Revista de Biología Tropical, ISSN: 2215-2075, Vol. 74 (S1): e20267074, abril 2026 (Publicado Abr. 23, 2026)

Small-scale variability in benthic habitat shapes size structure differences

in the sea urchin Echinometra lucunter

Ruber Rodríguez-Barreras1*; https://orcid.org/0000-0001-7790-6108

Nohelys M. Ruiz-Chevres1; https://orcid.org/0009-0003-3118-6143

José E. Rojas-Torres1; https://orcid.org/0009-0007-2708-1305

Eduardo A. González-González1; https://orcid.org/0009-0005-5702-7807

Milly N. Cortés-Silva1; https://orcid.org/0009-0003-7583-1888

1. Department of Biology, University of Puerto Rico at Bayamón, Bayamón, Puerto Rico, USA; ruber.rodriguez@upr.edu

(*Correspondence), nohelys.ruiz@upr.edu, jose.rojas17@upr.edu, eduardo.gonzalez20@upr.edu, milly.cortes@upr.edu

Received 19-XI-2025. Corrected 15-XII-2025. Accepted 05-III-2026.

ABSTRACT

Introduction: Coral reefs have undergone widespread degradation, reducing ecosystem structural complexity

and altering benthic habitat conditions. These changes can affect herbivorous invertebrates such as the sea urchin

Echinometra lucunter, a key herbivore and bioeroder that can function as a bioindicator of environmental quality.

Objectives: This study evaluated whether reef condition influences the population structure of E. lucunter by

comparing benthic composition, rugosity, density, and test diameter between a healthy and a degraded zone at

Isla de Cabra, Puerto Rico.

Methods: Two reef zones at Isla de Cabra were sampled. Benthic cover was quantified using photo quadrats

analyzed with CPCe software. Rugosity was calculated using the chain-and-tape method, and urchin density and

test diameter were obtained from replicated quadrats and morphometric measurements.

Results: The healthy zone showed higher coral and turf algae cover and greater rugosity, whereas the degraded

zone was dominated by macroalgae. Although E. lucunter density did not differ significantly between zones,

individuals from the healthy zone were significantly larger (4.3 ± 0.8 cm) than those from the degraded zone

(3.5 ± 0.9 cm). The degraded zone exhibited a size distribution skewed toward smaller classes, suggesting growth

limitation under lower habitat quality conditions.

Conclusions: Local variability in benthic habitat structure influences the size structure of E. lucunter, while

population density remains relatively stable. Size-based metrics reflect small-scale habitat differences and

complement traditional benthic descriptors when assessing local habitat condition in shallow Caribbean reefs.

Key words: Echinometra lucunter; benthic habitat; rugosity; size structure; coral reef degradation; Caribbean.

RESUMEN

La variabilidad a pequeña escala en el hábitat bentónico determina las diferencias en la estructura

del tamaño del erizo de mar Echinometra lucunter

Introducción: Los arrecifes coralinos han sufrido una degradación generalizada, lo que ha reducido la compleji-

dad estructural de los ecosistemas y alterado las condiciones del hábitat bentónico. Estos cambios pueden afectar

a invertebrados herbívoros como el erizo Echinometra lucunter, un herbívoro clave y bioerosionador que puede

funcionar como bioindicador de calidad ambiental.

https://doi.org/10.15517/pxdpcr84

SUPPLEMENT

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 74 (S1): e20267074, abril 2026 (Publicado Abr. 23, 2026)

INTRODUCTION

Coral reefs are among the most biodiverse

and productive ecosystems on Earth, providing

food, shelter, and nursery grounds for thou-

sands of marine species (Hughes et al., 2017;

Pereira et al., 2024; Spalding et al., 2017). These

ecosystems deliver crucial ecosystem services,

such as coastal protection, fisheries productiv-

ity, and tourism opportunities, which collec-

tively sustain millions of livelihoods in tropical

regions (Burke et al., 2011; Moberg & Folke,

1999). Yet, despite their ecological and socio-

economic significance, coral reefs have declined

sharply over the past four decades, with global

coral cover decreasing by nearly half due to

multiple anthropogenic stressors (Jackson et

al., 2014). In the Caribbean, this decline has

been particularly pronounced, driven by coastal

urbanization, eutrophication, overfishing, sedi-

mentation, and mass bleaching events linked

to ocean warming (Alvarez-Filip et al., 2011b).

