Size, sex and quantity of Scylla serrata and Portunus pelagicus on

Size, sex and quantity of Scylla serrata and
Portunus pelagicus on Inhaca Island, Mozambique
2007-2008
.
Johan Florentzson
Degree project for Master of Science (One Year) in
Biology
60 hec
Department of Marine Ecology
University of Gothenburg
Contribution number
530
0
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
Abstract
Inhaca Island, located in Southern Mozambique, is a relatively unpolluted site and is often used as reference
point in scientific investigations, as well as extensive research of the island itself because of its undisturbed
status. This paper focuses on questions concerning two edible and commercially important crabs of the family
Portunidae, with the main incentive being an estimation of their current status (population) and health
(physiology). The investigations were performed in a creek surrounded by mangroves and mudflats, called Saco
da Inhaca, where crabs were fished using baited cages. Results revealed larger amount of Scylla serrata being
caught during the warm season compared to the cold. When looking at distribution over different substrates S.
serrata showed no preferences, but Portunus pelagicus seemed to prefer substrates that did not dry out
completely during low tides. The sexual distribution showed a slight dominance of males. Analysis of proteins in
the blood indicated better health of crabs caught on Inhaca compared to Maputo, and plausible reasons for is
discussed. P. pelagicus showed a faster degradation of haemolymph protein when starved, compared to S.
serrata. A small literature study concerning aquaculture of these crabs is included. In conclusion a number of
differences were detected both between sites, seasons and species but more extensive research is suggested
should any commercial harvest of crabs begin and/or if aquaculture is possible on Inhaca.
Introduction
The purpose of this study was to investigate the
amount and condition of two large portunid crabs,
the Mud crab Scylla serrata (Forskål, 1755) and the
Blue crab Portunus pelagicus (L. 1766) from two
sites in southern Mozambique: Inhaca Island and
Bairro dos Pescadores. These two species are
commercially important in many parts of the Indowest pacific (e.g. Potter et al., 1983, Williams and
Hill, 1982) and are subjected to increased
exploitation in Mozambique (Macia et al. unpubl.).
Scylla serrata is a common species all over the
Indo-west pacific (Williams & Hill, 1982;
Demopoulos, 2007), and the genus itself contains
four species as determined by Keenan et al. (1998).
The average internal carapace width (ICW) of S.
serrata is previously reported to be 138 mm, with a
maximum of 192 mm (Keenan et al., 1998). S.
serrata is commonly associated with estuarine
mangrove forests, but is also the most dominating
of the Scylla species when salinities are above 34
i.e. more oceanic environments (Keenan et al.
1998). It is considered a nocturnal animal, and
remains buried during the day, emerging at dusk,
feeds during the night (provided the tide is in) and
burrows again come dawn (Hill, 1976). According
to investigations of gut-content (Hill 1976) over 50
percent contained mollusks and contents of fish
was rarely found. This indicates that fish is not a
natural diet for S. serrata but given the common
usage of fish as bait for crabs one can suggest this
to be a preferable food source to the crabs and
therefore it attracts crabs in a satisfactory way.
Use of fish could be considered preferable
compared to their natural diet since this prevents
surrounding food-sources to compete with what
the cages have to offer (Fielder (1965).
Portunus pelagicus (Portunidae) carries an
important role in the fishing industry at many
locations (Smith (1982) as cited by Potter et al.
(1983)). The average carapace width (CW) for P.
pelagicus was found to be 128 mm with a
maximum size of 164 mm (Krishna and
Balakrishnan, 1971). The species is often
considered a benthic carnivore and eats mainly
sessile mollusks and other invertebrates (Potter
and Lestang, 2000; Hill 1976), but has also been
known to prey on gobies (Potter and Lestang,
2000). An investigation performed in Australia
showed that commercial fishing of P. pelagicus is
seasonal and the largest catches are landed
between January and April (Potter et al., 1983),
which represents the summer season months. This
was concluded to depend on the decreasing
salinities which drove the crabs to migrate. P.
pelagicus prefers salinities between 30 and 40 ppt
(Potter et al. 1983) with the lower critical survival
limit of juveniles being 5 ppt. Higher salinities
resulted in faster growth rates with an optimum
growth between 30 and 40 ppt; however when
salinity above 40 ppt was tested decreases in
growth was detected (Romano and Zeng, 2006).
