ORIGINAL ARTICLE

Habitat provision of barnacle tests for overcrowdedperiwinklesAna C. F. Silva1, Vanessa Mendonc�a2, Rita Paquete1, Nuno Barreiras1 & Catarina Vinagre2

1 Centro de Geo-sistemas/CVRM, Instituto Superior T�ecnico, Universidade de Lisboa, Lisbon, Portugal

2 Faculdade de Ciencias, Centro de Oceanografia, Universidade de Lisboa, Lisbon, Portugal

Keywords

Barnacle; colonisation; microhabitat;

morphometric; shell; snail.

Correspondence

A. C. F. Silva, Centro de Geo-sistemas/CVRM,

Instituto Superior T�ecnico, Universidade de

Lisboa, Av. Rovisco Pais, 1049-001 Lisbon,

Portugal.

E-mail: ana.c.f.silva@tecnico.ulisboa.pt

Accepted: 18 February 2014

doi: 10.1111/maec.12161

Abstract

In habitats where competition for space is a shaping force of animal distribu-

tion such as in the intertidal rocky ecosystem, new habitats are readily taken

by colonising species. We examined the importance of empty Chthamalus spp.

tests as a habitat for the intertidal common periwinkle Melaraphe neritoides on

Portuguese rocky shores. The role played by the space between neighbouring

barnacles as a habitat for other species has been largely studied with regard to

how an ecosystem functions, whereas the equivalent role of empty barnacle

tests remains largely unknown. The small periwinkle is one of the most abun-

dant snails in European rocky shores and is an important prey for key mobile

predators. Biological facilitation is common in the rocky intertidal zone, where

biological structures often potentiate the abundance of other species. The role

played by barnacles as biological facilitators through habitat provision is not

fully understood. In this study, the abundance and morphometric features of

empty barnacle tests and their occupants were examined across shore levels

and shores with differing exposures, as these are important gradients explaining

barnacle distribution. The availability of empty barnacle tests was also experi-

mentally manipulated to examine the percentage, time and length of occupa-

tion. Empty barnacles were more abundant on the midshore of sheltered

shores and barnacle tests were wider on the upper shore but taller on the mid-

shore. The minimum barnacle test occupation rate by the periwinkle was 70%.

Barnacle shell height was an important factor determining snail occupation,

where taller barnacles harboured significantly more, but not necessarily larger,

snails. Snails outside the barnacle tests were significantly larger than those

found within, suggesting that this crustacean group has an increased impor-

tance as a habitat for juvenile snails, thus potentially influencing the population

dynamics of M. neritoides. We found that occupation of experimentally created

empty barnacles was extremely high (70%) on the day after, and remained at

100% after 3 months of monitoring. Our study is the first of its kind to focus

on the features of barnacle tests that snails occupy and their occupying snail

traits. According to our results, it is clear that barnacles have an important role

in providing additional habitat for young gastropods. The small periwinkle is

an important grazer and prey for intertidal and subtidal foraging predators;

hence, the potential refuge role of barnacle tests for juvenile M. neritoides may

be important in the dynamics of intertidal communities.

Marine Ecology (2014) 1–11 ª 2014 Blackwell Verlag GmbH 1

Marine Ecology. ISSN 0173-9565

Introduction

Biological facilitation has been advocated as a community

structuring force, comparable to other factors such as pre-

dation and competition (Bruno et al. 2003). Facilitation

often occurs when one species increases the survival and/

or abundance of another. The available rocky intertidal

substratum is most often covered in dominating algae and

mussel or barnacle matrices, making space limited and a

key resource for the establishment of intertidal popula-

tions, typically subject to strong competitive interactions

(Menge & Sutherland 1976; Connell 1983). Barnacles are

facilitators of snail survival in mangroves by providing ref-

uge from predation (Catesby & McKillup 1998). Barnacles

can thus be perceived as biologic generators of habitat

and/or refuge through their empty tests that remain

attached to the rock after death. Other biologic habitat/

refuge providers, such as algae, greatly minimise the envi-

ronmental stress and predation pressures which intertidal

organisms experience during the tidal cycle (Menge 1978).