The cumulative effect of these pressures has

simplified the structural complexity of reefs,

reduced coral recruitment, and altered benthic

trophic interactions (Gardner et al., 2003).

Habitat degradation often leads to shifts from

coral-dominated to algae-dominated systems,

fundamentally altering reef architecture and

ecological dynamics (Hughes, 1994; Norström

et al., 2009). The proliferation of macroal-

gae limits coral settlement, reducing substrate

availability for crustose coralline algae, and

modifies the microhabitats used by a variety

of invertebrates and fishes (McCook, 1999;

Mumby & Steneck, 2008). Furthermore, reduc-

tions in structural complexity influence refuge

availability, predation pressure, and hydrody-

namic conditions near the substrate (Alvarez-

Filip et al., 2011a). These changes directly

affect species that depend on small crevices or

irregular surfaces for protection and feeding,

including many echinoids that play key roles in

maintaining benthic balance (Lawrence, 2013).

Echinoderms, and particularly sea urchins,

are among the most important invertebrates in

coral reef ecosystems due to their dual func-

tion as grazers and bioeroders (Bak, 1990; Fong

et al., 2024; McClanahan, 1988). By remov-

ing epilithic algal turfs and eroding carbonate

substrates, they regulate algal growth, facilitate

coral recruitment, and contribute to calcium

carbonate cycling (Lawrence, 2013; Sammarco,

1982). Their ecological importance became

evident after the massive mortality of the long-

spined sea urchin Diadema antillarum in the

1980s, an event that caused widespread mac-

roalgal overgrowth and long-lasting changes in

Objetivos: Este estudio evaluó si la condición arrecifal influye en la estructura poblacional de E. lucunter, com-

parando la composición del bento, la rugosidad, la densidad y el diámetro de la testa entre una zona saludable y

otra degradada en Isla de Cabra, Puerto Rico.

Métodos: Se muestrearon dos zonas arrecifales en isla de Cabra. Se cuantificó la cobertura bentónica mediante

foto-cuadrantes analizados en el software CPCe. La rugosidad se calculó mediante el método de cadena y cinta, y

la densidad y el diámetro de los erizos a partir de cuadrantes replicados y mediciones morfométricas.

Resultados: La zona saludable presentó mayor cobertura de coral y tapetes de algas y una rugosidad más elevada,

mientras que la zona degradada estuvo dominada por macroalgas. Aunque la densidad de E. lucunter no difirió

significativamente entre zonas, los individuos de la zona saludable fueron significativamente más grandes (4.3 ±

0.8 cm) que los de la zona degradada (3.5 ± 0.9 cm). La zona degradada mostró una distribución sesgada hacia

clases pequeñas, lo que sugiere limitaciones en el crecimiento bajo condiciones de menor calidad del hábitat.

Conclusiones: La variabilidad local en la estructura del hábitat bentónico influye en la estructura de tallas de E.

lucunter, mientras que la densidad poblacional se mantiene relativamente estable. Las métricas basadas en tallas

reflejan diferencias del hábitat a pequeña escala y complementan los descriptores bentónicos tradicionales al

evaluar la condición local del hábitat en arrecifes someros del Caribe.

Palabras clave: Echinometra lucunter; hábitat bentónico; rugosidad; estructura de tallas; degradación arrecifal;

Caribe.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 74 (S1): e20267074, abril 2026 (Publicado Abr. 23, 2026)

Caribbean reef structure (Edmunds & Carpen-

ter, 2001; Lessios, 1988). Since then, other echi-

noids, including Echinometra lucunter, have

partially filled the grazing niche once dominat-

ed by Diadema, thereby assuming an increas-

ingly critical role in the maintenance of reef

resilience (Alvarado, 2011; McClanahan, 1995).

