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PRIMARY RESEARCH PAPER

Local consumers are the first line to control biologicalinvasions: a case of study with the whelk Stramonitahaemastoma (Gastropoda: Muricidae)

A. Giacoletti . A. Rinaldi . M. Mercurio .

S. Mirto . G. Sara

Received: 30 October 2015 / Revised: 8 January 2016 / Accepted: 9 January 2016

� Springer International Publishing Switzerland 2016

Abstract The increasing spread of invasive species

in the Mediterranean Sea determines several alter-

ations in local food webs, changing the feeding habits

of native organisms. The whelk Stramonita haemas-

toma is a widespread Mediterranean gastropod that

consumes bivalves, barnacles and limpets. Previous

studies showed a shift in its diet from the bivalve

Mytilaster minimus to the invasive mussel Brachidon-

tes pharaonis, presumably due to a higher energy gain.

Here we tested whelks’ preference among natives and

a novel prey, calculating the profitability ratio, and

integrating those results with biochemical analysis on

prey tissues and the routine metabolism of the whelks.

Further, we used the scaled functional response as a

theoretical tool to describe whelk ability to obtain

energy from their environment by using four different

prey species: B. pharaonis, Mytilus galloprovincialis,

M. minimus and Patella caerulea. Whelks evidenced a

Type II functional response for all prey, while

Brachidontes displayed a lower attack rate and a

higher handling time. Stramonita showed a greater

preference for Brachidontes, that resulted as the prey

with the higher energetic content, and the second most

profitable after Patella. This suggests that the higher

energy gain is behind the change in the predator’s diet,

with possible effects on its energy budget.

Keywords Invasive species � Functional response �Gastropod � Mussels � Stramonita haemastoma �Brachidontes pharaonis

Introduction

There are only so many ways in which local commu-

nities can control biological invasions. Autochthonous-

predators (sensu Schoener, 1986) are usually the first in

line to influence the likelihood of invasion (Ricciardi

et al., 2013). Invaders—often r-strategists—deploy

their pronounced ability to compete for space—by

exploiting local resources and promptly addressing

local establishments. The importance of both predators

and top-down control in structuring ecological com-

munities has to date been widely discussed by ecolo-

gists, and the loss of predators may provide an

important example of how they shape marine environ-

ments (Terborgh, 2010). In particular, the common

Indo-Pacific mussel Brachidontes pharaonis (Fischer

P., 1870), which entered the Mediterranean Sea after

the opening of the Suez Canal (1869), is slowly

Handling editor: Vasilis Valavanis

A. Giacoletti � A. Rinaldi � M. Mercurio � G. Sara (&)

Dipartimento di Scienze della Terra e del Mare,

University of Palermo - Local UO CoNISMa, Viale delle

Scienze Ed. 16, 90128 Palermo, Italy

e-mail: gianluca.sara@unipa.it

A. Rinaldi � S. Mirto

IAMC-CNR, via Giovanni da Verrazzano 17,

90194 Castellammare del Golfo, Trapani, Italy

123

Hydrobiologia

DOI 10.1007/s10750-016-2645-6

colonising near all rocky upper subtidal and lower

intertidal substrates (Safriel & Sasson-Frosting, 1988)

in the south-western Basin (Sara et al., 2013), thereby

outcompeting the native bivalve Mytilaster minimus

(Poli, 1795) (Safriel & Sasson-Frosting, 1988). In sea

locations, however, it seems that the initial Brachidon-

tes colonisation process is constrained by predators,

such as Muricid gastropods (Rilov et al., 2002), which

are able to regulate the population dynamics of their

prey (Safriel et al., 1980). In the Mediterranean Sea, the

whelk Stramonita haemastoma (Linnaeus, 1767; Gas-

tropoda: Muricidae) is a widely distributed intertidal

top consumer that adopts opportunistic strategies to

consume a large number of species (Basedow, 1994).

Before dense beds of the invading mussel B. pharaonis

were established along the Israeli Mediterranean coast,

indigenous barnacles, small native mussels, Vermeti-

dae gastropods and limpets were the main potential

food items for S. haemastoma (Rilov et al., 2001).

Previous laboratory experiments have shown that B.

pharaonis was the food item preferred by S. haemas-

toma over the indigenous mussels and barnacles (Rilov,

1999), and that in sites where it was abundant (densities

[25 % of the available prey), it constituted the whelks’

main food, except when energetic limitations ‘‘forced’’

the predator to prey on the more abundant, but

presumably less profitable prey (Rilov et al., 2002).