For example, the predation on phytal meiofauna by blen-

nies is greatly reduced within the complex structure of the

alga Corallina officinalis Linnaeus, 1758 (Coull & Wells

1983), mussel recruits also survive better amidst macroal-

gae (Moreno 1995), and fucoid algae ameliorate desicca-

tion stress for limpets (Moore et al. 2007). Experimental

studies aiming to assess the importance of interspecific

facilitation are still considered to be insufficient (Bertness

& Leonard 1997). The main aim of the present work was

to examine the role of empty but fixed barnacle tests as a

biologically created habitat for the highly abundant peri-

winkle Melaraphe neritoides (Linnaeus, 1758). Where bar-

nacles die but their shell structure remains attached to the

rock, a new colonising space is created within its test. If

the space created within empty barnacle tests is then col-

onised, barnacles can be considered facilitators in the

intertidal ecosystem by providing additional habitats.

Studies on the ecology of this common European inter-

tidal snail have almost exclusively been based on sampling

snails among barnacle matrices (e.g. Raffaelli & Hughes

1978; McGrath 1997), and its usage patterns of empty bar-

nacle tests remain unknown.

In the NE Atlantic and Mediterranean, the chthamalids

Chthamalus stellatus (Poli, 1791) and Chthamalus monta-

gui Southward, 1976 dominate the surface of the rocky

shore (Boaventura et al. 2002; Jenkins 2005; Monaco &

Helmuth 2011), and can be key competitors for space with

algae and mussels (see Benedetti Cecchi et al. 2000). These

barnacles are small (maximum basal size ~ 5–6 mm, Relini

1983) and are known to provide habitats for snails,

bivalves and isopods (Fish & Fish 1996). Several

species colonise empty barnacles or thrive in their matrix,

in search of a refuge from detrimental forces such as

predation, wave dislodgment and desiccation (see Cardona

et al. (2013); Catesby & McKillup 1998). The periwinkle

M. neritoides is commonly found on Portuguese

shores within barnacle tests and among neighbouring

barnacles.

Several factors are known to promote post-settlement

barnacle mortality, resulting in empty barnacle tests. Pre-

dation by fish, starfish, whelks and crabs (e.g. Connell

1961; Fairweather 1987; Navarrete & Castilla 2003; Silva

et al. 2010a,b) and competition for space with algae are

important biological factors, but environmental effects

such as thermal stress and wave action are also known to

increase mortality rates (e.g. Shanks & Wright 1986; Bert-

ness et al. 1999; Chan et al. 2006). Predation by fish and

whelks is more likely to produce rock-attached empty

tests, as these predators respectively eat only the cirri and

drill the opercular plates. In the locations of the present

study, drilling gastropods are very rare. The distribution

of chthamalid barnacles varies within shores and in Eur-

ope; C. stellatus is more abundant on the lower tidal level

of exposed shores, whereas C. montagui is more abundant

on the upper shore of sheltered locations (Jenkins 2005;

O’Riordan et al. 2011). Chthamalus montagui is the dom-

inant species on Portuguese shores (Monaco & Helmuth

2011). Although these distribution patterns are known

for live chthamalids, there is very limited knowledge on

the availability and distribution patterns of empty barna-

cles. Barnacle mortality can be expected to be higher on

the lower shore due to increased predation pressure from

crabs and fish (Little & Kitching 1996; Silva et al. 2010a,

b). The variability in the mortality of barnacles is more

complex to predict between shores, as predation is typi-

cally stronger on sheltered than on exposed shores (e.g.

Silva et al. 2010a,b), and wave action can also affect

barnacle populations (Raffaelli & Hawkins 1996). The

importance of barnacle tests as habitats for occupying

snails and the characteristics of their morphometric rela-

tionship remain largely unknown.

