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Effects of autotomy compared to manual declawing on contests between males for females in the edible crab cancer pagurus: implications for fishery practice and animal welfare.

ABSTRACT In many decapod fisheries, claws are removed and the animal is returned to the sea with the assumption that there is little impact on the fitness and welfare of the animal, or on the productivity of the population. Here, the impact of claw loss, by two methods of claw removal, is examined during competition between males for access to females in the crab Cancer pagurus. Males induced to autotomize a claw showed little reduction in their competitive ability; however, those subject to the fishery practice of manual declawing showed a marked decrease in their competitive ability. Compared with autotomized males, these declawed crabs displayed activities that suggest an awareness of the wound caused by the appendage being twisted off and the data are consistent with an impaired welfare for these animals. They were also less likely to display to their opponent compared with autotomized crabs. Intact males showed high aggression toward declawed males, which showed low aggression in return. Further, declawed crabs showed particularly high levels of submissive acts. The declawed crabs thus rarely gained the female compared with autotomized crabs. The present study demonstrates that manual declawing has a major detrimental impact on fitness and welfare of edible crabs and we suggest that this method of harvesting should be replaced with induced autotomy of a single claw.

KEY WORDS: autotomy, Cancer pagurus, manual declawing, contests, welfare


Manual declawing of crabs is practiced in many fisheries, including the southern Florida stone crab Menippe mercenari (Ehrhardt 1990), the northeast Atlantic deep-water red crab Chaceon affinis, the southern Iberian fiddler crab Uca tangeri (Oliveira et al. 2000), and in northern Europe, the edible crab Cancer pagurus (Patterson et al. 2009). After declawing, the animal is released and the practice of manual declawing is defended because crabs may naturally autotomize a claw or walking leg, for example, when grasped by a potential predator, and then regenerate the lost limb (Juanes & Smith 1995). It has thus been argued that manual declawing offers a sustainable resource within the fishery (Carroll & Winn 1989).

The fishery practice of manual declawing by twisting and breaking the limb from the body in the edible crab, however, typically breaks some of the exoskeleton of the main body around the point of articulation of the limb (Patterson et al. 2007). This causes a stress response that includes a marked elevation of glucose within 10 min and increased lactate within 1 min. The ratio of glucose to glycogen altered significantly after 10 min, indicating mobilization of glycogen energy stores typical of the crustacean stress response (Patterson et al. 2007). Claw ablation of the freshwater prawn Macrobrachium rosenbergii produced a similar increase in glucose (Manush et al. 2005). In the edible crab, however, induced autotomy, which results in a clean severance of the limb without damage to the adjacent exoskeleton, does not cause physiological stress (Patterson et al. 2007).

Manual declawing under experimental conditions also results in high mortality. In the stone crab Menippe mercenari, 47% of individuals that had both claws removed died within 24 h and 28% died after a single-claw removal (Davis et al. 1978). Patterson et al. (2007) found that wound sizes of manually declawed Cancer pagurus that died were larger than those that survived, suggesting that the extent of wounding is a major factor in crab mortality. Those that had claws removed by induced autotomy had significantly lower mortality than did those manually declawed. A lower mortality rate was noted when claws were broken along the natural fracture plane in M. mercenaria (Simonson & Hochberg 1986) and mortality depended on the severity of the wound, and how the claw was broken off (Juanes & Smith 1995).

Further, the loss of one or both claws by either method places crabs at a distinct disadvantage in terms of feeding. For example, although loss of one claw does not alter the feeding motivation of Cancer pagurus, it does decrease ability to feed on bivalves (Patterson et al. 2009). This reduced food choice due to claw loss is also seen in Cancer productus and Carcinus maenas, which are constrained to handling smaller prey (Elner 1980, Brock & Smith 1998) and growth and regeneration may be reduced (Savage & Sullivan 1978, Elner 1980, Juanes & Smith 1995, Seed & Hughes 1995, Brock & Smith 1998). Thus, claw loss can affect the long-term fitness of these animals (Smallegange & Van Der Meer 2003).