Trophic interactions are generally well described by

functional response models quantifying consumer per

capita consumption rate depending on local prey

abundance (Holling, 1959). Several authors have

measured the number of prey eaten by single predators

in small ‘‘arenas’’ (Hassel, 1978; Lang et al., 2012;

Toscano et al., 2014), in order to eliminate the

possibility of seeing anything other than prey depen-

dence (Abrams & Ginzburg, 2000). Using the func-

tional response can explain and predict the impact of

predators on prey populations (Juliano, 2001; Dick

et al., 2013). In particular, Twardochleb et al. (2012)

showed that the functional response might predict,

along with simple population growth models, whether a

predator will provide biotic resistance against non-

native preys at different prey densities. This prediction

is made by determining the functional response shape

and parameters (a, h). Predator functional responses

interact with prey birth rate and abundance, and the

magnitude of this interaction is reflected in the shape of

the curve (Type II or III) and the intensity of the attack

rate (a) (Twardochleb et al., 2012). According to the

current literature, functional response experiments in

the natural field are quite rare. We thus decided to focus

on laboratory feeding trials at different prey densities.

Our specific aims were to investigate the prey choice

process by studying prey profitability and assimilation

efficiency, and comparing experimental results to their

respective energetic content derived from biochemical

analyses. We further estimated the routine metabolism

of S. haemastoma by measuring the oxygen consump-

tion rate. At the same time, we derived the functional

response of S. haemastoma over B. pharaonis, com-

paring it with some of their most common indigenous

prey, the model species used many times in companion

studies: mussels Mytilus galloprovincialis (Lamark,

1819) (Sara et al., 2011; Montalto et al., 2014) and M.

minimus (Sara & De Pirro, 2011), and the limpet

Patella caerulea (Linnaeus, 1758) (Prusina et al., 2014)

in order to analyse predator–prey trophic interactions in

greater detail. We examined whether (i) whelks exhib-

ited a Type II or Type III functional response; (ii)

functional responses differed between natives and a

novel prey and (iii) the new prey determines changes in

the whelk’s ecology by altering functional response

parameters such as the attack rate (a) and prey handling

time (h), affecting the duration and the cost of the

predatory event, with a probable different energy intake

from the new prey.

Materials and methods

Sampling and experimental design

Specimens of S. haemastoma were collected alive at

low tide during February 2014 from the intertidal

shores near San Vito Lo Capo and the natural reserve

of Monte Cofano (Castelluzzo, Trapani). Whelks were

brought back to the laboratory and placed in a 60-l

aquarium at room temperature (18–20�C) and seawa-

ter salinity (37–38%) and they were allowed to

acclimate for 1 week to reduce stress generated by

manipulation and transport (Garton & Stickle, 1980).

Specimens were then gradually transferred from room

temperature, at daily increase of *1 to 24�C, the

optimal experimental temperature to elicit maximum

feeding rates in the lab (Rilov, 1999; Brown & Stickle,

2002). Having reached this experimental temperature,

whelks were constantly acclimated there for 10 days,

with no feeding (Garton & Stickle, 1980) prior to the

Hydrobiologia

123

start of the experiment. We thereby standardised the

hunger level (Garton & Stickle, 1980). Aquaria used in

the present study comprised n = 10-1l independent

plastic compartments (15 9 8 cm base, 10 cm

height), each containing a single whelk. We decided

to use individual arenas so that feeding would not be

affected by scents from surrounding treatments, as

gastropods use chemosensory cues to find food (Smith,

1983). Each aquarium was aerated, and kept under

constant light, while seawater was changed after each

trial. All the experiments were repeated twice for each

treatment.

Prey selection

While previous studies have determined the prefer-

ence of the Lessepsian species B. pharaonis over the

indigenous mussel M. minimus, the barnacle Balanus

perforatus, and the limpet P. caerulea (Rilov et al.,

2002), we conducted two further laboratory experi-

ments, both aiming for greater detail of the S.

haemastoma prey preferences. In particular, we com-

pared the preference of S. haemastoma feeding on B.

pharaonis vs. the mussel M. galloprovincialis and

repeated trials with the limpet P. caerulea, following

the experimental design proposed by Underwood &

Clarke (2005). Each experiment was replicated twice

for each treatment, with prey of standard shell length

chosen as these corresponded to the most common

prey upon which the whelks were observed feeding in

the field (author’s pers. obs.):B. pharaonis (25–35 mm),

M. galloprovincialis (25–35 mm), and P. caerulea

(20–25 mm) were offered to medium size

(40–45 mm) whelks. To reduce the likelihood that

further encounters with the same type of prey may

increase the predator experience by artificially affect-

ing the handling efficiency (Rovero et al., 1999), these

authors suggested simultaneously offering two differ-

ent prey species (B. pharaonis and M. galloprovin-

cialis, B. pharaonis and P. caerulea) to each

individual whelk. The trial ended as soon as whelks

had made their choice of prey. Attacks were consid-

ered to start when we inserted prey in the arena and

ended when a feeding S. haemastoma crawled away

from its first prey. To meet the independence criterion

(Underwood, 1997), each whelk was used in only one

experimental trial and then killed by gentle freezing.

