JOURNAL OF CRUSTACEAN BIOLOGY, 24(1): 178–187, 2004
SPATIAL AND TEMPORAL DISTRIBUTION OF SHORE CRABS
CARCINUS MAENAS IN A SMALL TIDAL ESTUARY
(LOOE ESTUARY, CORNWALL, ENGLAND)
Kim Rewitz, Bjarne Styrishave, Michael H. Depledge, and Ole Andersen
(KR, BS, [correspondence], OA) Department of Life Sciences and Chemistry, Roskilde University, P.O. Box 260,
4000 Roskilde, Denmark ([email protected]);
(MHD) Plymouth Environmental Research Centre, University of Plymouth, Drake Circus,
Plymouth PL4 8AA, United Kingdom
ABSTRACT
Tidal, diel, and spatial variations in numbers, sex, size, and colour morphology of shore crabs Carcinus
maenas caught in baited drop nets during tidal periods, at neap tide and spring tide, were studied in the
strongly tidal Looe Estuary, Cornwall, Southwest England. Depth, salinity, temperature, pH, and oxygen
tensions were measured simultaneously. High numbers of both genders were caught in the estuary. In
total, 61% of adult crabs caught were females. However, the sex ratio (males over females) in the catches
significantly increased (P , 0.05) from approximately 0.2 at the station nearest to the mouth of the estuary
to approximately 4 at the innermost station. Due to the well-established relationship between carapace
colouration and intermoult duration, catches were analysed with regard to red and green colour forms,
besides for sex and size. Green crabs were caught throughout the estuary and constituted 79% of total
catches. Green males dominated the shallow stations, whereas green females dominated the deep stations.
The catch per unit effort (CPUE) was related to tidal and diel phases, with most adult crabs being caught
during high tide and only few during low tide. Also, more adult crabs were caught during night time than
during day time. The CPUE increased with increasing depth, and crabs were never caught at salinities
below 15& and rarely at salinities below 20&. Oxygen tension, temperature, and pH exerted no effect on
the distribution of shore crabs. Even though conclusions based on these data depend on whether catchdata analysis reflects true population abundances, sex ratios, and colour morphology compositions, the
data suggest that the small size, strong current, and high salinity characteristics of the Looe Estuary allow
both genders and colour forms to migrate into the estuary during high tide and to return to the shore before
low tide, thereby exploiting a marginal feeding habitat.
The shore crab, Carcinus maenas (Linnaeus,
1758), inhabits a variety of nearshore habitats
including tidal estuaries. Intraspecific competition for food and mates is an important
determinant of shore crab behaviour and
physiology, and trade-offs between growth and
reproduction are believed to determine moulting
frequency and intermoult duration (Kaiser et al.,
1990; Reid et al., 1994; Reid et al., 1997).
Previous investigations have shown that shore
crab populations differ in several population
characteristics in relation to habitat characteristics: size, sex, and colour morphology can
differ in relation to depth, salinity, and tidal
regimes (Crothers, 1968; McGaw and Naylor,
1992a; Aagaard et al., 1995). Together, these
studies indicate that male crabs are more
migratory than female crabs, and that green
crabs are more explorative than red crabs and
more often migrate into intertidal habitats.
The colour morphology is related to moulting. The new carapace formed during moulting
is green and only slowly turns orange and
eventually red (Crothers, 1968). Coloration is
therefore related to intermoult duration (McGaw
et al., 1992). Large red males are most likely to
get access to receptive females (Reid et al.,
1994). Colour variation analysis of (especially
male) crab populations can therefore indicate
whether the overall strategy for shore crabs
within a population is growth or reproduction.
Estuaries are highly productive habitats
suited for foraging. As water depth, salinity,
and temperature vary extensively and rapidly in
estuaries, the physiology of shore crabs may,
however, be compromised. Estuaries are therefore marginal habitats for shore crabs. Hence,
exploitation most likely depends on migration
with the tide, which would minimize stress due
to abiotic factors.
Based on the previous studies cited above,
one would expect that green male crabs would
be the most frequent sex and morph in shore
crab populations in intertidal areas of estuaries,
178
179
REWITZ ET AL.: DISTRIBUTION OF CARCINUS MAENAS IN A TIDAL ESTUARY
estuaries, this would conceivably make the Looe Estuary
an unfavourable habitat for shore crabs during low tide, due
to low salinity and risk of avian predation. However, shore
crabs may be able to forage intensively in the estuary during
high tide and return to the open sea before low tide rather
than remaining within the estuary during low tide.
Sampling and Data Analysis
Fig. 1. Location of the nine sampling stations in the Looe
Estuary, Cornwall, England.
and that abundance would depend heavily on
the tidal phase. This hypothesis was investigated in the present study of the Looe Estuary,
Cornwall, England.
