Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e59627, enero-diciembre 2025 (Publicado Mar. 07, 2025)

Motile macroinvertebrates and fishes sheltering in burrows with

and without the Indo-Pacific Rock-boring Urchin Echinometra mathaei

(Camarodonta: Echinometridae), at Mana Island, Fiji

Floyd E. Hayes1*; https://orcid.org/0000-0003-3032-8405

Antonio I. Robles1,2; https://orcid.org/0000-0002-0024-617X

John C. Duncan1; https://orcid.org/0000-0002-2408-8022

1. Department of Biology, Pacific Union College, 1 Angwin Ave., Angwin, CA 94508, United States of America;

floyd_hayes@yahoo.com (*Correspondence), airobles@icloud.com, jduncan@puc.edu

2. Pacific Union College Preparatory School, 1 Angwin Ave., Angwin, CA 94508, United States of America.

Received 23-IV-2024. Corrected 22-XI-2024. Accepted 18-II-2025.

ABSTRACT

Introduction: Sea urchins of the genus Echinometra are ecosystem engineers that burrow into lower intertidal

and subtidal rocks, creating shelter for themselves and a variety of marine animals.

Objective: To test the hypothesis that motile macroinvertebrates and fishes shelter proportionately more fre-

quently in burrows with Echinometra mathaei, whose spines potentially offer additional protection, than in

burrows without E. mathaei.

Methods: We studied the motile macroinvertebrates and fishes sheltering in the burrows of the Indo-Pacific

Rock-boring Urchin E. mathaei (Type C) at Mana Island, Fiji.

Results: Burrows with E. mathaei averaged shorter in length than burrows without E. mathaei. We observed 42

motile macroinvertebrates of at least eight species and 49 fishes of at least eight species in burrows with E. mathaei

(n = 1 127), and four motile macroinvertebrates of three species and one fish of one species in burrows lacking

E. mathaei (n = 243). Motile macroinvertebrates occurred with statistically equal frequencies in burrows with

E. mathaei (3.6 %) and in burrows without E. mathaei (2.5 %). Fishes occurred significantly more frequently in

burrows with E. mathaei (4.3 %) than in burrows without E. mathaei (0.4 %).

Conclusion: Fishes, but not motile macroinvertebrates, gain more protection from predators by sheltering in

burrows with sea urchins, whose spines offer additional protection, than in burrows without sea urchins.

Key words: coastal ecosystems; community ecology; defensive behavior; ecosystem engineers; Pacific Ocean.

RESUMEN

Macroinvertebrados móviles y peces que se refugian en madrigueras con y sin el erizo perforador

de rocas del Indo-Pacífico Echinometra mathaei (Camarodonta: Echinometridae), en isla Mana, Fiji

Introducción: Los erizos de mar del género Echinometra son ingenieros de ecosistemas que excavan en rocas

intermareales y submareales inferiores, creando refugio para ellos y una variedad de animales marinos.

Objetivo: Probar la hipótesis de que los macroinvertebrados móviles y los peces se refugian proporcionalmente

con más frecuencia en madrigueras con Echinometra mathaei, cuyas espinas ofrecen potencialmente protección

adicional, que en madrigueras sin E. mathaei.

Métodos: Estudiamos los macroinvertebrados móviles y los peces que se refugian en las madrigueras del erizo

perforador de rocas del Indo-Pacífico E. mathaei (Tipo C) en la isla Mana, Fiji.

https://doi.org/10.15517/rev.biol.trop..v73i1.59627

AQUATIC ECOLOGY

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e59627, enero-diciembre 2025 (Publicado Mar. 07, 2025)

INTRODUCTION

Sea urchins (Echinodermata: Echinoidea)

enhance the biodiversity of shallow benthic

marine communities by providing microhabi-

tats, shelter, and nutrients for a wide variety of

organisms (Lawrence, 2020; Steneck, 2020). Sea

urchins of the genus Echinometra are ecosystem

engineers (Jones et al., 1994) that often burrow

into subtitdal rocks, creating shelter for them-

selves and a variety of invertebrates and fishes

(Asgaard & Bromley, 2008; McClanahan &

Muthiga, 2001; McClanahan & Muthiga, 2020).