The small periwinkle Melaraphe neritoides is one of the

most abundant grazers on European rocky shores, and the

most common snail on the upper shore level in the Portu-

guese coast (Boaventura et al. 2002). These periwinkles

have been found to be an important intertidal grazer as

they can control the establishment of ephemeral algae dur-

ing the early stages of ecosystem succession (Byers 1990).

Melaraphe neritoides has also been found to be an impor-

tant prey of key intertidal resident predators such as the

blenny Lipophrys pholis (Jacinto et al. 2013) and the crab

Pachygrapsus marmoratus (Silva et al. 2009), and other

subtidal crustacean foragers (Silva et al. 2010a,b). Because

the periwinkle undertakes vertical migrations on the shore

(McGrath 1997), both intertidal and subtidal predators

have easy access to this snail. This study aims to assess the

2 Marine Ecology (2014) 1–11 ª 2014 Blackwell Verlag GmbH

Habitat provision of barnacle tests Silva, Mendonc�a, Paquete, Barreiras & Vinagre

importance of barnacles in their role as facilitators for the

establishment of the European distributed snail M. nerito-

ides in the form of an additional habitat. Ecological facili-

tation has scarcely been considered in studies of

community dynamics, despite playing a crucial role in the

functioning of the intertidal ecosystem (Bertness &

Leonard 1997). There is a lack of information on the scales

at which facilitation occur (Wright & Jones 2006). We

considered tidal height and shore exposure to be two of

the most important factors determining barnacle mortality

and thus, the creation of barnacle tests as habitat.

Specifically, we examined whether there was a signifi-

cant relation between: (i) availability of empty barnacle

tests across tidal heights and shore exposures and

M. neritoides occupation rates of those tests; (ii) barnacle

shell size and the snail occupant abundance and size; and

tested experimentally whether there is a limitation in the

availability of barnacle empty tests, whereby new tests are

readily colonised.

Methods

Sampling

All surveys and experimental manipulation (correspond-

ing to sections A–C below) were done sequentially

between August–October 2011.

A1: Barnacle test availability across shore levels and exposures

To determine at what spatial scale the variation in the

abundance of dead and live barnacle tests occurs, the sur-

vey design described in Table 1 was used, where barnacles

were sampled within shores at differing shore levels and

between shores of differing exposure. In this design, the

upper shore and the midshore were examined in both

sheltered and exposed replicated shores. The abundance

of live barnacles was included as these represent the

future dead barnacle tests, hence allowing the identifica-

tion of their potential abundance as habitats. The exposed

shores (Cabo Raso, Raio Verde) are located in a cape fac-

ing predominant wave action and, the sheltered shores

(Paimogo, Vale Pombas) represent bays (for more infor-

mation on exposure differences between these shores see

Silva et al. 2010a,b).

A minimum of 10 photographs covering a 10 9 10 cm

area of the rock surface where barnacles were conspicu-

ously present were taken at low tide for all factor combina-

tions (i.e. two shore levels for each shore, a total of two

sheltered and two exposed shores). An image analysis of

each photograph was made in the laboratory based on

screen counts and the following response variables were

measured per image: the number of live barnacles – oper-

cular plates visible and closed, the number of dead barnacle

tests – barnacle structure intact but opercular plates miss-

ing – and, the total number of barnacles present.

The size variation of barnacle tests was also assessed in

situ by measuring the aperture length of all dead tests

(Barnes & Gonor 1973), using the same design as above,

but this time only considering the dead tests present

within 10 replicates of 10 9 10 cm (Table 1). The soft-

ware PRIMER+ was used to analyse the significance of

data and explore variation patterns (Clarke & Warwick

2007; Anderson et al. 2008). A minimum of 10000

permutations were used, based on the Bray–Curtis simi-

larity matrix of non-transformed abundance data for all

PERMANOVA tests. The Euclidean distance was also used on

PERMANOVA size tests and when data was univariate, as this

is equivalent to non-permutated ANOVA (Anderson 2005).

The Monte Carlo correction and the corresponding

P-value were used whenever there were fewer than 100

unique permutations (Anderson et al. 2008).