The method employed to estimate crab
abundance was baited drop nets. Hence, the
data obtained show numbers of caught, actively
feeding crabs. Conclusions based on these data
therefore depend on whether catch numbers can
be considered valid estimates of total crab
abundance, and whether catch efficiency is
similar for different sizes, sexes, and colour
forms. As few published data are available for
evaluating this question, our conclusions are
preliminary and should be further tested in other
experimental situations.
MATERIALS AND METHODS
Study Site
The Looe Estuary, situated on the southern coast of
Cornwall, Southwest England, is a small estuary. The
estuary is heavily influenced by the tide, and the inner part
almost empties of seawater during low tide, receiving only
a limited freshwater input from two rivers merging about
1 km upstream from the shore. The substrate is sandy in the
outer part of the estuary, but changes gradually and is
muddy in the inner part of the estuary. The Looe Estuary is
essentially a channel, approximately 1 km long and 60 m
wide. The depth varies about two metres at neap tide and
about five metres at spring tide with a maximum depth of
about six metres (Neil, 1994). Due to the limited freshwater
input, only a small stream (less than 10 m wide and 0.5 m
deep) remains during low tide. As opposed to other
Two separate samplings of shore crabs, Carcinus maenas,
were conducted in the estuary over two consecutive tides,
during a neap tidal period, and during a spring tidal period.
During the two samplings, crabs were caught at nine
stations. Five stations were located along the estuary
(stations 1–5) with station 1 located where the estuary
opens toward the sea (permanent high salinity) and station 5
located in the inner part of the estuary. Crabs were also
caught at four stations across the estuary at locations close to
station 4 (stations 4a–4d). Figure 1 and Table 1 show the
location of the stations and the mean depth and distance
from the seashore. The first sampling was conducted from
August 28 to August 30. The second sampling was
performed one week later from September 6 to September
7. Shore crabs were caught once every 90 min at each station
during a 26-h period. A probe (Hydrolab, Datasonde 4) was
submerged along with the drop net to obtain data for abiotic
parameters such as depth (m), salinity (&), oxygen
concentration (mg Ll), pH, and temperature (8C).
During each sampling, crabs were caught using a baited
drop net as described by Aagaard et al. (1995). On each
station, the drop net was submerged on the seabed for 5 min
at the exact same location. The drop net was constructed
from an iron frame (70 cm in diameter) covered with a net
having a 1-cm mesh size. Fresh raw mackerel was used as
bait and was placed in a fine-meshed bag, which made it
inaccessible to the crabs. Catch per unit effort (CPUE) is
defined here as the number of shore crabs caught per
sampling.
The catches at each station were assigned to one of four
categories: 1) high tide day time; 2) high tide night time; 3)
low tide day time; 4) low tide night time. High tide was
defined as 3 h before and after maximum high tide and low
tide as 3 h before and after minimum low tide. Night was
defined as being from 1 h before sunset to 1 h after sunrise.
Before the crabs were released at the location where they were
caught, the catches were analysed for total numbers and
individual crab characteristics: sex, size (mm carapace width,
CW), and developmental stage (juveniles defined as CW , 35
mm; Crothers, 1967), to allow calculation of sex ratios defined
as males/females. Crabs were further analysed for coloration
of the abdomen and separated into two colour categories,
green and red. Green crabs included crabs with a light or dark
green appearance. Red crabs included ‘‘non-green’’ crabs
ranging in colour from orange to red or dark brown.
Statistical Analysis
A Kolmogorov-Smirnov test of normality was carried out
for all data, and ANOVA and Tukey post hoc tests were
used to test for significant differences. If the KolmogorovSmirnov test indicated that data were not normally
Table 1. Distance from the coast (m) and water depth (m) at mid-tide level for each sampling station.
Station
1
2
3
4
5
4a
4b
4c
4d
Distance (m)
Depth (m)
0
2.2
200
1.3
350
1.9
760
1.4
960
0.6
750
1.8
750
1.6
750
1.5
750
1.4
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JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 24, NO. 1, 2004
Table 2. Total catch during two tidal periods (26 h) for the different groups of crabs at each station, arithmetical averages of
CPUE values, and the percentage of green to red males and females for each station. Also shown is mean carapace width
(CW). G#: green males; G$: green females; R#: red males; R$: red females; and Juv: juveniles. Numbers in horizontal lines
(different stations) with different letters are significantly (P , 0.05) different for longitudinal stations 1–5 and transverse
station 4a–4d, respectively, based on statistics on the original data.
