www.elsevier.com/locate/jembe

Journal of Experimental Marine Biolo

Agonistic interactions between invasive green crabs, Carcinus

maenas (Linnaeus), and sub-adult American lobsters, Homarus

americanus (Milne Edwards)

P.J. Williams *, T.A. Floyd, M.A. Rossong

Biology Department, St. Francis Xavier University, PO Box 5000, Antigonish, NS, Canada B2G 2W5

Received 23 June 2004; received in revised form 9 August 2005; accepted 11 August 2005

Abstract

The invasive green crab, Carcinus maenas, has recently expanded its range into the Southern Gulf of St. Lawrence, where there

is potential for substantial niche overlap with juvenile American lobsters, Homarus americanus. We used two experiments to elicit,

record and analyze the agonistic interactions of adult green crabs (carapace width of 63–75 mm) and sub-adult (carapace length of

55–70 mm) lobsters. The first experiment gave each animal equal access to a limited food resource. The green crabs were first to

the food in significantly more trials, spent a significantly greater proportion of time with the food, and were able to successfully

defend the food from attacks by the heavier lobsters. In the second experiment, we allowed the lobsters to gain possession and

initiate feeding on the food before releasing the green crabs. In these trials, the lobsters spent significantly more time with the food,

and were able to defend the food from the green crabs. The results of both experiments are discussed in the context of game theory.

The different behaviour of the crustaceans in the two experiments is consistent with the bbourgeoisQ strategy in a hawk and dove

game simulation. With this strategy, an animal acts like a hawk if in possession of a resource, but acts like a dove if the other animal

is in possession of the resource. The fact that the green crabs were able to physically compete with, and in many cases dominate the

larger, heavier lobsters supports the potential for competitive impacts of green crabs on sub-adult lobsters.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Agonistic interactions; Carcinus maenas; Competition; Crustaceans; Homarus americanus; Invasive species

1. Introduction

The European green crab, Carcinus maenas (Lin-

naeus), has established populations in many estuarine

and coastal regions outside of its native distribution in

the eastern Atlantic. The majority of research exploring

the impact of green crabs has focused on the prey of

0022-0981/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jembe.2005.08.008

* Corresponding author. Tel.: +1 902 867 3320; fax: +1 902 867

2389.

E-mail address: jwilliam@stfx.ca (P.J. Williams).

this omnivorous crustacean, and significant changes at

the individual, population and community level of prey

species have been attributed to the establishment of

green crab populations (Hughes and Elner, 1979; Ver-

meij, 1982; Grosholz et al., 2000; Trussell and Smith,

2000; Walton et al., 2002; Floyd and Williams, 2004).

However, recent studies have begun to concentrate on

the potential of green crabs as a competitor with native

crustaceans.

The southern Gulf of St. Lawrence is a relatively

shallow body of water on the eastern coast of Canada.

Green crabs are a recent addition to this area, with the

gy and Ecology 329 (2006) 66–74

P.J. Williams et al. / J. Exp. Mar. Biol. Ecol. 329 (2006) 66–74 67

first confirmed sightings in 1994 in the eastern part of

the Gulf (Audet et al., 2003). Green crabs have since

spread westward along the coast, and reached the

western part of the Gulf in 2002 (Audet et al.,

2003). Typically, green crabs first become established

in estuaries and bays, where the resultant population

serves as a source of larvae and emigrants for further

range expansion (Cohen et al., 1995). The intervening

coastal regions eventually also become populated with

green crabs. The southern Gulf, with its abundant

estuaries and warm summer temperatures, offer ideal

conditions for green crabs (Gillis et al., 2000; Audet et

al., 2003). As green crab populations increase, the

potential exists for competition between them and

native crustaceans, such as the rock crab, Cancer

irroratus, and the American lobster, Homarus amer-

icanus. The present study examines agonistic behav-

iour between adult green crabs and sub-adult

American lobsters.

