Freshwater Crabs - IUCN

INTEGRATION OF FRESHWATER BIODIVERSITY INTO AFRICA’S
DEVELOPMENT PROCESS:
MOBILIZATION OF INFORMATION AND DEMONSTRATION SITES
Demonstration project of the Gambia River Basin
Mobilization of Information and Demonstration Site
Training of Trainers module
Demonstration Project of the Gambia On River Basin (West Africa)
Freshwater crabs
Dr. Ndiaga THIAM & Anis DIALLO
September 2010

INTEGRATION OF FRESHWATER BIODIVERSITY INTO AFRICA’S
DEVELOPMENT PROCESS:
MOBILIZATION OF INFORMATION AND DEMONSTRATION SITES
Demonstration project of the Gambia River Basin
Training of Trainers module
on
Freshwater Crabs
Wetlands International Afrique
Rue 111, Zone B, Villa No 39B
BP 25581 DAKAR-FANN
T EL. : (+221) 33 869 16 81
FAX : (221) 33 825 12 92
EMAIL : wetlands@orange.sn
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Table of Content
Introduction ............................................................................................................................................................. 4
Goal and objectives of the module ....................................................................................................................... 5
Module content .................................................................................................................................................... 6
Training needs ...................................................................................................................................................... 7
Expected results ................................................................................................................................................... 8
Topic 1. Presentation of the study area ................................................................................................................... 9
Topic 2. general information on crabs .................................................................................................................. 11
2.1.- Classification of crabs ................................................................................................................................ 11
2.2.- Origin of crabs ........................................................................................................................................... 11
2.3.- Morphology and anatomy .......................................................................................................................... 12
2.3.1.- General organization of a crab ........................................................................................................... 12
2.3.2.-Circulatory System ............................................................................................................................. 13
2.3.3.- Respiratory System ............................................................................................................................ 14
Topic 3. Eco- biology of crabs .............................................................................................................................. 15
3.1.- biological diversity of crabs in the gambia river basin .............................................................................. 15
3.2.- Reproduction and life cycle ....................................................................................................................... 16
3.3.- feeDing behavior of crabs .......................................................................................................................... 17
Topic 4. Importance of crabs ................................................................................................................................ 19
4.1.- economical importance .............................................................................................................................. 19
4.2.- intermediate host ........................................................................................................................................ 19
4.3.- Toxicity ...................................................................................................................................................... 19
Topic 5. protocol to monitor crabs in the gambia river basin ............................................................................... 20
1. Materials ............................................................................................................................................... 20
5. 1.1.- Scientific team .................................................................................................................................. 20
5.1.2.- Collection material ............................................................................................................................. 20
5.1.3.- Sample periods .................................................................................................................................. 21
5.1.4.- Study site ........................................................................................................................................... 21
5. 2.- Methods ..................................................................................................................................................... 21
5.2.1.- Study and collection methods ............................................................................................................ 21
5.2.2.- Data processing .................................................................................................................................. 22
bibliography .......................................................................................................................................................... 23
ANNEXES ............................................................................................................................................................ 24
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INTRODUCTION
The Gambia River Basin Development Organization (OMVG), which is composed of The
Gambia, the Republic of Guinea, Guinea Bissau and Senegal, plans to build a dam on the
Sambangalou site. The construction of this dam will involve a significant disturbance to
biodiversity as proven by the impact studies conducted by OMVG. In order to mitigate the
negative impact of this project, and to simultaneously improve on those that are positive,
Wetlands International Africa, in partnership with the IUCN-Species Survival Commission
and the OMVG, through Phase 2 entitled "Demonstration Project of the Gambia River (West
Africa)" within the program "Integration of Freshwater Biodiversity into Africa’s
Development Process: Mobilization of Information and Demonstration Sites", agree on the
implementation of a plan to monitor the biodiversity of freshwater ecosystems in the Gambia
River basin.
