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2011 "The status and distribution of freshwater crabs [Central Africa]" Neil Cumberlidge Northern Michigan University
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Recommended Citation Cumberlidge N. 2011. Chapter 6. The ts atus and distribution of freshwater crabs, pp. 71-78. In: Brooks, E.G.E., Allen, D.J. and Darwall, W.R.T. (Compilers). 2011. The tS atus and Distribution of Freshwater Biodiversity in Central Africa. Gland, Switzerland and Cambridge, UK: IUCN: i-ix+140pp.
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Cumberlidge, N.¹
¹ Department of Biology, Northern Michigan University, Marquette, Michigan 49855, USA
The Purple March Crab Afrithelphusa monodosa (Endangered) which lives in swamps and year-round wetland habitats in north-western Guinea where it is known from only a few specimens from two localities. This species is clearly a competent air-breather and has a pair of well-developed pseudolungs. It is mainly threatened by habitat loss and degradation. © PIOTR NASKREKI
An unidentifi ed freshwater crab species within the family Potamonautes. This specimen was collected in central Africa, a region noted for its limited fi eld sampling. © DENIS TWEDDLE The Purple March Crab Afrithelphusa monodosa (Endangered) which lives in swamps and year-round wetland habitats in north-western Guinea. © PIOTR NASKREKI
Potamonautes lirrangensis (Least Concern), a relatively abundant and widespread species found in large slow fl owing rivers in rainforests across central and eastern Africa. © DENIS TWEDDLE
CONTENTS 6.1 Overview of the African freshwater crab fauna 180 6.1.1 Biogeographic patterns 182 6.2 Conservation status 183 6.3 Patterns of species richness 184 6.3.1 All freshwater crab species: interpretation of distribution patterns 186 6.3.2 Threatened species 187 6.3.3 Restricted range species 188 6.3.4 Data Defi cient species 190 6.4 Major threats 191 6.4.1 Habitat destruction 191 6.4.2 Pollution 192 6.4.3 Natural predators and competition with introduced species 192 6.4.4 Taxonomic issues 193 6.5 Research actions required 193 6.6 Conservation recommendations 195 Species in the spotlight – A unique species fl ock of freshwater crabs in Lake Tanganyika: a model for studies in evolution and extinction 196 Species in the spotlight – The East African tree hole crab – spectacular adaptation 198 180 CHAPTER 6 | FRESHWATER CRABS crab insmallstreamsthroughoutthe rainforest andsavannazonesofwesternAfrica. 1955; Cumberlidge1999;Yeo and smallerwaterbodies(Rathbun1921;Balss1936;Bott habitats, from mountainstreams tolarge lowlandrivers conspicuous crustaceansare present inalmostallfreshwater inhabiting Africaninlandwaters,andtheselarge and Freshwater crabsare amongthemostimportantinvertebrates 6.1 The CommonCreekCrab 2006; Cumberlidge Potamonautidae (Cumberlidge1999,2009a,b;Daniels of freshwater crabsin10genera thatallbelongtothe Afrotropical zone.ThispartofAfricaincludes119species (Socotra, theSeychelles,andMadagascar)thatalsoliein IndianOcean Desert, butexcludestheislandsofwestern here includesall43Africancountries southoftheSahara et al 1896 (Yeo Potamonautidae Bott,1970,andPotamidaeOrtmann, genera ( where there are three speciesoffreshwater crabsintwo freshwater crabsare Morocco, Algeria,Tunisia, andEgypt, Afrotropical Africathathost zone.Thecountriesofnorthern zone, andtherest oftheAfricancontinentthatliesin AfricainthePalaearcticincludes northern zoogeographical focuses onthefreshwater crabsofcontinentalAfrica,which and Ng2009;Cumberlidge . 2009).ThepartoftheAfrotropical zonedealtwith crab fauna Overview oftheAfricanfreshwater Potamonautes et al . 2008; Cumberlidge 2009a,b; Cumberlidge . 2008;Cumberlidge2009a,b; et al and . 2008;Yeo Liberonautes latidactylus et al etal Potamon . 2009). . 2009). . 2008). The present work . 2008).Thepresent work et al ) and two families, ) andtwofamilies, . 2008; Cumberlidge . 2008;Cumberlidge (Least Concern)isthemostcommon andmostfrequentlycaughtfreshwater et al . et al Tavares 2006;CumberlidgeandDobson2008; 2004; CumberlidgeandReed2004,2006; et al 1998, 1999,2009a,b;CumberlidgeandBoyko2000;Corace less diverseregional fauna(Bott 1955;Cumberlidge1997, Africa(fourspecies,twogenera)eachhavea and northern Africa(20species,onegenus) six genera),whilesouthern (33 species,threeAfrica(24 genera),andwestern are Africa(44species,three eastern genera),centralAfrica more thanoneregion. Forexample,themostdiverseregions composition, withonlyafewspecies(14outof120)foundin and eachofthesehasadistinctfreshwater crabfaunal purposes ofthisproject, subdividedintothefi Cumberlidge family, thePotamonautidae(Bott1955;Cumberlidge1999; and specieslevelswith100%endemicityforthepredominant freshwater crabfaunaishighlyendemicatthefamily, genus, al (154 genera,849species,twofamilies)regions (Yeo genera, 298species,twofamilies)andOriental/Palaearctic seen inthefreshwater crabfaunasoftheNeotropical (50 120 species,twofamilies)(Table 6.1)ismarkedlylowerthan The totaltaxonomicdiversityofcontinentalAfrica(11genera, . 2008;Cumberlidge . 2001;Cumberlidge . 2009) . 2009) et al © PIOTRNASKREKI . 2009). The African continent is, for the . 2009).TheAfricancontinentis,forthe et al et al . 2002;CumberlidgeandVanini . 2009).ContinentalAfrica’s ve main regions, ve mainregions, et et CHAPTER 6 | FRESHWATER CRABS 181 Potamonautidae, Potamonautinae Potamonautidae, Hydrothelphusinae Potamonautidae, Hydrothelphusinae Potamonautidae, Hydrothelphusinae Potamonautidae, Hydrothelphusinae Potamonautidae, Hydrothelphusinae Potamonautidae, Hydrothelphusinae E Potamonautidae, Potamonautinae E Potamonautidae, Potamonautinae E Potamonautidae, Potamonautinae N Potaminae Potamidae, W Potamonautidae, Potamonautinae W Potamonautidae, Potamonautinae W Potamonautidae, Potamonautinae 1 1 1 1 2 1 2 1 1 1 1 1 1 1 No. Genera Region subfamily Family, N = northern Africa, W = western C = central Africa, E Africa, 2 1 2 1 1 2 1 No. Species Country 12 Morocco3 Algeria4 Tunisia Egypt56 Nigeria Liberia7 Guinea89 1 d’Ivoire Côte 10 Ghana 111 Togo 1 Sierra Leone12 1 1 13 Benin 1 1014 Chad 1 8 15 Mali N16 Mauritania 5 17 N 3 6 Potamidae, Potaminae Burkina Faso N18 Niger Potamidae, Potaminae 19 