Conflicts between animals are resolved by agonistic

behaviour, a term that encompasses a spectrum of

behaviour ranging from escape at one extreme to phys-

ical combat that may result in injury or death of the

combatants. Game theory (Maynard Smith, 1974) has

provided a conceptual framework for the analysis of

animal conflicts and agonistic behaviour. With this

approach, the conflict becomes an optimization game,

in which the potential benefits (food, shelter, access to

mates) are balanced against the potential costs (injury

or death, exposure to predators, increased metabolic

output) of conflict. Game theory predicts that if fighting

is costly (due to risk of injury, etc.), then conflicts

should be resolved at an early stage, based on some

indicator of fighting ability (Glass and Huntingford,

1988). In addition, agonistic interactions should esca-

late from non-injuring displays to behaviour that has the

potential to cause injury (Huntingford et al., 1995).

Most of the described agonistic behaviour of lob-

sters pertains to intraspecific interactions. Lobsters are

solitary animals (Karavanich and Atema, 1998) and

will fight each other if placed in close proximity

(Zeitlin-Hale and Sastry, 1978; Atema and Voigt,

1995; Huber and Kravitz, 1995). These fights seem

to fit the general predictions for game theory, begin-

ning with threatening displays, progressing to some

pushing with claws, followed by unrestrained combat

(Atema and Voigt, 1995). The threat displays, which

involve conspicuous posturing with the chelae, are

often sufficient to resolve conflicts (Atema and

Voigt, 1995; Karavanich and Atema, 1998). In the

laboratory, lobsters quickly establish dominant–subor-

dinate relationships, and subsequent encounters be-

tween these animals do not involve combat, but are

resolved by retreats and avoidance by the subordinate,

thereby minimizing risk and injury to the combatants

(Karavanich and Atema, 1998; Spanier et al., 1998).

In general, size is the determining factor affecting the

outcome of intraspecific interactions.

Little is known about agonistic behaviour of lobsters

in interspecific interactions, although Miller et al.

(1971) suggests that interspecific competition for food

in kelp beds is intense. Experimental work by Cobb et

al. (1986) explored habitat use and shelter occupancy

with lobsters and two species of crab, the rock crab,

Cancer irroratus, and the jonah crab, Cancer borealis.

Regardless of relative size, lobsters dominated compe-

tition for the limited resource, shelter. Richards and

Cobb (1986) found similar results with lobsters and

jonah crabs and suggest that the high cost of not

obtaining a shelter provides the motivation for the

lobsters to win the battles for shelter, even against a

larger opponent.

Agonistic behaviour of the green crab appears quite

different than that of lobster. Whereas lobsters tend to

be solitary, green crabs can be found at relatively high

densities (up to 5/m2; Young et al., 1999). Over 200

crabs were captured in one cylindrical trap (60 cm�30

cm) in a 2-h set in Pomquet Harbour, Nova Scotia

(Campbell, 2001), so the presence of con-specifics is

not a deterrent to green crab foraging. While lobsters

make extensive use of displays and show an escalation

of aggression before actual combat, green crab battles

do not begin with displays, but rather go directly to

intense physical fighting (Sneddon et al., 1997a,b). This

study investigates the interspecific interactions of the

American lobster and the invasive green crab.

We set up two experiments that used competition

for a limited food resource to elicit agonistic interac-

tions between relatively large, sub-adult lobsters and

adult green crabs. The first experiment was designed

to test the following hypotheses: 1) the larger lobsters

will spend significantly more time with the food; and

2) the lobsters will bwinQ a significantly greater pro-

portion of agonistic interactions. The results of the

first experiment suggested the need for another exper-

iment, in which the lobsters were permitted to gain

possession of the food before the green crabs. In this

experiment, we tested the following hypotheses: 3) if

allowed to feed first, lobsters would spend significant-

ly more time with the food; and 4) the lobsters will

bwinQ a significantly greater proportion of agonistic

interactions. In addition to the specific hypotheses, we

wished to describe the agonistic behaviour of the two

species when paired together, and compare that be-

P.J. Williams et al. / J. Exp. Mar. Biol. Ecol. 329 (2006) 66–7468

haviour with published information concerning intra-

specific interactions.

2. Materials and methods

Lobsters used in this study were obtained in June

2002, from a commercial fisherman operating in St.