A2: Barnacle shell height across shore levels and exposures

Preliminary surveys indicated that there was a potential

difference in the height of barnacle tests between shore

levels. To examine this, the shell height of 100 dead bar-

nacles was compared and tested with PERMANOVA using

the design detailed in Table 1. To measure shell height,

each barnacle was detached from the rock with fine twee-

zers, which then supported the shell while measuring the

largest plate height with a digital calliper (0.001 mm).

B: Shell occupancy across shore levels and exposures

The number and size of Melaraphe neritoides found

within each barnacle test were assessed in situ using the

same survey design (Table 1) as for the previous section

(shell availability), but for a new set of replicates. On the

midshore, and because often more than 50 tests were

available per replicate and an exhaustive sampling would

be impractical for the duration of the low-tide phase, 10

subsampled dead barnacle tests were haphazardly chosen

within a replicate, their aperture length was recorded and

the test plates subsequently removed with tweezers in

order to access and measure its occupants. For the upper

shore the same procedure was applied, but the small

number of empty tests available allowed surveying all

tests within the replicate. The shell length and width of

all M. neritoides found within the barnacles tests were

measured using a digital calliper (0.001 mm). The infor-

mation gathered allowed us to characterise the percentage

of shell occupancy, the identity and morphometric fea-

tures of the occupants and examine the relationship

between aperture length and occupant identity and size

across several spatial scales.

To establish whether there was a M. neritoides size/

ontogenic segregation between the snails inhabiting

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Silva, Mendonc�a, Paquete, Barreiras & Vinagre Habitat provision of barnacle tests

Table

1.Su

rvey

designforallexperim

ents

Survey

Factors

Relationbetween

factors

(1)Sh

ore

level(fixed,

upper

andmid)

(2)Sh

.Exposure

fixedsheltered

endexposed)

(3)Sh

ore

(C.Raso,

R.Verde,

Paim

ogo,

V.Pombas)

(4)Site

(ran

dom,

site

1+2)

(5)Location(fixed,

outside+inside

barnacle

shells)

A1 Shellavailability

(deadbarnacles)

XX

X(ran

dom)

X–

(3)nestedin

(2)

(4)nestedin

(3

A1 Shellavailability

(live

barnacles)

XX

X(ran

dom)

X–

(3)nestedin

(2)

(4)nestedin

(3)

Variationin

aperture

length

XX

X(ran

dom)

X–

(3)nestedin

(2)

(4)nestedin

(3)

A2 Shellheight

XX

X(ran

dom)

X–

(3)nestedin

(2)

(4)nestedin

(3)

B Shelloccupan

cy

XX

X(ran

dom)

X–

(3)nestedin

(2)

(4)nestedin

(3)

Size

orsnails

outsidevs

inside

barnacle

shells

XX

X(ran

dom)

XX

(3)nestedin

(2)

(4)nestedin

(3)

C Occupan

t

response

X–

X(fixed)

X–

(4)nestedin

(3)

4 Marine Ecology (2014) 1–11 ª 2014 Blackwell Verlag GmbH

Habitat provision of barnacle tests Silva, Mendonc�a, Paquete, Barreiras & Vinagre

barnacle tests and those living outside amongst neigh-

bouring barnacles, the length and width of 15 M. nerito-

ides present amidst the barnacle matrix (but not inside

barnacles) were measured within and around (when nec-

essary) each replicate, and compared with that of those

refuged within barnacle tests. No comparison was made

between the number of M. neritoides found within barna-

cle tests and that of those among the barnacle matrix as

this study focus on the habitat role of empty tests, not on

the already much studied space between tests.

Shell height may also influence the number and size of

occupants and we hypothesized that taller tests corre-

sponded to more and/or larger occupants. To examine

this, the height of 80 empty randomly chosen barnacles

and their corresponding number and size of M. neritoides

occupants were measured. The tests were easily removed

intact from the substrate with laboratory tweezers, and

the height of the tallest plate was measured with digital

callipers. For this hypothesis, the following experimental

design was used and tested with PERMANOVA where snail

number per shell was used as a covariate: shore level

(fixed, two levels: upper and mid levels), shore (fixed,

two levels: Cabo Raso and Raio Verde), site (random,

nested in shore, two levels: site 1 and site 2).