In addition, the sharp spines of Echinometra

sea urchins may protect associating organ-

isms from predators (Cheh et al., 2021; Nunes

et al., 2019). The motile macroinvertebrates

and fishes sheltering in burrows excavated by

Echinometra sea urchins have been studied for

two species: the Atlantic Rock-boring Urchin

Echinometra lucunter in the Western Atlantic

Ocean (Hayes et al., 2016; Hayes et al., 2019;

Monroy-López & Solano, 2006; Nunes et al.,

2019; Schoppe, 1991; Schoppe & Werding,

1996; Wirtz et al., 2009; Yamarte et al., 2019)

and the Central American Rock-boring Urchin

Echinometra vanbrunti in the eastern Pacific

Ocean (Hayes et al., 2022; Schoppe & Werding,

1996; Vallejo, 2007).

The Indo-Pacific Rock-boring Urchin

Echinometra mathaei is widely distributed

throughout the Indian Ocean and Western

Pacific Ocean (Clark & Rowe, 1971). It typi-

cally inhabits burrows excavated in rock and

coral substrates in lower intertidal and shallow

subtidal (usually < 10 m) depths along the coast

and outer coral reefs, but it does not burrow

into softer sediments in reef flats and seagrass

beds (Khamala, 1971; McClanahan & Muthiga,

2001; McClanahan & Muthiga, 2020; Neill,

1988; Russo, 1980; Suzuki, 2005; Yamamori

& Kato, 2017). In soft mudstone in Japan, E.

mathaei occupies burrows excavated and sub-

sequently abandoned by the Burrowing Fine

Spine Urchin Echinostrephus molaris, sharing

the burrows with 18 species of invertebrates

and one species of fish (Yamamori & Kato,

2017; Yamamori, 2022). In burrows excavated

by E. mathaei, two species of crustaceans, Sea-

urchin Snapping Shrimp Arete indicus and the

shrimp Tuleariocaris holthuis (Brasseur et al.,

2018; Dabbagh et al., 2019a, Dabbagh et al.,

2019b; Gherardi, 1991; Gherardi & Calloni,

1993; Ghory et al., 2018) and one species of fish,

Dusky Gregory Stegastes nigricans (Cheh et al.,

2021), associate with E. mathaei.

In this study, we provide the first descrip-

tion of the motile macroinvertebrate and fish

communities sheltering in burrows excavated

by E. mathaei. Furthermore, we test the hypoth-

esis that motile macroinvertebrates and fishes

shelter proportionately more frequently in bur-

rows with E. mathaei, whose spines potentially

offer additional protection, than in burrows

without E. mathaei.

MATERIALS AND METHODS

Study area: Mana Island is a small island

(1.75 km2) with a maximum elevation of 70 m

Resultados: Las madrigueras con E. mathaei promediaron una longitud más corta que las madrigueras sin E.

mathaei. Observamos 42 macroinvertebrados móviles de al menos ocho especies y 49 peces de al menos ocho

especies en madrigueras con E. mathaei (n = 1 127), y cuatro macroinvertebrados móviles de tres especies y un

pez de una especie en madrigueras sin E. mathaei (n = 243). Los macroinvertebrados móviles se presentaron con

frecuencias estadísticamente iguales en madrigueras con E. mathaei (3.6 %) y en madrigueras sin E. mathaei (2.5

%). Los peces se presentaron significativamente con mayor frecuencia en madrigueras con E. mathaei (4.3 %) que

en madrigueras sin E. mathaei (0.4 %).

Conclusión: Los peces, pero no los macroinvertebrados móviles, obtienen más protección contra los depreda-

dores al refugiarse en madrigueras con erizos de mar, cuyas espinas ofrecen protección adicional, que en madri-

gueras sin erizos de mar.

Palabras clave: ecosistemas costeros; ecología comunitaria; comportamiento defensivo; ingenieros de ecosiste-

mas; océano Pacífico.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e59627, enero-diciembre 2025 (Publicado Mar. 07, 2025)

in the Mamanuca Islands of western Fiji (Fig. 1).