Georges Bay, Nova Scotia, Canada. The lobsters ranged

from 55 to 70 mm carapace length (CL) (mean CL=62.4

mm, standard deviation (S.D.)=5.72 mm, n =25). Al-

though the lobsters were not weighed, published weight–

CL relationships suggest a weight range from 135 g to

300 g for 55–70 mm CL lobsters (Aiken, 1980; Hudon

and Lamarche, 1989; Miller and Addison, 1995). The

lobsters were held in a fiberglass tank (60 cm�215

cm�60 cm deep) at 10 8C. A 12 :12 h day–night cycle

was established, with daytime illumination provided by a

central 25 W incandescent bulb. Nighttime light was

provided by a 15-W red-coated incandescent bulb locat-

ed over each holding tank, with the light intensity further

reduced by a translucent plastic cover, as in Lawton

(1987). Experiments were carried during the nighttime

part of the cycle, as both lobsters (Lawton and Lavalli,

1995; Spanier et al., 1998) and green crabs (Naylor,

1958) forage more during the night. Lobsters were not

fed during the course of the experiments.

Three lobsters selected for testing the following day

had their bands removed, and were placed in

30�20�20 cm mesh (1 cm2 plastic-coated wire)

box suspended in the holding tank. Due to the con-

straints in obtaining sub-legal lobsters, the lobsters

were used in both experiments described below. Lob-

sters were tested once in each experiment, with a

minimum 2-week period between testing. This period

of time is sufficient for lobsters to lose the ability to

chemically recognize an individual with which they

have fought (Atema and Voigt, 1995; Karavanich and

Atema, 1998).

Green crabs were captured during the same week

from Pomquet Harbour, Nova Scotia, Canada, using

baited cylindrical wire-mesh traps. Crabs ranged in

size from 63 to 75 mm carapace width (mean

CW=69.5 mm, S.D.=3.48, n =25), and were held in

a second tank in the same temperature-controlled room

that housed the lobsters. Green crabs weighed between

70 and 130 g. All dimensions and holding conditions in

the two holding tanks were identical, except that the

crabs were not banded. Prior experience holding green

crabs at our facility indicated that little mortality or

injury occurred at these densities. As with the lobsters,

the same green crabs were used in each experiment and

were not fed during the course of the experiments.

2.1. Test arena and video recording

Video recordings of green crab and lobster inter-

actions were made in a fiberglass tank (90 cm diam-

eter, 60 cm depth). The bottom of the tank was

layered with 4 cm of fine quartz gravel, and the

tank was filled to a level of 45 cm with filtered,

UV sterilized seawater. Water in the tank was aerated

except during filming. Complete water changes were

carried out each night. Two PanasonicR CCD video

cameras (WV-BP334, minimum illumination of 0.08

lx) were used, one mounted on the edge of the tank

70 cm from the substrate, and one suspended centrally

110 cm above the tank bottom. Both cameras were

directed at a cable tie anchored in the center of the

tank. Signals from both cameras were directed to an

ElmoR MonoQuad-4 beam-splitter, and then to a

videocassette recorder. The beam-splitter provided a

video image with both camera views, and a time

record.

2.2. Experiment 1: simultaneous release

This experiment paired green crabs and lobsters

with a limited food source. Trials were completed

during the night portion of the light cycle. A hole

was drilled through a live commercially reared blue

mussel (Mytilus edulis L.) with 5–7 cm total length,

and the mussel was attached with a plastic cable tie to

the central area of the tank (as in Jensen et al., 2002).

A bucket equipped with two plastic flaps was inverted

over the mussel, and served to separate the two halves

of the tank and isolate the mussel. One green crab and

one lobster were placed on either side of the partition,

and left for 15 min. At the end of the acclimatization

period, the bucket was lifted and filming commenced.

If neither animal approached the food and initiated

feeding within 30 min, the trial was terminated. Trials

where feeding took place were terminated after both

animals left the food area for at least 10 min. If one

animal had dominated time with the food, the operator

removed the dominant, and observed whether or not

the remaining animal went to the food. At the end of

each trial, both animals were measured, a small plastic

numbered tag glued to the carapace using a super-glue

and accelerant, and animals were returned to holding

tanks. At the end of the experiment, both green crabs

and lobsters were fed to satiation with finely chopped

fresh fish fillet.