In keeping with many other decapods, the claws of Cancer pagurus are sexually dimorphic, being larger in males, and are used during competition between males for access to females (Lee 1995). This dimorphism is even greater in fiddler crabs Uca tangeri, in which the major claws of males are used for signaling to females to attract them to their breeding burrows and to defend their burrows from other males. The removal of this vital appendage biases the operational sex ratio toward females, as clawless males are treated as females by other males and females (Oliveira et al. 2000). Thus, removing the major claw of male fiddler crabs has potential consequences at the population level. In hermit crabs Pagurus minutus and Pagurus nigrofascia, males show precopular guarding of females and often fight intruder males to retain the female. Intruders with a naturally missing, presumably autotomized, major cheliped were as likely as intact intruders to escalate a contest but were less successful in gaining the female compared with intact males (Yasuda et al. 2011, Yasuda & Koga 2016). Deficits in contests for females have also been noted in other decapods that have missing chelae (Smith 1992, Daleo 2009). These studies, however, have only examined how claw loss affects contest behavior and outcome and have not examined how the nature of claw loss might mediate contest behavior, outcome, and fitness.

It is expected that the loss of a major weapon will adversely affect contest performance in that damaged male when fighting an intact male (Arnott & Elwood 2009). This could be due to a decrease in how that damaged male assesses its own fighting ability, often called resource holding potential (RHP) (Parker & Stuart 1976). In addition, it might reduce the intact opponent's estimation of the damaged male's RHP. It is likely, however, that there is a greater loss of RHP to the damaged male if the claw is removed by manual declawing rather than by induced autotomy, because the former causes considerably more injury and physiological stress (Patterson et al. 2007). It is possible that the intact opponent could detect the greater injury caused by manual removal of its opponent's claw. Alternatively, it is possible that the intact male may simply detect the loss of the claw in its opponent and not the manner of that loss. By comparing the contests involving either manually declawed or autotomized males competing against intact males insights into the assessment processes during contests may be gained.

Two major practical concerns associated with claw harvesting in Cancer pagurus and other decapods are addressed. First, the practice might not be as benign to the population as previously suggested and thus future productivity may be compromised (Carroll & Winn 1989). Here, the fitness consequences of having a missing claw and on the nature of the claw loss of males competing for females is examined. If claw loss and the nature of claw loss impact fitness, it would be expected that there would be differences in the ability to compete for key resources. Second, some methods of claw removal may affect the welfare of individual crabs more than others (Sherwin 2001, Patterson et al. 2007, Elwood et al. 2009). Thus, activities indicating an awareness (not necessarily conscious) of the wounds arising from declawing are recorded.


Collection, Maintenance, and Experimental Procedure

Male and female Cancer pagurus of 140-180 mm carapace width were collected by commercial fishermen in April and May 2012, using baited pots, in the Irish Sea, off the Ards Peninsula of County Down, and maintained on deck in fish trays and baskets. Crabs were transported from the harbor slip at Portaferry, County Down, Northern Ireland, to the adjacent Queens University Marine Laboratory in storage boxes.

Morphometric data were collected from each animal [sex, wet weight (g), and carapace width (cm)] and the crab was tagged with "Queen Bee" tags (Thorne, UK), small colored plastic dots numbered 1-100, attached to the carapace with nontoxic, waterproof epoxy glue. Crabs were then maintained for 5 days to recover from the stress of capture (Patterson et al. 2007, Barrento et al. 2011) in outdoor 5,500-1 circular, low-profile water tanks (76 cm depth, 175 cm diameter), with a continual supply of sand-filtered water piped directly from the sea (8-9[degrees]C). Tanks were equipped with overflow outlets to allow for water circulation, and an air diffuser was used to aerate the water. To control feeding and provide shelter and protection, crabs were kept in individual lidded storage boxes (71 cm X 44 cm X 38 cm), with approximately 20 X 3 cm diameter ventilation holes for water and oxygen circulation. Approximately 10-13 boxes were kept in each outdoor tank. The outdoor tanks were kept covered and secured with blue/green-coated woven polyethylene tarpaulins, to ensure minimum light intensity/disturbance. The crabs were not fed during this period.

Observations of contests were made in a tank (Fig. 1) of 9.5-mm-thick plate glass measuring 80 cm X 50 cm X 50 cm. It comprised three chambers separated by removable rigid Perspex partitions (3 mm thick), blackened using marine paint (Krylon Fusion). These tank dividers were perforated (3-cmdiameter holes) to allow the movement of water and any chemical cues released by the crabs, including hemolymph leaking from wounds, throughout the tank. A continuous supply of Strangford Lough sea water at approximately 9.5[degrees]C and air [via an Airstone (BiOrb)] was pumped into the tank.