1
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
Inhaca Island, my primary study site, lies in
0
0
southern Mozambique (25 56´40’’-26 04´57’’ S,
0
0
32 54´36’’-32 57´33’’ E) approximately 30 km
offshore from the capital Maputo. It is a small
Island with a population estimate of roughly 5000
(Olavo pers. com.). The island is 14 km long and 7
km wide (map, appendix I) and with a marine
research station located on the west side. The
second site outside of Maputo consist of large
mangrove forests along the shore i.e. they have
direct contact with the ocean. S. serrata and P.
pelagicus are both harvested in these two study
areas and most of the collected animals are used
as food locally (de Boer and Longamane, 1996).
Since S. serrata is a highly attractive food-source all
over the Indo-west Pacific, the populations are
feared to be put under dangerous exploitation
pressure in places were regulations of the fisheries
does not exist (Robertson and Kruger, 1994, Vay,
2001). S. serrata has been shown to move and feed
less as temperature drops below 20 °C, resulting in
decreased catches during the cold season (Hill
1980), and similar variations have been shown in
Australia (Hill 1982). However, there might be
other factors than temperature influencing the
abundance of S. serrata, one of which is suggested
to be the migration of berried females offshore in
times of spawning (Hill 1975 & 1994). The offshore
spawning was suggested to increase dispersal of
larvae (Hill 1994). Temperature was shown to be a
critical factor for growth and survival of juvenile S.
serrata (Ruscoe et al., 2004). However in their
study they refer to previous experiments showing
salinities to have effect on juvenile survival, so the
area is still debated of sorts.
Genetic stability and mixing of the gene-pool have
previously been investigated at several sites along
the South African coast, also one site in southern
Mozambique and one on Madagascar. The results
showed that populations from estuaries close to
each other share a large proportion of their genes.
This is likely to be because of the offshore release
of eggs by gravid females (Davis et al., 2003), which
strengthens the argument concerning offshore
spawning since mixing of larvae would provide
different estuaries with genetic material from
other nearby estuaries.
Hyland et al. (1984) argued that S. serrata with
access to mud flats and mangroves don’t migrate
since they have everything they need in their
habitat. It has also been suggested that S .serrata
moves onto mudflats with the high tides in order
to forage (Hill et al. 1982). It is, however, not
known whether they have any substrate
preferences. Concerning the habitats themselves
the harvesting of large predators have shown to
lead to an increase in the number of opportunistic
species, which in turn might cause changes in the
compositions of the habitats i.e. heavy harvesting
of S. serrata and P. pelagicus might change the
habitat in which these are found (de Boer and
Prins, 2002).
Aquaculture is commonly used to enhance the
market value of crabs sold in different ways.
Different aquaculture methods for Mud crabs are
being used around the world, and a study from
South Africa shows good possibilities to keep and
breed S. serrata in captivity (Davis, 2004).
Currently no aqua culturing of S. serrata (only
existing Scylla species in Mozambique) exists in
southern Mozambique. Aqua culturing, however, is
still dependant on wild caught crabs which are
used mainly in three different ways and only one of
these were caught crabs are used for egg rendering
decreases the stress on the natural population. The
different ways of using crabs in aquaculture are
fattening, soft shell timing and as breeding stock.
Fattening is when undersized crabs are caught and
kept in cages or pens until they reach the market
demands for size and weight. Secondly, for the soft
shelled market crabs are caught and kept until they
molt at which point they are preserved and sold
before their shells hardens (Shelley, 2008). Last is
as breeding stock, egg producers providing starting
batches for cultivation of juvenile crabs. One goal
of aqua culturing is to lessen the stress induced by
harvesting of the existing population and since
humans increase in number the harvesting is
bound to increase which is why aqua culturing is
thought to help secure the continued existence of
natural crab population.
The primary objective of this study was to
investigate whether there are any seasonal
variations in catches and whether the crabs show
2
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
any
preference
towards
certain
habitats/substrates. Physiological studies were also
included into the investigations and crabs were
collected from a second site outside Maputo called
Bairro dos Pescadores.