C: Occupant response to refuge availability

We hypothesized that the very high percentage of occupa-

tion of barnacle tests by Melaraphe neritoides was possibly

due to the increased advantage that a refuge provides.

That percentage was the focus of the previous section B.

Next, to determine whether occupation rates were depen-

dent on the availability of barnacle tests, a manipulative

test was done whereby the opercular plates and live tissue

of barnacles were removed with tweezers, thus creating a

new empty shell. This procedure was randomly applied

to 10 barnacles within each 10 of the 10 9 10 cm repli-

cates. We replicated this at the upper and mid-tidal levels

and between shores (Table 1). The monitoring of the

shell occupation was done 3 days after manipulation and

then every week for 12 weeks.

Results

Shell availability and size variation across shore levels and

exposures

The number of empty barnacle tests varied significantly

only at the site level (Table 2). However, a closer exami-

nation of the PCO ordination indicated a trend for

higher abundances of tests to occur at the midshore level,

with the sum of the axis explaining more than 90% of

the total variation (Fig. 1). The combination of the origi-

nal PCO and the bottom bubble plot allows us to see that

more dead barnacle tests were found at midshore levels,

but no clear pattern was detected for differences between

sheltered and exposed shores. The average number of

dead barnacles on: (i) sheltered shores was 4.6 individuals

� 0.7 (mean � SE) and on exposed shores was 4.0 indi-

viduals � 0.3 and (ii) on the midshore was 5.9 individu-

als � 0.2. On the upper shore was 2.8 individuals � 0.3.

The trend of higher abundances of dead barnacles occur-

ring on the midshore (as shown by the PCO) was further

supported by the SIMPER analysis (average abundance of

dead barnacles upper shore = 2.24; average abundance of

dead barnacles midshore = 6.02). SIMPER also showed

that barnacle tests are more abundant on sheltered (aver-

age abundance of dead barnacles sheltered = 4.55) than

on exposed shores (average abundance of dead barnacles

exposed = 3.89). The apparent contradicting result

between PERMANOVA and PCO techniques suggests that

Table 2. PERMANOVA results for sections A, B and parallel experiments where only significant results are shown

Survey

PERMANOVA results

Significant terms Pseudo-F p(PERM) Degrees freedom Factor Residual

A1

Shell availability (dead barnacles)

Site 2.84 0.004 1.144

A1

Shell availability (live barnacles)

None – – –

Variation in aperture length Shore level 13.23 0.03 1.144

A2

Shell height

Shore level 9 Shore 4.18 0.01 1.144

B

Shell occupancy (no snails/barnacle shell)

Site 5.47 0.002 2.288

B

Shell occupancy (size snails/barnacle shell)

Site 9 Shore level 6.06 0.002 2.288

Size of snails outside vs inside barnacle shells Location 29.42 0.03 (p Monte Carlo) 1.108

Marine Ecology (2014) 1–11 ª 2014 Blackwell Verlag GmbH 5

Silva, Mendonc�a, Paquete, Barreiras & Vinagre Habitat provision of barnacle tests

more efficient replication is required in further studies to

be able to prove the apparent emerging pattern regarding

shore level differences.

There was no significant variation in the abundance of

live barnacles between shore exposures and shore levels

(Table 2). The average numbers were almost identical

between shores of differing exposure (SIMPER: shel-

tered = 18.12 individuals; exposed = 17.61 individuals).

However, although not statistically significant, SIMPER

results indicate that the average abundance of live barna-

cles is on average larger on the midshore than on the

upper shore (upper = 13.60 individuals; mid = 21.77

individuals).

The aperture length of barnacles was significantly larger

on the upper shore (mean aperture length = 1.5 mm)

than on the midshore (mean aperture length = 1.1 mm).

The height of empty barnacle tests was significantly dif-

ferent between shore levels, but interacted with the site

spatial scale (Table 2). The SIMPER analysis showed that

barnacle tests were taller on the mid (average size = 3.47)

than on the upper shore (average size = 1.39).