The red sea urchin Echinometra lucunter

(Linnaeus, 1758) is widely distributed across

shallow rocky and coral reefs in the tropical

Western Atlantic and Caribbean (Alvarado,

2011; Kroh & Mooi, 2025). This species usu-

ally inhabits crevices and cavities in calcareous

and basaltic substrates, where it actively grazes

on algal turfs and organic films (Hendler et

al., 1995; Shulman, 2020). Through its feed-

ing activity, E. lucunter contributes to both

bioerosion and benthic stability, making it a

key determinant of reef surface composition

(Perry & Harborne, 2016). Abundance and size

structure of this genus tend to vary with habitat

conditions, including substrate hardness, refuge

availability, and algal composition (McClanah-

an & Shafir, 1990). Because the abundance and

morphometric traits vary according to habitat

characteristics, E. lucunter may represent a

useful model to assess environmental condi-

tions and reef habitat quality in shallow coastal

systems (Belford, 2020; Rodríguez-Barreras et

al., 2024). Morphometric characteristics, such

as test diameter, spine length, and volume,

can provide indirect information on energy

allocation and environmental stress in urchin

populations (Rogers-Bennett, 1994). Individu-

als living in optimal habitats with abundant

turf algae and complex topography tend to

exhibit larger sizes, while those in degraded

or macroalgae-dominated areas often remain

smaller, possibly due to limited food resources

and increased metabolic costs (McClanahan,

1995; Perry & Harborne, 2016). Therefore,

analyzing size distributions rather than sim-

ple density measures offers a more integrative

approach to understanding how habitat condi-

tion affects population dynamics and growth

patterns in E. lucunter.

In Puerto Rico, coastal degradation associ-

ated with sedimentation, eutrophication, and

physical disturbance has led to the decline of

many nearshore reefs, particularly along the

north coast (Camargo et al., 2009; Hernández-

Delgado & Ortiz-Flores, 2022). Broad-scale

pressures frequently manifest through fine-

scale, localized processes that generate strong

spatial heterogeneity in benthic condition

over distances of only a few meters. Empirical

evidence from Caribbean reefs indicates that

patchy sediment deposition, localized coral

mortality, microtopographic variability, and

spatial differences in herbivory intensity can

rapidly shift benthic dominance between coral-

and algae-dominated states at very small spa-

tial scales (Hernández-Fernández et al., 2019;

Simmons et al., 2022). As a result, adjacent

reef patches within the same continuous reef

structure may exhibit sharply contrasting ben-

thic composition and structural complexity. Isla

de Cabra, located at the entrance of San Juan

Bay, represents a microcosm of these processes,

where zones of contrasting coral cover, rugosity,

and algal dominance coexist within short spa-

tial scales. Prior studies have shown significant

spatial heterogeneity in benthic composition

and rugosity across this area (Rodríguez-Barre-

ras et al., 2024), yet the biological consequences

of habitat changes for invertebrate assemblages

remain poorly understood. This study aimed to

evaluate how small-scale variability in benthic

habitat condition relates to the size structure

and local density of the sea urchin Echinometra

lucunter within a shallow reef at Isla de Cabra,

Puerto Rico.

MATERIALS AND METHODS

Study area: This study was conducted in

September 2025 at Isla de Cabra, located at the

entrance of San Juan Bay in northern Puerto

Rico (18°28’31” N, 66°08’09” W), ranging from

1–2 m in depth. Sampling was performed

within a shallow patch reef located along the

northeastern margin of the island’s northwest-

ern zone (Fig. 1A).

Within this patch reef, we defined two

spatially independent microhabitat zones based

on contrasting benthic conditions (Fig. 2).

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 74 (S1): e20267074, abril 2026 (Publicado Abr. 23, 2026)

Each zone was sampled using independent,

non-overlapping transects, ensuring spatial

independence between sampling units. The

two zones were separated by a sandy channel

covered by Thalassia testudinum, which served

as a natural boundary between microhabitats

and facilitated clear spatial delimitation dur-

ing field sampling. Zones were operationally

defined as: (1) Healthy Reef Zone (HRZ): char-

acterized by higher live coral cover and greater

structural complexity, and (2) Degraded Reef

Zone (DRZ): dominated by macroalgae, turf

algae, and unconsolidated substrates. These

zones represent contrasting benthic conditions

within the same reef structure but were treated

as independent sampling areas for all analy-

ses. The classification of both zones was car-

ried out using criteria applied in other studies

(Mujiyanto et al., 2020; Widodo et al., 2021),

where a healthy reef exhibits at least 40%

Fig. 1. Study site at Isla de Cabra and rugosity method. A. Location of the two zones surveyed within the reef: Healthy Reef