In order to return to the purpose of the work
performed here additional physiological studies
were performed during winter season. Blood was
sampled and analyzed for total content of protein,
in order to investigate the general health of
individuals. During well fed conditions crabs build
up protein and store as reserves that can be used
later if needed (Subhashini and Ravindranath,
1982). Specimens for this analysis were sampled at
Inhaca Island and Bairro do Pescadores outside of
Maputo, to see if health was site related. The
haemolymph system is complex and regulated by
many factors, for e.g. the molting cycle. when the
haemolymph is diluted, as water is accumulated in
the crabs e.g. after molting (Chen & Chia, 1997). All
crabs investigated where hard-shelled and actively
foraging, and thus assumed to be in intermoult
stage. A small experiment, following the basic
principles from the study by Subhashini and
Ravindranath (1982), was set up with the attempt
to enlighten possible differences in blood protein
degradations rates between S. serrata and P.
pelagicus. The aim of this experiment was to clarify
whether the two species investigated differed in
adaptation to starvation and capture (i.e.
immediate and more long-term response to
human handling and lack of food). Reason for this
experiment was based mainly on morphological
differences between S. serrata and P. pelagicus,
and since the latter is more shaped to move in the
open water to collect food and has the ability to
move to better areas should this become scarce.
More active species are likely to have higher
metabolic rates and therefore require more
energy. It was therefore believed that they would
be less adaptable to starvation.
Hypotheses
1. There are seasonal effects on crab
abundance due to different climate
conditions such
as
rainfall
and
temperature
fluctuations
following
different seasons.
2. There is a difference in blood protein
content in crabs on Inhaca Island
compared to those from Maputo.
3. Scylla serrata and Portunus pelagicus
shows habitat preference because the
species prefers a certain substrate to
reside on and use as feeding grounds.
4. There is a difference in blood protein
degradation rate between S. serrata and
P. pelagicus because the species have
different physiological adaptations to
their environment.
Method
Field collection
Scylla serrata and Portunus pelagicus were caught
on Inhaca Island during two periods, November December 2007 and April – June 2008, correlating
to the warm and cold season (average water
temperatures: 27,4°C & 21,9°C) and hereby
referred to as summer and winter seasons
respectively, using cages locally produced in
Maputo. The cages were built as rectangular
parallelepipeds with two entrances, one at each
short side. Nine cages were used during the
summer season and twelve during winter season.
The cages varied somewhat in construction
between seasons due to different manufacturers
(details: appendix II). Cages were placed randomly
on three different substrates (mangrove, mud and
river) in the Saco bay area on Inhaca Island
0
0
(26 01’56’’S, 32 54’53’’, details: appendix I) and
were baited with small fish bought locally on the
island. Efforts were made to keep the size of bait
similar and when possible also stick to the same
species of fish (Terapon jarbua were commonly
used). The cages were left to fish for one high tide
(or on occasion two due to inaccessibility caused
by weather). The catches were collected, cages rebaited and randomly placed on their substrates.
Crabs from Bairro dos Pescadores near Maputo
were bought due to inability to fish in this area,
partly from a local fisherman and partly at the fish
3
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
market. No P. pelagicus were investigated from
this area since they were sold dead and would not
have given any reliable blood samples.
Crabs were measured in two ways over the dorsal
carapace using slide calipers with mm accuracy,
CW (Carapace Width) & ICW (Internal Carapace
Width) as described by Keenan et al. (1998) (se
appendix III).
The measurement ICW was determined to be most
trustworthy for S. serrata since a number of
specimens had lost one or two of their ninth
anterolateral spines which made the CW measure
less reliable.
P. pelagicus have centimeter long spines
(compared to millimeters for S. serrata) and none
of the animals in this study had broken spines.
Since CW is commonly used in other studies as well
as in the fishing industries it was determined to use
the CW measurements for the P. pelagicus. Sex
was determined by observing abdomen width,
which differs between females and males (se
appendix III).
Crab haemolymph was collected in the field by
inserting a 1 ml syringe into the joint of the most
posterior leg. The blood was transferred from the
syringe to eppendorph vials containing an
1
anticoagulant buffering solution . Syringes were
also flushed with the buffer prior to sampling in
order to prevent coagulation in needle or syringe.
Initially 0,5 ml of blood was mixed with equal
amount of anticoagulant, however this proved
insufficient, since coagula formed in many samples,
and the proportions were changed to 0,3 ml blood
with 0,6 ml anticoagulant. The samples were
immediately put on ice to keep them cold during
transport, which took approximately 45 minutes.
Upon arrival at the laboratory samples were frozen
at -18 °C.
Some of the crabs caught were tagged with cable
ties around one of the legs to get an estimate on
recapture rates. These crabs were only sampled for
blood at the first capture. Water temperature in
the river was measured during sampling using Oxi
330/SET probe equipment.
1
Anticoagulant: 0,45M NaCL, 0,1M Glucose, 30mM
Tri Na Citrate *2 H2O, 26mM Citric acid, 2ml 0,5M
EDTA/100ml solution.