D. R. Congo 6 2 Potamidae, Potaminae 20 Cameroon W 4 321 Congo22 Potamonautidae, Potamonautinae Gabon23 3 2 Central African Republic24 W 2 Equatorial Guinea/Bioko 225 2 Angola (Cabinda) 1 2 Potamonautinae Potamonautidae, 26 Sudan W 2 Somalia 24 W27 1 Potamonautidae, Potamonautinae 1 428 1 13 1 Ethiopia 2 Potamonautidae, Potamonautinae 3 Kenya W29 11 3 W30 8 1 Uganda Potamonautidae, Potamonautinae W 2 4 W 131 W Rwanda Potamonautinae Potamonautidae, 132 Potamonautidae, Potamonautinae Burundi Potamonautidae, Potamonautinae C 3 Potamonautidae, Potamonautinae Tanzania W33 2 C C 1 3 Potamonautinae Potamonautidae, 34 C 1 Malawi Potamonautidae, Potamonautinae 35 Potamonautidae, Potamonautinae Potamonautinae Potamonautidae, C Angola36 Potamonautidae, Potamonautinae 6 Botswana C C37 Potamonautinae Potamonautidae, 2 13 Lesotho38 Mozambique Potamonautidae, Potamonautinae 12 Potamonautidae, Potamonautinae 39 Namibia 1 440 South Africa E41 1 Swaziland 24 42 2 Zambia Potamonautidae, Potamonautinae E Zimbabwe 1 4 1 Potamonautidae, Potamonautinae 5 2 E 3 1 E Potamonautinae Potamonautidae, 1 E 14 1 4 Potamonautinae Potamonautidae, 1 Potamonautidae, Potamonautinae 1 1 1 E 3 1 S 3 S 1 Potamonautidae, Potamonautinae S S 1 Potamonautidae, Potamonautinae Potamonautidae, Potamonautinae S Potamonautinae Potamonautidae, 2 Potamonautinae Potamonautidae, 1 S S Potamonautidae, Potamonautinae Potamonautinae Potamonautidae, S Potamonautidae, Potamonautinae S Potamonautinae Potamonautidae, Potamonautidae, Potamonautinae Table 6.1. Numbers of species, genera, families and subfamilies, and their regions of occurrence, for each of for of occurrence, regions and their subfamilies, families and genera, of species, Numbers 6.1. Table crabs. freshwater in Africa that has the 43 countries = eastern Africa, S = southern Africa.” 182 CHAPTER 6 | FRESHWATER CRABS Africa, E = eastern Africa, N = northern Africa. Africa,N=northern Africa, E=eastern occurrence, andasummary of theirconservationstatus. Table 6.2.Thethreatened speciesoffreshwater crabsfoundincontinentalAfrica,withtheirregions of andfreshwaterpatterns ecoregion boundariesfoundinthe no closecorrelation betweenfreshwater crabdistribution et al Temperate freshwater ecoregions) (Thieme andCoastal,Cuanza,Zambezi, andSouthern Eastern Upper Guinea,West CoastEquatorial,Congo,Great Lakes, freshwater ecoregions (e.g.,theNorthAfrican,Nilo-Sudan, crabs are alsorepresented inmostof the continent’s representation inAfrica’s aquaticecosystems,freshwater and drainforested highlands.Becauseoftheirwidespread Africa, especiallythosethatfl highest intheriversandstreams ofthemajorriverbasins freshwater resources aboveground. Speciesdiversityis savanna rivers,andevenxericsystemswhere there are mountain systems,large lakes,large riverrapids,dry moist forest rivers,Mediterraneansystems,highlandand continental Africa,includingfl Freshwater crabsare foundinallmajorhabitattypes 6.1.1 Biogeographicpatterns 7 6 5 4 3 2 1 8 9 10 11 14 13 12 18 17 16 15 19 20 21 22 23 24 26 25 . 2008).However, withonlyafewexceptions,there is Species Louisea balssi Liberonautes rubigimanus Liberonautes nanoides Globonautes macropus Afrithelphusa monodosa Liberonautes lugbe Liberonautes grandbassa Louisea edeaensis Potamonautes gonocristatus Potamonautes idjiwiensis Potamonautes mutandensis Potamonautes choloensis Liberonautes nimba Potamonautes platycentron Potamonautes lividus Potamonautes infravallatus Potamonautes ignestii Potamonautes gerdalensis Potamonautes montivagus Potamonautes pilosus Potamonautes raybouldi Potamonautes reidi Potamonautes triangulus Potamonautes unisulcatus Potamonemus sachsi Potamonautes xiphoidus ow through lowlandrainforest oodplains, swamps,lakes, et al . 2005;Abell itiuinRdLs aeoyRedListCriteria RedListCategory Distribution Regional / UB1ab(iii)+2ab(iii) VU W/C / NB1ab(i,iii)+2ab(i,iii) EN C/E / NB1ab(iii,v)+2ab(iii,v);C1+2a(ii) EN C/E NB2ab(iii) B1ab(iii)+2ab(iii) B1ab(iii)+2ab(iii);C2a(i) B1ab(iii)+2ab(iii);C2a(i) B1ab(iii)+2ab(iii) EN EN EN EN CR B1ab(iii) W CR W W W W W UB1ab(iii)+2ab(iii);D2 VU W UB1ab(iii)+2ab(iii) VU W UB1ab(iii)+2ab(iii) VU W NB1ab(i,ii,iii,iv,v) EN C NB1ab(i,ii,iii,iv,v) EN C NB1ab(i,ii,iii,v) EN C UB1ab(iii)+2ab(iii) VU S UB1ab(i) B1ab(iii)+2ab(iii) VU EN E E UB1ab(i);D2 B1ab(i);D2 B1ab(i);D2 VU VU VU E E E UB2ab(i) VU E UB1ab(i,iii);D2 VU E UB2ab(iii) VU E UB1ab(i,ii,iii);D2 VU E UB1ab(iii)+2ab(iii);D2 VU E C = central Africa, W = western Africa, S = southern Africa,S=southern C=centralAfrica,Wwestern species thathavearestricted distribution. distribution coincideswithecoregion boundariesare those African region. Theonlyinstanceswhere freshwater crabs least twofreshwatercrabsspecies. The WhiteVolta Riverasanexampleofsuitablehabitatforat
© KLAAS-DOUWEDIJKSTRA CHAPTER 6 |
FRESHWATER CRABS
The Purple March Crab Afrithelphusa monodosa (Endangered) which lives in swamps and year-round wetland habitats in north- western Guinea where it is known from only a few specimens from two localities. This species is clearly a competent air-breather and has a pair of well-developed pseudolungs. It is mainly threatened by habitat loss and degradation. © PIOTR NASKREKI
6.2 Conservation status with 26 (21%) of the 120 species assessed as globally Threatened (Table 6.2 and 6.3, and Figure 6.1a). There is The conservation status of Africa’s freshwater crab fauna insuffi cient information to assess the status of 27 species was assessed by Cumberlidge et al. (2009) using the which were categorised as Data Defi cient (DD) due to a lack IUCN Red List Categories and Criteria at the global scale specimens, and locality and population data (Cumberlidge (IUCN 2003), and individual species assessments are et al. 2009; Table 6.3, and Figure 6.1a). If all Data Defi cient freely available through the IUCN Red List website (www. (DD) species also proved to be Threatened, the level of iucnredlist.org). Results reveal current high levels of threat threat could be as high as 44%.
Figure 6.1. The proportion (%) of freshwater crab species in each regional IUCN Red List Category in mainland continental Africa (Cumberlidge et al. 2009, Appendix 1): a) all 120 species from the region; and b) following removal of the 27 DD species.