Sand and small pebble substrate, collected from the Strang-ford Lough tidal area, was provided (approximately 3-4 cm deep). The exterior rear and sides of the tank were also blackened using black marine paint. The area surrounding the tank was cordoned off using black plastic sheeting to control for light interference during the observation period. Red light (OS RAM Fireglow Effect 60 W, 170 lumen) was used to enable observations without natural and/or artificial light intrusion, and to obscure the observer. At the end of each observation, the sea water was drained from the tank, and refilled for the next subjects to eliminate chemical cues and leaked hemolymph.

For each replicate, two male crabs and one female crab were randomly selected for each contest. From these, one of the males was randomly selected (by drawing tokens from a cup) to have either the right or the left claw forcibly removed (manually declawed), or the male crab was induced to autotomize a claw. The other male crab and the female crab remained intact. The experimental replication was intact versus autotomy, n = 34, and intact versus manually declawed, n = 26; and animals were used only once. Manual declawing involved holding the body of the crab in one hand and grasping and sharply twisting the claw with the other (Patterson et al. 2007). Autotomy involved making a small cut at the joint at the top of the merus, the claw is then cast off by the crab at the joint that attaches to the body (Patterson et al. 2007). Males were individually placed in the two small chambers and the female in the large chamber (Fig. 1), for 1 h to acclimatize in red light. The tank partitions were then removed and all three crabs were free to move throughout the tank. Continuous recording, using a mounted digital camera above the tank, was used to capture all occurrences of behavior during the 60-min observational period. The winner was the male in physical contact with the female at the end of the contest. Some were on top of the female in the typical guarding posture. Others remained next to the female, using a claw to hold her by the carapace or a leg, or the male placed a claw or walking legs on the female. Other winners simply stayed next to or in front of the female, but remained in physical contact. Females did not show resistance to the presence of the male. The contest losers were not in close proximity to the winning male or the female at the end of the contest. From preliminary observations, a number of activities were identified. These were classified into five broad categories, to avoid excessive analyses (Table 1). In addition, "frothing" from the mouth parts was recorded as occurring or not immediately after the claw treatments were performed, before the crab was placed into the water. It was characterized by a bubbly foam coming out of the mouth parts. Further, when the nonintact males were first put into the individual sections of the observation arena, it was noted whether hemolymph was visible in the water. Finally, touching the wound by the nonintact crab, with its remaining claw and/or walking legs, was recorded during the 60-min observational period.

Ethical Consideration

No license was required for this experiment because invertebrates other than cephalopods are not regulated under the UK Scientific Procedures Act. Nevertheless, sample sizes were kept as low as possible for contingency analyses, and fewer replicates were used in the treatment considered to be the more extreme, as recommended (Elwood 1991). Manual claw removal is an extreme procedure, but one that is used in many fisheries on very large numbers of animals. It is possible that the data from the present study might guide future fisheries practice. On this basis, the procedures used in the experiment were considered justified.

Data Analyses

Effects of claw loss and the nature of that loss on which animal initiated the contests, produced the first display, success in obtaining the female, and self-directed behavior toward the wound of nonintact males, were analyzed using contingency tests and/or binomial tests. Logistic regression was used to analyze the effect of relative size of contestants that were successful in obtaining the female. For the categories aggression, defensive, dominant, and submission, the occurrence of each activity for each category was noted without respect to duration and totaled as the number of such acts in each category. An activity was deemed to have occurred twice (or more), if separated by the occurrence of another activity.

We used a one between and one within repeated measures analysis of variance (ANOVA) to determine the effects of claw removal procedure (between observations factor: declawed or autotomized) and claw removal status (within observations factor: missing claw or intact) on the agonistic behaviors. We also included the interaction term between these factors. Repeated measures are used because two animals within one contest do not act independently of each other (see Briffa & Elwood 2010 for statistical rationale). All data in the ANOVA were [log.sub.10](x + 1) transformed to improve normality. Multiple tests were not adjusted by Bonferroni correction because that has been criticized for too easily rejecting real effects (Nakagawa 2004). All statistical analyses used the Statview package.