Zone (blue) and Degraded Reef Zone (yellow). B. Diagram of the chain-and-tape method used to estimate rugosity (R),

where Lc represents the contour length of the chain and Lh the horizontal linear distance.

Fig. 2. Representative examples of reef condition categories used in this study based on published classification criteria. A.

(Healthy reef) illustrates a site with ≥40% live coral cover, characteristic of a healthy reef. B. (Degraded reef) shows a site

with low live coral cover and a high abundance of macroalgae and dead coral skeletons associated with algal mats, typical

of degraded reefs.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 74 (S1): e20267074, abril 2026 (Publicado Abr. 23, 2026)

live coral cover, whereas a degraded reef is

characterized not only by low coral cover but

also by a high abundance of macroalgae and

dead coral skeletons associated with algal mats

(Gudka et al., 2023).

The HRZ was characterized by a higher

relative dominance of scleractinian corals, pri-

marily Porites astreoides, with additional contri-

butions from Siderastrea radians and Agaricia

agaricites, and a benthic matrix composed main-

ly of turf algae and calcareous macroalgae (e.g.,

Halimeda spp.). This assemblage reflects a reef

state with greater structural complexity and

carbonate-producing organisms (Ceccarelli et

al., 2018). In contrast, DRZ was dominated

by fleshy macroalgae such as Dictyota spp.,

the presence of coralline red algae such as

Amphiroa spp., and low coral cover, indicative

of a macroalgae-dominated reef and altered

ecosystem functioning (Ceccarelli et al., 2018).

Although the two study zones were located in

close proximity (<5 m apart), this spatial jux-

taposition reflects the fine-scale heterogeneity

and patchy spatial patterns commonly observed

in shallow Caribbean reefs, where localized

disturbances, herbivory pressure, and micro-

environmental conditions can generate sharply

contrasting benthic states over relatively short

distances (Hernández-Fernández et al., 2019).

Rugosity, benthic composition: To esti-

mate the benthic complexity of both zones, we

used the chain-and-tape method (Storlazzi et

al., 2016). A 5 m stainless-steel chain was gently

draped across the natural contours of the reef

surface, following all depressions and crests

without stretching or compressing the chain.

The straight-line horizontal distance between

the two chain endpoints was measured with a

measuring tape (Fig. 1B). The ratio between the

chain length (C) and the linear distance (L) was

calculated as the Rugosity Index (RI = C / L).

Five independent replicates were taken per

zone, ensuring that each transect was indepen-

dent and representative of the overall habitat

structure. Higher RI values indicate greater

surface complexity and potential availability of

microhabitats for reef organisms (Fig. 1B).

Benthic composition was assessed using

photo quadrats and the random point count

technique in Coral Point Count (CPCe) version

4.1 (Kohler & Gill, 2006). A total of ten 50 x

50 cm PVC quadrats were randomly placed in

each zone, and each quadrat was photographed

using a Pentax WG-8 waterproof camera (20

MP). All images were uploaded and analyzed to

determine the percentage of points of the ben-

thic community. Fifteen randomly distributed

points per image were used to classify substrate

cover into seven benthic categories: C = Coral,

CJ = Coral juvenile, MA = Macroalgae, TF =

Turf algae, CA = Coralline algae, SPR = Sand,

pavement, and rubble, and U = Unknown. Ben-

thic composition was expressed as the percent-

age of points assigned to each benthic category

relative to the total number of points per image,

((points per category × 100)/total points), and

reported as mean ± SD.