183 184 CHAPTER 6 | FRESHWATER CRABS species offreshwater crabsfrom Africacouldbeconfi species (3%)were assessedasNearThreatened (NT).No (VU) (14species)(Figure 6.1bandTable 6.3). Justthree (two species),Endangered (EN)(10species)orVulnerable threatened categories,eitherasCriticallyEndangered (CR) of the94assessedspecieswere listedinoneofthree species of eight speciesof eight oftheninespecies Potamonemus of Cumberlidge streams intheforested partsofAfrica(Cumberlidge1999; of theseliveinrivers,marshylowlands,ormountain and were (LC),andmost judgedtobeLeastConcern (65 species,69%,Figure 6.1b)belongtosevengenera Of the94speciesthatcouldbeassessed,majority found in both C + S). C = central Africa, W = western Africa, S = southern Africa, E = eastern Africa, N = northern Africa. Africa,N=northern Africa,E=eastern Africa,S=southern found inbothC+S).=centralAfrica,Wwestern both E+C,E+S,andSCE);ENincludestwospeciesthatare foundinbothC+E);DDincludesonespeciesthatis fi with an*=totalspeciesperregion includingthosespeciesthatare foundinmore thanoneregion. Forexample,LCincludes Appendix 1). Table 6.3. Aubry’s Crab of breathingair. during rainstormsitisoftenfoundon land becauseitiscapable and ponds,digsburrowsnearwaterways; atnightand Equatorial Guinea.Thislargespecies inhabits streams,rivers, border oftheCampoMaanNationalPark ontheborderto Central Africa.Thisspecimenwascollected onthewestern widespread speciesfoundthroughoutmuchofwesternand ve speciesthatare foundinbothW+C,twospeciesthatare foundinbothE+N,andonespecieseachthatis UNRdLs aeoyNWCES E C W N IUCN RedListCategory es ocr 01 41 64 14 2 10 2 15 1 0 0 24 9 2 16 4 1 0 0 2 10 3 0 0 4 2 0 0 0 0 2 0 Least Concern Near Threatened Vulnerable Endangered Critically Endangered aaDficet0 25 25 117 4 5 20* 12 44* 5 31* 0 24* 2* Total speciesperregion* Data Defi cient Potamonautes
The numberofAfricancrabspeciesineachIUCNRedListcategorybyregion (Cumberlidge Sudanonautesaubryi Erimetopus The totalnumberofspeciesperCategorydoesnotincludefoundinmore thanoneregion. Numbers et al , eightofthe11species © JENSKIPPING are LC,asare oneofthethree speciesof Liberonautes . 2009). and
Forty-three outofthe76species Potamon Platythelphusa , andoneeachofthetwo (Least Concern)isa . Twenty-six (28%) Sudanonautes , three ofthe rmed REGION , central Africa(Brooks (Smith Africa Africa (CumberlidgeandDaniels2008),western Africa(Darwall available foreastern Detailed assessmentsofspecies’regional statusare also Extinct untilexhaustivesurveyshavebeencarriedout. be notedthataspeciescannotformallyassessedas Extinct (EX)orintheWild(EW).However, itshould species numbers(Cumberlidge1999). Forexample,there identical forest areas that haveagradientofdeclining areas ofhighbioticdiversitysurrounded byapparently shown bythedistributionofspecies inAfricanforests is the fewestinsavannaecosystems. Oneinteresting pattern rainforest ecosystems,especiallyinhighlandareas, and of water, with the highestnumberofspeciesoccurringin appears todependonvegetationcoverandtheavailability in highlandregions (Cumberlidge1999).Speciesdiversity are noticeablymore abundantintherainforest, especially found inallofthemajorecosystemscontinentalAfrica,but and vegetationcover. Forexample,freshwater crabsare region, andvarieswithecosystem,aquaticdrainagebasins, crab faunainAfricaisnotuniform,changesfrom region to available dataindicatethatthecompositionoffreshwater are necessarytoascertaintheiractualdistributions.The just afewrecords, andinthesecasesfurthercollections that are knownonlyfrom eitherthetypelocalityorfrom African speciesare quitewell studied, there are stillsome are stilllikelytobeincomplete. Althoughamajorityofthe have beenderivedfrom allavailable specimenrecords but (Cumberlidge 1999;Table 6.1).Distributiondatausedhere endemic tothecontinent,anddistributedover43countries recognisable freshwater crabfauna,withallspecies Continental Africahasarich,highlydiverse,anddistinctly ofspeciesrichness 6.3 Patterns et al . 2009), northern Africa(Garcia. 2009),northern Red ListCategory et al Total speciesper . 2011). et al . 2005), southern . 2005),southern et al et al . 2010),and . 2009, . 2009, CHAPTER 6 | FRESHWATER CRABS 185 are are . 2000), is found et al Erimetopus has a number of Deckenia REGION 6641 and Potamon Louisea DENIS TWEDDLE © 9 00990 8 07000 4 04000 3 01200 2 00020 2 00200 2 00200 1 10000 1 01000 N = northern W = western Africa, 76 2 4 23 33 19 1105510 Potamonautes Genus # Species N W C E S Sudanonautes Platythelphusa Liberonautes* Afrithelphusa* Potamonemus Deckenia* Erimetopus* Louisea* Potamon Globonautes* Genera /region 2 endemic to western Africa, only in eastern Africa. Although both found only in central Africa, and wide distributional ranges in the Mediterranean and Middle wide distributional ranges in the Mediterranean East as far east as the Himalayas (Brandis Palaearctic within Africa this genus is found only in the zone of northern Africa. Table 6.4. Number of species and genera found in and genera species Number of 6.4. Table in Africa. each region central Africa, E = easternAfrica, C = southern Africa, S = endemic. Africa. * = regional
, and are all are For example,
Sudanonautes Potamonautes Platythelphusa Liberonautes include some widespread include some widespread ve regions, and (Least Concern) is a relatively abundant and widespread species found in large slow fl owing rivers in (Least Concern) is a relatively abundant and widespread species found in large slow fl Liberonautes occur in two regions. occur in two regions. , and is found in all fi ve African regions has genera with species that has ve African regions species, and some species that are endemic to a particular species, and some species that are restricted contrast, six genera have a relatively In region. endemic to a particular region. range and are Sudanonautes Afrithelphusa, Globonautes Potamonautes lirrangensis shery Malawi. in rainforests across central and eastern Africa. It is subject to a small commercial fi Potamonemus is found in three regions, while regions, is found in three are three relatively small areas of the African rainforest that of the African rainforest areas small relatively three are high number of species having an unusually stand out as of endemic species: the than normal number and a higher in western the Lower Guinea forest Africa, Upper Guinea of eastern and the forests in Nigeria and Cameroon, forest (Cumberlidge 1999).D. R. Congo taxonomic diversity is highest in At the genus level the western lower in eastern and central Africa (six genera), lowest in northernAfrica (four genera), and Africa (two 6.4). genera) and southern(Table Africa (one genus) (with Although eastern region Africa is the most species rich genera and two subfamilies), western 44 species in three most diverse (with 24 species, Africa stands out as the 6.4). Distribution (Table six genera and two subfamilies) that each at the genus level indicate patterns considered of the fi that have have a wide distribution, and many other species distribution. Five out of eleven genera have a a restricted For example, than one region. wide distribution in more Potamonautes 186 CHAPTER 6 | FRESHWATER CRABS Stewart 1997;Corace et al species inAfrica(Bott,1955,1959,1969,1970;Stewart Potamonautes 6.3.1 western Africa; the western limitofthegenus isSenegal, Africa;thewestern western The eightspeciesof 1999; CumberlidgeandBoyko2000). have arestricted distributionincentralAfrica(Cumberlidge the othertwospecies, are widespread throughoutandcentralAfrica,while western Sudan