Upon review of the videotapes, we noted which

animal first went to the food and initiated feeding,

and used the non-parametric binomial test to determine

0

20

40

60

80

100

20

0

40

60

80

Perc

enta

ge o

f T

otal

Tim

e W

ith B

ait

Experiment 1:SimultaneousRelease

Experiment 2:Lobster FeedsFirst

*

*

CrabLobster

A

B

Fig. 1. Bars indicate mean percentage of time (error bars: 95% C.I.

that green crab or lobster spent with the food, expressed as a percent

age of the total time either animal spent feeding. Open bars represen

percentages for lobsters, hatched for green crabs. Panel A shows the

results from Experiment 1, in which both green crab and lobster were

released simultaneously. Panel B summarizes the results from Exper

iment 2, in which the lobster was allowed to initiate feeding before the

green crab was released. *Significant differences at the 0.05 level.

P.J. Williams et al. / J. Exp. Mar. Biol. Ecol. 329 (2006) 66–74 69

whether one species was significantly first to the food.

The total time each animal spent with the mussel during

the trial was recorded, and the mean time each crusta-

cean spent with the mussel, expressed as a percentage

of total feeding time for the trial, was compared using a

Wilcoxon signed ranks test of arcsine, square root

transformed data. This non-parametric test was used

because a normal probability plot of the data was not

linear, indicating the data were not normal. The bino-

mial test was used to determine if one species spent

more time with the mussel in more trials than the other.

The results of these tests were used to evaluate hypoth-

esis 1, that the lobster would dominate time with the

food.

We also examined the interactions that took place

during the trials. An interaction was deemed to have

occurred when one animal was feeding, and the other

approached the first, and physical contact took place.

The interaction ended when one animal left the area.

The interactions were graded as successful if the ag-

gressor took over the food, and the other animal

retreated. An unsuccessful interaction involved the ag-

gressor backing away, with the other animal maintain-

ing possession of the food. Fisher’s Exact test was used

to compare the proportion of successful interactions by

green crabs and lobsters. The result of this test was used

to evaluate hypothesis 2, that the lobsters would win

significantly more interactions. The duration of the

interactions was also recorded.

2.3. Experiment 2: lobster feeds first

This experiment was designed to allow the lobster to

take possession of the food source before the green

crab. A mussel was anchored in the tank as described

above. A green crab was placed in the tank, confined

under a small (20 cm diameter) plastic bucket. The

lobster was then placed in the tank. When the lobster

had spent at least 5 min feeding on the mussel, the

bucket covering the green crab was removed, and the

video recording initiated. As in the first experiment, the

trial was terminated when both animals abandoned the

mussel for at least 10 min. Lobsters and green crabs

were fed after the experiment, and were held for an

additional 2 weeks to determine if they moulted. Any

trials in either experiment that involved an animal that

moulted were discarded, as it has been shown that molt

status affects aggressiveness (Thorpe et al., 1994;

Atema and Voigt, 1995).

Upon review of the videotapes, we compared the

mean time each crustacean spent with the mussel,

expressed as a percentage of total feeding time for the

trial, using the paired t-test on arcsine square root

transformed data. The non-parametric binomial test

was used to determine if one species spent more time

with the mussel in more trials than the other. The results

of these tests were used to evaluate hypothesis 3, that

the lobster would dominate time with the food, if

allowed to feed first.

Fisher’s Exact test was used to compare the propor-

tion of successful interactions by green crabs and lob-

sters. The result of this test was used to evaluate

hypothesis 4, that the lobsters would win significantly

more interactions. The duration of the interactions was

also recorded.

3. Results

3.1. Experiment 1: simultaneous release

Of the twenty trials conducted, five were discarded

either because of a lobster moulting or neither animal

going to the bait. Green crabs were first to the bait in 13

of the 15 trials, significantly more than the lobster

)

-

t

-

P.J. Williams et al. / J. Exp. Mar. Biol. Ecol. 329 (2006) 66–7470

( p =0.007, binomial test). The mean time until the

green crab initiated feeding was 7.8 min (S.D.=7.1

min, range=4 sec to 24 min). Green crabs spent pro-

portionally more time with the bait (expressed as a

percentage of total time either animal fed during a

trial) in 11 of 15 trials (approaches significance at

p =0.058, binomial test). The proportion of time green

crabs spent with the bait was significantly greater than

with lobster (81.2% versus 18.8%, respectively, Wil-

coxon signed ranks test; p =0.035, Fig. 1A). In the trials

when both animals had some time with the food, the

lobsters typically gained possession at the end of the

trial (Fig. 2A). These results suggest that hypothesis 1,

that the lobsters would dominate the food, should be

rejected.