Shell occupancy across shore levels and exposures

The average barnacle shell occupancy by Melaraphe neri-

toides, considered to be at least one snail inside each shell,

was 100% for the midshore and 70% for the upper shore

across shores and sites.

The number of snails found within tests varied signifi-

cantly at the site level, between site 1 and 2 of all shores

(Table 2). There was also a significant interaction

between site and shore level for the response variable size

of snails (Table 2). The snail M. neritoides was larger on

the upper shore at several sites of the four sampled

shores. Barnacle height, but not aperture length, was a

significant predictor of the number and size of M. nerito-

ides within tests: number: pseudo-F1,639 = 75.69; p

(perm) = 0.001; size: pseudo-F1,639 = 2144.9; p

(perm) = 0.001. Barnacle tests were on average higher on

the midshore than on the upper shore and taller tests

corresponded on average to more snail occupiers (Fig. 2).

The SIMPER results showed that the average number of

snails found within barnacle tests on the midshore (six

Fig. 1. Principal component analysis of the

variation of dead barnacles between shore

exposure and shore levels. From the

combination of the original PCO and the

bottom bubble plot, the higher number of

dead barnacle tests can be observed at

midshore levels.

6 Marine Ecology (2014) 1–11 ª 2014 Blackwell Verlag GmbH

Habitat provision of barnacle tests Silva, Mendonc�a, Paquete, Barreiras & Vinagre

individuals) was larger than on the upper shore (two

individuals). Although not significant (pseudo-F > 0.05),

snails were larger on the upper shore (average

length = 1.55 mm) than on the midshore (average

length = 1.47 mm).

Snails outside the barnacle tests were significantly larger

than those found within (pseudo-F1,79 = 29.42; p

(MC) = 0.03), for both snail length (SIMPER: average out-

side length = 2.62 mm, average inside length = 1.74 mm)

and snail width (SIMPER: average outside width =1.77 mm, average inside width = 1.22 mm). This is shown

visually by the separation of samples in the MDS ordina-

tion plot (Fig. 3). The overlaid vectors in Fig. 3 (‘pie chart’

overlay) further shows that snail length, and not height, is

the morphological feature that was best related to the sepa-

ration mentioned above because: (i) the longer the vector

line the best correlated is the factor with the sample separa-

tion in the MDS-snail length has a long vector line and (ii)

the vector ‘length’ is aligned in direction with the horizon-

tal separation of the samples of snail outside and inside

barnacle tests. All other factors and interactions were non-

significant (P > 0.05).

Occupant response to refuge availability

This experiment indicated that empty tests are readily

colonised by M. neritoides. Occupation occurred a day

after the creation of the additional refuge, with average

occupation rates of 70% found for the upper shore (Raio

Verde, site 1 = 60%, Raio Verde,site 2 = 80%, Cabo

Raso, site 1 = 75%, Cabo Raso, site 2 = 70%) and 50%

for the midshore (Raio Verde, site 1 = 70%, Raio Verde,

site 2 = 40%, Cabo Raso, site 1 = 55%, Cabo Raso, site

2 = 50%). Within 3 days of refuge creation, more than

90% of the vacant tests had been occupied by the snail

on both shore levels and shores. These rates reached

100% within 2 weeks of refuge creation and were similar

until at least 3 months after.

Discussion

This study shows that empty Chthamalus spp. barnacles

are important microhabitats and a potential refuge for

the periwinkle Melaraphe neritoides in the highly competi-

tive intertidal zone. The coupled survey-experimental

approach allowed us to describe in detail the role played

by empty barnacles as a microhabitat for snails in general,

and specifically size variation of M. neritoides. In this

study, the gastropod use of barnacle tests was responsive

to small-scale variation in habitat availability, as seen for

other intertidal gastropods (Chapman & Underwood

2008).