savanna.Eightofthe11species monodi savanna inNigeria,whilethesemi-terrestrial air-breathing savanna. and occur exclusivelyinrainforest habitats,while granulatus, S.orthostylis,chavanesii rivers) andonland. most aquatichabitats(standingwater, streams andmajor rainforest, GuineaandSudan savanna), andare foundin andcentralAfrica(tropical major ecosystemsofwestern Bioko. Speciesof Nigeria, south-westCameroon, andontheislandof species 1999 (Cumberlidge 1989,1991,1993b,c,1994b,1995,a,b,c,d, Africa as fareastUgandaandSudanineastern and centralAfrica,withonespecies( Sudanonautes Niger Riverfl is evenfoundintheSaharadesertMali,butonlywhere the along theNileRiver).Onespecies( Africa(bothwidespread and twoinnorthern speciesfound species of (33, 24,and20speciesrespectively), andthere are onlyfour Potamonautes forested CongoRiverbasin.The vastmajorityofspecies species of are endemictoaveryrestricted area. Thelargest numbersof breathing habit,whileothershave anarrow distributionand the continent,somehaveadoptedasemi-terrestrial air- and are associatedwiththemajorriverbasinsthroughout region. Somespeciesof (the Maghreb) thatliesinthePalaearctic zoogeographical does notoccurisinthecoastalfringeofnorth-westAfrica Africa (Figure 6.2).Theonlyregion where from theCapetoCairo, andfromof SenegaltotheHorn Africa, andare distributedthroughout sub-SaharanAfrica species ofthislarge genusare foundinallfi and Tavares 2006;CumberlidgeandDobson2008).The76 2004, 2006;CumberlidgeandBoyko2000; Cumberlidge andVanini 2004;ReedandCumberlidge S.fl ) ( . 1995; Cumberlidge 1997, 1998, 1999, 2009a,b; . 1995;Cumberlidge1997,1998,1999,2009a,b; distribution patterns All freshwater crabspecies:interpretation of Figure 6.2 is the only species found in both Guinea and in dry istheonlyspeciesfoundinbothGuineaanddry
oweri are foundintheforested region ofsouth-east Sudanonautes kagoroensis Potamonautes Potamonautes ows through Timbuktu(Cumberlidge1999). are foundinbothrainforest andwoodland includes 11 species distributed in western includes11speciesdistributedinwestern includesbyfarthelargest numberof occur in eastern, central, and southern Africa Africa central,andsouthern occur ineastern, ) . Thelargest numbersof Sudanonautes africanus, S. nigeria, S. Sudanonautesafricanus,S.nigeria, Sudanonautes Liberonautes et al S. orthostylis occur in eastern Africa and in the Africaandinthe occurineastern in western Africa (all endemic), Africa(allendemic), inwestern . 2001;Cumberlidge Potamonautes occur in most of the occurinmostofthe Potamonautes ecorssei and occurs only in Guinea occursonlyinGuinea are allrestricted to S. fl and S. sangha oweri are widespread Potamonautes Sudanonautes Sudanonautes Sudanonautes ve regions of S. faradjensis et al ) reaching S. aubryi S. aubryi . 2002; . 2002; , each , each S. S. )
population growthandindustrialagrariandevelopment. threats suchashabitatlossanddegradationlinkedto Tunisia. ItisLeastConcernandaffectedbyhumaninduced endemic tothestreamsandriversofMorocco,Algeria © The freshwatercrab Africa. partoftheUpperGuinea forestin thewestern of 1996a,b; 1999).Thefourspeciesof forest (CumberlidgeandSachs1991;Cumberlidge1991, in smallwaterreservoirs intree holesinclosedcanopy of theUpperGuineaforest inLiberiaandGuinea,lives the LiberianTree part HoleCrab,occursonlyinthewestern 1992; Cumberlidge1993a;1999). leave thewateratnighttofeed(CumberlidgeandClark 6.2). Thesespeciesoccurinsmallstreams andprobably Nigeria, south-westCameroon,Togo andwestern (Figure and centralAfricaintheforested highlandsofsouth-east The three speciesof nanoides nimba L. paludicolis are foundinaquatichabitats(streams ormajorrivers)while in bothGuineaandSudansavannazones.Mostspecies mostly inrainforest, although and Côted’Ivoire. Speciesof species eachhavealimiteddistributioninLiberia,Guinea, entire rangeofthegenus,while theremaining seven 6.2). Onespecies( (Cumberlidge 1999;CumberlidgeandHuguet2003 limitisMali limitisGhana,andthenorthern the eastern other species, is relatively widespread inthe lowerCongoRiver, whilethe 1999). Oneofthetwospecies and themoistlowlandrainforests (Cumberlidge1994a, only insouth-westCameroon intheforested highlands 1996a,b; 1999).Thetwospeciesof only knownfrom SierraLeone(Cumberlidge1987,1991; found onlyinGuinea,while al distribution (CumberlidgeandReed 2004;Cumberlidge . 2009).Allninespeciesof JEAN-PIERRE BOUDOT are foundathighaltitudes,and Afrithelphusa gerhildae are foundexclusivelyinthemajor rivers. ismore terrestrial and E. vandenbrandeni, Potamonalgeriense L. latidactylus Potamonemus Platythelphusa A. afzelii L. latidactylus and ) iswidespread overthe Liberonautes Globonautesmacropus Erimetopus Afrithelphusa occur in only in western occurinonlywestern is more restricted inits A. monodosa L. rubigimanus and (Least Concern)is Louisea L. chaperi A. leonensis are endemicto isalsofound ( are found are found E. brazzae) occur only are both ) ( and and Figure are et L. L. ,
CHAPTER 6 | FRESHWATER CRABS 187
) ), ). ), four P. lividus P. L. balssi P. idjwiensis, P. and P. choloensis, P. choloensis, P. P. P. xiphoidus P. P. gonocristatus P. and and L. edeaensis P. reidi, P. triangulus, Potamonemus reidi, P. P. ), one from southern), one from Africa ( ), four from eastern), four from Africa ( A. monodosa, G. macropus, L. nanoides, L. nimba and L. rubigimanus from western Africa ( from sachsi, central Africa (the most speciose region) and just one and just one central Africa (the most speciose region) no threatened are in southern 6.3). There Africa (Figure species in the northern Of the threatened Africa region. from assessed as EN, of which four are species, 10 were western Africa ( and platycentron, mutandensis, P. P. central Africa ( and two from southernNone of the species from Africa was assessed assessed as VU, as EN. A further fourteen species were eastern from Africa ( of which nine are montivagus, P. infravallatus, P. ignestii, P. gerdalensis, P. unisulcatus, raybouldi, P. pilosus, P. et
2005). One . 2009; Figure . 2009; Figure are endemic to are et al. et al . 1999; Marijnissen ) is found in continental Deckenia et al . 1995; Marijnissen P. algeriensis P. ( et al . 2000; Cumberlidge 2009a). Potamon et al 2004), while both species of 6.1a; Table 6.5). Most of these (20 species) are found in these (20 species) are 6.5). Most of 6.1a; Table either eastern Africa (11 species) or western Africa (nine species in threatened only three are species), and there Lake Tanganyika (Cumberlidge Lake Tanganyika al. eastern Africa (Ng species of in northernAfrica and is endemic to the Mahgreb Africa (Brandis species 6.3.2 Threatened Some 28% (26 out of 94 species) of African freshwater threatened assessed as being in one of the three crabs were categories (CR, EN, VU) (Cumberlidge Figure 6.2. The distribution of freshwater crab species across mainland continental Africa. Species richness = Africa. Species mainland continental across crab species The distribution of freshwater 6.2. Figure sub-catchment. species per river/lake number of 188 CHAPTER 6 | FRESHWATER CRABS IUCN RedListCategory. Table 6.5.Numberofspeciesfrom eachgenusin richness =numberofspeciesperriver/lakesub-catchment. Figure 6.3. eu CN UE RDD CR EN VU NT LC Genus oa pce 431 0227 2 10 14 3 64 Total species Globonautes Potamon Louisea Erimetopus Deckenia Potamonemus Afrithelphusa Liberonautes Platythelphusa Sudanonautes Potamonautes
The distributionofThreatened freshwater crabspeciesacross mainlandcontinentalAfrica.Species 430124 018 000100 010000 000200 100001 020000 101001 000103 301220 800001 800003 IUCN REDLISTCATEGORY