Adaptation/starvation experiment
A total number of 23 crabs were brought back to
the laboratory on Inhaca (9 S. serrata & 14 P.
pelagicus) and placed in an aquarium with running
seawater. Salinity and temperature varied since
the water was pumped from the ocean into a large
elevated tank from which altitude pressure made
the water flow. The crabs were starved for eight
days. Blood was sampled from each crab at the
following intervals; field, arrival at the station
followed by interval testing on hours 2, 6, 12, 24,
48, 96 (4 days) and 184 (8 days). The blood
samples were frozen immediately after extraction.
Analysis of blood
The blood samples collected on Inhaca Island were
analyzed at the Eduardo Mondlane University in
Maputo. Haemolymph analysis were performed
using a revised version of the Coomassie Blue
method (Spector 1978, modified by Hernroth) In
case of coagula formation in the blood sample the
liquid to be investigated was taken with care to
avoid this. Samples were diluted with distilled
water to fit within the standard curve. The
Coomassie reagent was mixed with the diluted
blood samples in the proportion 50µl sample to
950µl reagent, after five minutes the samples were
placed in a spectrophotometer (JENWAY
6105UV/Vis Spectrophotometer)) and read at
λ595nm. To get the absorbance of the reagent a
blank of 50µl distilled water and 950µl reagent was
also
measured.
Results
from
the
spectrophotometric analysis were compared to
that of a standard curve constructed from a
dilution of BSA (Bovine Serum Albumin- base
solution 500mg BSA/50ml sterile water) which was
corrected to a pH level of 4,6. Coagulation in
samples was treated with a sonic homogenizer
with the attempt to dissolve clots, however this
proved rather insufficient.
Statistics
Statistical comparisons were performed using
single factor ANOVA for comparing differences in
haemolymph and also degradation of these. Multifactorial ANOVA (MANOVA) was used to compare
catches over tides, seasons and different
substrates. Histograms were used to compare
classes of animals and to draw result figures.
4
No. Scylla serrata
Results
Catch and size
18
Summer 2007 Spring tide
16
Summer 2007 Neap tide
14
Winter 2008 Spring tide
12
Winter 2008 Neap tide
10
8
6
4
2
0
5
7
9
11
13
15
Size-class ICW (cm)
Figure 1. Histogram based data showing distibution of Scylla serrata between different size-classes (5-7cm, 7-9cm, 9-11cm, 11-13cm,
13-15cm internal carapace width (ICW)) on Inhaca Island , Mozambique, for different seasons and tides.
Fig. 1 compares total number of Scylla serrata
caught in different size-classes caught during
spring/neap tide and summer/winter season. More
crabs were caught during spring tides than neap
tides of the summer season (p=0,003). Differences
in size showed borderline significance between
spring and neap-tides during the summer season,
and between spring tides for the two seasons
(p=0,057 & p=0,058 respectively). Average size
(and Standard Deviation (SD)) of S. serrata was
99,4 (±20) mm and the largest individual measured
was 148 mm (ICW). Average catch, not comparing
-1
tides or substrates, was 1,5 crabs day for the
-1
summer season and 0,93 crabs day for the winter
season, this difference was not significant
(p=0,064).
No. Portunus pelagicus
12
10
8
6
4
2
0
5
7
9
11
13
15
Size-class CW (cm)
Figure 2. Histogram data showing distribution of Portunus pelagicus between different size classes (5-7cm, 7-9cm, 9-11cm, 11-13cm,
13-15cm caparapace width (CW)) on Inhaca Island , Mozambique for different seasons and tides. Note the reverse order of series
compared to those of S. serrata, this due to eaiser visibillity .
As can be seen in fig. 2 there are no large
differences between total catches of Portunus
pelagicus during the four different periods. A slight
peak in catch can be seen for spring tide of the
summer season, when larger crabs appear to be
more frequent and the opposite is noted for spring
tide during winter season. Average size (±SD) of P.
pelagicus was 113 ±16 mm, with a maximum size
of 144 mm (CW). Average catch, without
-1
comparing substrates or tides, was 0,73 crabs day
-1
for the summer season and 1,76 crabs day for the
winter season (p=0,045).
5
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
Substrates
1
Catch day-1 Scylla serrata
Mangrove
0,8
Mud
River
0,6
0,4
0,2
Catch day-1 Portunus pealgicus
1
Mangrove
0,8
Mud
River
0,6
0,4
0,2
0
0
Summer
Summer
Winter
Figure 3 a). Comparisons of catches (no. crabs/day) between
different substrates for Scylla serrata on Inhaca Island ,
Mozambique, during summer (2007) and winter (2008). Error
bars represented by standarad deviation.