The shores are formed of volcanic rock head-

lands alternating with sandy beaches. A barrier

reef and lagoon occur to the west and South of

the island, patch reefs occur within the lagoon,

and a fringing reef occurs along the north and

east shores. The coral reefs of the Mamanuca

Islands, including Mana Island, support a rich

diversity of marine life but are threatened by

human activities (Fenner, 2006; Harborne et

al., 2001).

Sampling methods: We studied the motile

(non-sessile, free moving) macroinvertebrates

(large enough to see without the aid of a micro-

scope) and fishes sheltering in the burrows of

E. mathaei along 150 m of the shore of a rocky

promontory and on adjacent submerged rocks

on the north coast of Mana Island (17°40’14”

S, 177°6’34” E, Fig. 1) during 21-24 June 2019,

25 March 2023, and 11 September 2023. Three

morphs of E. mathaei, Type A (white-tipped

or entirely white spines), Type C (entirely

dark spines), and a rare type undesignated by

a letter (entirely maroon spines), occur in Fiji

and possibly represent distinct species (Appana

et al., 2004). All E. mathaei at our study site

represented Type C. We used snorkeling gear

in shallow water < 6 m deep to search for

motile macroinvertebrates and fishes within

each burrow. Not all burrows were occupied

by E. mathaei. We sampled organisms in bur-

rows with E. mathaei and in burrows without E.

mathaei (Fig. 2). We measured the length (near-

est cm) of a subset of burrows of both types

with a wooden ruler. Underwater writing slates

were used to record data. Each species was

identified as the surveys were taking place or

photographed and subsequently identified with

the assistance of published field guides (Allen

et al., 2003; Humann & Deloach, 2010; Rosen-

stein, 2019) and experts identifying our photos

posted at iNaturalist (www.inaturalist.org). All

organisms were observed in situ; we did not

attempt to remove any organisms for subse-

quent identification. The English and scien-

tific names follow iNaturalist, which frequently

incorporates taxonomic revisions.

Statistical analyses: A Mann-Whitney U

test (z statistic) (Zar, 2010) was calculated to

compare the lengths of burrows with and with-

out urchins. We calculated the number of indi-

viduals and the percent of burrows occupied

for each species of motile macroinvertebrate

and fish observed in burrows with and without

Fig. 1. Mana Island, Fiji, with study site along north coast indicated in red.

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e59627, enero-diciembre 2025 (Publicado Mar. 07, 2025)

E. mathaei. Chi-square analyses of contingency

tables (X2 statistic) (Zar, 2010) were calculated

to compare the proportions of burrows with

motile macroinvertebrates and fishes between

burrows with E. mathaei and burrows without

E. mathaei. The statistical tests were calculated

with Statistix 10 software (Nimis, 2013).

RESULTS

Burrow characteristics: The burrows of E.

mathaei comprised irregularly shaped grooves

on rock surfaces, usually wider at the ends than

in the middle (Fig. 1), and varied greatly in

length, ranging from 4 to 57 cm (mean = 16.5,

SD = 9.8). The burrows with urchins averaged

shorter in length (mean = 15.9 cm, SD = 10.6,

range = 4-57 cm, n = 94) than burrows without

urchins (mean = 17.6 cm, SD = 7.8, range =

6-46 cm, n = 47; Mann-Whitney U test, z =

2.04, p = 0.04).

Motile macroinvertebrates: We observed

42 motile macroinvertebrates of at least eight

species in burrows with E. mathaei (n = 1 147),

and six motile macroinvertebrates of three spe-

cies in burrows lacking E. mathaei (n = 243;

Table 1). Motile macroinvertebrates occurred

with statistically equal frequencies in burrows

with E. mathaei (3.6 %) and in burrows without

E. mathaei (2.5 %; Yates corrected X2 = 0.51, d.f.

= 1, p = 0.48; Table 1).

Adult gastropod molluscs of three species

comprised 19.0 % of the motile macroinver-

tebrates in burrows with E. mathaei (Table 1),

including the Mulberry Whelk Tenguella mar-

ginalba (family Muricidae, 2.4 %), Striped Engi-

na Engina mendicaria (family Pisaniidae, 4.8 %,

Fig. 3A), and unidentified cone snails Conus sp.