Profitability of prey

We also compared the biomass of the different prey by

calculating their individual dry weights (oven 95�C for

24 h) and the ash content of their flesh (muffle furnace

at 450�C for 4 h) to estimate the organic matter as

Ash-Free Dry Weight (AFDW) to the nearest 0.001 g.

Then, to assess the profitability of different prey, we

divided per capita dry tissue by handling times for

each prey species, according to Brown & Richardson

(1987).

Biochemical analysis of prey

A sample of n = 10 specimens for each prey (B.

pharaonis, M. minimus, P. caerulea) was analysed to

determine the protein, carbohydrate and lipid content

(Modica et al., 2006). The values for M. galloprovin-

cialis were derived from the literature (Okumus et al.,

1998; Orban et al., 2002). Protein (PRT) determination

was carried out according to Hartree (1972) and

modified by Rice (1982), using bovine serum albumin

(BSA) as a standard. Carbohydrates (CHO) were

determined according to Dubois et al. (1956) using

D(?)glucose as a standard. Lipids (LIP) were

extracted using the method of Bligh & Dyer (1959)

and measured according to Marsh & Wenstein (1966).

Tripalmitine was used as a standard. The caloric

content of mussel flesh was calculated using the

equation:

Kcal g�1 ¼ 0:041%CHO þ 0:055%PRT

þ 0:095%LIP

where CHO, PRT, and LIP are, respectively, the

percentage content of carbohydrate, protein, and lipid

(Winberg, 1971). Caloric values expressed in Kcal g-1

were finally converted into J g-1 and related to the

average tissue of each prey species.

Assimilation efficiency

As part of the manipulation, we also obtained a

qualitative estimate of the whelk’s assimilation effi-

ciency (AE). For every prey, AE was measured by the

ratio method of Conover (1966):

AE ¼ F � Eð Þ=½ 1 � Eð ÞF�

Hydrobiologia

123

where F is the ratio between ash-free dry weight

(AFDW) and dry weight (DW) for food, and E is the

same ratio for the faeces; this represents the efficiency

with which organic material is absorbed from the

ingested food material. Here, 15 specimens of S.

haemastoma feeding on each prey, acclimated as

above, were used to estimate whelk AE in a simple

feeding trial, where one prey was offered to a single

predator. According to experiments conducted on

other whelks, such as Rapana venosa (Valenciennes,

1846), the emptying of 1 g fresh mussel started soon

after food was digested and lasted for about 8 h

(Seyhan et al., 2003). In this study, we were not

interested in estimating the evacuation time for S.

haemastoma, so the whelks were given 24 h after the

predation event for the complete collection of faeces.

Once collected by pipettes, faeces were washed with

0.5 M ammonium formate (purest grade) to remove

adventitious salts (Widdows & Staff, 2006), placed on

small aluminium disposable dishes, dried in the oven

(95�C for 24 h), and then incinerated in a muffle

furnace (450�C for 4 h). After each step, the samples

were weighted using a precision balance (Sartorius BL

120S ± 1 lg). For the calculation of AE, together

with the faeces collected from the whelks, 10 prey of

each type were weighted, after removal of tissues, then

dried and incinerated as above.

Respiration rate

Rates of oxygen consumption by single whelks were

measured in a respirometric glass chamber (0.3 l) in a

temperature-controlled water bath. Air-saturated fil-

tered seawater was added to each respirometric cham-

ber and stirred with a magnetic stirrer bar beneath a

perforated glass plate (e.g. small Petri dish with holes)

supporting each individual (Widdows & Staff, 2006).

The decline in oxygen concentration was measured by a

calibrated oxygen fibreglass sensor connected to a data

logger (PyroScience Firesting O2), capable of four

sensor connections. We used a total of n = 10 animals

for this experiment, acclimated as above, and then fed

ad libitum for one week until the day before the

experiment. The decline was continuously recorded for

at least 1 h, excluding the initial period (*10 min)

when there is a more rapid decline in oxygen caused by

a disturbance of the sensor’s temperature equilibration.

Respiration rate (RR, lmol O2 h-1) was calculated

according to Widdows & Staff (2006):

RR ¼ Ct0 � Ct1ð Þ � Volr � 60 t1 � t0ð Þ�1

where Ct0 is oxygen concentration at the beginning of

the measurement, Ct1 is the oxygen concentration at

the end of the measurement, and Volr is the volume of

water in the respirometric chamber. After the mea-

surement, each animal was killed by gentle freezing

and dissected, and shell was separated from body

tissue, in order to calculate their individual dry

weights and standardise respiration rates to body

weights. Respiration rates were finally converted into

an energetic cost of routine activities, expressed in

J g-1 day-1 for a cost/benefit analysis.