To this end, Wetlands International, in collaboration with its partners, is interested in
developing this educational module on malacologic fauna within the framework of
biodiversity monitoring in the Gambia River basin.
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GOAL AND OBJECTIVES OF THE MODULE
This module is designed for the state’s technical services, NGOs and the local
communities of the Gambia River Basin for practical implementation of a
preliminary monitoring plan of freshwater biodiversity of the Gambia River basin.
It offers a precise and operational methodology to monitor the status and
dynamics of freshwater crabs. The development of this type of course involves
choices that must eventually be adjusted in the field. Ultimately, this course will
enable:
• Provision of general information (on the systematics, eco-biology of crabs,
etc.).
• Grasping of general concepts related to the ecology of freshwater crabs;
• Strengthening of the capacity of trainers in the monitoring of species for the
conservation of species biodiversity;
• Provision of information on the impacts of the construction of the Sambangalou
hydroelectric dam;
• Establishment of a plan for the monitoring of crab biodiversity;
• Evaluation of the educational means and training duration of target groups.
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MODULE CONTENT
It contains various chapters presented as topics.
• Topic 1 provides a general introduction of the study area and the issue.
• Topic 2 provides general information on freshwater crabs.
• Topic 3 discusses the eco-biology of freshwater crabs
• Topic 4 presents the importance of freshwater crabs
• Topic 5 describes the methodology that should be used to monitor
freshwater crabs in the Gambia River basin
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TRAINING NEEDS
Human resources:
- 1 facilitator (specialist that will provide the training)
- People in charge of ecosystem conservation in the countries that share
the Gambia River basin
Needs in material
- Room (training site)
- Training support (PowerPoint format)
- Copies of the course
- Maps on the area’s status, study sites and maps on the distribution of key
species;
- Boards to identify key species;
- Video projector
- Flip chart and writing support
- Notebooks, pens, pencils, erasers.
Financial resources
- Facilitator fees
- Participants’ perdiem
- Other organization-related expenses
Timing of the course
The time required for the training is 15 hours and is broken down as follows:
- Introduction and presentation of the study area (3 h)
- General information on crabs (3 h)
- Eco-biology of freshwater crabs (3 h)
- Importance of freshwater crabs (2 h)
- Monitoring plan of crabs in the Gambia River basin (4)
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EXPECTED RESULTS
The main expected outcomes in developing this module are the training of
technicians and the provision of a method to monitor freshwater crabs:
1.- Expected outcomes in biodiversity monitoring
- Listing and mapping of species is elaborated;
- The community structure of specific sites is well defined;
- The spatial and temporal distribution of crabs is known;
- The comparative study of the specific composition upstream and downstream
of the dam is effective;
- The study of species’ weight is done.
2.- Expected outcomes of the training session
After the training session, the future trainers will:
• Know the biological and abiotic impacts generated by the dam;
• Have mastered the methodology for monitoring crabs’ biological diversity;
• Have the ability to train other targeted groups for wider dissemination on
the need for biodiversity conservation and valorization of collected species.
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TOPIC 1. PRESENTATION OF THE STUDY
AREA
The Gambia River has its source in the high rainy mountains of the Fouta Djallon in the
northern part of Guinea’s central region. The total amount of water leaving Guinea for
Senegal is estimated at around 3 km³ / year. The river then flows north to enter The Gambia in
the country’s extreme east. The total area of the Gambia River Basin (Figure 1) is 77,850 sq.
km. There is a large fluctuation in river flow between the wet (2,000 m 3 / s) and dry season
(10 m 3 / s). Due to this reason, and given the flat topography of The Gambia, salt water can be
found at about 70 km upstream during the rainy season and 250 km upstream in the dry
season. This has an effect on the distribution of species and habitats at the river’s mouth. All
changes in river flow have an effect on the composition and structure of zones near the river’s
mouth.