Initiation, Display, and Success

Of the 60 staged encounters, 57 resulted in one male obtaining the female but in six of these there was no overt interaction between the males. In the other 51 cases, the males interacted before one obtained the female and, of these replicates, intact crabs were more likely than nonintact crabs to win the contest (binomial 35 versus 16, P = 0.003). Autotomized males were as likely to get the female as intact males (binomial 14 versus 17, P = 0.72), but manually declawed crabs were less likely to obtain females compared with intact crabs (binomial 2 versus 18, P = 0.0004). Further, autotomized crabs were more likely to win the contest compared with manually declawed crabs (autotomized 14/31 versus manually declawed 2/20, G = 7.76, P = 0.005). Logistic regression showed that relative size of competing crabs did not affect whether the intact crab won the female ([X.sup.2] = 0.008, [df.sub.1,50], P = 0.93).

Of the 51 contests, 44 involved cheliped displays by one or both opponents. Intact crabs were more likely than nonintact crabs to be the first to display (binomial 33 versus 11, P -0.0013). Nevertheless, intact crabs did not differ from autotomized crabs in displaying first (binomial 17 versus 10, P = 0.24), but were more likely to display first if placed with a manually declawed male (binomial 16 versus 1). Further, autotomized crabs were significantly more likely than declawed crabs to initiate displays (autotomized 10/27 versus manually declawed 1/17, [X.sup.2.sub.1], = 5.5, P = 0.02).

There was no difference between autotomized crabs and manually declawed crabs in the probability of initiating the contest (autotomized 14/31 versus manually declawed 7/20, G = 0.52, P = 0.47). Contest initiators were more likely to win than were noninitiators (binomial 35 versus 16, P = 0.003). Further, intact initiators were more likely than nonintact initiators to win the contest (intact 25/31 versus nonintact 10/20, G = 5.26, P = 0.022). Autotomized crabs that initiated the contest against their intact opponent were more likely to win compared with manually declawed crabs that initiated the contest against their intact opponent (autotomized 9/14 versus manually declawed 1/7, G = 5.07, P = 0.024).


There was no overall difference between contests involving autotomized or manually declawed crabs in the number of aggressive activities ([F.sub.(1,49)] = 0.42, P = 0.5; Fig. 2). Intact crabs exhibited more aggressive behavior than did the nonintact crabs ([F.sub.(2,49)] = 65.13, P < 0.0001; Fig. 2). Importantly, there was a significant interaction effect between type of contest (involving autotomized or manually declawed) and the intact/nonintact status of the contestants ([F.sub.(2,49)] = 9.80, P = 0.003; Fig. 2). This is because intact crabs competing against manually declawed crabs showed a particularly high number of aggressive activities, whereas the manually declawed crab showed the least number (Fig. 2).


More defensive activities occurred in contests involving manually declawed crabs than those with autotomized crabs ([F.sub.(1,49)] = 4.22, P = 0.045; Fig. 3) and nonintact crabs displayed considerably more defensive behavior compared with intact crabs ([F.sub.(2,49)] = 24.62, P < 0.0001; Fig. 3), but there was no significant interaction effect between these factors ([F.sub.(2,49)] = 2.SI, P = 0.096).


Contests involving autotomized or manually declawed crabs did not differ in the overall number of dominance activities ([F.sub.(1,49)] = 3.70, P = 0.06; Fig. 4) but intact crabs exhibited more dominance activities than did nonintact crabs ([F.sub.(2,49)] = 14.53, P = 0.0004; Fig. 4). There was no significant interaction effect ([F.sub.(2,49)] = FIT, P = 0.19).


A higher number of submissive activities occurred in contests involving manually declawed crabs than those involving autotomized crabs ([F.sub.(1,49)] = 9.32, P = 0.004; Fig. 5). Nonintact crabs exhibited more submissive behavior than did intact crabs ([F.sub.(2,49)] = 22.47, P < 0.0001; Fig. 5). Importantly, there was a significant interaction effect ([F.sub.(2,49)] = 8.19, P = 0.006; Fig. 5). This arose because of the exceptionally high number of submissive activities performed by the manually declawed crabs compared with the other groups.

Other Observations

Crabs that were manually declawed were more likely to froth at the mouth than autotomized crabs (manually declawed 17/23 versus autotomized 9/34, G = 12.88, P < 0.001), hemolymph from the wound was visible in the water in more replicates with manually declawed crabs than autotomized crabs (manually declawed 16/23 versus 6/34, G = 16.07, P < 0.0001), and manually declawed crabs were more likely to touch the wound with its remaining claw or front walking legs than did autotomized crabs (manually declawed 15/23 versus autotomized 7/34, G = 15.93, P < 0.0001).