Predation time

Predatory activity is usually described by two mutually

exclusive phases comprising the functional response of

Holling (1959): searching time and food handling.

These components of feeding behaviour can be

integrated in a cost-benefit analysis, to determine the

optimum maximising the net rate of energy intake

(Pyke & Charnov, 1977). Four experiments were

conducted in order to investigate and measure the

foraging behaviour of S. haemastoma towards four

different prey species: B. pharaonis (25–35 mm), M.

minimus (*10 mm), M. galloprovincialis (20–30 mm)

and P. caerulea (*20 mm). In each session, one

specimen was offered to a single whelk acclimated as

above, and the trial was repeated for each prey species,

in order to compare the predator’s responses. Whelk

behaviour was recorded with a USB web camera placed

in front of the experimental aquarium, connected to a

laptop computer running video capture software (Debut

video capture software, http://www.nchsoftware.com/

capture/). Searching and handling times were then

recorded to the nearest second from the acquired videos

through a digital chronometer. The response period

began when the siphon protruded through the siphonal

canal, the foot was extended, and the shell lifted to

move towards the prey. Searching time ended once the

whelk was on top of its prey; at this point, the handling

time began. This then ended when the whelk crawled

away from the prey at the end of the predatory event. As

we were only interested in studying the handling time of

prey, searching times were analysed only to verify the

efficiency of the arena in standardising the searching

component, as long as it requires a different study

approach and probably field observations. During

Hydrobiologia

123

experimental trials, in order to simulate a natural

predatory event, the prey was introduced first, and left

acclimating for *1 h, before introducing the predator

and starting the recording session.

Functional response

Four experiments were conducted in order to measure

the functional response of S. haemastoma on different

prey species. Whelks involved in these experiments

were divided into four groups, and each group was

tested for a different kind of prey: the mussels B.

pharaonis, M. galloprovincialis and M. minimus, and

the limpet P. caerulea. Each experiment was repli-

cated twice: an increasing number of B. pharaonis

(25–35 mm; n = from 1 to 6) or M. galloprovincialis

(25–35 mm; n = from 1 to 7), or M. minimus

(*10 mm; n = from 1 to 11), or P. caerulea (20–

25 mm; n = from 1 to 6) were offered to 40–45 mm

predators, and the consumed items were counted after

a period of 24 h, without replenishing them. Data from

these experiments were useful to investigate if whelks

displayed Type II or Type III functional response, and

to do this a phenomenological analysis was used

according to Juliano’s (2001) and Paterson’s (2015)

methods. All statistical analyses were performed using

R v. 3.1.3 (R Core Team, 2015). For each prey, logistic

regressions of proportion of amount killed against

density were performed using a function (frair::

frair_test) provided in an integrated package for

functional response analysis in R (frair, Pritchard,

2014). Type II functional responses were indicated by

a significant negative first-order term, whereas Type

III responses were indicated by a significant positive

first-order term followed by significant negative

second-order term (Juliano, 2001; Pritchard, 2014).

The dynamics of predator–prey systems are described

by several equations assuming either a constant prey

density (i.e. a ‘‘replacement’’ experimental design) or

a diminishing one (i.e. a ‘‘non-replacement’’ experi-

mental design). While Hollings original Type II

functional responses assume a constant prey density,

in the case of a reduction in prey density due to

consumption, Roger’s (1972) ‘‘random predator’’

equation is more appropriate (Pritchard, 2014). This

equation accounts for prey depletion and their non-

replacement over time:

Ne ¼ N0 1 � exp a Neh� Tð Þð Þð Þ ð1Þ

where Ne is the number of prey eaten, N0 is the initial

prey density, a and h are the attack rate and handling

time, respectively, and T is the total time available.

Coefficients a, h used in (1) were derived from

individual predation experiments for each prey (de-

scribed in the Predation time section); functional

response models were fitted using the maximum-

likelihood estimation (bbmle:: mle2, version 1.0.52,

Bolker and R Development Core Team, 2014).

Statistical analysis

In order to test for significant differences in handling

times between prey species and AE, a Permutational

Univariate Analysis of Variance (PERMANOVA)

was used due to different sample sizes. Analyses were

performed considering the prey species as a fixed

factor (4 levels), and variables were ln (y ? 1)

transformed to retain information on relative concen-

trations but reduce differences in scale among the

variables. The Euclidean similarity measure was used,

and all P-values were calculated using 9999 permu-

tations of the residuals under a reduced model

(Anderson, 2001). Data from prey choice experiments

were analysed using t-tests. ANOVA was performed

on biochemical data, considering the caloric content as

a fixed factor (4 levels). When significant differences

were detected, the Student–Newman–Keuls (SNK)

post hoc pair wise comparison of means was used

(Underwood, 1997). Cochran’s test was used prior to

ANOVA to test the assumption of homogeneity of

variance (Underwood, 1997).