A total of 45 interactions took place during the 15

trials, with a maximum of 12 interactions in 1 trial, and

0 interaction in 4 trials. As green crab dominated time

with the bait in the first experiment, most of the inter-

actions (37 of 45) consisted of the lobster as the ag-

gressor trying to displace the green crab from the bait.

The duration of the interactions ranged from 2 to 150

Elapsed Tim

****nn***nn**************n*++++++++++*nnnnn**n**+**********nnnnnnn***********++++*nnn+**n**nnnnnnnnn++++++********nnn**************nnnnnnn**********nnn*nnnnn****************n*********nn****nnnn*nn***nn*nnn*****************+++++++++++++++++++++++++++***nnnn*****+++*+++++++***nn++n+*++++*+*nnnnnnnn**n*nnn*nnnnnn+*nnnnnnnnnnn****n*nnnnn********nnn********n***************

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++**+++++++++++++++++++++++***++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++***nnn*++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++***nnnnnnnnnnnnnnnn****++++++************************n+++***nnnnnn****************************nnnn+*nnnnnnnnnnn++++++++++++++++++++++++++++++*nnnn**************

Exp

erim

ent 1

Tri

als

Exp

erim

ent 2

Tri

als

0 10 20 30 40

Fig. 2. Timelines of individual trials in which green crab and American l

horizontal line of characters represents one trial within an experiment, with

denotes neither on bait) representing approximately one minute. Panel A pro

were released simultaneously. Panel B summarizes the results from Experim

green crab was released.

sec for lobster (mean=25.5, S.D.=30.1) and from 4 to

24 sec for green crab (mean=12.5, S.D.=7.7). Green

crabs and lobsters had similar success in displacing the

other animal, with percentage of successful interactions

at 25% and 29%, respectively (Fisher’s Exact test,

p =0.580). These results lead us to reject hypothesis

2, that the lobsters would win a significantly greater

proportion of interactions.

3.2. Experiment 2: lobster feeds first

Some non-experimental mortality, coupled with

three lobsters disqualified by moulting, resulted in 12

valid trials for Experiment 2. The lobsters spent a

greater proportion of time with the bait in 8 of the 12

trials (not significantly different than green crab,

p =0.388, binomial test). Lobsters spent a significantly

greater proportion of time with the bait than did green

crabs, averaging 78.6% versus 21.4% for green crabs

(proportions significantly different, p =0.038, paired t-

test on arcsine square root transformed percentages;

Fig. 1B). The timelines of the trials (Fig. 2B) show

e (Minutes)

******nnnnn*nnnnnnn***nn****

"+" Crab on Bait"*" Lobster on Bait"n" Neither on Bait

++++++n+++++++*

++++++++++++++++++ to 109 min.

++++++++++++++++++++++++++++++++++

nn++

++++++++*

"+" Crab on Bait"*" Lobster on Bait"n" Neither on Bait

Experiment 1:

Simultaneous

Release

Experiment 2:

Lobster Feeds

First

B

A

50 60 70 80

obster were given access to a limited food resource (mussel). Each

each character (+ denotes crab on bait, * denotes lobster on bait, n

vides timelines for Experiment 1, in which both green crab and lobster

ent 2, in which the lobster was allowed to initiate feeding before the

P.J. Williams et al. / J. Exp. Mar. Biol. Ecol. 329 (2006) 66–74 71

the dominance of lobster with the bait. In contrast to the

timelines of Experiment 1 (Fig. 2A), the trials in which

lobster dominated the bait are punctuated with periods

of time when there is no animal feeding on the mussel.

These results support hypothesis 3, that the lobsters

would dominate time with the food, if they are allowed

to feed first.

There were 37 interactions that took place during the

12 trials, with a maximum of 10 in 1 trial, and 0

interaction in 3 trials. Lobsters initiated 21 interactions,

with 9 successful (43%), while green crabs initiated 16,

with 8 successful (50%). There was no significant

difference in proportion of successful interactions be-

tween species (Fisher’s Exact test, p =0.460). There-

fore, we reject hypothesis 4, that the lobsters would win

significantly more interactions.

4. Discussion

In Experiment 1, green crabs typically were first (13

of 15 trials) to discover and begin feeding on the bait.