(family Conidae, 4.8 %, Fig. 3B). Decapod crus-

taceans of three species comprised 23.8 % of the

motile macroinvertebrates in burrows with E.

Fig. 2. Burrows with and without Echinometra mathaei at Mana Island, Fiji. Photograph by Floyd E. Hayes.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e59627, enero-diciembre 2025 (Publicado Mar. 07, 2025)

mathaei (Table 1), including a juvenile Painted

Spiny Lobster Panulirus versicolor (family Panu-

liridae, 2.4 %) and adults of the Gaimard’s Her-

mit Crab Calcinus gaimardii (family Calcinidae,

11.9 %, Fig. 3C) and Small White Hermit Crab

Calcinus minutus (2.4 %, Fig. 3D). Unidentified

juvenile sea urchins Diadema or Echinothrix

spp. (family Diadematidae) comprised 40.5 %

of the motile macroinvertebrates in burrows

with E. mathaei (Table 1, Fig. 3B). Feather stars

(order Comatulida) comprised 16.7 % of motile

macroinvertebrates in burrows with E. mathaei

(Table 1), including an adult Bennett’s Bushy

Feather Star Oxycomanthus bennetti (family

Comasteridae, 2.4 %). An unidentified Conus

sp. and a juvenile Diadema or Echinothrix

TABLE 1

Number (% of burrows occupied) of motile macroinvertebrates and fishes observed in burrows with (n = 1 127)

and without (n = 243) the sea urchin Echinometra mathaei at Mana Island, Fiji

Phylum, class, order

Family, genus and specific epithet

Burrows with

E. mathaei

Burrows without

E. mathaei

Mollusca, Gastropoda, Neogastropoda

Muricidae, Tenguella marginalba 1 (0.09) ‒

Pisaniidae, Engina mendicaria 2 (0.18) ‒

Conidae, Conus ebraeus ‒3 (1.23)

Conidae, Conus sp. 2 (0.18) ‒

Unidentified 3 (0.27) ‒

Mollusca, Gastropoda, Littorinomorpha

Cypraeidae, Monetaria annulus ‒1 (0.41)

Mollusca, Cephalopoda, Octopoda

Unidentified ‒1 (0.41)

Arthropoda, Malacostraca, Decapoda

Panuliridae, Panulirus versicolor 1 (0.09) ‒

Calcinidae, Calcinus gaimardii 5 (0.44) 1 (0.41)

Calcinidae, Calcinus minutus 3 (0.27)

Calcinidae, unidentified 1 (0.09) ‒

Echinodermata, Echinoidea, Diadematoida

Diadematidae, Diadema or Echinothrix sp. 17 (1.51) ‒

Echinodermata, Crinoidea, Comatulida

Comasteridae, Oxycomanthus bennetti 1 (0.09) ‒

Unidentified 6 (0.53)

Chordata, Actinopterygii, Gobiiformes

Gobiidae, Eviota teresae 3 (0.27) ‒

Gobiidae, Eviota fasciola 3 (0.27) ‒

Chordata, Actinopterygii, Bleniiformes

Tripterygiidae, Enneapterygius sp. 1 (0.09) ‒

Blenniidae, Cirripectes castaneus 4 (0.41) ‒

Blenniidae, unidentified 3 (0.27) ‒

Chordata, Actinopterygii, Ovalentaria

Pomacentridae, Chrysiptera leucopoma 6 (0.53) ‒

Pomacentridae, Chrysiptera unimaculata 1 (0.09) ‒

Pomacentridae, Plectroglyphidodon leucozona 8 (0.71) ‒

Pomacentridae, Stegastes nigricans 17 (1.75) 1 (0.41)

Chordata, Actinopterygii

Unidentified 3 (0.27) ‒

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sp. were the only motile macroinvertebrates

observed sharing a burrow together with an E.

mathaei (Fig. 3B).

