indicates that, in the

absence of flow (0 m/s), the expected abun-

dance is around two individuals, assuming the

observation is not a structural zero. Therefore,

increased water velocity is a factor modulating

the abundance of Plecoptera larvae.

It is noteworthy to mention that the DO

concentration slightly increased with each

velocity category (low: mean DO 8.6 ± 0.6

mg/L; moderate: mean DO 8.8 ± 0.7 mg/L; fast:

mean DO 9.1 ± 0.7 mg/L). A similar trend was

observed with the preferences of water depth.

Fig. 2. Fuzzy numbers used to determine optimal, regular and inadequate habitat for Anacroneuria nymphs (Plecoptera:

Perlidae) and Corydalus larvae (Megaloptera: Corydalidae) based on flow velocity in the Savegre River, Costa Rica.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

Two extremes were found: very shallow (< 10

cm) and deep waters (> 100 cm), both with

a reduced number of organisms, while the

greatest abundances of stoneflies were found

between 23 and 36 cm. However, there was

no difference between the depth categories

(Kruskal-Walis = 6.05; p > 0.83).

For Megaloptera larvae, a total of 97 indi-

viduals of the genus Corydalus (Corydalidae)

were collected. Larvae. were most abundant

and frequently found within moderate current

velocities (0.5-1.0 m/s), however, we did not find

a difference between velocity categories (Krus-

kal-Wallis = 0.87; p = 0.97; Fig 2). As well as for

Anacroneuria, there is an increase in abundance

with increasing current velocity, until it reaches

a maximum point between 0.6 and 0.8 m/s,

and then the abundance decreases. The count

model indicated a slight but significant nega-

tive effect of the velocity (β = -0.333, p = 0.075),

suggesting that abundance may decrease with

increasing velocity. The zero-inflation model

revealed a significant base probability of struc-

tural zeros (logit intercept = -1.600, p < 0.001),

corresponding to a 17 % probability of excess

zeros. This supports the presence of two pro-

cesses: one generating true absences and anoth-

er generating counts. In terms of depth, the

greatest abundance of Corydalus was recorded

between 16 and 29 cm, but without a differ-

ence with respect to the other depth categories

(Kruskal-Walis = 1.04; p = 0.31).

Also noteworthy is the fact that a regres-

sion analysis showed that most large-sized

Corydalus larvae were collected near the river-

side, while small larvae were found far from the

bank (R2= -0.64; p = 0.00) (Fig. 3).

With these preferences, the classification

of the velocity values into the optimal, regu-

lar and inadequate categories, was established.

However, preferences were not as clear for

depth and, therefore, the opinion of the panel

of experts (theoretical rule) was used for the

establishment of the categories (Table 4).

Case Study, habitat availability: Habitat

availability modeling was carried out using the

habitat classification shown in Table 4. Fig. 4

shows the percentile distribution of the opti-

mal, regular or inadequate habitats in Savegre

River for Anacroneuria, under different flow

scenarios (3, 8, 13, 18, 23 and 28 m3/s). The

different flows were modeled in a stretch of

75 m long, named S1. In this application, the

most favorable flows for Anacroneuria nymphs

are those between 8-18 m3/s, because those are

the ones that provide the highest percentage of

optimal habitat.

All modeled flows had a predominance

of inadequate and regular areas along the first

40 m, since the stretch morphology created

low current velocity and deep areas, which are

not suitable for Anacroneuria spp. nymphs.

Fig. 3. Relationship between the body size of Corydalus

larvae and their distance from the riparian forest. The blue

line is the Regression line, red lines are lower and upper

confidence limit (95 %).

Table 4

Perlidae and Corydalidae velocity and depth water habitat preference in the Savegre River.

Classification

of habitat

Velocity (m/s); empirical rule Depth (cm); theoretical rule

Anacroneuria Corydalus Anacroneuria Corydalus

Optimal 0.8-1.2 0.4-1.3 0.5-1.7 0.2-1.8

Regular 0.35-0.73; 1.04-1.80 0.1-0.6; 1.1-1.7 0.1-0.05; 1.4-3.1 0.01-0.2; 1.4-3.2

Inadequate 1.6-3.0 0.0-0.2; 1.6-3.0 2.7-6.0 0.1-0.01; 2.7-6.0

10 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

However, from 45 to 75 m, river morphol-

ogy changes, creating optimal habitats for the

nymphs, with rapids and riffles. Under a flow

of 3 m3/s, the river becomes narrower, and

consequently, the area of fast-flowing waters is

smaller. With higher flows, fast water areas are

increased and so is the availability of habitats

for Anacroneuria spp. nymphs. However, with

flows higher than 18 m3/s, the percentage of

optimal habitats slightly decreases, due to an

increase in depth. A flow of 13 m3/s can be

considered as the flow that benefits the larvae of

Anacroneuria spp., because it generates the set

of velocities and depths that the larvae prefer

Fig. 4. Above view of the modeled critical dam site of the Savegre River for different flow scenarios. The optimum (green),

regular (yellow) and inadequate (red) percentage of habitats for Anacroneuria spp. larvae are presented for each modeled flow

(The x-axis corresponds to the width and the y-axis to the length of the river stretch).