Although intact crabs were more successful than were nonintact crabs in competing for females, those induced to autotomize a claw were considerably more successful than crabs that were manually declawed. Indeed, autotomized crabs fared no worse than intact crabs when just those contests were examined. That is, it is not the absence of a claw that reduces the ability of a male to obtain a female, at least under the present conditions, rather it is the manner of claw loss. Manual declawing clearly places males under a severe intraspecific competitive disadvantage. Negative effects of claw loss have been noted in other studies (Sekkelsten 1988, Abello et al. 1994), but the manner of claw loss has received little or no attention with regard to such competition. To understand how the outcome of contests is influenced by the nature of the claw loss, the activities used in the competitive process are considered.

Activities that occur early in the encounter should indicate how the males assess themselves in terms of RHP rather than indicating how the opponent perceives them (Elwood & Arnott 2012). Intact crabs were more likely than nonintact crabs to initiate the contest by moving toward the opponent; however, manually declawed and autotomized crabs did not differ in the probability of initiation of contests. Initiating the contest gives an advantage to that crab because those that initiated were more likely to obtain the female. Autotomized crabs that initiated, however, were more likely to win access to the female than were manually declawed crabs, suggesting that the latter did particularly poorly in the ensuing fight.

Intact males were also more likely than nonintact males to be the first to display. Here, there was a marked effect of the nature of claw loss because, whereas the autotomized crabs were as likely as the intact to display first, the manually declawed crabs very rarely displayed first. This suggests that it is not the lack of a claw that is dissuading the crab to engage in display but, rather, it was due to the poor condition of the declawed crabs (Patterson et al. 2007). Further, the raising up and stretching out of the claw or claws is likely to be energetically expensive (Doake et al. 2010) and perhaps beyond the capability of a manually declawed crab.

Manually declawed crabs were more likely than autotomized crabs to lose hemolymph in amounts that could be seen in the water. Frothing at the mouth was also more common in declawed crabs than in autotomized crabs, such frothing in crabs being attributed to stress (Deshai et al. 2012). Manual declawing also results in elevated concentrations of lactate compared with intact and autotomized crabs (Patterson et al. 2007). High lactate concentrations during contests cause fatigue (Briffa & Elwood 2005) and alter behavior such as defensive actions (Stoner 2012). Manually declawed crabs may therefore be unable to engage in fighting, and may withdraw from the contest based on assessment of their internal state.

Manually declawed crabs appeared to be aware of their wound, as indicated by their much higher incidence of touching the wound compared with autotomized crabs. Although not part of the recording protocol, a number of manually declawed crabs showed a "shudder" response when touching the wound. The remaining claw or a leg was brought to the wound site and either inserted directly into the wound or probed the edges of the wound site. The "shudder" response was only observed when the wound was being touched and the crab's body was seen to give a little shake or tremble. Touching at the site of the application of a noxious stimulus has been noted in glass prawns (Barr et al. 2008) and hermit crabs (Appel & Elwood 2009) and is considered to indicate an awareness of the location of a wound. Shaking of a claw has been noted following injection of formalin into that appendage (Dyuizen et al. 2012), but the present study is the first to note shaking/shuddering of the entire body.

Some manually declawed crabs shielded their wound by positioning the remaining claw in front of the wounded area. This protected the wound from contact by the intact opponent, but impeded the ability of wounded crabs to engage in the normal activities seen in crab fights. These observations indicate that declawed crabs were aware (not necessarily conscious) of their wound and that the wound resulted in marked changes in behavior that are not merely reflexive but consistent with the idea of pain (Elwood 2011, Sneddon et al. 2014). These crabs also appeared to be in poor condition and incapable of effective competition. How this resulted in losing the encounter may be determined by examining the specific groups of activities that comprise the competitive interaction, that is, aggression, defense, dominance, and submission.