Station
1
2
3
4
5
4a
4b
4c
4d
Neap tide
Total catch
G#
G$
R#
R$
Juv
Average CPUE
15
79
3
25(a)
13
7(a)
42
97
6
8(a)
28
10
44
182(b)
21
82(b)
43
20(b)
48
36(a)
9
6(a)
3
5(a)
19
4(a)
15
4(a)
5
2(a)
80
166(a)
21
35(a)
9(a)
18(a)
97
131(a)
29(a)
29
6(a)
16(a)
53
36(b)
9(b)
6(b)
41
8(b)
51
26(b)
9(b)
8(b)
56(b)
8(b)
Spring tide
Total catch
G#
G$
R#
R$
Juv
Average CPUE
23
65
2(b)
15
7
7
13
29(a)
5(b)
10
9
4(a)
42
143(b)
13
48
16
16(b)
50
29(a)
30(a)
12
15
9
25
6(a)
27(a)
6
13
5(a)
69
147(a)
35(a)
49
29
21(a)
93
112
23
44
35
19(a)
65
10(b)
10(b)
13
22
10
50
4(b)
4(b)
8
49
9(b)
Green:Red
G# : R#
G$ : R$
88:12
78:22
83:17
88:12
72:28
71:29
72:28
78:22
51:49
50:50
73:27
79:21
79:21
77:23
86:14
71:29
89:11
65:35
Mean CW (mm)
G#
G$
R#
R$
Juv
47
46
58
49
30(b)
45
45
57
47
29(b)
48
46
54
49
29(b)
48
49
59
50
23(a)
52
48
61
49
21(a)
50
47
59
48
27
49
48
59
51
25(b)
distributed, a nonparametric Kruskal-Wallis ANOVA by
ranks or Mann Whitney U test was used.
RESULTS
Data were analysed with respect to catch efficiency, sex ratio, size, and colouration in relation
to station location, tidal, and diel regimes.
Catch Efficiency
The CPUE for shore crabs varied along the
estuary, with the highest average CPUE value at
station 3 and decreasing both downstream
(stations 1 and 2) and upstream (stations 4 and
5); the lowest average CPUE value occurred at
the inner station 5. During both neap tide and
spring tide samplings, average CPUE values
were significantly lower (P , 0.05) at stations
1, 2, and 5 compared to station 3 (Table 2).
Across the estuary, the highest average CPUE
values (about 20) were found at the deeper
stations located on the eastern side of the
estuary (stations 4a and 4b), decreasing to
approximately 10 at shallower stations on the
western side of the estuary (station 4c and 4d).
Average CPUE values were significantly higher
52
48
59
50
29(a)
51
47
59
48
28
at stations 4a and 4b than at station 4d, with no
significant differences between neap tide and
spring tide samplings. Compared to neap tide,
there was a tendency towards higher average
CPUE values at the inner stations 4 and 5 and
lower average CPUE values at the outer stations
2 and 3 during spring tide (Table 2).
Figure 2 shows the total catch of adult and
juvenile shore crabs for all stations over a 26-h
period during neap tide. Most adult shore crabs
were caught during high tide at all stations with
the exception of station 1 where most crabs
were caught during the beginning of low tide.
Crab Sex Ratios
The sex ratio (male over female) varied
significantly between catches at the different
stations. Overall, the average sex ratio of adult
shore crabs was 0.64 (1:1.56), which is significantly (P , 0.01) lower than the theoretical
average of 1, demonstrating that females were
the most frequently caught sex in the estuary.
The sex ratio for adults significantly increased
(P , 0.001) from about 0.25 in catches at the
REWITZ ET AL.: DISTRIBUTION OF CARCINUS MAENAS IN A TIDAL ESTUARY
181
Fig. 2. Carcinus maenas. Catch per unit effort (CPUE) for adults (open bars) and juveniles (solid bars) collected over two
tidal phases. Also shown are average carapace width (e) and depth (—). Horizontal bars indicate night time. Note the
difference in scale.
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JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 24, NO. 1, 2004
Fig. 3. Carcinus maenas. Sex-ratio (males over females) at each sampling station. Open symbols: Neap tide. Closed
symbols: Spring tide. Asterisks indicate significant differences from a theoretical 1:1 male : female sex ratio (P , 0.05).
station located near the mouth of the estuary
(station 1), to about 4 in catches at station 5 (Fig.
3). The sex ratio was significantly lower at
stations 1, 2 and 3 (P , 0.05) and significantly
higher at stations 4 and 5 (P , 0.05) than what
would be expected from a theoretical average
sex ratio of 1 over the entire sampling area. No
significant differences in sex ratios were observed between neap tide and spring tide or night
and day. During neap tide, the sex ratios were
significantly lower at station 4a and significantly
higher at station 4c and 4d than the theoretical
ratio of 1 (P , 0.05). During spring tide, station
4a revealed a significantly (P , 0.05) lower sex
ratio than 1 (Fig. 3).