Three main types of wetlands can be found in the basin namely mangroves near the mouth,
small flood zones in the middle, and thick riparian forests in the mountains of Guinea. These
wetlands provide habitats to about 1,500 species of plants, 80 species of mammals, 330
species of birds, 26 species of reptiles, about 150 species of freshwater fish and 481 other
species found in coastal lagoons. Several endangered species such as chimpanzees, crocodiles
and the Egyptian plover can still be found in these regions.
There are about 3 million people living in the Gambia basin, who are involved chiefly in
agriculture (70 to 90% of the population). Other activities include fishing, livestock, forestry
and trade.
The Gambia River Basin Development Organization (OMVG) was founded in 1978 to foster
the development of the basin for the provision of irrigation and hydroelectric power. In order
to meet the ever increasing need for clean energy production, a feasibility study was
conducted for the construction of a dam on the upper Gambia River. The construction of a
hydroelectric dam has recently been approved in a location near Sambangalou. The dam will
have an impact on the hydrological, ecological and biotic aspects of the river. The main
impacts are:
• The reduction of maximum flood flow by 50 to 60%
• The reduction of the water depth to an overall average of about 10 cm
• The intrusion of saline water in a range of 150 km, resulting in:
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- The loss of animal biodiversity and mangroves along the river bank
- Possible changes in the configuration of morpho-sedimentation and
microbiology
- A decrease in irrigation water levels
- A decline in fish production
There will be a significant impact on freshwater biodiversity, not only on commercial fish
species but also on endangered animals such as the West African manatee. As recommended
in the assessment of environmental impacts, compensation must be given to local
communities for the loss of income due to changes in the environment. Changes must be
monitored continuously in order to detect any obvious change in biodiversity that requires a
management response.
Figure 1: Gambia River basin
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TOPIC 2. GENERAL INFORMATION ON
CRABS
2.1.- CLASSIFICATION OF CRABS
Crabs belong to the kingdom Animalia and further to the phylum Arthropoda (from the
Greek arthron "articulation" and podos "foot", also called "articulated") which groups
together invertebrate animals. Arthropods bodies are composed of articulated segments (or
metameres), covered with a rigid cuticle, which forms an exoskeleton that, in most cases, is
made of chitin. The phylum Arthropoda has by far the most species and individuals of all the
animal kingdom (80% of known species). There are over a million and a half current species
of arthropods. Crabs are part of the huge class known as crustaceans (2 pairs of antennae,
essentially aquatic and gill-breathing), of the subclass Malacostraca (higher crustaceans), the
superorder Acaridae, the order Decapoda , and the suborder Brachyura. Crabs have antennas
the first pair of which is oral appendages turned into mandibles. That is the reason why it is
part of the group Mandibulata. They have several families.
2.2.- ORIGIN OF CRABS
More than 6,800 living species and nearly 1,800 fossils have been described so far (De Grave
et al., 2009) spread over the entire planet. Crabs colonize a wide variety of environments:
aquatic of course, but also continental, some species spend almost their entire lifecycle
outside of water. It is especially in the tropical zones that one finds, indiscriminately, marine
crabs, freshwater crabs and land crabs.
The appearance of crabs goes back to the secondary era, the oldest fossil crab known dating
from the Mid-Jurassic about 170 Ma ago.
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2.3.- MORPHOLOGY AND ANATOMY
2.3.1.- General organization of a crab
One distinguishes Brachyura, true crabs (Figure 2), from Paguroidea or hermit crabs.
Brachyuran are crustaceans with five pairs of legs, the first being modified to form a pair of
claws, a rather flat shell and a short and broad abdomen under the thorax.