Results

Prey selection

Stramonita haemastoma showed a larger significant

preference (4 times more) for the invasive mussel B.

pharaonis with respect to the blue mussel of similar

size (t test, P\ 0.05), while no significant preference

was exhibited by whelks between B. pharaonis and

limpets (t test, P[ 0.05; Fig. 1).

Profitability of prey

The Brachidontes pharaonis dry weight (0.15 ±

0.01 g) was similar to that of P. caerulea (0.13 ±

Hydrobiologia

123

0.02 g) and they both resulted higher than M. gallo-

provincialis (0.05 ± 0.01 g) and M. minimus

(0.0049 ± 0.0005 g) (ANOVA P\ 0.001; Table 1).

When dry masses of prey were divided by handling

times, S. haemastoma showed greater profitability

ratios for Patella (PERMANOVA P\ 0.01; Table 2;

Fig. 2a) then in order for Brachidontes (P\ 0.001),

Mytilus (P\ 0.01) and Mytilaster (P\ 0.001).

Biochemical analysis

Brachidontes pharaonis resulted in a significantly

higher energetic content (20,983 ± 1,375 J/ind.)

compared to native prey, that resulted, respectively,

in: 580 ± 28 J/ind. for M. minimus (*10 mm), and

12,870 ± 2,188 J/ind. for P. caerulea (20–25 mm),

while the energetic content of M. galloprovincialis

derived from the literature was equal to 10,052 ±

669 J/ind. (ANOVA P\ 0.001; Table 1; Fig. 2d).

Assimilation efficiency

AE showed no differences between B. pharaonis

(0.70 ± 0.02), M. galloprovincialis (0.66 ± 0.01) and

P. caerulea (0.77 ± 0.03) (PERMANOVA P[ 0.05;

Table 3; Fig. 2c), while M. minimus (0.57 ± 0.04)

resulted in a significantly lower AE (P\ 0.05). P.

caerulea further resulted in an higher AE compared to

M. galloprovincialis (P\ 0.001) and M. minimus

(P\ 0.01), while the two native mytilid species

showed a non-significantly different AE (P[ 0.05).

Respiration rate

The average energetic cost of routine metabolism

derived from the measurement of oxygen consumption

resulted equal to 194.29 ± 27 J g-1 day-1.

Predation time

Searching times within experimental arenas resulted

not different among different prey (PERMANOVA

B. pharaonis P. caerulea0

10

20

30

40

50

60

70

80

90

100%

Pre

y ch

oice

B. pharaonis M. galloprovincialis

Prey species

Fig. 1 Results of prey choice experiment on different prey

items in laboratory

Table 1 ANOVA table of results (dry weights and energetic content) of different prey species consumed by S. haemastoma

(* p\ 0.05; ** p\ 0.01; *** p\ 0.001; ns not significant)

Dry weight Energetic content

Source df MS F P Source df MS F P

DW 3 0.049 30.79 *** Joule 3 26.131 861.38 ***

Res 36 0.0016 Res 36 0.0303

Table 2 PERMANOVA table of results (profitability) and group analysis of different prey species consumed by S. haemastoma

(* p\ 0.05; ** p\ 0.01; *** p\ 0.001; ns not significant)

Source df MS Pseudo-F P(perm) Groups (Ht) t P(perm) Unique perms

Sp 3 0.000000607 23.881 *** Bp, Pc 3.408 ** 2867

Res 21 0.000000025 Bp, Mg 3.72 ** 2887

Bp, Mm 9.765 *** 2884

Pc, Mg 3.785 * 126

Pc, Mm 5.697 ** 126

Mg, Mm 7.238 ** 126

Bp = B. pharaonis, Pc = P. caerulea, Mg = M. galloprovincialis, Mm = M. minimus

Hydrobiologia

123

P[ 0.05) (32.1 ± 9.9 min. for B. pharaonis, 14.0 ±

6.4 min. for M. galloprovincialis, 61.8 ± 14.8 for M.

minimus and 44.4 ± 21.7 min. for P. caerulea). By

contrast, the main component of feeding measured in

this study, handling time, showed significant differ-

ences (PERMANOVA P\ 0.001; Table 4; Fig. 2b)

(301.2 ± 35.8 min. for B. pharaonis, 151.5 ± 11.1

min. for M. galloprovincialis, 134.2 ± 40 for M.

minimus, and 131 ± 32.24 min. for P. caerulea). In

particular, handling times with B. pharaonis were

greater than those exhibited by whelks feeding on the

other prey species. Whelks did not show significantly

Table 3 PERMANOVA table of results and group analysis for assimilation efficiency (AE) of different prey species consumed by S.

haemastoma (* p\ 0.05; ** p\ 0.01; *** p\ 0.001; ns not significant)