Urchin density and size structure: Within

each zone, ten 50 x 50 cm PVC quadrats were

randomly placed to count all visible E. lucunter

individuals. For morphometric measurements,

a total of 50 individuals of E. lucunter were

manually collected from crevices and cavities

within each reef zone. Individuals were care-

fully handled and measured in situ using a

Vernier caliper (± 0.05 mm precision). The

diameter test was recorded as the maximum

horizontal axis across the peristome. After

the measurement, all sea urchins were imme-

diately returned to their original location to

minimize disturbance.

Statistical analysis: Normality of data dis-

tributions was evaluated using the Shapiro–

Wilk test, and Levene’s test was applied to assess

homogeneity of variances. Student’s t-tests was

used to compare rugosity and urchin den-

sity between the Healthy Reef Zone (HRZ) and

Degraded Reef Zone (DRZ). Sea urchin density

and rugosity data met the assumptions of nor-

mality and homogeneity of variances and were

analyzed without transformation. Test diam-

eter data were log₁₀-transformed to meet nor-

mality assumptions and to allow comparisons

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 74 (S1): e20267074, abril 2026 (Publicado Abr. 23, 2026)

of size structure between reef zones. Benthic

cover data were not normalized or transformed

because no inferential statistical tests were con-

ducted. All statistical analyses were performed

in R v4.5.0 (R Core Team, 2025), using a signifi-

cance threshold of α = 0.05.

RESULTS

Spatial community structure: Benthic

composition (frequency of points) differed

between the Healthy Reef Zone (HRZ) and

the Degraded Reef Zone (DRZ) at Isla de

Cabra (Fig. 3). In the HRZ, the substrate was

dominated by live coral (C) and juvenile coral

colonies (CJ), which together accounted for

approximately one third of the total points

recorded (Fig. 3A). Turf algae (TF) and coral-

line algae (CA) were also present but in moder-

ate proportions, while macroalgae (MA) and

sand, pavement, and rubble (SPR) contributed

minimally. This composition reflects a structur-

ally more complex reef surface. In contrast, the

DRZ was characterized by a strong dominance

of turf and macroalgae, with the combined

categories (MA + TF) representing over 70 %

of the frequency of points (Fig. 3B). Coral and

juvenile colonies were scarce (< 10 %), and the

substrate consisted largely of sand and rubble.

When the main benthic categories were

grouped, the contrast between reef zones

became even more evident. The combined mac-

roalgal cover (MA + TF) was markedly higher in

the Degraded Reef Zone (DRZ), exceeding two-

thirds of total substrate coverage and reflecting

a benthic surface dominated by filamentous and

fleshy algae (Fig. 3C). In contrast, the Healthy

Reef Zone (HRZ) displayed substancially great-

er coral cover (C + CJ), averaging close to

40–45 % of total substrate (Fig. 3D).

Fig. 3. Frequency of point (%) by benthic category and reef zones at Isla de Cabra, Puerto Rico. A. Healthy Reef Zone (HRZ).

B. Degraded Reef Zone (DRZ). Categories: C = Coral; CJ = Coral juvenile; MA = Macroalgae; TF = Turf algae; CA = Coralline

algae; SPR = Sand, pavement, and rubble; U = Unknowns. C. Combined macroalgal cover (MA + TF) showing higher values

in DRZ. D. Combined coral cover (C + CJ) showing higher values in HRZ. Error bars represent standard deviations. Colors

denote reef condition: blue = HRZ, yellow = DRZ.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 74 (S1): e20267074, abril 2026 (Publicado Abr. 23, 2026)

Sea urchin abundance and size struc-

ture: The density and body size of Echino-

metra lucunter exhibited contrasting patterns

between the two reef zones at Isla de Cabra

(Fig. 4A). The HRZ exhibited a mean den-

sity of 20.27 ± 7.25 ind. m-2, while the DRZ

showed a slightly lower value of 17.87 ± 7.58

ind. m-2. Mean density did not differ signifi-

cantly between zones (t = 0.89, p = 0.383; Fig.

4A). Regarding mean size, individuals from

the HRZ had a larger mean test diameter

(mean = 4.3 ± 0.8 cm), whereas those from

the DRZ were significantly smaller (mean =

3.5 ± 0.9 cm). Average size showed significant

differences between the two zones (t = 4.50, p

= 1.88 × 10-⁵; Fig. 4B).