Intact crabs were more aggressive than nonintact crabs and they were particularly aggressive when encountering a manually declawed crab rather than one that had autotomized. In return, the manually declawed crabs showed very few aggressive acts. It is possible that the intact crab was responding to either the wound or the behavior of declawed opponents and increasing aggression above that normal for crab fights. Alternatively, the intact crab might be fighting normally without information being gathered about the wound of the nonintact crab. It is clear, however, these contests are highly asymmetric with respect to the number of aggressive acts shown. Intact crabs also showed more acts of dominance than did the nonintact crabs, but the lack of a significant statistical interaction shows that, in contrast to aggressive acts, this was not affected by the nature of claw loss. With dominance activities, there is no evidence that the intact male can discriminate between the two types of claw loss in an opponent. Thus, the behavior of the intact crab does not distinguish whether these contests are based on self-assessment, where each contestant acts according to its own abilities, or by mutual assessment, where each incorporates information about the ability of the opponent (Elwood & Arnott 2012).

Defensive acts were shown less often by the intact crabs compared with nonintact crabs. Both types of claw loss resulted in high numbers of defensive acts in the affected males, and the lack of a significant statistical interaction indicates that the nature of claw loss did not have a marked effect on defensive behavior. Crabs with a missing claw also showed more submissive acts than did those with both claws. In this case, submissive acts were much more frequent by manually declawed crabs compared with autotomized crabs. This indicates that the declawed crabs are attempting to avoid the agonistic encounter, presumably because they are aware of their poor condition. Thus, judging from the observation on submission, manually declawed crabs are not attempting to fight but rather are attempting to limit damage. Thus, the data on the nonintact crabs indicate self-assessment is affecting how they compete (sensu Taylor & Elwood 2003).

It is clear that intact crabs were more motivated to fight compared with those missing a claw. Further, autotomized crabs were more motivated to engage in a fight than manually declawed crabs. This is evidenced by crabs that were autotomized showing fewer submissive acts and winning more contests than manually declawed crabs. It is possible that autotomized crabs engaged in dishonest signaling to convey a greater aggressive intent and fighting ability, a common trait among crustaceans (Steger & Caldwell 1983, Backwell et al. 2000, Elwood et al. 2006, Laidre 2009). Indeed, male hermit crabs that lack the major claw (presumably by autotomy) are just as likely to escalate contests for females, but were much less likely to win than intact intruders (Yasuda & Koga 2016). In the mud crab Cyrtograpsus angulatus (Dana, 1851), crabs missing claws by induced autotomy were also able to win contests when competing against intact crabs (Daleo 2009).

One surprise in the present study was that body size did not have a significant effect on the outcome of contests because body size has been shown to be important in numerous other taxa (Arnott & Elwood 2009). In the present study, however, a narrow range of crab sizes was used as no crab below the minimum legal landing size of 140 mm carapace width was included in the experiment. With a wider size range of opponents, those crabs with a missing claw might effectively compete against much smaller opponents. Thus, if autotomized animals are released in the sea, they would encounter a broader range of crabs than in the experiment and might have an increased chance of winning a contest for females, as well as other resources, when facing much smaller opponents. Further, it is possible that manually declawed crabs might also fare better with much smaller opponents. That is not to suggest that these crabs might do well if released because previous studies have shown a high mortality of manually declawed crabs (Patterson et al. 2007). It is important to note that in this experiment a maximum of one claw was removed, whereas in some fisheries two may be removed. The consequences of losing both claws by manual declawing would be severe from the point of view of survival (Davis et al. 1978) and even if lost by autotomy, there would be major detrimental effects on ability to feed (Juanes & Smith 1995) and undoubtedly on competitive ability.


It is clear that the ability to compete against intact crabs is severely affected by the nature of claw removal. Crabs that have a single claw manually removed by twisting have very poor success in male-male contests compared with those that lose a claw by induced autotomy. This major fitness impact is likely due to the hemolymph loss seen immediately after manual claw removal but much less after induced autotomy. Wounds are much larger after manual declawing (Patterson et al. 2007) and these crabs showed the stress response of frothing from the mouth (Deshai et al. 2012). Manual declawing rather than autotomy also results in rapid increases in hemolymph lactate and glucose that is typical of a marked physiological stress response (Patterson et al. 2007). Further, the observation of repeated touching and picking at the wound after manual declawing, as well as guarding of wounds, suggests an awareness of the wound. Thus, there are concerns for the welfare of crabs subject to manual declawing (Elwood 2011). There must also be concerns that returning crabs to the sea after manual declawing will not enhance population sustainability, because of the loss in competitive ability, the loss of feeding ability (Patterson et al. 2009), and the substantial mortality (Patterson et al. 2007) seen in these animals. It is suggested that manual declawing is discontinued in those fisheries in which it still occurs. An alternative would be training fishermen to induce autotomy in one claw, followed by return of the crab to the sea.