Source df MS Pseudo-F P(perm) Groups T P(perm) Unique perms

Sp (species) 3 5.8615E-2 7.014 *** Bp, Pc 1.708 ns 9843

Res 59 8.3567E-3 Bp, Mg 1.787 ns 9824

Bp, Mm 2.676 * 4181

Pc, Mg 3.667 *** 9818

Pc, Mm 3.187 **

Mg, Mm 1.976 ns

Bp = B. pharaonis, Pc = P. caerulea, Mg = M. galloprovincialis, Mm = M. minimus

P. caerulea B. pharaonis M. galloprovincialis M. minimus0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012 SNK: Patella > Brachidontes > Mytilus > Mytilaster aPr

ofita

bilit

y In

dex

(DW

/ H

T of

pre

y)

B. pharaonis M. galloprovincialis M. minimus P. caerulea0

20406080

100120140160180200220240260280300320340360 b

Prey species

Han

dlin

g tim

e (m

in. ±

s.e

.)

P. caerulea B. pharaonis M. galloprovincialis M. minimus0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0c

Ass

imila

tion

effic

ienc

y (A

E, a

dim

ensi

onal

)

B. pharaonis P. caerulea M. galloprovincialis M. minimus0

150030004500600075009000

1050012000135001500016500180001950021000225002400025500

SNK: Brachidontes > Patella > Mytilus > Mytilaster d

Ener

getic

con

tent

(J/p

rey

± s.

e.)

Fig. 2 a Profitability Index of prey (dry tissue consumed per

prey/handling time); b handling times recorded from video

analysis for different prey species; c assimilation efficiency

(AE) calculated for the different preys; d energetic content (J/

prey) calculated from LPG analysis of tissues from the different

species

Hydrobiologia

123

different handling times among native prey species

(P[ 0.05).

Functional response

Phenomenological analysis showed that S. haemas-

toma met the Type II functional response for each prey

species, as revealed by the significant negative first-

order term (Table 5; Fig. 3a–d). Brachidontes resulted

in a significant lower attack rate (0.103 h; Table 5)

respect to M. minimus (0.267 h), while it resulted in

the higher handling time (5.8 h) followed by M.

galloprovincialis (5.696 h), P. caerulea (4.438 h) and

M. minimus (2.799 h). Rogers’ analysis further con-

firmed that the whelks spent significantly more time

handling B. pharaonis than the other native prey

(Table 6).

Discussion

Foraging for food in the rocky intertidal zone involves

a number of potential dangers, such as exposure to

predators and the increased likelihood of being

dislodged or affected by thermal stress (Burrows &

Hughes, 1989; Denny et al., 2009). Longer handling

times imply greater risks to the predator that must be

compensated at least by a higher energy gain, in order

to justify the choice and fulfil energetic requirements

for routine metabolism, as measured in the present

study. Before the dense beds of the invading mussel B.

pharaonis were established along the Mediterranean

coast, the indigenous mussels, limpets and barnacles

constituted the main food items of S. haemastoma.

Here, according to our results, the invasive species

showed, in the following order, the highest edible

tissue content, the highest energy gain from consump-

tion and generated the best assimilation efficiency in

whelks, thereby justifying the switch to the alien

species as showed by Rilov et al. (2002). This change

in preference may also be part of some ecological

processes controlling the abundance and biogeograph-

ical range size of an invasive species, and even its

impacts within a community (Maron & Marler, 2008;

Ricciardi et al., 2013). From surveys conducted during

the same period along the intertidal zone of Castel-

luzzo (Trapani), not affected by noticeable anthro-

pogenic activities as trampling (Milazzo et al., 2004)

or whelk harvesting, and Capo Gallo - Isola delle

Femmine, where trampling and harvesting are usually

much more intense due to the proximity to the city of

Palermo, S. haemastoma densities, recorded along

100 9 1 m transects, resulted *0.56 ind./m2 and

*0.03 ind./m2, respectively (A. Giacoletti, pers. obs.).

The presence ofP. caeruleawas recorded in both sites,

while B. pharaonis, occasional and rare in Castel-

luzzo, reached more than 9000 individuals ± 4033

ind. per m2 in the second site (Isola). M. minimus

reached densities of *19,753.3 ± 9,445 ind./m2 (G.