Size-frequency distribution of E. lucunter

varied between reef zones (Fig. 5). Diameters

ranged from 2.5 to 6.2 cm, with a modal class

around 4.5–5.0 cm in the HRZ (Fig. 5A). The

distribution was relatively symmetrical, with

frequent representation of medium and large

size classes. Larger individuals (> 5 cm) were

frequent in this zone. Conversely, the distribu-

tion was skewed toward smaller size classes

in the DRZ, with most individuals measuring

between 2.5 and 4.0 cm and few exceeding 5

cm (Fig. 5B).

Fig. 4. Density and test diameter of the sea urchin Echinometra lucunter in two reef zones at Isla de Cabra, Puerto Rico. A.

Mean density (individuals m-²) of E. lucunter showing no significant difference between the Healthy Reef Zone (HRZ) and

Degraded Reef Zone (DRZ). B. Test diameter (cm) illustrating significantly larger individuals in the HRZ (t = 4.50, p = 1.88

× 10-⁵). Boxes represent interquartile ranges, horizontal lines the medians, whiskers the minimum and maximum values, and

crosses the means. Colors denote reef condition: blue = HRZ, yellow = DRZ.

Fig. 5. Size–frequency distribution of the sea urchin Echinometra lucunter by reef zone at Isla de Cabra, Puerto Rico. A.

Healthy Reef Zone (HRZ); B. Degraded Reef Zone (DRZ). Bars represent the frequency of individuals per 0.5 cm test

diameter class. Colors denote reef condition: blue = HRZ, yellow = DRZ.

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DISCUSSION

Small-scale habitat variability and sea

urchin size structure: Our results demonstrate

that small-scale benthic heterogeneity within

a single reef system influences the size struc-

ture of the sea urchin Echinometra lucunter,

even when population densities remain stable

between zones. Zones defined as “healthy”

and “degraded” are used here in an operational

sense, reflecting local differences in structural

complexity and benthic composition, rather

than broad reef-scale condition. Greater rugos-

ity and coral cover in the Healthy Reef Zone

created a more heterogeneous habitat, offering

microrefuges and substrates better suited for

the foraging activity of E. lucunter. A previous

study demonstrated the importance of struc-

tural complexity as a key determinant of popu-

lation density and size structure in this species

(Rodríguez-Barreras et al., 2024). The pre-

dominance of large individuals in the Healthy

Reef Zone aligns with observations from other

Echinometra populations, where complex habi-

tats promote higher epilithic turf availability

and lower environmental stress (McClanahan

& Shafir, 1990).

In contrast, the Degraded Reef Zone exhib-

ited a significantly higher dominance of mac-

roalgae (e.g., Dictyota spp., Sargassum spp.)

and lower structural complexity (rugosity), as

documented by our benthic surveys. Because

macroalgae are not the primary food resource

of Echinometra lucunter, which mainly feeds

on epilithic turf algae, this shift in benthic

composition likely reduces the availability of

preferred food resources. In combination with

reduced habitat complexity, these conditions

may increase intraspecific competition for suit-

able feeding and refuge microhabitats, poten-

tially generating bioenergetic constraints that

limit individual growth. This interpretation

is supported by a recent study in Puerto Rico

showing that substrate characteristics (artificial

basaltic vs. natural) directly influence body

size in E. lucunter (Rodríguez-Barreras et al.,

2024), as well as by previous studies highlight-

ing the role of habitat structure in regulating

sea urchin performance (McClanahan & Shafir,

1990; Perry & Harborne, 2016). Consequently,

the reduced sizes observed in the Degraded

Reef Zone reflect are consistent with the docu-

mented differences in benthic composition and

habitat complexity.