Our study is the first of its kind to examine in detail

the use of empty barnacle shells by M. neritoides as a

habitat, by considering the morphological features of bar-

nacle tests and snail shell features. Littorina snails actively

search for beneficial microhabitats during low-tide to

avoid desiccation and thermal stress (Jones & Boulding

1999). Barnacles may provide refuges similar to crevices

by altering the complexity of the substratum (Jernakoff

1985). In terms of microhabitats, crevice size is a key fac-

tor for the populations of Littorina rudis and M. nerito-

ides, with the size range of occupants increasing with

microhabitat size (Raffaelli & Hughes 1978). Our results

contradict this when barnacle tests are considered as a

microhabitat for M. neritoides, as no snail size differences

were found between barnacles of differing heights.

Fig. 3. Multidimensional analysis comparing the length of the snail

Melaraphe neritoides found within and among the barnacle tests. A

vector overlay of the snail width and length was superimposed to the

MDS to identify visually which size feature characterised the

horizontal separation between the size of snails outside and inside

barnacle tests.

Fig. 2. Average height of barnacle tests on the upper and the

midshore and the number of snails found in tests. Taller tests were

found for the midshore and had on average more snails in them.

Because the graph depicts mm and counts simultaneously, the y-axis

units are detailed in the x-axis categories. SE, standard error.

Marine Ecology (2014) 1–11 ª 2014 Blackwell Verlag GmbH 7

Silva, Mendonc�a, Paquete, Barreiras & Vinagre Habitat provision of barnacle tests

Although the exploitation of crevices is known for M.

neritoides, the use of barnacles as microhabitats has only

been studied for Littorina neglecta Bean, which lives

almost exclusively within barnacle tests and shows a size

trade-off whereby snails have smaller shells in order to

exploit barnacles as microhabitats (Reid 1993). Other

snails are known to undertake allometric downscale resiz-

ing to fit this microhabitat (Reid 1993; Hughes 1995).

The aperture length of barnacles was also found to be

larger on the upper shore. The majority of snails outside

barnacle tests were larger than the average aperture length

of the tests, showing that snail size is a determining factor

in the exploitation of this microhabitat. Larger barnacles

are able to better endure the desiccation stress associated

with the high shore (Hughes 1995). Littorinids are

reported to be larger higher on the shore (Vermeij 1972)

and M. neritoides to be one of the most resistant to water

loss by evaporation (Britton 1995), enabling it to live

high on the shore. Our study supported the results of

Vermeij (1972), as the average size of snails was slightly

larger on the upper shore than on the midshore. How-

ever, our study also shows that not only barnacle aperture

length is important to its inhabitants, but also test height.

In fact, we show that shell height is the determining fac-

tor for barnacle occupation in terms of snail number and

size. On the midshore, and potentially due to the higher

competition for space and food, taller presumably allow

more barnacles to coexist in dense matrices and place cir-

ri in an advantageous higher feeding position in relation

to their shorter counterparts. Taller tests on the midshore

can potentially allow more snails to colonise them, but it

is likely that there is a balance between the smaller aper-

ture length associated with taller tests and the size of col-

onising snails – the precise nature of such balance

remains to be known.

Predation is a contradicting force in shaping barnacle

test availability since the number of available tests increases

when only the barnacle cirri are consumed, as is the case

due to predation by fish (Harvey et al. 2003). Hence, pre-

dation enhances the availability of refuges for snails, but

also creates more space for barnacle spats to settle when

the entire barnacle (and not only the cirri) is consumed

(Minchinton & Scheibling 1993; Hunt & Scheibling 1997).