Sara, unpublished data) in Castelluzzo, while very low

Table 4 PERMANOVA table of results (handling time) and group analysis of different prey species consumed by S. haemastoma

(* p\ 0.05; ** p\ 0.01; *** p\ 0.001; ns not significant)

Source df MS Pseudo-F P(perm) Groups (Ht) t P(perm) Unique perms

Sp 3 1.48 6.7366 ** Bp, Mg 3.532 ** 2887

Res 22 0.21 Bp, Pc 3.763 ** 5623

Bp, Mm 3.505 ** 2890

Mg, Pc 1.001 ns 461

Mg, Mm 0.928 ns 462

Pc, Mm 0.019 ns 461

Bp = B. pharaonis, Pc = P. caerulea, Mg = M. galloprovincialis, Mm = M. minimus. Ht handling time

Table 5 Phenomenological analysis of the functional

responses for each prey species (* p \ 0.05; ** p \ 0.01;

*** p\ 0.001; ns not significant)

Prey species Type II

N0 P

B. pharaonis -0.2937 *

M. galloprovincialis -0.4788 ***

M. minimus -0.4036 ***

P. caerulea -1.1440 *

Hydrobiologia

123

densities were evident in site 2 (Isola), probably due to

the competition with the invasive mussel. The lack of

an important predator may be one of the factors,

together with other abiotic characteristics of the site

not considered in this study, that explain this remark-

able difference in density and abundance of B.

pharaonis between the two sites. Under natural

conditions, whelks tend to select prey of optimal size,

such as those chosen for this study, thereby exhibiting

an active choice of an easily accessible or energeti-

cally rich species of prey over others (Garton, 1986;

Brown & Richardson, 1987; Brown, 1997), gaining

Table 6 Phenomenological analysis of the functional responses: attack and handling parameters estimate for each species

(* p\ 0.05; ** p\ 0.01; *** p\ 0.001; ns not significant)

Prey Parameter Estimate SE z value P (z)

B. pharaonis a 0.103 0.051 2.008 *

h 5.807 2.074 2.800 **

M. minimus a 0.267 0.064 4.179 ***

h 2.799 0.277 10.096 ***

M. galloprovincialis a 0.322 0.191 1.687 ns

h 5.696 0.945 6.027 ***

P. caerulea a 0.504 0.315 1.602 ns

h 4.438 0.758 5.854 ***

0

1

2

3

4

5

6 aM

ean

B. p

hara

onis

eat

en (±

SE)

0

1

2

3

4

5

6 b

Mea

n M

. gal

lopr

ovin

cial

is e

aten

(±SE

)

0

1

2

3

4

5

6

7

8 c

Mea

n M

. min

imus

eat

en (±

SE)

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8

0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 70

1

2

3

4

5

6 d

Prey density, n

Mea

n P.

cae

rule

a ea

ten

(±SE

)

Fig. 3 a–d Functional response: relationship between prey eaten (items consumed in 24 h) and prey density

Hydrobiologia

123

the most energy with the least handling effort. Our

profitability data confirmed this statement, with our

whelks actively selecting prey characterised by the

higher tissue content such as P. caerulea and B.

pharaonis. Palmer (1983, 1984) demonstrated that prey

selection in three thaidid species could be explained by

the food value of prey provided in an enclosed arena.

Food value integrates several complex factors deter-

mining prey choice: energy content, abundance and

vulnerability (handling time) of a prey species. Accord-

ing to this theory, and as demonstrated by our results,

the change in feeding preferences is driven by the

higher profitability (energy gain relative to energy

expenditure, Garton, 1986) of the invasive mussel

compared to the native prey, and suggests the whelk’s

ability to assess the prey’s value, regardless of their

feeding history, as hypothesised by Rilov et al. (2002).

Accordingly, the present S. haemastoma significantly

preferred the invasive species to their regular food item,

such as barnacles and mussels (Rilov, 1999) with the

exception of, in our study, the limpet P. caerulea. This

suggests that the prey’s energetic content was important

in the choice of species singularly because whelks, with

the invasive species, could attain foraging optimisation

(Krebs & Davies, 1993). The ability of S. haemastoma

to assess food value has also been shown by other

authors (Richardson & Brown, 1990; Brown, 1997), but

the current study has finally demonstrated that the

content of edible tissue (biomass) plays a more

important role than handling time in determining

profitability. Our study supported this hypothesis, i.e.

that whelks actively preferred mussels with a greater

amount of edible tissue like B. pharaonis to M.

galloprovincialis and M. minimus, despite an appar-

ently longer handling time being necessary. Our trials,

however, revealed no such preference between B.

pharaonis and the limpet P. caerulea, in contrast to

what has been previously demonstrated (Rilov et al.,

2002), probably because of the similar edible biomass

of the two species, that led to an equal choice. Rilov

et al. (2002) recorded handling times between 3 and

16 h on the same type and size of prey (B. pharaonis

25–35 mm), while handling times from our experi-

mental trials ranged between 2 and 9 h. Such handling

times varied widely for different kinds of prey: 2–3 h

for M. galloprovincialis, and 1–4 h for M. minimus and

P. caerulea (Fig. 2b).

One of the most important ecological extrapola-

tions from our results on functional response is the

effect of a predator on prey at low prey density. The

pattern of food consumption in relation to food density

in our whelks met the Type II Holling functional

response for both invasive and indigenous prey. While

in the Type II function, as in the Type III, the number

of killed prey approaches the asymptote following a

sigmoid function, the first part of the Type II curve is

exponential. Such a fact is crucial as it describes a

consumption rate that increases exponentially for low

densities of prey even though it gradually slows down

to an asymptote. This type of function is usually

defined as destabilising at low densities (Dick et al.,

2013). The main consequence is that handling time

can be the limiting factor in the feeding process.