In fig. 3 the average catch per day of S. serrata (a.)
and P. pelagicus (b.) caught on different substrates
(Mangrove, Mud & River) is shown, comparing the
two investigated seasons. Analysis of S. serrata
reveals no significant differences between
substrates or season, high variances hid possible
differences. P. pelagicus differs between all
substrates comparing seasons, mud &river showed
p=0,000 and mangroves p= 0,031.
Winter
Figure 3 b). Comparisons of catches (no. crabs) between
different substrates for Portunus pelagicus on Inhaca Island,
Mozambique, during summer (2007) and winter (2008). Error
bars represented by standard deviation.
Ten previously caught S. serrata were re-caught
during this investigation of which seven had moved
onto another substrate compared to where they
were first caught, for P. pelagicus four were caught
again of which two had moved onto other
substrates. No statistics were performed on these
data.
Sex
100%
% of sex
18
5
4
5
6
7
6
7
female
male
50%
25
12
8
9
17
12
4
8
0%
S:ST
S:NT
W:ST
Scylla serrata
W:NT
S:ST
S:NT
W:ST
Portunus pelagicus
W:NT
Figure 4. Distribution between sexes for catches of Scylla serrata and Portunus pelagicus on Inhaca Island, Mozambique.
S:ST- Summer Spring Tide, S:NT- Summer Neap Tide, W:ST- Winter Spring Tide, W:NT- Winter Neap Tide.
As shown in fig. 4 a majority of males were caught
on all occasions apart from one for P. pelagicus
during the summer season neap tides. There were
no significant differences in sex distribution for the
two species. Similar results were shown in studies
where only slightly higher number of male P.
pelagicus were caught (Potter and Lestang 2000)
and equal amount male and female S. serrata (Hill
et al. 1982).
6
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
Blood
mg protein per ml haemolymph
160
In fig. 5 the amount of blood proteins were
compared between areas and species. These data
were based on samples with no (or small amounts
of) coagulum. No statistical difference was
detected between the three groups (p=1)
Scylla
serrata
140
120
Portunus
pelagicus
100
80
60
n=10
40
20
n=12
n=27
0
Maputo
Site
Figure 5. Differences in Blood protein concentration (mean
mg/ml ±SD) of Scylla serrata between Maputo and Inhaca Island,
and between Scylla serrata and Portunus pelagicus on Inhaca
Island, Mozambique.
Blood protein concetration
(mg/ml)
Inhaca Island
200
180
160
140
120
100
80
60
40
20
0
y = 0.28x + 116
a)
0
160
b)
R² = 0,0003
2
4
c)
6
0
6
0
d)
50
100
150
200
50
100
150
200
y = -5.12x + 110
140
R² = 0.23
120
100
80
60
40
20
0
0
2
4
Hours after capture
Figure 6. Degradation of blood protein for Scylla serrata (a & b) and Portunus pelagicus (c & d) held in captivity without food over
a maximum period of eight days at Inhaca research station. Blood samples were taken at predetermined intervals (X-axis) to
compare the haemolymph concentration. Figures a) & c) describes what occurred during the initial six hours of the experiment
and figures b) & d) shows the results of the whole experiment.
Fig. 6 show the degradation of haemolymph
proteins for S. serrata (a & b) and P. pelagicus (c &
d). S. serrata show a slower degradation rate of the
proteins, as can be seen by comparing trend line
slope. However this could only be seen as a trend
for the whole 180 hour captivity, but for the first
six hours the difference in degradation is
statistically significant (p=0,005).
7
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
Coagulum
% samples with different coaugla
ammount
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
None
Some
Half
Much
Very much
Fig. 7 describes the amount of coagula formed
from proteins in the samples. The samples from
Inhaca, that had been stored in a freezer for 1-5
weeks, contained a lot more coagula compared to
samples taken in Maputo, which were only frozen
and immediately defrosted to create a similar
treatment effects.
Unkown
Inhaca P.
Pelagicus
Inhaca S. Serrata
Maputo S.
Serrata
Site and species
Figure 7. amount of coagulum in Scylla serrata and Portunus
pelagicus, Inhaca island (29april-13june 2008) and for Scylla
serrata from Bairro dos Pescadores, Maputo (26/6 2008)
Temperature
Measurements of temperature showed a
difference (p=0) between seasons where the daytime water temperature in the summer season
averaged 27,4 ± 1,4 °C (n=28) and during winter
season an average of 21,9 ± 1,2 °C (n=15) was
measured.