In trials pitting green crab against the Asian shore crab,

Hemigrapsis sanguines, Jensen et al. (2002) also found

that green crabs were first to find the bait, but they were

ultimately displaced by the grapsid. Lobsters were able

to displace green crabs from the bait in some trials, but

generally this occurred near the end of the trial when

the green crabs had already consumed most of the flesh

from the mussel. Green crabs clearly dominated time

with the limited food resource in the first experiment.

This is not the result one would have predicted based

upon the relative size of the animals, with lobsters

much larger than the green crabs, and possessing

much larger chelae, both in relative and absolute

terms. Typically, in intraspecific combat in crustaceans,

the larger individual wins (Hyatt, 1983; Richards and

Cobb, 1986; Glass and Huntingford, 1988; Thorpe et

al., 1994). There are exceptions to this general rule,

based on relative weapon size (Sneddon et al., 1997a),

or asymmetries in walking legs (Sneddon and Swaddle,

1999) for example, but these exceptions usually apply

to bouts where there is little difference in size between

opponents. If one animal is more than twice the weight

of another, as was the case in our experiments, it would

be expected to win agonistic interactions. A general

observation from review of the taped trials is that

green crabs won the physical contests with lobsters

because the crabs were more agile, moved more quick-

ly, and showed more aggression. Hazlett (1971) showed

that increased aggression can allow a smaller individual

of one species of Brachyuran crab to win contests

against larger individuals of another species. In our

experiments, the lobsters appeared ponderous and

clumsy in comparison, and the green crabs easily evad-

ed the chelae of the lobster by darting over or between

them to strike at the anterior portion of the lobster.

In the second experiment, where we allowed lobster

to initiate feeding on the bait prior to the release of

green crab, the lobsters dominated time with the bait,

and aggressively defended it, often leaving the bait to

chase the green crab. It seems that first possession of

the resource conferred some advantage in these two

experiments.

Enquist and Leimar (1987) point out that the owner

of a resource has more information about its value than

an intruder seeking to obtain the resource, and in fact

game theory provides an explanation for the different

behaviour of the lobsters in the two experiments. Ar-

cher (1988) discusses a variation of the hawk–dove

game in which there is an additional conditional strat-

egy called bourgeois, in which the organism responds

like a hawk if it is in possession of a resource, or like a

dove if the other animal possesses the resource. The

bourgeois strategy has been shown to be an evolution-

ary stable strategy in a mixed population of all three. It

could be argued that both green crabs and lobsters

exhibited behaviour in our experiments that was con-

sistent with the bourgeois strategy.

Neither species made extensive use of displays dur-

ing interactions in either experiment. Game theory pre-

dicts that animals should utilize displays to resolve

conflicts in a low-risk manner (Huntingford and Turner,

1987; Huntingford et al., 1995), and indeed much of the

research investigating lobster agonistic behaviour

shows that lobsters routinely use displays in intraspe-

cific interactions and in interactions with fish predators

(Wahle, 1992). There is, however, little evidence that

crustaceans use displays in interspecific interactions,

especially between species with such different body

types as crabs and lobsters. Certainly studies of inter-

specific agonistic behaviour in crustaceans are not

common (Hazlett, 1971), and most of the work that

has been done utilizes closely related, morphologically

similar species (Dingle, 1983; Smith et al., 1994; Hyatt

and Salmon, 1978). Krekorian et al. (1974) report

extensive use of displays by American lobster interact-

ing with the clawless California spiny lobster (Panuliris

interruptus); however, these two species are of similar

body types. The value of a display in communicating

information obviously depends on the other animal

understanding the signal, possibly reducing their utility

in interspecific interactions. Unlike lobsters, green

crabs do not use displays at the beginning of intraspe-

cific interactions, although they may display at various

P.J. Williams et al. / J. Exp. Mar. Biol. Ecol. 329 (2006) 66–7472

times during a fight (Sneddon et al., 1997a,b), and we

have observed that they make extensive use of the

meral spread displays during routine handling. The

ineffectiveness of initial displays has the potential to

make intraspecific interactions more risky, with a

higher potential for injuries to occur.

Whether or not these experimental results represent

what may occur in a natural setting depend on a number

of factors, including the degree of overlap between the

two species (McDonald et al., 2001). Certainly in New

England, there is considerable evidence that green crabs

and sub-adult lobster are found in the same areas,

including the lower intertidal and upper subtidal regions

(Wahle and Steneck, 1991; Cowan, 1999; Ellis and

Cowan, 2001; Cowan et al., 2001). In southwestern

Nova Scotia, lobsters may be found in depths of 3 m

or less (Elner, 1981). In the Southern Gulf of St.