Ecological repercussions will occur if local recipient

communities have predators displaying Type II

Holling functional response at their top, but the

colonisation of new invaders can result in different

fates if they have Type II or Type III predators as, in

the second case, they are not able to exclude invasive

species when they are at low density. This is an

important ecological aspect that has only recently been

considered in the biological invasion literature (Dick

et al., 2013, 2014; Dodd et al., 2014; Paterson et al.,

2015). Indeed, according to a common scheme (Drake

& Lodge, 2006), an invasive species reaching new

recipient sites with habitat-suitable conditions for its

establishment (Peterson & Vieglas, 2001; Sutherst,

2003) and with a sufficient propagule pressure (Drake

& Lodge, 2006), starts to increase the population

density, intercepting local resources, as shown by

Leung et al. (2004) and Mack et al. (2000). In most

invaders, the first period of colonisation is delicate: the

initial small nucleus of an invasive species should

contrast with the suppressing effects of environmental

stochasticity and with the biotic resistance exerted, for

instance, by the local presence of predators (e.g. all

components of the Allee effect). Thus, if local

predators possess Type II or Type III functional

response characteristics, this becomes crucial in

addressing the new establishment of the introduced

species (sensu Drake & Lodge, 2006). From recent

surveys conducted at site 1 (Castelluzzo), whelks seem

able to control densities of possible invaders such as B.

pharaonis, and may have sufficient plasticity to

modify feeding patterns in order to accommodate this

new species in their diet. Predation rates derived from

functional response modelling seem somewhat

reduced on Brachidontes (lower attack rates and

Hydrobiologia

123

higher handling times) when compared to native prey,

but this may be attributed to either longer manipula-

tion time or satiety, eventually leading to an higher

energy gain. There is currently limited knowledge

about the absorption efficiency of marine predatory

gastropods, so our results can only be compared to a

few others. These do report similar absorption effi-

ciency for predatory gastropods of the same class-size

(e.g. Huebner & Edwards, 1981). Other comparisons

have also been made via experiments performed on

scavenger or herbivorous gastropods (Dozey-Stenton

et al., 1995; Huebner & Edwards, 1981), producing

results that do not differ greatly from our experiments.

Significant differences in the absorption efficiency

(AE) resulted between B. pharaonis and the native M.

minimus, while such differences were not detected

between Brachidontes and Patella or Brachidontes

and Mytilus. Our results demonstrate that M. minimus

showed at the same time lower tissue and energetic

content, resulting in the lower AE, this being for

reasons classified as the less profitable prey for S.

haemastoma through the profitability index (Fig. 2a).

Conclusion

Our mesocosm study confirmed that a higher energy

gain, probably due to a greater amount of edible tissue

from the novel prey, can lead the predator to a shift in

diet preference, with possible effects on its energy

budget, and implications for allocation of resources.

On the other hand, a shift such as that shown by our

whelks could also be part of an ecological process

controlling the spread and impact of an invasive

species in the Mediterranean Sea. On a small scale, on

our intertidal shores, whelks actively contributed to

containing the density of the alien species in site 1,

while harvesting whelks for human consumption

positively influenced the proliferation of alien species

in site 2, affecting the natural dynamics of food webs.

The Type II functional response has recently been

cited in the literature as able to contain the colonisa-

tion of new invaders, and the current study highlights

the importance of taking this ecological functional

trait into consideration when studying invasion pro-

cesses. Certainly, there remain other factors to be

considered in examining impacts on natural ecosys-

tems, such as habitat complexity and competition;

nevertheless, it would seem that laboratory-derived

functional response may be considered a simple, cost-

effective and accurate tool to predict ecological

impacts of invasive species on aquatic systems.

Acknowledgments PRIN TETRIS 2010 Grant (n.

2010PBMAXP_003) funded to Gianluca Sara by the Italian

Minister of Research and University (MIUR) supported this

research. We thank Dr. Francesca Ape for her help in the

biochemical analysis of prey and Anna Lossmann for the

English fine tuning. The authors declare no competing financial

interests.

Author contributions: AG and GS conceived the idea and led

the writing, AG and MM carried out all experiments in meso-

cosms, AG & AR performed modelling work and analysed

output data, SM provided grant funds for AG while GS provided

lab facilities at DISTEM and research funds.

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