8
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
Discussion
Local fishermen on Inhaca refer to spring tides as
“La mare vida”, an expression that translates to
“the living ocean”, and describes how maritime
harvest and fishing give higher yields during these
periods. This statement describes a part of the
current investigation as to whether Scylla serrata
or Portunus pelagicus were predominant during
any specific tide.
Scylla serrata – Mud crab
S. serrata was more abundant during the spring
tides compared to neap tide during the summer
season. Animals that were caught during spring
tides were also slightly larger than animals caught
at neap tides. In addition trends suggest larger
crabs being caught during the spring tides of the
summer season compared to spring tides in the
winter season. This appears to concord with an
earlier study from South Africa where lower
catches were found during their “winter season”
(Hill 1975). Given the proximity between
Mozambique and South Africa one could expect
seasons to be called the same, but the article does
not discern which months is considered winter or
summer seasons. However temperature were
lower in South Africa during both seasons, with
winter season temperatures ranging from 13 to
0
0
15 C and summer season between 20 and 25 C
(Hill 1975). The largest crabs, which were females,
were as previously mentioned caught during the
summer season, which according to Hill (1994)
should be the time for large females to migrate
offshore in order to spawn. However since no
gravid crabs were caught in this study they would
have little use of migrating offshore, since the main
reason for this would be better larval dispersal.
The size of the Scylla serrata in this study (mean
99.4 mm) was smaller compared to the mean of
138.4 mm found by Keenan et al (1998) when
samples from the whole Indo-West Pacific were
taken. No significant differences could be detected
between catches of S. serrata on different
substrates, however trends suggest that more
crabs were caught during the summer season.
Concerning the substrates no conclusions can be
made but other authors have suggested that S.
serrata move onto mudflats and mangroves in
order to feed (Hill et al., 1982). There are studies
showing S. serrata to be relatively sedentary, with
movement limited to 1 square kilometer, provided
they have sufficient living conditions (Demopoulos
et al., 2007) and the currently investigated area
was a little over one square kilometer. Also as
mentioned in the introduction crabs with access to
mud flats and mangroves, both of which are
present in the Saco, show no great movement
(Hyland et al. 1984). Population size estimates and
movement patterns are calculated on amount of
recaptured animals which was the original plan of
this investigation. However since there was a low
amount of recapture no conclusions could be
drawn from these data. When regarding the
continuous catch found in this study, it appears as
though the population of S. serrata in Saco bay is
stable with current harvest pressure. However, no
predictions can be made for what increased
exploitation pressure might lead to.
Haemolymph
Investigations
comparing
blood
protein
concentration of S. serrata populations on Inhaca
Island with those of Bairro dos Pescadores
provides no reliable results since variations were
too high. However one could argue that Inhaca
Island should be a healthier place for the crabs to
live due to its less polluted state. When looking at
water movements, Maputo Bay receives inland
water from three large rivers (Espírito Santo,
Maputo River and N´komati), emptying large
amounts unprocessed water into the bay.
Therefore Maputo Bay is thought to be more
polluted compared to Inhaca which receives large
parts of its surrounding waters directly from the
Indian Ocean (de Boer et al. 2000). Also Inhaca
Island is often used as a reference site in local
scientific investigations (Macia pers. comm.) which
might say something about its cleanliness.
Pollutants have been suggested to affect multiple
factors in crustaceans such as blood glucose and
hormone regulation (Fingerman et al. 1998), direct
effect on haemolymph by pollutants have been
reported by Watson et al. 2004 who detected
changes in the haemolymph of Carcinus maenas
after these were exposed to PAH's (Poly Aromatic
Hydrocarbons). However the results found for
9
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
haemolymph were not as great as from urinary
analysis after the same treatment and therefore
the latter was considered a more reliable
investigative parameter. Since no specific pollution
parameters were tested in this investigation and
also since no results differentiating Maputo from
Inhaca Island were detected little can be said
about the pollution in Maputo bay other than the
hypothetical problems mentioned above.
Portunus pelagicus – Blue crab
The abundance of P. pelagicus shows tendencies to
be smaller during the summer season (NS) which
might be explained by its preference for higher
salinities of 30-40 ppt (Potter et al. 1983). Since the
amount of freshwater runoff during the summer
season is larger due to seasonally heavier rainfall P.
pelagicus could be believed to relocate to habitats
less estuarine.