Crab Size
Juveniles were generally caught throughout the
estuary, but more frequently at the shallower
stations (Table 2). Even during neap tide, 56
juveniles were caught at station 4d, the shallowest
station across the estuary. This was significantly
(P , 0.05) more than the nine and six juveniles
that were caught at the deeper stations 4a and 4b,
respectively. Juveniles and green males dominated the shallow station 4d, each constituting
more than 30% of the total catch. The size
distribution of juveniles depended on station
depth (Table 2), hence, the average size of
juveniles decreased significantly (P , 0.05) from
station 1 to station 5. Also, juvenile crabs were
significantly larger at station 4a than at station 4d
(P , 0.05). No significant difference in size was
observed between the stations for the four adult
morphotypes. However, size differences were
observed between colour forms. The average CW
of red males was 58 mm, significantly more (P ,
0.001) than the average CW of 49 mm for green
males. Red females were slightly but significantly (P , 0.001) larger (average CW 49 mm)
than green females (average CW 47 mm).
In general, more juvenile shore crabs were
caught during low tide (not significant), in
particular at the beginning of the incoming tide,
especially at stations 4c and 4d. Juveniles were,
however, not caught at stations 4 and 5 at
minimum low tide. During low tide, a tendency
towards a decrease in mean carapace width was
observed, reflecting the higher proportion of
juveniles caught during this period (Fig. 2). The
average CPUE value of adult shore crabs was
significantly (P , 0.01) higher during high tide
(maximum 19.2) than during low tide (maximum 7.8) (Table 3).
More adult shore crabs were caught during
night time than during day time. During high
tide, the average CPUE value for adults was
significantly higher (P , 0.001) during night
time (average CPUE ¼ 18.2) than during day
time (average CPUE ¼ 11.8) (Table 4).
Crab Colouration
Green crabs (79% of total catch) dominated
catches in the estuary. Green females dominated
REWITZ ET AL.: DISTRIBUTION OF CARCINUS MAENAS IN A TIDAL ESTUARY
catches at the deep stations (stations 1, 3, 4a,
and 4b). Significantly more green females were
caught at station 3 than at stations 4 and 5 (P ,
0.05) and at stations 4a and 4b compared to
stations 4c and 4d (P , 0.05) (Table 2).
The catch pattern of red females was similar
to that of green females, with the highest total
catch on the deep stations, whereas only few red
females were caught at the shallow stations 5, 4,
4c, and 4d (Table 2). Except at station 5, red
females never constituted more than 35% of the
total catch of females. The average CPUE values
were higher for green males than for red males,
but the green male : red male ratio decreased in
catches from station 1 to station 5. In contrast to
other morphs, no significant differences in the
total catch of green males were observed among
the stations, demonstrating that green males
were the most evenly distributed morphotype in
catches throughout the sampling area and the
only morphotype frequently caught at shallow
stations (Table 2). Across the estuary, red males
were mainly caught at the deep stations 4a and
4b (Table 2). Compared to stations 4c and 4d,
significantly (P , 0.05) more red males were
caught at station 4a during spring tide and
station 4b during neap tide.
The average CPUE values of green males,
green females, and red females were significantly (P , 0.05) higher during high tide than
during low tide in all catches. Also, significantly
(P , 0.001) more red males were caught during
high tide compared to low tide in three out of
four catches (Table 3). There was no significant
difference in the number of green shore crabs
caught during neap and spring tide, whereas red
males were caught in greater numbers at the
inner stations (stations 4 and 5) during spring
tide than during neap tide (Table 2). The
average CPUE value for red males increased
from 9 at station 4 during neap tide to 30 during
spring tide (P , 0.05). Although more red
females were caught at the inner stations during
spring tide, this was not significant.
For both green male and green female shore
crabs, average CPUE values were significantly
higher during night time than during day time
(G#: P , 0.01; G$: P , 0.001). Diel variations
were not observed for red shore crabs and
juveniles (Table 4).
Influence of Depth, Salinity, Oxygen Tension,
Temperature, and pH on CPUE
Only depth, salinity, and oxygen tension varied
significantly between samplings and stations.
183
For shallow stations such as stations 2, 4, and 5,
a linear correlation between depth and CPUE
was observed, CPUE increasing with increasing
depth (P , 0.05; data not shown). No correlation
between depth and CPUE was observed for the
deeper stations 1 and 3. The distribution of shore
crabs within the estuary was related to salinity,
but not in a linear manner. At the inner stations,
salinity fluctuated dramatically during the tidal
phase, from 35& during high tide to 7.2& and
3.5& during low tide at stations 4 and 5,
respectively. Variations were less pronounced
toward the open sea and at stations 1, 2, and 3,
where salinity never decreased below 17&.