The body of a brachyuran typically possesses a large cephalothorax, depressed and housing
all the organs. The cephalothorax consists of the head (pre-oral lobe + 4 segments) and
pereion (8 segments). The whole is enveloped in a continuous shell formed by pleurae of the
last cephalic segment and the pereion tergites. The pereion (7 segments) is reduced and folded
under the cephalothorax. The first 3 segments of the pereion are merged with the head their
appendages are mouthparts or maxillipeds associated with mouthparts (mandibles, maxillules,
maxillae). The appendages of the last 5 segments of the pereiopods are pereion. The first pair
of pereiopods are the chelipeds (claw bearing), and are usually well developed. The claw
includes mobile digits articulated on the propodus composed of the manus and the polex. The
other pereiopods are locomotory. The appendages of arthropods are usually biramous
composed by a more powerful ventral endopodite and a more delicate dorsal exopod, often
foliaceous and having a role in respiration. These two branches are supported by a basal
segment, the protopodite, which in crustaceans, is divided into the precoxopodite, the
coxopodite and the basipodite (having an exo-and endopodite). Among the brachyurans, the
exopodite of the pereiopods are housed in the gill cavities located in portions of the
cephalothorax.
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Figure 2 : The body plan of a crab (Brachyura): fr.wikipedia.org/wiki/Fichier:Crabe_anatomie.jpg
2.3.2.-Circulatory System
In crustaceans, the heart is located in the cephalothorax and is suspended in a pericardial
pouch (the pericardium is a membrane that surrounds the heart). The blood which enters
through small holes called ostioles is sent into the arteries that branch off to different organs.
Flowing then in a system of lacunae (That is to say, in the empty spaces between cells and
organs), it is taken to the gills and then is brought into the pericardial cavity.
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2.3.3.- Respiratory System
Like many aquatic animals, crustaceans (and crab) have gills. The gills are precisely the most
efficient structures to capture oxygen in water.
The crab’s ventilation is via its aquatic gill. Movements of the mouthparts circulate and
refresh water in the gill cavities that are formed by the cephalothoracic carapace on either side
of the body; water enters through an inhalation orifice at the base of the claws and exits
through an exhalation orifice located on either sides of the mouth.
It can withstand long emersions as long as the gills remain moist. Also, in the mediolittoral
zone, changes in tides force these organisms to tolerate dry low tide zones. The crab has
therefore developed specialized structures enabling it to consume gaseous O 2 . The crab’s
breathing apparatus allows consumption of dissolved or gaseous O 2 .
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TOPIC 3. ECO- BIOLOGY OF CRABS
3.1.- BIOLOGICAL DIVERSITY OF CRABS IN THE GAMBIA RIVER
BASIN
Very few species of freshwater crabs have been reported in the area, which seems quite
normal, given the natural distribution of this group, which is far better represented at sea and
in brackish water. Thus, only two species have been recorded, including Potamautes ecorssei
Liberonautus latidactylus and both belong to the Potamonautidae family. These typical
freshwater species have been identified in Niokolo Koba Park (Figure 2). It seems that the
first species is more widespread in the area. It would be interesting to do an inventory to have
a much more comprehensive listing of the number of crab species in the Gambia River basin.
One must also note that some species can move from freshwater habitats into brackish water
environments.
Figure 3 : Distribution of crabs in the Gambia River basin
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3.2.- REPRODUCTION AND LIFE CYCLE
Crabs molt regularly in order to grow. Once the female molts, its skeleton falls off. It can lay
thousands of eggs that get attached under its belly until they hatch.
Most brachyuran are gonochoric, only a few species are hermaphroditic. For many species of
crabs, internal reproduction is only possible after the female molts. Fertilized eggs remain
attached to the female’s pleopods. The abdomen is then "detached" from the cephalothorax
and provides a protective space to lay eggs. The eggs are incubated for a duration, which
varies according to the species, then hatch at the protozoea or zoea stage; the number of zoeal
stages varies among species. After the last zoea stage, the crab goes through a final larval
stage, which is also pelagic, the megalope
; its morphology is intermediate between the zoe shape and that of the crab. After a certain
period of time, the larva migrates to the substrate where it performs its last larval molt that
leads to the first crab stage (Figure 4). The duration of larval life varies, for instance it is 65 ±
11 days on average for the Cancridae crabs and 29 ± 16 for the family Ocypodidae. However,
there are times of larval development that are much longer, 4 to 12 months for Cancer
magister, or much shorter for Tunicotheres moseri for which the development lasts between 3
and 7 days. Finally, direct development, where the individual coming out of the egg
resembles a miniature adult, is rare. This occurs, for example, in crabs in the Xanthidae of the
family: Pilumnus lumpinus, P. novaezelandiae, and P. vestitus.