Potter and Lestang (2000) found that P. pelagicus
was more likely to spawn in the bay area outside
their investigated estuary as suggested by Anon
(1983); this might be because of better larval
dispersal and better survival of juveniles in higher
salinities (Romano and Zeng 2006). Like for S.
serrata berried females were scarce, with only one
ovigerous P. pelagicus caught during both seasons,
thus making patterns of migration for females
impossible to determine. As for S. serrata P.
pelagicus gave no re-catches reliable enough to
conclude anything about these.
P. pelagicus seems to prefer the river and to some
extent mud substrates compared to the
mangroves. Only 1,5% of total P. pelagicus catches
(i.e. summer and winter seasons combined) were
caught in the mangroves. S. serrata showed no
such habitat preference, being evenly distributed
over all three substrates. The reason behind this
difference between the two species could be that
P. pelagicus is less adapted to desiccation than S.
serrata. P. pelagicus caught in cages above the
water at low tide were less active and in a few
cases even dead, while S. serrata seemed to bury
despite the mesh and did not seem less alert. Also
P. pelagicus caught in dry cages regained activity
quickly when submerged. Another indication of the
difference in desiccation tolerance was that no
living P. pelagicus were to be found at the fish
market in Maputo while the S. serrata were still
lively. Therefore it is suggested that P. pelagicus
prefers substrates that are lower relative to the
zero-tide level compared to that of S. serrata.
Concerning P. pelagicus caught in the two other
substrates the largest amount was caught in the
river, but the largest specimens was found on the
mud. This might be explained by the higher
exposure to predators on the mud flats, i.e. birds,
and could be a reason for smaller animals to stay
away from this area. Not only other species prey
on small crabs, cannibalism is also frequently
occurring (Marshall et al. 2005).
Scylla serrata & Portunus pelagicus
Looking at sex distribution for both S. serrata and
P. pelagicus, overall more males were caught. This
might be because of the assumed migration of
female’s offshore (Hill, 1994; Potter et al. 1983),
but since this migration is seasonally dependant it
cannot explain the overall majority of males caught
during all periods of this investigation. However
since the difference is mostly minute there is no
reason to interpret too much into these results.
Haemolymph degradation
The proteins in the haemolymph of starved
Portunus pelagicus degraded faster compared to
Scylla serrata. The biggest difference was found
during the initial six hours after capture where P.
pelagicus showed a significantly greater loss of
haemolymph proteins compared to S. serrata. This
is of interest because it indicates the different
species immediate reaction to capture/captivity.
This in turn shows that testing of S. serrata in the
field is less important since they remain relatively
stable during transport, while P. pelagicus needs
immediate blood extraction in order to obtain
reliable values.
P. pelagicus is more active when compared to S.
serrata which mostly stays still and endures, this
might be the reason the previous shows greater
drops in blood protein levels. Faster more active
animals have a higher metabolism and therefore P.
pelagicus is likely to metabolize the proteins faster
than S. serrata. Some relations to this could be
seen in the work by Paterson & Spanoghe (1997)
who found that Rock lobsters (Panulirus cygnus)
shows metabolic degradation of haemolymph
10
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
when exposed to stressful environments such as
transportation. Low levels of oxygen and poor
environmental conditions in the aquarium, as well
as the lack of food, could lead to similar effects and
depending on the species said degradation should
differ. The oxygen levels of the water flowing
through the aquarium was unknown and in wild S.
serrata used burrows which are likely to be
subjected to decreased oxygen levels and
increased temperature during low tide. Therefore
suggestions that S. serrata is more adapt to stress
caused by oxygen depletion is suggested since
their health was better than that of P. pelagicus.
However since P. pelagicus is a smaller crab they
might be affected by the amount of blood sampled
even though the volumes were small
Coagulum
Even though blood samples were mixed with
anticoagulant buffering solution many samples
formed coagula. The main reason for this is the
storage time which was longer for samples taken
on Inhaca du to the lack of proper equipment for
immediate testing. This was strengthened by the
observation that blood samples from Maputo
formed very small amounts of coagula, despite
similar treatment, with the only big difference
between the sites being said storage time.
Triggering factors of clotting are suggested to
depend on the levels of fibrinogen in the blood,
which sets of the coagulation process and is
suggested to have similar chemical composition
and exist in different amount in all decapods
(Ghidalia et al. 1980). However, since the attempt
to solve the coagulum by sonic homogenization
failed, the conclusion concerning blood samples is
that they should not be stored frozen for extended
periods of time and if possible should be processed
immediately.