a lackofknowledgeorunder-collection. Thesespeciesare are notsimplyaproduct ofomissionerrors stemmingfrom Africa (Figure 6.4).Thelimiteddistributionsofthesespecies restricted rangespeciesare irregularly distributedthroughout western Africa).Twowestern species( and onefrom centralAfrica( have arestricted range(<20,000km Excluding DDspecies,some29specieswere foundto 6.3.3 Restrictedrangespecies habitat degradationandpollution. and agriculturaldevelopmenttheassociatedaquatic threats tothesespecieswere identifi (both fromAfrica)were eastern assessedasNT. Themain Africa. Justtwospecies, were assessedasCR,bothofwhichare from western
Deckenia imitatrix P. sachsi L. grandbassa 2 ed asurban,industrial, ) (Table 6.6),andthese , which is also found in , whichisalsofoundin and and L. lugbe D. mitis )
CHAPTER 6 | FRESHWATER CRABS 189 WINKS EMMERSON © (Vulnerable) is a (Vulnerable) Potamonautes lividus The Blue River Crab relatively restricted range species endemic to swamp forests in north-eastern Kwa-Zulu Natal, South Africa. (and 2 or less). 2 LC < 20,000 <10 N C LC < 800 <5 N C LC < 20,000 8 N C LC < 5,000 7 N W VU < 20,000 <10 N C VUVUVUVUVU < 20,000VU < 20,000VU < 20,000VU < 20,000VU 4 < 20,000VU <10 < 20,000VU <5 < 20,000VU <5 < 20,000 YVU <10 N < 2,000 <10 < 20,000 N <10 < 20,000 N <5 N < 20,000 W E <10 Y < 2,000 <10 E N E 6 N E <5 N S <5 Y E N E N E N W W E E ENENENENEN < 5,000 < 5,000 < 5,000 < 20,000 2 < 5,000 5 1 3 N 2 N N Y W N W W W C EN < 5,000 3 N C ENENENEN < 5,000 < 500 < 500 < 500 <10 <5 <5 <5 N N N N E E E E CRCR ~ 100 100 ~ 1 1 N N W W RL = IUCN Red List status; LC = Least Concern; VU = Vulnerable; EN = Endangered; CR = Critically CR = Critically EN = Endangered; Least Concern; Red List status; LC = RL = IUCN VU = Vulnerable; cient. cally recorded, through many surveys conducted through cally recorded, Globonautes macropus Liberonautes nanoides Liberonautes rubigimanus Louisea edeaensis Liberonautes grandbassa Liberonautes lugbe Afrithelphusa monodosa Species RL Category Range (km2) # Loc PA Region Louisea balssi Potamonautes gonocristatus Potamonautes idjiwiensis Potamonautes mutandensis Potamonautes platycentron Erimetopus brazzae Potamonautes margaritarius Potamonemus mambilorum Potamonemus sachsi Sudanonautes kagoroensis Liberonautes nimba Potamonautes choloensis Potamonautes gerdalensis Potamonautes ignestii Potamonautes infravallatus Potamonautes lividus Potamonautes montivagus Potamonautes pilosus Potamonautes raybouldi Potamonautes reidi Potamonautes triangulus Potamonautes unisulcatus Potamonautes xiphoidus Despite the dangers of population fragmentation, current Despite the dangers of population fragmentation, current population levels of stenotopic species assessed as LC or estimated to be stable because they have been NT were Table 6.6. Species of freshwater crabs of Africa restricted to single river basins, excluding those considered to considered those excluding river basins, to single restricted of Africa crabs of freshwater Species 6.6. Table be Data Defi Endangered; Range = an estimation of the species distribution range based on all known specimens; #Loc = Number of specimens; #Loc = based on all known species distribution range an estimation of the Range = Endangered; of = yes, N = no; regions Y area; in a protected = found PA the species was collected; which localities from discontinuous = northern noted as N W = western Africa, Africa are Africa, E = eastern Africa, C = central Africa, S = southern Africa. specifi they may over the years, as being absent in localities where Any disruption to the habitats have been expected to occur. or development, pollution, of these species (either from could have serious consequences, given that political unrest) as range species have been assessed 24 of these restricted range is 6.6). Any species with a restricted (Table Threatened population fragmentation potentially vulnerable to extreme a a rapid decline, and even extinction, in and could suffer short time should dramatic changes in land-use relatively of immediate concern its habitat. It is therefore suddenly affect that 24 of the 94 crab species that could be assessed are distribution ranges of less than 20,000km known from some of these have an estimated range of 5,000km 190 CHAPTER 6 | FRESHWATER CRABS are from centralAfrica( be DD(Table 6.3,Figure 6.1a).ThirteenoftheDDspecies Some 27ofthe94species(28%) assessed were judgedto 6.3.4 DataDefi al species nowindecline(Cumberlidge2008;Cumberlidge their beingtheremnant populationsofformerlywidespread (marginal) habitat orthrough islandcolonization, ratherthan relatively recently inresponse toisolationinaspecialised unknown, butitisthoughtlikelythattheyhavespeciated the restricted rangesofthestenotopicspeciesare largely and endangertheirlong-termexistence.Thereasons for threats thatwouldimpactthehealthofthosestreams collected recently andthere are noidentifi mainland continentalAfrica.Speciesrichness=numberofspeciesperriver/lakesub-catchment. Figure 6.4.Thedistributionoffreshwater crabspecieswith severely restricted distributionalrangesacross . 2009). cient species E. vandenbrandeni,P. acristatus, able immediate immediate able et et in viewoftheabsencerecent informationontheirExtent found inrecent years.ThesespecieshavebeenlistedasDD thatinmanycasestheseDDspecieshavenotbeen concern Cumberlidge 1999; the distributionoftheselittle-known species(Bott1955; available, ascarcity thatcontinuestofueluncertaintyabout of DDspeciesrefl Africa were foundtobeDD.Therelatively highproportion macrobrachii three are fromAfrica( southern bipartitus, P. didieri,P. rodolphianus, are fromAfrica( eastern gerhildae, A.leonensis,P. senegalensis, and schubotzi, P. semilunaris,Potamonemus asylos,S.orthostylis P. adeleae,P. adentatus,P. lueboensis,P. punctatus,P. S.sangha ) (Figure 6.5).Noneofthespeciesfrom northern ), fi ve are fromAfrica( western ects thegenerallackofspecimens P. praelongata,P. P. amalerensis, P. kensleyi,P. dubius, et al and . 2009).Itisofgreat P. rothschildsi and
S.nigeria), A. afzelii, A. A. afzelii, and ), and ), and P. six six
CHAPTER 6 | FRESHWATER CRABS 191 owing 6.4 Major threats 6.4 Major threats crabs include water to African freshwater The main threats pollution, urban, industrial, and agricultural development, human from resulting and habitat loss and deforestation population and urban and agricultural expansion (Figure 6.6). 6.4.1 Habitat destruction to the endemic species include habitat destruction Threats driven by timber extraction, in the form of deforestation the demands of increasing agriculture, mining, increasing industrial development, the alteration of fast fl and the power, of hydroelectric rivers for the creation drainage of wetlands for farming and other uses (Figure E. S. cient and cient freshwater crab species across mainland continental Africa. Species mainland continental across crab species cient freshwater P. schubotzi, S. orthostylis P. or where they are known from either only from known they are or where , ) ) and the full range extent is uncertain. It is possible . 