During high tide, the salinity remained largely
constant for several hours, reaching 35& at all
stations. Crabs were rarely caught at salinities
below 15&, and at all stations, the highest
CPUE values occurred at maximum salinity.
Oxygen concentrations varied from 5.3 mg
L–1 to 8.2 mg L–1 with a mean of 7.3 mg L–1
during neap tide and 6.5 mg L–1 during spring
tide (P , 0.001). Even though oxygen tensions
fluctuated substantially within the estuary,
significant effects on CPUE were not observed
at any station. Temperatures ranged between
13.98C and 19.78C, and pH varied from 7.6 to
8.5, having no significant effect on variations in
CPUE at any station.
DISCUSSION
That energy-demanding behavioural traits almost always have positive effects on Darwinian
fitness, either directly on reproductive success
or indirectly through, e.g., faster growth is
widely accepted. The intertidal segment of the
Looe Estuary is an unsuitable habitat for
residence, moulting, and egg development of
shore crabs. The very high CPUE values
observed during high tide, which, given the
catch technique used, are indicative of actively
foraging crabs, therefore indicate that the
estuary must be an important feeding ground
for shore crabs. It is theoretically possible, yet
not very plausible, that the CPUE values
obtained are heavily biased and represent
neither true abundance nor sex and colour
composition of the crab populations. That is,
a large population of red female crabs would
also migrate but not be caught, or alternatively,
that high numbers of adult shore crabs remain
within the estuary during low tide but suspend
foraging. The CPUE could potentially be
influenced by the varying strength of tidal
currents, resulting in varying dispersal of scent
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JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 24, NO. 1, 2004
Table 3. Arithmetical averages of CPUE values for all longitudinal or transverse stations during high tide (HT) and low tide
(LT) for neap tidal and spring tidal periods. High tide is defined as three hours before and after maximum high tide, and low
tide is defined as three hours before and after minimum low tide. G#: green males; G$: green females; R#: red males; R$:
red females: Juv: juveniles; ns: not significantly different.
Neap tide
Spring tide
LT
P
1–5
G#
G$
R#
R$
Adults
Juv
2.8
5.6
0.7
1.8
10.9
1.1
0.6
2.6
0.4
0.8
4.4
1.2
,0.001
,0.05
ns
,0.05
,0.01
ns
3.2
4.7
1.8
1.8
11.4
1.1
0.6
2.1
0.1
0.5
3.2
1.0
,0.001
,0.001
,0.001
,0.001
,0.001
ns
4a–4d
G#
G$
R#
R$
Adults
Juv
5.5
6.7
1.3
1.5
15.4
1.3
2.6
3.6
0.6
0.7
7.8
2.4
,0.001
,0.05
,0.001
,0.001
,0.001
ns
6.8
7.5
1.9
3.0
19.2
2.4
1.8
3.2
0.3
0.5
5.9
3.1
,0.001
,0.001
,0.001
,0.001
,0.001
ns
Stations
HT
LT
P
HT
from the baited drop nets, yet extensive
variations in CPUE values were observed, and
we therefore believe that the CPUE values
obtained reflect extensive tidal migrations.
By migrating into the estuary during high tide
and leaving before low tide, shore crabs increase
their available food resources while limiting
environmental stress (Reid and Aldrich, 1989;
Kaiser et al., 1990; Reid et al., 1993). The
estuary is essentially a small freshwater stream
during low tide. Consequently, the large number
of crabs present during high tide should require
many crabs to reside above the water surface
during low tide if they remained within the
estuary and avoid being exposed to low salinity,
which would compromise their physiology. The
salinity was observed to be as low as 3.5& at
station 5 during low spring tide. At such low
salinity, the ability of shore crabs to maintain
haemolymph homeostasis and osmoregulation
would be severely challenged (Reid et al.,
1989).
Because shore crabs hide under stones and in
crevices and dig into the sediment, an intensive
search was conducted at low tide and a total of
seven adult crabs were recovered. Hence, the
high CPUE during high tide and the low CPUE
during low tide are likely to reflect extensive
migrations, and hence extensive changes in
shore crab abundance. This is further supported
by the high catch of shore crabs at station 1 at
the beginning of low tide, indicating that the
crabs leave the estuary during low tide. In the
present study, most shore crabs were caught at
the deeper stations in the middle part of the
estuary (station 3) and the fewest were caught at
the innermost station 5. It is possible that there
is a limit as to how far into the estuary shore
crabs can migrate if they are to return to the
open sea before low tide.