Surrounding factors
The introduction mentions lower activity caused by
lower temperature with the critical level being 20
°C (Hill 1980), however there is still a possibility
that the water temperature gave a lowered activity
during the winter season when mean measured
0
temperature was 21,9 C . Lowered activity may
results in fewer catches which could be a reason
for lower catches during the winter season.
The investigation of recapture collided somewhat
with the continuous blood sample investigation
since the latter required crabs to be brought to the
research station and kept there, while recapture
needed these animals to be set free in order for
possible recapture to occur.
Information that cages placed within 100 meters of
each other (Williams and Hill, 1982) was
discovered after the investigation was done and
thereby no actions to avoid this was taken.
And lastly since levels of water filtration for the
aquarium on the research station was unknown
starvation might be questioned since food particles
could possibly be transported with the water. Loo
et al. (1993) investigated the clearing rate of
suspended food particles by Nephrops norvegicus
and Hommarus gammarus and found that some
nutritional benefits can be detected when animals
are allowed only to filter for food. Since these two
also are large decapods one could suspect similar
results should they be performed on S. serrata and
P. pelagicus.
Aquaculture
Since no aqua culturing of S. serrata is presently
applied in southern Mozambique the following text
is based solely on literature research. As
mentioned in the introduction there are three
currently working ways of keeping S. serrata (only
species existing in Mozambique). Of these the
rearing of larvae should be the most economically
viable since it would produce more crabs, while the
other two methods only deliver the crabs already
caught in the wild. Should interest of aqua
culturing on Inhaca shown up the subject must be
thoroughly investigated beforehand.
Acknowledgments
My thanks to everyone who helped me with this
thesis, to mention a few: my supervisor Susanne
Eriksson for great help with lab work and writing.
Adriano Macia who supervised my work in
Mozambique. Stina Larsson who helped me with
pretty much everything. Olavo and Olga for
practical help at the research station on Inhaca
Island and Peter Tiselius for help with statistics.
11
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
Cited Literature
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serrata in relation to size and molt
cycle Journal of Experimental Marine
Biology and Ecology, 217, 93-105
(1997).
3. Davis J.A., van Blerk L.L., Kirby R and
Hecht T. Genetic variation in the mud
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(Crustacea: Portunidae) in South
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4. Davis A. J., Churchill G. J., Hecht T and
Sorgeloos P. Spawning characteristics
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serrata (Forskål) in captivity. Journal
of the world of aquaculture society.
Vol. 35 No. 2 (2004).
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Humans. Biological conservation 78,
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University of Gothenburg
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and Mann D. Cannibalism in juvenile
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availability. Applied Animal Behaviour
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(1982).
Potter C., Chrystal P. J., and
Loneragan N. R. The biology of the
blue manna crab Portunus pelagicus
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Williams G. R. The combined effects
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Web pages
1.
http://www.hkfish.net/eng/database/crabs/structur
al.htm 2008-10-15 Sex determination
Of Scylla serrata.
2. http://www.fishsa.com/crabs.php
2008-10-15 Sex determination of
Portunus pelagicus.
13
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
Appendix
I
Map of Inhaca
With details of Saco Bay
Mangrove
Mud
River
Sand
2
Scale
3
0
1
Fished area
k
m
Map modified from Salmaõ Bandeira’s map of Segrass dispersal on Inhaca Island (With permission)
14
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
II
40 cm
Detailed description of the cages
15cm
40
cm
D=20cm
20cm
90 cm
The nine cages used during fall were equipped with
a hatch on top which cages used during spring
lacked. This is not believed to affect the catches in
any way.
The twelve cages used during spring-season all had
shorter entrances as shown by the right entrance
(15cm) compared to that of the cages used during
the fall which measured 20cm in tube-length (left
side of picture).
The mesh size used was 2cm2tion of cages
15
Johan Florentzson
Master Thesis in Marin Ecology
University of Gothenburg
III
Description CW, ICW and Sex
Curtsey of Keenan et al. 1998
Scylla serrata male
Picture from http://www.hkfish.net/eng/database/crabs/structural.htm
Portunus pelagicus male
Picture from http://www.fishsa.com/crabs.php
Scylla serrata female
Picture from http://www.hkfish.net/eng/database/crabs/structural.htm
Portunus pelagicus female
Picture from http://www.fishsa.com/crabs.php
16