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

and, at the same time, the river widens in the

most turbulent section, generating a greater

supply of optimal habitat.

In the case of Corydalus larvae, a 13 m3/s

flow was also the best scenario, since it result-

ed in the highest percentage of optimal and

the lowest percentage of inadequate habitats.

As the water volume in the model increased (>

18 m3/s), the optimal area decreased. However,

as the flow increased, the sector of the mod-

eled river stretch increased its width, allowing

for higher availability of habitats and more

proximity to the riverbank (with ecological

advantages because of terrestrial-aquatic eco-

system interactions). In all modeled flows, the

percentage of inadequate habitat was lower

than the percentages of regular and optimal

habitats (Fig. 5).

Fig. 5. Above view of the modeled critical Dam site of the Río Savegre for different flow scenarios. The optimum (green),

regular (yellow) and inadequate (red) percentage of habitats for Corydalus spp. larvae are presented for each modeled flow

(The x-axis corresponds to the width and the y-axis to the length of the river stretch).

12 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

DISCUSSION

This research demonstrates the affinity of

Anacroneuria spp. and Corydalus spp. for mod-

erate and fast current velocities while also pre-

ferring low depths. Anacroneuria nymphs and

Corydalus larvae are known benthic rheophilic

organisms with the ability and anatomical traits

to colonize areas with fast and turbulent water.

They have a dorsoventrally compressed body

and hook nails to hold on firmly to the sub-

strate (Stark 2014). In this study, both Perli-

dae and Corydalidae, preferred habitats where

water velocity is considered moderate to strong

(0.6-1.8 m/s). It is noteworthy to mention that

such velocities influence the presence of certain

types of substrates, with dominance of rocks

and boulders (Leopold et al., 1992). Therefore,

microhabitat preferences related to substrate

choice or position of the insects in the benthic

zone (over or under rocks or in crevices) are

equally related to the velocity regime.

The abundance of both orders increased as

velocity increased until a maximum peak was

reached, thereafter, the abundance diminished

and at velocities higher than 2 m/s, none of

the orders were found. Waddle & Holmquist

(2011) found the same relation regarding the %

EPT (Plecoptera, Ephemeroptera and Trichop-

tera): the abundance of this group increased

asymptotically as velocity increased. However,

in that study, the modelled velocities were

relatively low, and they do not disregard the

fact that higher velocities might have changed

the upper end of the curves, as high velocities

are expected to decrease habitat suitability. In

North America, Gore et al. (2001), it deter-

mined that the greatest diversity of aquatic

macroinvertebrates was found at velocities

between 0.6 m/s and 0.7 m/s. The increased

abundance of other macroinvertebrates in this

velocity range might also play a part in the

presence of Anacroneuria and Corydalus, since

both genera are predators (Contreras & Har-

ris, 1998; Gutiérrez, 2010), however, it is clear

that preference for higher velocity is not solely

related to prey availability since many potential

preys inhabit very low velocities. In our study,

the maximum abundances of all four species of

Anacroneuria were registered at slightly higher

velocities (0.9 m/s).

With increasing velocity, an increment in

viscosity also occurs (Dodds, 2002), which

could make movement and displacement diffi-

cult for the organisms. This might be the reason

why aquatic insects are less frequently found in

extreme velocities. Gore et al. (2001) state that

although these organisms have the necessary

adaptations for living in sites with high veloci-

ties, they avoid environments where energy

loss exceeds the gain. On the other hand, at low

velocities (< 0.4 m/s) no Anacroneuria nymphs

and only a few Corydalus larvae were collected.

This is expected since these organisms depend

on high concentrations of DO, and higher

water velocities increase the concentration and

renovation of DO (Genkai-Kato et al., 2005;

Chapra et al., 2021), as also seen in this study,

with a slight increase in the DO concentration

with the velocity category.

Regarding neotropical rivers, similar

velocity preferences were observed when the

Threshold Indicator Taxa Analysis (TITAN)

was used in two studies: Hanh et al. (2018) in

Ecuador determined that the velocity threshold

for Corydalidae was between 0.2 m/s and 1.0

m/s, while Perlidae had a lower current veloc-

ity threshold between 0.4-0.6 m/s. Quesada-

Alvarado et al. (2020) in the Naranjo river

watershed, Costa Rica, determined that the

threshold for Corydalidae was between 0.3 m/s

and 1.5 m/s, while Perlidae had a threshold

between 0.5 m/s and 1.2 m/s.