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Nauplius de balane
Zoé de crabe
Mégalope de crabe
Crabe brachyoure
Figure 4: Different stages of crab development
3.3.- FEEDING BEHAVIOR OF CRABS
Most crab species are predatory and/or scavengers. However, there are also species that are
herbivores, omnivores and scavengers. They eat meat (fish, shellfish, mussels, etc.), but also
food left over by fish.
At different stages of its life, the blue crab is both prey and consumer of plankton, fish, plants,
molluscs, crustaceans, and organic debris.
In macrophages, the food is picked up with the chelipeds that transfer it to the 3rd pair of
maxillipeds; the maxillipeds push the food further towards the mandibles and maxillae, which
shred it before swallowing. For macrophages, the food sample is filtered. This can be
achieved by the bristle combs located in certain appendages, such as those at the level of
maxillipeds in the case of Pinnotheridae. The filtered particles are then turned in towards the
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mouth. The filter is either active (mouthparts), due to the beating of one of its appendages
(scaphognathite ), or passive, using a natural water stream or one generated by the host if the
crab is symbiotic. Depending on the crab’s food behavior, chelipeds may have varying
morphologies. Thus, crabs that eat encrusting algae have spoon-shaped claws to scrape and
collect food. Carnivore/scavenger crabs, such as Cancer pagurus (Linnaeus, 1758), which eat
shellfish, have blunt crusher claw, reminiscent of the grinding surface of molars, adapted for
crushing shells. Others have sharp pincer claws, in the shape of a blade, enabling flesh
cutting.
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TOPIC 4. IMPORTANCE OF CRABS
4.1.- ECONOMICAL IMPORTANCE
Crabs are an important food resource for men and are therefore the object of highly developed
economic activity. In the Mediterranean basin, the rock crab (green crab) is often one of the
components of traditional recipes such as fish soup. As for the edible crab it is generally
caught in Britain. The crab can be eaten just like a molting crab.
One estimates the global catch of crabs to 1.2 million tons per year (2003 data). However, it
should be emphasized that the coastal crab being an animal that is easily caught, this figure
does not include individual fishing and artisanal fishing, especially in poor countries, where
the crab represents a very economical source of protein. On the coast of equatorial Africa,
children easily catch blue crabs that they simply roast over charcoal.
4.2.- INTERMEDIATE HOST
The discovery of larvae and pupae of Simuliidae living in association with river crabs dates
back to 1928, as Edward pointed out the presence of what he thought was a simple variety of
Ethiopian species Simulium hirsutuin on the Potamon niloticum crab. He found resemblances
of the "complex neavei” (S. neavei S., S. nyasalandium), vectors of human onchocerciasis
fixed on freshwater crabs (Grenier & Moucher, 1958).
4.3.- TOXICITY
Some species are toxic especially very colorful crabs of the Xanthidae group. Some crab
species are poisonous and one must be careful. The crab with a white shell of the Cook
Islands and the reef crab from tropical regions for instance, are considered the most
poisonous.
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TOPIC 5. PROTOCOL TO MONITOR CRABS
IN THE GAMBIA RIVER BASIN
The monitoring of crabs in the Gambia River Basin will be focused on the riverbed. This
monitoring could be done through a stock assessment of crabs present in the area.