Crustaceans are ecologically very important
as they often constitute the basis of estuarine
food webs. This is illustrated by the great
number of crabs caught in the present experiment. In previous studies, male shore crabs have
been demonstrated to migrate more extensively
and into more shallow waters than female shore
crabs (McGaw and Naylor, 1992b; Hunter and
Naylor, 1993; Aagaard et al., 1995). Several
explanations for this behavioural difference
between male and female shore crabs have been
offered. One is that females may be less able to
undergo regular migrations due to the diversion
of energy into egg production. Also, females are
generally smaller than males and may therefore
have lower capacities for migrating great
Table 4. Arithmetical averages of CPUE values for all
longitudinal and transverse stations during high tide day
time (HTd), high tide night time (HTn), low tide day time
(LTd), and low tide night time (LTn) for all groups of crabs.
G#: green males; G$: green females; R#: red males; R$: red
females; Juv: juveniles; ns: not significantly different.
G#
G$
R#
R$
Adults
Juv
HTd
HTn
P
LTd
LTn
P
4.0
4.5
1.4
1.8
11.8
1.7
5.7
8.4
1.5
2.7
18.2
1.4
,0.01
,0.001
ns
ns
,0.001
ns
1.6
2.7
0.4
0.7
5.5
1.6
1.3
3.2
0.3
0.5
5.4
0.8
ns
ns
ns
ns
ns
ns
REWITZ ET AL.: DISTRIBUTION OF CARCINUS MAENAS IN A TIDAL ESTUARY
distances before the tide turns. Furthermore,
females have smaller and weaker chelae than
males and may therefore be less able to exploit
the intertidal zone (Warner and Jones, 1976;
Elner, 1980). Another suggestion is that
females, in general, may be less physiologically
tolerant than males (Reid et al., 1997). In the
present study, however, more females were
observed to migrate into the estuary than males.
At station 4a, which is located approximately
750 meters within the estuary, approximately
two females were caught for every male. This
indicates that females can migrate like males at
least in small estuaries, and are able to exploit
the intertidal zone extensively during foraging.
Female shore crabs were, however, far more
abundant at deep stations, whereas males
dominated the shallow stations. At 108C,
salinities above 26& are needed for normal
egg development (Crothers, 1967). Females
must therefore avoid shallow waters with low
salinity to ensure egg development. Due to the
very limited freshwater input, the Looe Estuary
is generally more saline than other estuaries in
Southwest England (Neil, 1994). This, in
combination with the small size of the estuary,
may allow a greater number of females to
undertake more extensive migrations into the
estuary without compromising egg development
than has been reported for other estuaries in the
area (McGaw and Naylor, 1992b).
In this study, the distribution of juveniles
among catches was clearly different from that of
adults, because juveniles were relatively more
abundant in catches from the shallow stations.
This is consistent with observation by Hines
et al. (1995) demonstrating that juvenile blue
crabs, Callinectes sapidus, move around in
nearshore areas within an estuary, whereas
adults move freely in and out of the estuary
but remain in deeper areas. Further, juvenile
crabs caught at shallower stations were significantly smaller than at deeper stations. By
staying in these shallow areas with comparatively few adult crabs, juvenile crabs are able to
exploit the shallowest areas while avoiding
intraspecific competition from larger crabs
(Reid et al., 1997). If juveniles, because of
their size, can hide from avian predators even
during low tide, there is little need for investing
energy on tidal or diel migration. This may
explain why the overall catch of juvenile crabs
was not significantly affected by tidal and diel
cycles. The juveniles were caught throughout
the tidal phase, indicating that many juveniles
185
remained within the estuary during low tide.
Juvenile shore crabs may reside permanently
within the estuarine environment, which then
act as a nursing ground similar to that observed
for other crustaceans, including the blue crab,
C. sapidus (Kneib and Knowlton, 1995; Dı́az
et al., 2003). However, juvenile shore crabs
were not caught at stations 4 and 5 at minimum
low tide, most likely because of the very low
salinity.
The high ratio of red to green shore crabs in
catches in the inner part of the estuary during
high tide most likely reflects that large numbers
of red crabs migrate into the estuary during high
tide. This is in contrast to previous studies in
which red shore crabs were observed to be
restricted to subtidal areas (McGaw and Naylor,
1992b; Hunter and Naylor, 1993; Reid et al.,
1997). Unlike green crabs, however, red crabs
were never caught at the inner stations during
low tide. The reason for this may be that red
crabs are more sensitive to low salinities than
are green crabs (McGaw and Naylor, 1992a).