Adults of the Black-and-white Cone Snail

Conus ebraeus (family Conidae) comprised 50

% of the motile macroinvertebrates in burrows

without E. mathaei, followed by an adult Money

Cowry Monetaria aunnulus (family Cypraeidae,

16.7 %), a small unidentified octopus (order

Octopoda, 16.7 %), and an adult C. gaimardii

(family Calcinidae, 16.7 %, Table 1).

Fishes: We observed 49 fishes of at least

eight species in burrows with E. mathaei (n =

1 147), but only one fish of one species in bur-

rows lacking E. mathaei (n = 243, Table 1). The

fishes occurred significantly more frequently

in burrows with E. mathaei (4.3 %) than in

burrows without E. mathaei (0.4 %; Yates cor-

rected X2 = 7.50, d.f. = 1, p = 0.006, Table 1).

Adult gobies (family Gobiidae) of two spe-

cies comprised 12.2 % of the fishes in burrows

with E. mathaei (Table 1), including the Terry’s

Shrimpgoby Eviota teresae (6.1 %, Fig. 4A)

and Barred Eviota Eviota fasciola (6.1 %, Fig.

4B). An adult triplefin, possibly a Blackcheek

Threefin Enneapterygius rufopileus (family

Tripterygiidae, Fig. 4C), comprised 2.0 % of

the fishes in burrows with E. mathaei (Table 1).

Adult blennies (family Blenniidae) of one or

more species accounted for 14.3 % of the fishes

in burrows with E. mathaei (Table 1), including

the Chestnut Blenny Cirripectes castaneus (8.2

%) and unidentified blennies (6.1 %). Juvenile

and adult damselfishes (family Pomacentridae)

of four species accounted for 65.3 % of the fishes

Fig. 3. Examples of motile macroinvertebrates in the burrows of Echinometra mathaei: A. Engina mendicaria; B. Conus sp.

(left center) and Diadema sp. or Echinometra sp. (lower right); C. Calcinus gaimardii; D. Calcinus minutus. Photographs by

Floyd E. Hayes (A, C, D) and John C. Duncan (B).

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e59627, enero-diciembre 2025 (Publicado Mar. 07, 2025)

in burrows with E. mathaei (Table 1), including

the Pacific Surge Demoiselle Chrysiptera leuco-

poma (12.2 %, Fig. 4D), Onespot Demoiselle

Chrysiptera unimaculata (2.0 %), Whiteband

Damsel Plectroglyphidodon leucozona (16.3 %),

and Dusky Gregory Stegastes nigricans (34.7 %).

The remaining 6.1 % of fishes in burrows with

E. mathaei were unidentified cryptobenthic

species (Table 1).

An E. fasciola and an unidentified fish

were the only fishes observed sharing a burrow

together with an E. mathaei. We did not observe

any fishes cohabiting a burrow with any motile

macroinvertebrate other than E. mathaei.

The only fish observed in a burrow without

E. matthaei was an adult C. leucopoma (Table 1).

The adults of all fishes were relatively small,

never exceeding 10 cm in length.

DISCUSSION

The burrows of E. mathaei are usually

inhabited by a single individual, gradually

increase in size through bioerosion by the sea

urchin, and are reused by multiple generations

of sea urchins (Russo, 1980). The preference

by E. mathaei for shorter burrows in our study

suggests that the conditions of the longest bur-

rows are suboptimal for foraging or defense

from predators, which needs further study.

Of the eight species of motile macroinver-

tebrates and eight species of fishes observed

sheltering in burrows of E. mathaei, only one

species, the damselfish S. nigricans, had been

previously reported associating with E. mathaei

(Cheh et al., 2021). None of the organisms

sheltering in E. mathaei burrows associated

Fig. 4. Examples of fishes in the burrows of Echinometra mathaei: A. Eviota teresae; B. Eviota fasciola; C. Enneapterygius sp.,

possibly E. rufopileus; D. Chrysiptera leucopoma. Photographs by Floyd E. Hayes.

8Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e59627, enero-diciembre 2025 (Publicado Mar. 07, 2025)

exclusively with E. mathaei or appeared to be

specialized for associating with sea urchins,

indicating that the associations are facultative

rather than obligatory.