2009). It is also of concern that many of these species sangha that in some cases the DD status may be due to under- sampling but, as mentioned above, this is not thought to is be the case for many of the DD species. Further research needed on all of these species because, at least, they may range endemics vulnerable to habitat to be restricted prove loss. one or only a few localities ( vandenbrandeni information either on their taxonomic distinction (e.g. of Occurrence, habitat, ecological requirements, population habitat, ecological requirements, of Occurrence, (Cumberlidge and long-term threats size, population trends, et al a few individuals collected many years known only from are ago, and that no new specimens have been found recently. is insuffi there The DD status is also assigned where Figure 6.5. The distribution of Data Defi 6.5. The distribution Figure richness = number species per river/lake sub-catchment. river/lake sub-catchment. number species per richness = 192 CHAPTER 6 | FRESHWATER CRABS Figure 6.6. towns inAfricaincludepollutionbysewageandindustrial aquatic communitiesinriversassociatedwithcitiesand to freshwater ecosystems.Potential future threats to and soilerosion thatcontributes environmental damage movements, whichare oftenaccompanied bydeforestation habitat destructionincludepoliticalunrest andrefugee the forest. Otherthreats thatresult indeforestation and access toremote andpreviously undisturbedpartsof is furtherexacerbatedbyloggingroads thatprovide 6.6). Destructionoftheforests inmanypartsofAfrica region oftheUpperGuineanRainforest. Deforestation nearthevillageSimpain westernGhana,inthe
Major threats tofreshwater crabspeciesinmainlandcontinental Africa. © JOHANNES FÖRSTER egrets, herons, giantkingfi necked otters,watermongooses, Africancivets,kites, A large numberofAfricanpredators –includingyellow- 6.4.3 health ofpeoplethatmayeatthem. metals incrabscouldposeanincreasing problem forthe in theseareas, notleastbecausethebioaccumulationof should begiventotheimprovement ofthewaterquality contain highlevelsofcontaminants.Immediateattention feeders thatingestotherinvertebratesanddetritusmay freshwater crabpopulationsbecausetheyare benthic rivers andotherfreshwater bodies.Thesepollutantsimpact sewage systemsinurbanareas inAfricacanaccumulate iron ore, andcoltan,from organic wastesfrom leaking Pollutants from miningactivities fordiamonds,gold,bauxite, 6.4.2 Pollution survival. the future willhavethebiggest impactontheirlong-term management ofAfrica’s forests andwaterresources in to range-restricted endemic species, andthecareful factors combinetoincrease the overalllevelofthreat more research needstobecarriedout.Alloftheabove farmers mayprove tobelethal tofreshwater crabs,but and generalwaste.Someagriculturalpesticidesusedby introduced species Natural predators andcompetitionwith shers, monitorlizards, CHAPTER 6 | FRESHWATER CRABS 193 et S. cation and c ecologies and c ecologies P. lirrangensis P.
© RANDALL BRUMMETT ed and taxonomic advances ed and taxonomic advances P. perlatus, P. that are all currently assessed as LC primarily currently all that are cant areas of this vast continent still remain of this vast continent still remain cant areas ciently explored, and new species of freshwater and new species of freshwater ciently explored, . 2002). keys. Although taxonomic knowledge has advanced and museum collections of considerably recently, deal of work a great crabs have improved, freshwater is a need for further surveys still needs to be done. There recently, some taxa currently assumed to be widespread assumed to be widespread some taxa currently recently, to be complexes of several and common may prove distinct cryptic taxa, each with specifi conservation action. Three direct distributions requiring such possibilities are actions required 6.5 Research Signifi insuffi in if collection efforts discovered to be sure crabs are intensifi are areas remote available in the form of identifi readily become more granulatus However, on account of their wide distributional ranges. the distribution patterns of these species consist of many deal isolated subpopulations that show a great relatively of morphological variation, and further investigations may show these species to be species complexes (Daniels al
is rapidly Procambarus Platythelphusa sh ( sh (Foster and Harper Procambarus clarkii . 2009). The ecological importance of et al ) in the rivers of western Kenya, which out-competed spreading elsewhere in Eastern elsewhere Africa and may even reach spreading populations of it could threaten where Lake Tanganyika, crabs in the endemic genus freshwater pers. comm.). (Howard, issues 6.4.4 Taxonomic crabs may prove The evolving taxonomy of freshwater to be a challenge for conservation planning in the future has been made because, although a lot of progress 2006, 2007; Ogada 2006). Recent reports indicate that indicate that 2006, 2007; Ogada 2006). Recent reports the highly competitive and crocodiles – depend on freshwater crabs as vital – depend on freshwater and crocodiles crabs components of their diet. This is because freshwater in African aquatic macro-invertebrates the largest are food chain ecosystems and form an integral part of the (Cumberlidge crabs in food webs in Africa is underlined by freshwater the impact of alien North American crayfi clarkii crab in freshwater the native crabs. The drop and replaced populations in these systems led to a subsequent decline in clawless otter populations that fed on crabs, most competition with other predators likely due to increased attracted by the abundance of crayfi Pollution from urban and domestic sources is a major threat to threatened freshwater crab species. Pollution from urban and domestic sources is a major The Louisiana Crayfish Procambarus clarkia has become seriously invasive throughout inland waters in Africa. It is omnivorous and will hunt and compete with the native freshwater crab species. © GEOFFREY HOWARD CHAPTER 6 | FRESHWATER CRABS 195 ne line rst step . 2009) represents a fi . 2009) represents et al c habitat requirements and a restricted and a restricted c habitat requirements cation of threatened species within the threatened cation of eld survey. The scarcity of available specimens The scarcity eld survey. cient fi cient and streams. The increasing loss of natural vegetation and of natural vegetation loss The increasing and streams. agriculture land development and of a result pollution as lowland rivers in the the likely to affect however, are, aquatic species that and many of the wholly long term, Even species be vulnerable. could eventually live there declines should catastrophic LC could suffer assessed as development, hydrology, be abrupt changes in land there known how the highland It is not or pesticide-use regimes. disturbance and pollution but, taxa will cope with habitat it is habitat requirements, considering their specialised as species will not adapt as readily likely that most of these countries lowland species. In many widespread the more often only a fi with a rapid pace of development, one assessed as LC from separates a species assessed that is assessed as EN. one from as VU, or a VU species a dramatic impact on could have Development projects species with specifi be therefore distribution. Conservation activities should the integrity of sites and aimed primarily at preserving key habitats while at the same time closely monitoring crab populations. freshwater crabs judged to be The 27 species of African freshwater of to this category primarily as the result assigned DD were insuffi is in some parts of the continent due to the long-term is in some parts of the continent due to the little is known of poor security situation, and as a result to these or threats the habitat needs, population trends, it is information has been gathered, species. When more a relatively expected that almost all DD species will have distribution and be endemic to the river basin restricted found. are they where crabs in The conservation assessment of freshwater Africa (Cumberlidge identifi the toward the development of a conservation toward and region range of strategy for endemic species. The restricted many species, together with the on-going human-induced primary are loss of habitat in many parts of the region, causes of concern for the long-term survival of this fauna. of endemism, crabs have a high degree freshwater Africa’s with many species living in specialised habitats such as highlands, and river rapids, lowland marshes, forested to determine is recommended islands. Additional research areas size and design of protected the minimum effective species such as crabs. for freshwater et ed to c threats to Africa’s to Africa’s c threats . 