The high salinity during high tide and the
relatively short distance between the upper part
of the estuary and the open sea may allow red
shore crabs to migrate into the estuary and
return to the open sea before low tide. Red crabs
are generally larger and more visible than green
crabs and therefore likely to be more vulnerable
to avian predation, which is considered an
important factor controlling population densities
of shore crabs (Dumas and Witman, 1993). The
tidal migration of all groups of adult shore
crabs, indicated by the tidal variations in catch
patterns, would reduce predation from birds, for
deep water provides protection from avian
predation. The foraging on shore crabs by gulls
and large wading birds is based upon vision and
therefore takes place during day time. Green
shore crabs, the morph most frequently caught
at shallow stations, were caught in greater
numbers during night time than during day
time. This diel variation in CPUE indicates that
shore crabs are well adapted to avoid avian
predators as salinity and depth are unaffected by
diel cycles.
Salinity influenced the CPUE values for shore
crabs in the Looe Estuary. Salinity is known to
entrain circatidal rhythms in shore crabs (Bolt
and Naylor, 1985). However, field studies have
demonstrated that shore crabs express clear tidal
rhythms also in habitats where changes in
salinity are minor (Warman et al., 1993;
Styrishave et al., 1999). Hydrostatic pressure
186
JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 24, NO. 1, 2004
is also known to entrain circatidal rhythms in
shore crabs by increasing activity during periods
of high hydrostatic pressure (Atkinson and
Parsons, 1973). However, because of the limited
freshwater input, salinity is tightly coupled to
depth, and in the present study, CPUE clearly
increased during periods with increasing hydrostatic pressure.
The oxygen tension varied between stations
and samplings but appeared not to influence the
distribution of shore crabs in the estuary. The
reason for this may be that oxygen tensions
never decreased to levels that stressed the
general physiology of shore crabs. Laboratory
experiments demonstrate that escape behaviour
is initiated and aerobic metabolism is affected
only at oxygen tensions below approximately
4–5 mg L1 (Taylor, 1976; Reid and Aldrich,
1989). Such low oxygen tensions were never
observed in the present study. Consequently,
only depth and salinity appeared to affect the
distribution of shore crabs in the estuary.
In conclusion, the present study suggests that
adult shore crabs, independently of size, sex,
and colour, undertake extensive migrations into
the estuary during high tide, leaving again
before low tide, probably to avoid osmotic
stress, risk of avian predation, and desiccation
during low tide. In addition, green shore crabs,
which were frequently caught at shallow
stations, exhibit a diel pattern of migration.
More female shore crabs where caught than
male shore crabs, indicating that in this estuary,
females migrate at least to the same extent as
males. This is at variance with previous reports
from other estuaries, indicating that female
shore crabs do not migrate as extensively as
male shore crabs. Even though both red and
green shore crabs migrate into the estuary, red
crabs were never caught during low tide. The
results corroborate and extend earlier studies
and support the hypothesis that shore crabs do
exploit even marginal habitats/econiches/ecosystems.
ACKNOWLEDGEMENTS
We are grateful to the Caradon District Council for access to
the study site, and to Dr. Shaw Bamber, Plymouth
Environment Research Centre, University of Plymouth, for
help and collaboration. This work was supported by grants
from the Danish Natural Sciences Research Council to B.S.
and O.A.
LITERATURE CITED
Aagaard, A., C. G. Warman, and M. H. Depledge. 1995.
Tidal and seasonal changes in the temporal and spatial
distribution of foraging Carcinus maenas in the weakly
tidal littoral zone of Kerteminde Fjord, Denmark.—
Marine Ecology: Progress Series 122: 165–172.
Atkinson, R. J. A., and A. J. Parsons. 1973. Seasonal
patterns of migration and locomotor rhythmicity in
Carcinus maenas.—Netherland Journal of Sea Research
7: 81–93.
Bolt, S. R. L., and E. Naylor. 1985. Interaction of
endogenous and exogenous factors controlling locomotor
activity rhythms in Carcinus exposed to tidal salinity
cycles.—Journal of Experimental Marine Biology and
Ecology 85: 47–56.
Crothers, J. H. 1967. The biology of the shore crab Carcinus
maenas (L.). 1. The background–anatomy, growth and
life history.—Field Studies 2: 407–434.
———. 1968. The biology of the shore crab Carcinus
maenas (L.). 2. The life of the adult crab.—Field Studies
2: 579–614.
Dı́az, H., B. Orihuela, R. B. Forward, and D. Rittschof.
2003. Orientation of juvenile blue crabs, Callinectes
sapidus Rathbun, to currents, chemicals, and visual
cues.—Journal of Crustacean Biology 23: 15–22.