Stable population dynamics despite deg-

radation: The absence of significant differences

in Echinometra lucunter density between the

healthy and degraded zones suggests that the

species can maintain relatively stable popula-

tions even under altered or low-complexity

environmental conditions (Rodríguez-Barreras

et al., 2024). This pattern is consistent with

observations from other sea urchin species that

exhibit ecological flexibility across contrast-

ing habitats. For example, the temperate sea

urchin Paracentrotus lividus maintains compa-

rable densities across a range of habitat types—

including rocky substrates, seagrass meadows,

and mixed sand–rock environments—and

shows marked trophic plasticity that allows it to

adjust to variable resource availability (Camps-

Castellà et al., 2020; Hereu et al., 2012). Similar-

ly, Centrostephanus rodgersii is known to persist

at high densities in depauperate “urchin bar-

ren” habitats, where structural complexity and

algal cover are greatly reduced, demonstrating

its capacity to thrive in ecologically degraded

conditions (Przeslawski et al., 2025). These

examples highlight that tolerance of habitat

modification is not unique to Echinometra but

rather a trait shared with other echinoids that

can tolerate and exploit marginal environments.

However, stable density does not necessar-

ily indicate a favorable population condition.

The dominance of small individuals in the

degraded zone suggests sustained recruitment

but limited growth. This combination—high

proportions of juveniles and small adults—has

been documented in sea urchins subjected

to nutritional stress or limited availability of

microrefuges in degraded reefs (Perry & Har-

borne, 2016). Moreover, the higher propor-

tion of macroalgae in the degraded area may

slow the transition of juveniles into larger size

classes by reducing access to epilithic turfs,

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 74 (S1): e20267074, abril 2026 (Publicado Abr. 23, 2026)

an important food resource for E. lucunter

(Sammarco, 1982). Functionally, a popula-

tion dominated by small individuals may exert

insufficient grazing pressure to counteract

algal expansion, thereby perpetuating the reef ’s

degradation state (Mumby & Steneck, 2008).

This negative feedback dynamic is common in

Caribbean reefs that have undergone structural

simplification (Alvarez-Filip et al., 2011b).

Functional implications: Our results have

important implications for understanding the

functional resilience of coastal shallow-water

reefs. Although Echinometra lucunter main-

tained similar densities between zones, the

marked reduction in size in the degraded reef

suggests a decline in its potential for bioero-

sion and grazing—key processes in benthic reef

dynamics. Previous studies have shown that

larger urchins are responsible for greater algal

removal and substrate bioerosion, contribut-

ing to the maintenance of suitable surfaces

for coral settlement (Bak, 1990; Sammarco,

1982). Therefore, communities dominated by

small individuals, such as those observed in

the degraded zone, may lose part of their func-

tional capacity, facilitating algal expansion and

perpetuating reef degradation.

Likewise, the predominance of macroal-

gae in the Degraded Reef Zone reinforces

a well-documented ecological pattern in the

Caribbean, in which coral loss and structural

simplification reduce habitat complexity and

create conditions favorable to algal prolifera-

tion (Alvarez-Filip et al., 2011b; Hughes et

al., 2007). This shift in the coral–algal balance

alters energy flows, reduces coral productivity,

and modifies trophic interactions. In systems

where Diadema antillarum remains reduced

or absent—as is the case across much of the

Caribbean—secondary herbivores such as E.

lucunter play an even more critical role in algal

control (Lessios, 2016). However, if these pop-

ulations are functionally limited by adverse

environmental conditions, their capacity to

compensate for the loss of primary herbivory

is also diminished.

Our findings show that Echinometra

lucunter exhibits changes in size structure across

locally contrasting benthic habitats within a

shallow reef, while population density remains

relatively stable. Rather than reflecting broad

reef-scale degradation states, the observed dif-

ferences emphasize the ecological relevance of

within-reef spatial variability in shaping popu-

lation size structure and potential grazing and

bioerosion capacity of E. lucunter. Accordingly,

size-based metrics derived from this species

may provide valuable information for detect-

ing localized habitat heterogeneity and assess-

ing ecological condition at the scale at which

key organisms–habitat interactions occur. By

complementing traditional benthic descriptors

such as coral cover and structural complexity,

this small-scale perspective provides a clearer

framework for interpreting reef functioning in

shallow, urban-influenced environments.

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 express our gratitude to the Depart-

ment of Biology at the University of Puerto

Rico at Bayamón for the logistical support

provided during the development of this study.

ChatGPT 5.2, OpenAI, was used for English

translation from Spanish and for grammatical

editing of the manuscript.

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