1. MATERIALS
5.1.1.- Scientific team
The ideal scientific team will consist of at least one expert researcher who will be assisted by
an experienced general systematic crab technician.
5.1.2.- Collection material
In order to conduct this study, two types of traps can be used. These traps are "kavel" cylinder
shaped and conical traps (Figure 5) of greater volume, each of which being well weighted for
stabilization. Moreover, most crabs are caught in traps. In some areas, it is the only authorized
form.
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5.1.3.- Sample periods
Figure 5: Conical trap
Sampling will be carried out both during the water increase and decrease and take into
account the circadian rhythm of crabs in different study sites.
5.1.4.- Study site
The study sites will be on the riverbed.
5.2.- METHODS
5.2.1.- Study and collection methods
The evaluation will be conducted both during water increase and decrease. The circadian
rhythm of crabs will be taken into account. The assessment of will focus on the areas
upstream and downstream from the dams.
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Stratified random sampling will be adopted. In light of previous work conducted in Africa, the
distance between traps will be set at 70 m. In general, in other species, it appears that for
immersion time of 12 hours, the radius of attraction for a trap is 35 m (Miller, 1975).
The choice of trapping depths will be based on knowledge of the depth distribution of
targeted crabs.
The traps will be dropped in the evening, then recovered in the morning after roughly 12
hours of immersion. They will then be replaced in the morning, and recovered in the evening,
and so on so forth.
For each retrieved trap, a number of parameters will be collected (trap number, trapping site,
station, etc. species, specific weight, overall weight, etc.).
5.2.2.- Data processing
The data collected will be entered and processed by data processing software (Excel, SPSS,
etc.). The results will be compared in terms of locations, stations, time, etc.
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BIBLIOGRAPHY
- Brusca (R.C.), Brusca (G.J.) 2003.- Invertebrates (2 éd.). Sinauer Associates. 702
p.
- David, M. & Steven, M. 1978. Principles of Paleontology (2 éd.). W.H. Freeman and
Co.. pp 4-5.
- Edward, F . W. 1928. Siinuliurn larva and pupa found on a crab. Entomologist. ,
61,42 p.
- Grenier, P. & Mouchet, J. 1958. Premières captures au Cameroun d’une simulie du
complexe Neavei sur des crabes de rivières et Simulium berneri sur des larves
d’éphémères. Remarques sur la signification biologique de ces associations.
Bul. soc. path. exo. Tome 5, N° 6, pp 968-980.
- Lecointre, G. & Le Guyader G. 2006. Classification phylogénétique du vivant, 3 e
édition, Belin, Paris.
- Miller, 1975.- Density of the commercial spider crab, Chionoecetes opilio, and
calibration of effective area fished per trap using bottom photography. J. Fish.
Res. Board Can., 32: 761-768.
- Romaric, F. 2006. « Malacologie », Dico de Bio, 2e éd. De Boeck Université.
- Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 eme éd.).
Brooks / Cole. pp 367-403.
- UICN, 2008. Biodiversité des eaux douces – une ressource cachée et menacée.
Commission de la sauvegarde des espèces
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ANNEXES
Annex 1 – Approximate budget
The approximate training budget is three million one hundred ninety six thousand FCFA
Francs.
DESIGNATION
Number
AMOUNT/
UNIT (fcfa)
TOTAL
AMOUNT (fcfa)
- Course support (Module) 20 20,000 400,000
- Edition of identification boards
and data collection cards
01 500,000 500,000
- Copy of boards 20 25.000 500,000
- Copy of data collection cards 100 50 5,000
- Markers 06 1,000 6,000
- Conservation liquid 10 50,000 50,000
- Computer rental (09 X 3) 15,000 270,000
- Rental of Power light (01 X 3) 20,000 60,000
- Facilitator’s fee 05 65,000 325,000
- Per Diem (participants) (18 X 3) 20,000 1,080,000
TOTAL 3,196,000
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