Despite a preference for high velocities for

the genus Corydalus, the bigger individuals (≥

60 mm) were found in areas with low velocity (<

0.4 m/s), and even in waters with no current at

all. This behavior can be explained since move-

ment of bigger individuals is more restricted by

density, viscosity and velocity of water (Sagnes

et al., 2008), but most importantly, larvae from

Corydalus pupate in terrestrial habitats, there-

fore, when they are ready to transition into

adults, they approach the riverbanks (Cover

& Resh, 2008). On the contrary, smaller larvae

are found in areas with higher velocities and

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

further away from the riverbanks, possibly to

protect themselves from predators and to pre-

date among other organisms, which are more

abundant in higher velocity habitats. Hence,

habitat selection of Corydalus depends on their

larval stage, and we recommend separating

Corydalus sizes for habitat modeling, similar

to what is done with fish (e.g. Yao et al., 2018).

In general, larvae and nymphs from the

two studied orders showed a higher dependen-

cy on velocity than water depth, since a wider

habitat selection range was observed for depth.

These results agree with those exposed by

Waddle & Holmquist (2011), who affirm that

larvae select their microhabitat as a function

of velocity and substrate, and only up to some

extent according to water depth. In our study,

in the Savegre river, both Anacroneuria and

Corydalus preferred water depths between 16 to

38 cm, similar to the results obtained by Gore

et al. (2001) where larvae were most frequently

found between depths of 20 to 25 cm. Further-

more, Shearer et al. (2015) recorded the great-

est abundance of aquatic macroinvertebrates

at depths less than 0.50 m, and Kim & Choi

(2018) registered the same pattern where shal-

low depths were the most preferred. However,

according to Alonso (2009), organisms can

colonize at water depths of up to 1.5 m in areas

of fast water current. This statement could not

be confirmed in the present study, because for

personal safety reasons, sampling in the Savegre

river was not executed at depths greater than 1

m. Therefore, we cannot conclude about micro-

habitat preferences at such depths. This is why

we decided to model the depth habitat prefer-

ence using only the information provided by

the panel of experts and the literature (Shearer

et al., 2015, Kim & Choi, 2018).

We conclude that, due to their narrow

range of optimal velocities and high ecologi-

cal requirements, Anacroneuria larvae can be

considered as an umbrella species if they are

to be used in habitat availability modeling for

flow studies. Eco-hydraulic models, in turn, are

a tool that allows us to observe how changes

in water volume can favor or reduce available

habitat for aquatic insect larvae.

Eco-hydraulic models show one of the

practical applications that can benefit from

information on aquatic insect habitat prefer-

ences and flow variations. We demonstrate how

watershed management can take advantage

of field-specific biological data to technically

support a regulatory decision, such as envi-

ronmental flow. For example, in Costa Rica,

legal regulations recommend the establishment

of an environmental flow that represents 10

% of the mean annual flow. For the Savegre

river this would represent ~ 8 m3/s. However,

with the preference information gathered by

this study for Anacroneuria and Corydalus,

and the habitat models created for site S1, we

estimated that a flow of 13 m3/s generates the

greatest amount of optimal habitat for both

larvae. Therefore, in the face of the possible use

of the water resource, it is necessary to consider

maintaining flows that provide the greatest

amount of optimal habitat needed by the spe-

cies that inhabit the rapids, since rapids and

waterfalls are the first habitats to be damaged by

a decrease in flow (Cortes et al., 2002).

This work contributes to expanding

knowledge on aquatic insects’ habitat prefer-

ences, which are typically scarce or nonexistent

in tropical regions. Similarly, it is a valuable

contribution to the field of ecohydraulics in

the tropics, as most studies originate from tem-

perate zones (Vázquez et al., 2020). Because

Anacroneuria nymphs respond positively to

increased water velocity and Corydalus lar-

vae change their habitat preference according

to their body size (with dependence on dis-

tance from the bank edge), it is necessary to

take into account aquatic macroinvertebrates

as bioindicators of flow to maintain ecosystem

health, especially when there are changes in the

hydraulic and hydrological regimes. The differ-

ent assemblages of aquatic macroinvertebrates

that use this type of habitat could be at risk

when there are hydraulic changes. For future

work on habitat preferences, methodologies

that allow sampling of the rapid zones at greater

depths (> 1m) should be implemented because

in the rainy season the supply of depths and

velocities of the river increases, and the insects

14 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e2025450, enero-diciembre 2025 (Publicado Nov. 03, 2025)

may prefer or use greater depths. This would

represent a change in the dynamics of prefer-

ence which ecohydraulic models are ignoring.

Finally, field studies on habitat preferences

involving native organisms are highly recom-

mended for modeling habitats in watershed

management and environmental flow.

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

a65v73n1-suppl. 1

ACKNOWLEDGMENTS

This study was financed by the Project

“Desarrollo de metodología para estimar caudal

ambiental fase 2.” UEN Projects and Associ-

ated Services of the Instituto Costarricense de

Electricidad. We thank Kimberly Rojas Ávila

and Julissa Romero for their support during

sampling. Furthermore, ICE staff collaborated

during the sampling and analysis of ecohy-

draulic conditions. We deeply thank Alexia

Pacheco for her most valuable comments on

this manuscript.

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