2008; Cumberlidge consider shery managers ne species distributions, species distributions, ne et al cally include the conservation and cally include the . 2009). et al c habitat requirements, describe population describe population habitat requirements, c ne specifi . 2009). 6.6. Conservation recommendations 6.6. Conservation patterns crabs The biology and distribution of the freshwater the potential as are becoming better known, of Africa are With 27 species (28%) to their long-term survival. threats Africa crabs from of freshwater of the 94 non-DD species risk of global extinction, assessed as being at currently endemic largely of the continent’s the long-term survival crab fauna is a concern. it is freshwater Nevertheless, plans for threatened hoped that conservation recovery identifi species will be developed for those species to discover new species, refi to discover this Red List be in need of conservation action through (Collen assessment process defi that specifi measures populations of river crabs. sustainable use of local levels and trends, and identify specifi and levels and trends, It is vital to the crab fauna. unique freshwater important and ecosystems that fi health of these al crabs The conservation of many species of freshwater of natural of areas depends primarily on preservation Although it enough to maintain water quality. habitat large freshwater is not yet known exactly how sensitive African is evidence to polluted or silted waters, there crabs are not likely to survive that similar crabs are Asia from 2007). Yeo when exposed to these factors (Ng and and exploitation of natural Development, agriculture in developing economies, necessary realities are products crab may have to be made if freshwater but compromises Judicious not to be extirpated in the future. species are unlikely to cause species is use of resources and careful not heavily extinctions as long as water drainages are and vegetation some forest polluted or redirected, respected are areas cover is maintained, and protected (Cumberlidge Common species assessed as LC have a wide distribution of the region in the rivers, wetlands, and mountain streams tolerant of changes to be relatively have proved and, so far, aquatic ecosystems. It is encouraging in land-use affecting adaptable species can persist in the already that these more disturbed and visibly polluted parts of the lowland rivers 196 CHAPTER 6 | SPECIES IN THE SPOTLIGHT C \mfclk`feXe[\ok`eZk`fe `eCXb\KXe^Xep`bX1Xdf[\c]fijkl[`\j`e 8le`hl\jg\Z`\jÕ UNDP/GEF ProjectonLakeTanganyika, RegionalCoordinationUnit, c/oSecretariatoftheLake Tanganyika Authority, RoutePri ¹ Platythelphusa maculata crevices underneathrocksandstones. that enablesittohideinsidenarrow has evolvedanextremelyfl shells. claw thattheyusetocrushsnail with onesignifi of instance, bothmalesandfemales al. adaptive evolution(Cumberlidge them idealsubjectsforstudiesin and ecologicalfeaturesmaking a strikingvarietyofmorphological Africa. Platythelphusidcrabshave species fl form theonlyknownlacustrine the genus platynotus crabs, including derived from freshwater ancestors. derived fromfreshwaterancestors. that allLakeTanganyika crabsare However, geneticevidenceindicates millions ofyears(Moore1903). sea andremainedunchangedover in thelakeafterarecessionof of marinecrabsthatgottrapped platythelphusids werearelictgroup Initially, itwasthoughtthatthe Glor 2010,andreferencestherein). processes ofadaptiveevolution(e.g. famed naturallaboratoryforstudying diverse lake,LakeTanganyika isa million years)andmostspecies tanganyicensis. large endemicsnail refuge withinemptyshellsofthe species, whichisoftenfoundtaking other hand,isatinyround-bodied Platythelphusaarmata As Africa’s oldest (estimated 9-12 As Africa’soldest(estimated9-12 1999;Marijnissen Bujumura, Burundi Platythelphusa conculcata endemic species of freshwater endemic speciesoffreshwater Lakes inharbouring10 among theAfricanGreat ake Tanganyika isunique Jg\Z`\j`ek_\jgfkc`^_k ock of freshwater crabs in ock offreshwatercrabsin Platythelphusa and nine species in andninespeciesin cantly enlarged Potamonautes Neothauma et al. ,
on the on the which which are armed are armed 2004).For at body et the et al. (Duftner including severalcichlidgroups 2000). Research on the crabs shows 2000). Researchonthecrabsshows niche diversifi result ofcompetitionandecological in theAfricanGreatLakesare the highlevelsofspeciesdiversity longstanding hypothesisisthat relatively shortperiodoftime.A seem tohaveevolvedwithina between theplatythelphusidcrabs million yearsago. during thesameperiod,about3.3-2.5 (Day andWilkinson 2006),diverged Marijnissen basins (CaneandMolnar2001; Tanganyika wassplitintoseparate dropped substantiallyandLake and drought,whenthelakelevels with aperiodofclimatechange the platythelpusidscoincides The evolutionarydivergenceof Potamonautes platynotus out ofthewatertofeedamongstrocksalonglakeshore. The morphological differences The morphologicaldifferences Synodontis 2004, 2005, 2007), as well as 2004,2005,2007),aswell fZbf]]i\j_nXk\iZiXYj etal. et al. cation (Schluter 2005; Koblmüller 2005;Koblmüller catfi 2006). Other taxa, 2006).Othertaxa, sh speciesfl (LC) is primarily aquatic but has been observed toclimb (LC)isprimarilyaquaticbuthasbeenobserved ock ock (Marijnissen that coexistinrockyshoreareas among platythelphusidspecies considerable ecologicalversatility However, thereisalsoevidenceof niche divergence(Marijnissen2007). could indeedbelinkedtoecological morphological differentiationthat endemic fi included inthedietofnumerous prey aswellpredators.Theyare components ofthefood-webas areas. Thecrabsareimportant the speciesarefoundinrocky these habitats,butthemajorityof of mud.Crabscanbefoundinall shells, sandybeachesanddeeplayers of empty stromatolite reefs,toextensivebeds cobble beaches,rockyoutcrops,and diversity ofhabitats,rangingfrom and catfi ( Mastacembelus plagiostomus Lake Tanganyika offersawide sh C. stappersi
( Chrysichthys brachynema Neothauma tanganyicensis sh, suchasspinyeels etal. © Marijnissen, S.A.E.¹ ), andcatfi HEINZ BUSCHER ncipale, KigobeNord, 2008). 2008). sh sh ), bagrid ), bagrid
CHAPTER 6 (Synodontis dhonti), as well as cichlids and breakdown of mate recognition Lake Tanganyika crab species (Boulengerochromis microlepsis systems as water visibility is reduced (Marijnissen et al. 2008). and Lamprolus lemarii). The crabs (Donohue and Molinos 2009). Lake Tanganyika’s crabs are themselves are top invertebrate Research in Lake Tanganyika has also threatened by invasion
predators, and their relative demonstrated negative impacts of of the non-native Louisiana |