Lawrence, diving surveys show that sub-adult lobsters

are usually found in less than 7 m depth (M. Comeau,

Department of Fisheries and Oceans, Moncton, New

Brunswick, Canada, personal communication). Hudon

(1987) caught the majority of lobsters b60 mm CL at

depths of b5 m. Although smaller green crabs may

frequent the intertidal area (Crothers, 1970), we rou-

tinely trap large green crab at 2–5 m depth in estuaries

and coastal marine areas. Further evidence of overlap

comes from lobster fishermen themselves. In Cheda-

bucto Bay, Nova Scotia, where green crabs have been

established since 1985 (Audet et al., 2003), lobster

fishermen routinely report catching green crabs in lob-

ster traps set at depths of up to 12 m. In a 2002

telephone survey of harbour authorities along the

coast of Nova Scotia (unpublished data, D. Garbary,

St. Francis Xavier University, Antigonish, Nova Scotia,

Canada), 9 of 73 respondents reported lobster fisher-

men catching very high numbers of green crabs in

lobster traps, in many cases to the extent that fishermen

had abandoned selected traditional trapping areas. It

seems likely then that there could be considerable

overlap between sub-adult lobsters and adult green

crabs.

Although crustaceans are generally good subjects for

laboratory study (Huntingford et al., 1995), there are

likely some qualitative and quantitative differences in

behaviour in the laboratory as compared to the natural

environment (Cobb et al., 1986; Karnofsky et al., 1989).

Experimental set-ups such as the one used in the present

study are designed to elicit agonistic behaviour, and the

resultant behaviour may not be directly applicable to

field situations (Jensen et al., 2002). In one of the few

published sets of in situ observations of lobster behav-

iour, Karnofsky et al. (1989) found that intraspecific

interactions occurred at a very low rate, 0.2 instances/

h of observation, and only 7% of the interactions in-

volved a high level of aggression. Similarly, in a very

large (180 m2) semi-natural experimental tank, Kar-

nofsky and Price (1989) only observed a high level of

aggression in 6% of interactions. The marked reduction

in agonistic encounters as compared to that suggested by

the literature (Scrivener, 1971) was interesting. With

respect to qualitative differences between behaviour in

the laboratory and the field, the fact that the bait was

anchored likely affected the interactions. Both lobsters

and green crabs will attempt to carry food away to

consume it (Lawton, 1987; Wahle, 1992; Lawton and

Lavalli, 1995; Spanier et al., 1998), and the inability to

do this may have altered some aspects of our results.

5. Conclusions

Our results suggest that adult green crabs have the

capability to physically compete with sub-adult lob-

sters, and can win contests for resources. There are,

however, interesting complexities in the relationship

between these two species, tied to their respective life

histories. There is considerable evidence that adult

green crabs catch and consume very small lobsters

(Elner, 1981; Wahle and Steneck, 1992; Barshaw et

al., 1994; Barshaw and Lavalli, 1988). We have pro-

vided evidence that adult green crabs can compete with

sub-adult lobsters. However, lobsters attain a much

larger maximum size than green crabs, with adults

weighing more than a kilogram not uncommon. The

presence of large lobsters in traps decreases the catch

rate of adult green crabs (Miller and Addison, 1995),

and there is evidence that large lobsters can prey upon

adult green crabs (Elner, 1981; Hirtle and Mann, 1978).

These facts suggest the potential for an ontogenetic

reversal of the dominance relationship between the

two species, as suggested by Richards and Cobb

(1986) for lobster and jonah crab, Cancer borealis.

We are presently carrying out further experimentation,

with the eventual goal of developing a mathematical

model for this relationship, which could generate test-

able hypotheses about varying green crab densities on

lobster population dynamics.

Acknowledgements

This research was supported by an NSERC operat-

ing grant to PJW, as well as a UCR grant from St.F.X.

University. Mr. Kevin MacDougall kindly supplied the

lobsters. Dr. Joe Apaloo provided statistical advice.

[AU]

P.J. Williams et al. / J. Exp. Mar. Biol. Ecol. 329 (2006) 66–74 73

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