This work was funded by the Department of Agriculture and Rural Development Northern Ireland (DARDNI) under their PhD Studentship Scheme.


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Institute for Global Food Security, School of Biological Sciences, Queens University Belfast BT7 9BL, Northern Ireland, United Kingdom

* Corresponding author. E-mail:

DOI: 10.2983/035.035.0426

Male competitive activities grouped into broad categories.

Activity                                 Description

Initiate contest *
  Approach              One opponent approaches the other opponent,
                            decreasing the distance between the
                            contestants. This is followed by:
  Mutual alignment        Opponents face each other; no contact
  Contact                 One opponent makes contact with other
  Contact alignment       Opponents face each other, claws in contact
  Claw stroke             One opponent uses claw(s) to stroke other
Aggressive behavior
  Display               One opponent extents claws out toward other
                          opponent, pinchers open; no contact
  Threat display        One opponent raises body high on walking
                          legs, extends claws, directed toward other
  Extend                One opponent swipes claw toward other
                          opponent; no contact
  Lunge                 One opponent, claw open and extended, thrusts
                          body forward at opponent; brief contact
  Manus contact         Opponents face each other in threat display;
                          claws in contact, pinchers open
  Pull in               One opponent uses claws to pull opponent,
                          decreasing distance between individuals
  Mutual push           One opponent uses claw(s) to push against
                          other opponent, other opponent pushes back
  Carapace grasp        One opponent grasps and holds other opponents
  Grip                  One opponent uses claw(s) to grip other
                          opponent, pinching/crushing observed
  Anterior strike       One opponent uses claw(s) to grip anterior
                          region of carapace of other opponent
  Wound grasp           One opponent uses claw(s) to grasp other
                          opponents wound site
  Repeated grasp        One opponent repeatedly grabs and grips
                          opponent; vigorous pushing and pinching/
                          crushing observed
  Grip back             One opponent uses claw(s) to return the grip
                          of other opponents' claw(s), pinching/
                          crushing observed
  Flip                  With interlocked claws or by grasp of
                          carapace, one opponent is lifted from the
                          substrate and held above opponent
Defensive behavior
    ([double dagger])
  Retreat               One opponent retreats rapidly from the other
  Withdraw              One opponent leaves the area of the other
                          opponent, increasing the distance between
                          the contestants
  Struggle              One opponent struggles to free itself from
                          other opponents grasp
  Push away             One opponent uses claw(s) to push other
                          opponent away, creating distance between
                          the opponents
  Dismount              One opponent climbs off other opponent
Dominant behavior
  Rise up               One opponent rises up on legs
  Pushdown              One opponent uses claw(s) to push down on
                          other opponent's carapace
  Tap                   One opponent uses claw(s) to "tap" on other
                          opponent's carapace
  Mount                 One opponent crawls on top of the other
  Push                  One opponent uses claws and/or body to push
                          against other opponent; contact
  Free                  One opponent releases other contestant from
Submissive behavior
  Motionless            One opponent freezes body position; no overt
                          sign of movement or response
  Submission            Opponent draws claws and walking legs in and
                          under body, lowers body
  Crawl under           One opponent attempts to position itself
                          under other opponent's body

* This is characterized by one crab decreasing the distance between
it and its opponent and one of four other activities.

([dagger]) Aggressive activities include "displays," incurring low
costs, followed by an "attack," and finally a "fight," that
presumably incurs the highest costs with the potential for injury to
both crabs.

([double dagger]) Defensive activities include one crab attempting to
repel and/or escape from its opponent.

([section]) Dominant behavior was observed when one crab appeared to
exert control over its opponent, typically with its opponent engaging
in subordinate behavior (below).

([paragraph]) This was observed by crabs typically in response to
dominant behavior by the opposing crab.


Please note: Some tables or figures were omitted from this article.
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Author:McCambridge, Claire; Dick, Jaimie T.A.; Elwood, Robert W.
Publication:Journal of Shellfish Research
Article Type:Report
Date:Dec 1, 2016
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