abundance can have important sedimentation on algal productivity Crayfi sh (Procambarus clarkii) and SPECIES IN THE SPOTLIGHT implications for organisms lower as well as on species diversity and the Red Claw Crayfi sh (Cherax in the food web, including algae, densities of ostracods, snails, and quadricarinatus). Both of these insect larvae, ostracods, and snails fi shes (McIntyre et al. 2004, and crayfi sh have been introduced to (Marijnissen et al. 2008, 2009). references therein). As crabs are Africa for aquaculture purposes and Lake Tanganyika is increasingly closely linked, either as predators are spreading rapidly throughout the under pressure from environmental or prey, to many other organisms continent (e.g. Arrignon et al. 1990; problems generated by human that are vulnerable to the impacts G. Howard, IUCN, pers. comm.). populations (reviewed in Lowe- of increased sedimentation, it is Considering their current presence McConnell 2003). The majority likely that they will respond by in catchments adjacent to Lake of species diversity is found shifting their foraging patterns (e.g. Tanganyika, it is likely that these in rocky near-shore areas, McIntyre et al. 2004). However, crayfi sh will move into the lake which are severely impacted by demonstrating and predicting the basin in the near future. Due to their sedimentation resulting from effects that habitat disruptions will resilience to different environmental ongoing deforestation in the lake have on the endemic crabs requires circumstances, high dispersal capacity catchment (Cohen et al. 2005). taking into account the complexity and opportunistic diet, crayfi sh Sedimentation can negatively affect of the ecosystem. Changes in the are capable of causing dramatic aquatic organisms through a range species fl ock may be delayed relative biodiversity shifts (e.g. Smart et al. of factors including deterioration of to those of other species as crabs 2002; Snyder and Evans 2006; Cruz habitat quality and heterogeneity, occupy multiple positions within and Rebelo 2007), and could cause an reduction of food intake, alteration the food web, and a considerable ecological disaster if they establish of competitive relationships, amount of overlap exists among the themselves in Lake Tanganyika. decrease of reproductive success, ecological niches of the different Without conservation effort to address environmental threats in Lake Tanganyika is the Lake Tanganyika basin, this could lead to the loss of a dynamic increasingly under pressure endemic species fl ock that provides from environmental problems unique insight into the evolution and ecological functioning of this generated by human populations unusually diverse lake system.
Platythelphusa armata (LC) is the largest and most distinctive species in the genus. Juveniles are often found sheltering within the empty shells of the gastropod Neothauma tanganyicense. © SASKIA MARIJNISSEN 197 Jg\Z`\j`ek_\jgfkc`^_k K_\ SPECIES IN THE SPOTLIGHT otamonautes raybouldi, where a tree branches. When snails. When it fi nd these snails it | the East African tree hole branching occurs it often creates a takes pieces of the snail shell back crab, is a bizarre example small hole, or bole, which fi lls with to its water-fi lled tree hole and of crab evolution. It is water. The East African tree hole masticates them, which releases Gexceptionally rare for a crab to crab has evolved to occupy this the calcium ions into solution. It live in a tree (an arboreal species), niche, adapting its behaviour and then ingests the calcium in order to CHAPTER 6 and most of those that do depend morphology accordingly. obtain suffi cient quantities to build on an external water supply – What is remarkable about this its own exoskeleton. It also eats its for example, several species of species is its natural history. This old exoskeleton when it moults. mangrove crab reside in trees for species has an apparent lack of As crabs have a hard exoskeleton protection but lay their eggs in dependence on any water body, they need to moult in order to be the sea. except that collected in tree holes able to grow and mature. Below the There are over 1,200 species as water run-off from the host tree. hard exoskeleton is soft tissue that of freshwater crab, out of which What is yet more remarkable is the expands over time. Eventually the only a handful exhibit some kind way in which it obtains the calcium hard exoskeleton will prevent any of arboreal association. The East in order to build its exoskeleton further expansion, at which stage African tree hole crab is unique (Bayliss 2002). It forages at night the process of moulting (or ecdysis) in this case. It lives solely in the or in heavy rain, when it leaves occurs. water fi lled tree holes formed its hole to search for leaf-litter It was fi rst observed and collected by Professor John Raybould in 1966, but it was not until 2004 when it was eventually described under the patronym Potamonautes raybouldi (Cumberlidge and Vannini 2004). It is a forest species found in just a few sites in east Africa, such as the East Usambara Mountains in northern Tanzania, and several sites in southern Kenya (e.g. the Shimba Hills). It has primitive lungs that enable it to spend time outside water, and elongated legs that allow it to climb up and down the tree trunks. The males have one claw that is larger than the other, which is presumably used for display and eventual mating. The females have been found protecting their young (parental care), living in the same tree holes with a brood of juvenile crabs that is presumably their own. These tree holes need Juvenile crabs in a communal tree hole. © JULIAN BAYLISS cleaning on a regular basis as leaves 198 ¹ Conservation Science Group, Department of Zoology, University of Cambridge CHAPTER 6 and other debris accumulate. The juvenile crabs are often too small to cope with moving larger leaves and debris, and would be unable to clean the tree-hole without | assistance from the adult female SPECIES IN THE SPOTLIGHT crab. They also need protection from predators, possibly including other crabs. The local people in the East Usambara Mountains use the water that Potamonautes raybouldi lives in as a medicinal potion. They often collect the water, which they call ‘Mazi yangodi’ (the water from holes with crabs), to give to pregnant women. The water contains calcium, which is required by pregnant women to reduce the risk of pregnancy- induced hypertension (PIH), high blood pressure (pre-eclampsia), and kidney failure, all of which can result in miscarriage. Exactly how and when the connection between the ‘crab water’ and calcium defi ciency in pregnant woman was arrived at is a mystery. The East African tree hole crab is a spectacular example of adaptation, essentially of an aquatic species not only to a terrestrial environment but also to an arboreal existence. It has lost its dependency on larger water bodies such a streams and rivers, and via the process of natural selection, through trial and error (otherwise known as evolution), it has managed to exploit a niche that crabs have rarely occupied. The further connection between the local population of the East Usambara Mountains and their use of the Mazi Yangodi further highlights this species scenario as an amazing example of natural history and a connection with a human society. However, this story of natural history is not fi nished. Exactly how the males and females fi nd each other in order to mate in closed canopy, dark forest, when each crab resides in a different tree and they are relatively short- sighted, remains an exciting mystery yet to be solved. Adult crabs defending their tree hole. © JULIAN BAYLISS 199