Dumas, J. V., and J. D. Witman. 1993. Predation by herring
gulls (Larus argentatus Coues) on two rocky intertidal
crab species [Carcinus maenas (L.) and Cancer irroratus
Say].—Journal of Experimental Marine Biology and
Ecology 169: 89–101.
Elner, R. W. 1980. The influence of temperature, sex and
chela size in the foraging strategy of the shore crab,
Carcinus maenas (L.).—Marine Behaviour and Physiology 7: 15–24.
Hines, A. H., T. G. Wolcott, E. González-Gurriarán, J. L.
González-Escalante, and J. Freire. 1995. Movement
patterns and migrations in crabs—telemetry of juvenile
and adult behavior in Callinectes sapidus and Maja
squinado.—Journal of the Marine Biological Association
of the United Kingdom 75: 27–42.
Hunter, E., and E. Naylor. 1993. Intertidal migration by the
shore crab Carcinus maenas.—Marine Biology: Progress
Series 101: 131–138.
Kaiser, M. J., R. N. Hughes, and D. G. Reid. 1990. Chelal
morphometry, prey size selection and aggressive competition in green and red forms of Carcinus maenas (L.).—
Journal of Experimental Marine Biology and Ecology
140: 121–134.
Kneib, R. T., and M. K. Knowlton. 1995. Stage-structured
interactions between seasonal and permanent residents of
an estuarine nekton community.—Oecologia 103: 425–
434.
McGaw, I. J., M. J. Kaiser, E. Nalyor, and R. N. Hughes.
1992. Intraspecific morphological variation related to the
moult-cycle in colour forms of the shore crab Carcinus
maenas.—Journal of Zoology London 228: 351–359.
———, and E. Naylor. 1992a. Salinity preference of the
shore crab Carcinus maenas in relation to coloration
during intermoult and to prior acclimation.—Journal of
Experimental Marine Biology and Ecology 155: 145–159.
———, and ———. 1992b. Distribution and rhythmic
locomotor patterns of estuarine and open-shore populations of Carcinus maenas.—Journal of the Marine
Biological Association of the United Kingdom 72: 599–
609.
Neil, J. 1994. Survey of the Benthic Macroinvertebrate
Infauna of the Looe Estuary. Cornish Biological Records
Units, Institute of Cornish Studies, Redruth, Cornwall,
England.
REWITZ ET AL.: DISTRIBUTION OF CARCINUS MAENAS IN A TIDAL ESTUARY
Reid, D. G., P. Abelló, M. J. Kaiser, and C. G. Warman.
1997. Carapace colour, inter-moult duration and the
behavioural and physiological ecology of the shore crab
Carcinus maenas.—Estuarine Coastal and Shelf Science
44: 203–211.
———, ———, I. J. McGaw, and E. Nalyor. 1989.
Differential tolerance of desiccation and hypoosmotic
stress in sub- and inter-tidal Carcinus maenas. Pp. 89–96
in J. C. Aldrich, ed. Phenotypic Response in Aquatic
Ectotherms. Japaga, Dublin.
———, ———, C. G. Warman, and E. Naylor. 1994. Sizerelated mating success in the shore crab Carcinus maenas
(Crustacea: Brachyura).—Journal of Zoology London
232: 397–407.
———, and J. C. Aldrich. 1989. Variations in response to
environmental hypoxia of different colour forms of the
shore crab Carcinus maenas.—Comparative Biochemistry and Physiology 92A: 535–539.
———, C. G. Warman, and E. Nalyor. 1993. Ontogenetic
changes in zeitgeber action in the tidally rhythmic
behaviour of the shore crab Carcinus maenas. Pp. 129–
187
133 in J. C. Aldrich, ed. Proceeding of the 27th European
Marine Biology Symposium, 1992. Japaga, Dublin.
Styrishave, B., A. Aagaard, and O. Andersen. 1999. In situ
studies on physiology and behaviour in two colour forms
of the shore crab Carcinus maenas in relation to
season.—Marine Ecology: Progress Series 189: 221–231.
Taylor, A. C. 1976. The respiratory responses of Carcinus
maenas to declining oxygen tension.—Journal of Experimental Biology 65: 309–322.
Warman, C. G., D. G. Reid, and E. Naylor. 1993. Variation
in the tidal migratory behaviour and rhythmic lightresponsiveness in the shore crab Carcinus maenas.—
Journal of the Marine Biological Association of the
United Kingdom 73: 355–364.
Warner, G. F., and A. R. Jones. 1976. Leverage and muscle
type in crab chelae (Crustacea, Brachyura).—Journal of
Zoology London 180: 57–68.
RECEIVED: 2 April 2003.
ACCEPTED: 17 July 2003.