Organisms inhabiting E. mathaei burrows

potentially benefit from reduced predation by

sheltering within burrows, by associating close-

ly with the sharp spines of E. mathaei, or syner-

gistically by both. Organisms may also benefit

by finding prey within the burrows. None of

the organisms that we observed in E. mathaei

burrows are known predators of E. mathaei

(McClanahan, 2000; McClanahan & Muthiga,

1989; McClanahan & Muthiga, 2001; McCla-

nahan & Muthiga, 2020). The largest carnivore

was the unidentified octopus, which sheltered

in a burrow without E. mathaei. Octopuses

have been reported preying on sea urchins else-

where (Smith, 2003; Villegas et al., 2014). All

of the fishes observed sheltering in E. mathaei

burrows were small, < 10 cm long, comprising

juveniles or adults of relatively small species.

Although several species of fishes comprise

the dominant predators of E. mathaei (McCla-

nahan, 2000; McClanahan & Muthiga, 1989;

McClanahan & Muthiga, 2001), small fishes

such as those that we observed in the burrows

pose no existential threat to sea urchins and are

more likely to benefit from the spines of sea

urchins than larger fishes (Hayes et al., 2019;

Hayes et al., 2022; Karplus, 2014).

Our data are too limited to evaluate wheth-

er macroinvertebrates benefit more by shelter-

ing in burrows or associating directly with E.

mathaei. Most species of molluscs and decapod

crustaceans possessed or lived within calci-

um carbonate shells, which provide protection

against predators. Juvenile Diadema or Echino-

thrix urchins, which were the most common

motile macroinvertebates, sheltered only in

burrows occupied by E. mathaei, but always in

opposite ends of the burrows, presumably to

reduce interspecific competition (McClanahan,

1988). Because Diadema and Echinothrix sea

urchins have proportionately longer spines than

E. mathaei (McClanahan, 1988), which presum-

ably provide more protection against predators

(Hayes et al., 2016; Hayes et al., 2019; Hayes

et al., 2022), Diadema sea urchins more likely

benefit from the shelter of a burrow than by

associating with the spines of E. mathaei.

Our data for all fish species combined

provide evidence that fishes prefer shelter-

ing in burrows with E. mathaei, even though

they averaged shorter in length than burrows

without E. mathaei, presumably because of

the potential additional benefit of associating

directly with the spines of E. mathaei. Cheh et

al. (2021) provided evidence that the damsel-

fish S. nigricans, which was the most common

fish species sheltering in E. mathaei burrows in

our study, perceived E. mathaei as providing

extra security from approaching threats, such

as larger predatory fishes or cephalopods. Simi-

larly, the Brazilian Fanged Blenny Ophioblen-

nius trinitatis perceived E. lucunter as providing

extra security from threats (Nunes et al., 2019).

In contrast with the burrows of E. mathaei,

fewer species of animals occurred in burrows

occupied by an urchin than in empty burrows

of the Pacific Purple Sea Urchin Strongylocen-

trotus purpuratus (Davidson & Grupe, 2013)

and Pot-hole Urchin Stomopneustes variolaris

(Chanket & Wangkulangkul, 2019). However,

these two species excavate a smaller burrow,

described as a “pit,” with less space available for

sheltering ectosymbionts. Thus, both the size

of a burrow and the presence or absence of an

urchin may affect the efficacy of an urchin bur-

row as a shelter from predators.

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 Acknowledgements

section. A signed document has been filed in

the journal archives.

ACKNOWLEDGMENTS

Our field work was funded by Herber Fam-

ily Faculty Development Grants, the Margaret

Hughes Faculty Research Fund and Faculty

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e59627, enero-diciembre 2025 (Publicado Mar. 07, 2025)

Development Fund of Pacific Union College

and by students taking a tropical field biol-

ogy course taught by Hayes. We thank Fabio

Maia for logistical assistance, Kimberly Lange

Velazquez for assistance with data collection,

Nathaniel Bell and Juan Manuel de Roux for

assistance with identifying mollusc photos, Eiji

Myorin for assistance with identifying hermit

crab photos and Francois Libert and Mark

Rosenstein for assistance with identifying

fish photos.

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