Scope and Intent:

Influence of Marine Protected Areas on ecosystem resilience and ecological processes.
Intraspecific competition between members of a species,
either where a high density population limits resources, or
through direct combative competition, often plays a major
role in the dynamics and distribution of a population.
Asymmetries in competitive ability can result from
differences in size, age, gender, condition or experience. In
any case, the outcome of competition for the inferior
competitor is usually detrimental.

In Moreton Bay there is a significant commercial and
recreational fishery targeting male mud crabs (Scylla serrata)
with carapace widths over 150mm; (undersized males and all
females are protected by Queensland law.)

Mud crabs are known to be aggressive scavengers; with large
males usually dominating any food source. The study tested
the hypothesis that the combination of gender and relative
size of a crab to its competitor determines its behaviour
during interference interactions. The premise of the study
therefore, was that in areas protected from fishing pressure, a
higher density of large males would result in altered
population dynamics and behaviour of individuals compared
to areas open to fishing pressure.
1
Mud Crabs, - Competition experiment and preliminary movement data.
R. Connolly, K. Pitt, D. Rissik, A. Olds, P. Maxwell, D. Burfeind, C. Beattie & R. Babcock.
Griffith University, Queensland & CSIRO Marine & Atmospheric Research.
December 2012.
Mean observed interaction time.
300
250
200
150
Results:

There were significant three-way interactions among the
crab’s gender, the competitors’ gender and the competitors’
relative size for the durations of attacking, avoiding and
feeding, (see Fig. 1.)

Large males appeared to dominate; both male and female
crabs spent the majority of observed interactions avoiding
larger males. Females placed in direct competition with a
larger male spent >80% of the interaction period avoiding
confrontation and, on average, 0% of the interaction period
was spent feeding, (see Fig. 2.)

Males spent more time feeding when with females of all
sizes compared to equal or larger sized males. Equal sized
males also spent a far longer period (275 seconds on
average, see Fig 1,) and a far higher proportion of time
fighting; (over 70%, see Fig. 2,) than any other gender or size
related interaction.
100
50
0
F vs SF
F vs EF
F vs LF
F vs SM F vs EM F vs LM M vs SM M vs EM M vs LM M vs SF M vs EF M vs LF
Fighting
Avoiding
Feeding
Fig 1, above.) Mean time (seconds) and observed behavioural interaction when male (M) and female (F)
mud crabs are confronted with a smaller (S,) larger (L) or equally sized (E) direct feeding competitor.




2
Fighting behaviour between females tended to be less
intense and shorter lived; consisting largely of pushing,
compared to fighting between males. A possible explanation
for these differences is that male mud crabs are inherently
more aggressive and are better adapted morphologically
and physiologically for intense and prolonged fighting.
There was a significant correlation among both genders for
time spent feeding to be negatively associated with time
spent avoiding other crabs.
The behaviours of mud crabs matched with an opponent of
the same gender but differing relative sizes were generally
similar to those observed in other species of crabs. Both
male and female mud crabs dominated smaller crabs of the
same gender, causing them to spend more time avoiding,
and crabs that were similar in size and gender spent the
most amount of time attacking.
In addition to the recorded relevant interactive behaviours,
all crabs, regardless of their gender and relative size of
competitor, spent similar amounts of time exhibiting
threatening behaviour (average 57 ±5.8 sec.) Crabs spent
the remaining time (average 64 ±1.6%) inactive on top of
the sand or slowly moving around the tank.
Influence of Marine Protected Areas on ecosystem resilience and ecological processes.
Mean proportion of observed crab interaction to competitor.
M vs LF
M vs EF
M vs SF
M vs LM
M vs EM
M vs SM
F vs LM
F vs EM
F vs SM
F vs LF
F vs EF
F vs SF
0%
Fig 2, right.) Proportion (%) of observed interaction time when male
(M) and female (F) mud crabs are confronted with a smaller (S,)
larger (L) or equally sized (E) direct feeding competitor.
10%
20%
30%
40%
Fighting
50%
Avoiding
60%
70%
80%
90%
100%
Feeding
Conclusions:

The results of the study supported the hypothesis that the size and gender of mud crabs have important consequences for how they respond to competitors in
the presence of food. Large Males did appear to dominate; any smaller male or female in a competitive interaction with a larger male tended to spend a large
portion of time avoiding confrontation and consequently spent less time feeding.

Despite potential differences in the behaviour of crabs in the laboratory and in more ‘natural’ conditions, a reasonable progression from the study results
might be that in a marine reserve where densities of large male mud crabs are shown to be significantly higher than surrounding areas, ecosystem processes
could be altered. Intense competition for food resources might result in deleterious competitive interactions for female and small male mud crabs. Smaller
males and females could consequently be forced to alter their foraging behaviour or migrate to areas where there is less chance of encountering high
densities of large males, such as open areas, beyond the boundaries of small marine reserves.

In addition to mud crabs, many other species of crabs are subjected to size and gender selective fisheries. Where size dependent sexual dimorphism occurs in
other crab species, interactions between large and small males and females could have similar outcomes to those observed in this study. Individual female
and small male crabs may therefore also be affected by more intense competition with large male crabs within protected areas. Finally, the results of this
study prompt further research into the potential indirect effects of manipulating the structure of populations through size and gender selective fisheries and
the establishment of marine reserves.