This increases the number of potential empty tests. We

argue that predation and desiccation contribute and

explain the observed size segregation between snails within

and outside barnacle tests. Young snails located within bar-

nacle tests are likely to obtain increased refuge from preda-

tors, compared with those remaining in the barnacle

matrix. However, this refuge was shown here to be size-

dependent, with snails larger than barnacle test aperture

being unable to exploit this habitat. However, it is also pos-

sible that, as for Littorina sitkana, young M. neritoides

move relatively less than the more desiccation-resistant

adults and simply remain longer on the barnacle refuge,

particularly higher on the shore, where thermal stress is

higher (Jones & Boulding 1999). Other gastropods are

known to exploit other predatory defences including size

refuge and shell morphological modifications (Vermeij

1976; Mitsch & Jørgensen 2003). Additionally, similar to

other snails and littorinids (McQuaid 1982; Britton 1995;

Hughes 1995), it seems likely that younger M. neritoides

seek refuge from desiccation pressures felt higher on the

shore. To confirm the importance of empty tests for snail

population dynamics it would be necessary to obtain longi-

tudinal data on populations to determine whether the tests

impact snail survivorship and growth. However, Jones &

Boulding (1999) have shown that in the long term, growth

rate and survivorship were significantly reduced when bar-

nacles were not available to the snails, especially when snail

densities were high.

The present study mainly examined the importance of

barnacles as refuges for littorinids at several spatial scales,

but also showed that these refuges are readily colonised the

day after the refuge was made available, and occupation

rates almost reach the maximum 4 months after creating

the refuge. Hence, there are very few non-occupied empty

barnacle shells at upper and midshores. In the literature,

there seems to be contradictory evidence accounting for a

correlation between the number of empty barnacle shells

and the number of juveniles for Littorina rudis (Emson &

Faller-Fritsch 1976) and Littorina neglecta (Fish & Sharp

1986). Our results indicate that M. neritoides is likely to

benefit from this microhabitat, potentially more so for

young gastropods, which are more vulnerable to environ-

mental and biological pressures. However, with similarities

to Boulding & Harper (1998), our work suggests that gas-

tropod abundance is limited by the barnacle microtopogra-

phy, and we show here that barnacle size is key to explain

the barnacle number and size found within tests. Our study

demonstrated that barnacle tests play a significant role in

the ecology of M. neritoides. This adds to our currently

limited knowledge on the importance of interspecific facili-

tation played by biologically generated habitats (Bertness &

Leonard 1997).

It remains unclear after the present study whether bar-

nacle tests can increase the local biodiversity. Other bar-

nacles such as Balanus and Megabalanus species colonise

the intertidal in Portugal and particularly the intertidal–sublittoral boundary. These are considerably larger barna-

cles than the Chthamalus species in terms of height and

they have a columnar shape (but not necessarily wider

apertures), potentially capable of harbouring more species

and larger individuals. We show here that barnacles can

play an additional role in ecosystem functioning by pro-

viding new habitats for snails, which otherwise would be

8 Marine Ecology (2014) 1–11 ª 2014 Blackwell Verlag GmbH

Habitat provision of barnacle tests Silva, Mendonc�a, Paquete, Barreiras & Vinagre

restricted by the crevice availability. This has also been

verified by Harley (2006) and Underwood & McFadyen

(1983) for Littorina spp. Barnacles greatly extended the

distribution of snails in space, including very early onto-

genetic life stages, which otherwise would have reduced

survival rates, particularly on the midshore where preda-

tors are abundant. Hence, by constructing new niches,

barnacles are maximising the ecological role of M. nerito-

ides, an important intertidal grazer and food item for sev-

eral vertebrate and invertebrate predators that are mainly

of sublittoral origin (Silva et al. 2010a,b). This study

would benefit from an experimental manipulation

designed to discover the effects of barnacle tests at differ-

ent locations on the shore with regard to different open-

ing sizes and test height (Macpherson & Scrosati 2008).

These differences are related to competition for space and

physiological stress factors, which concomitantly affect

the snails examined. Tests of different shapes and densi-

ties could be cross-transplanted and the habitat proprie-

ties independently tested.

In summary, barnacles are key ecosystem engineers in

the intertidal, allowing for the establishment of large pop-

ulations of M. neritoides.

Acknowledgements

We are grateful to Dr Awantha Dissanayake and four

anonymous reviewers who have greatly improved this

work. We thank Zara Reveley for the English language

revision. This study had the support of the Portuguese

Foundation for Science and Technology through the

grant SFRH/BPD/34934/2007 awarded to C. Vinagre.

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