-I ~ -.....•..• 375 Mus. Reg. Sci. Nat. Torino, 2011 IX Colloquium Crustacea Mediterranea TOrino, September 2-6, 2008: pp 375-385 Richard G. HARTNOLL' Ascension Island: contrasting biogeography of land and rock crabs ABSTRACT The geographical distribution is described for two common crabs on Ascension Island in (he South Atlantic: the Jand crab Johngarthia lagostoll/a, and the rock crab Grapslls adscen- sionis. The ronnel' appears to have originated from the West Atlantic, where it has conspecific populations. In contrast, the latter, G. adscC'nsionis, seems (0 be an East Atlantie species, with a different species III the West Atlantic. This dif1erence is discussed in relation to ditTerences in potential means of dispersal, the distribution of related species, and the ongin of other shallow water taxa on Ascension Island. Keywords: Ascension Island, biogeography. crabs, Johngarrliia, Grapslls Asccnsionls!and Ascension Island is a small and remote island in the central South Atlantic Ocean. It has an area of 97 kme, and is 1500 km from the Liberian Coast in West Africa, and over 2000 km from Brazil (Fig. 1). It is of relatively recent (~I million years) volcanic origin (sec Ashmole & Ashmole, 2000, for general back- 2 ground). The nearest land is the similar sized (122 km ) but much older (14 mil- lion years) island of St Helena, 1300 km distant. Ascension's remote position and recent origin pose interesting questions regarding the origin and affinities of intertidal and terrestrial species. In this paper these questions are explored by examining the two common crabs on Ascension which fall into those categories. Thirty four species of braehyurans are recorded from Ascension Island (Manning & Chace, 1990). However, thc majority of these are restricted to ei- ther the subtidal or lower intertidal: they are difficult to collect, and their com- plete geographic distribution is hard to detennine, Nevertheless there are two very prominent crab species. In the upper intertidal there is the rock crab, Grap- sus adsccnsiollis (Osbeck, 1765), common on all rocky shores around the is- land (Hartnoll, 2009). On land the gecarcinid land crab Johngarthia !agostoma • University of Liverpool and Marine Biologieal Association 376 (H. Milne Edwards, 1837) occurs throughout the year at altitudes above 200m, and at lower levels during the annual breeding migration (Hannoll et al., 2006). These two crabs are suited to the proposed study on several further grounds. The two genera are distributed across the Atlantic, but the land crabs and rock crabs on Ascension differ in distribution and affinities. FUl1hermorc, they differ in their potential means of dispersal. For each of the two species a series of ques- tions will be addressed: What are the affinities of the Ascension Island species? Where did they come from, and how did they travel? Are they endemic to Ascension? Are there implications for conservation strategies on Ascension? ECC SAG Fig. 1 - The location of Ascension Island. and the prevailing surface current systems in the South Atlantic Ocean (after Briggs, 1974). ECC, Equatorial Counter-CUlTent; SAG, South Atlantic Gyre. The hold line indicates the West African province. - 377 The land crabs Johngmihia lagostoma and J. weileri (Sendlel; 1912) The nomenclature of these species has a complex history (see Hartnoll et al., 2006), and this has produced some past confusion regarding their geographical distribution. Both were previously included within Gecarcinus lagostoma H. Milne Edwards, 1837. Gecarcinus was a genus with species occurring on both coasts of the American mainland, as well as on a variety of Atlantic and East Pa- cific Islands. Tiirkay ( 1970) divided the genus into two subgenera, Gecarcinus and Johngarthia. G. lagostoma was included in the latter, together with the Pa- cific species G. plana/us Stimpson, 1860 and G. malpilensis Faxon, 1893. A II three species were mainly restricted to small oceanic islands. Tiirkay (1973) subsequently separated G. lagos/om a and G. weileri (Sendler, 1912), in the cen- tral and West Atlantic, and East Atlantic respectively, making four species in the subgenus. Finally Tiirkay (1987) elevated Johngarthia to generic status, giv- ing the Ascension species its current name ofJohngarthia lagostoma (H. Milne Edwards, 1837). Despite some earlier confusion (reviewed in Hartnoll et al., 2006), the ge- ographic distributions of the two Atlantic species ofJohngarlhia are now clear (Fig. 2). Johngarthia lagos/oma occurs on three islands off Brazil (Trindade, Fernando de Noronha, and Atol das Rocas), and on Ascension Island (see Man- ning & Chace, 1990; Ashmole & Ashmole, 2000). It does not occur on Trinidad, West Indies, a persistent confusion in the literature: see for example Croizat et al. (1974). Johngarlhia v.'eileri is found on several islands off West Africa in the GulfofGuinea (Annobon, Sao Tome and Bioco), and from the African main- land at Bibundi in Cameroun (Tiirkay, 1973). No land crabs have been reported from Saint Helena, despite its latitude and climate being suitable. The rock crabs Grapsus adscensionis and G. grapsus (Linnaeus, 1758) Until recently all Atlantic specimens of Grapsus were attributed to Grapsus grapsus. However, Manning & Chace (1990) reviewed the situation, and deter- mined that the East Atlantic material, including that from Ascension Island and Saint Helena, should be attributcd to G. adscensionis (Osbeck, 1765). The West Atlantic material, including that from Fernando de Noronha and the Caribbean, remains G. grapsus (a species also found in the Eastern Pacific). The status of material from Trindade Island (Brazil) remains to be determined. So G. ad- scensiOflis occurs on Ascension Island, Saint Helena, the islands in the Gulf of Guinea, the Azores, Cape Verde Islands, the Canaries, and Madeira (Bouvier, 1940; Manning & Chace, 1990) (Fig. 3). Along the east Atlantic coast it is recorded from southern Portl.l,g,alto northern Angola (Chace & Hobbs, 1969). In the Atlantic G. grapsus is found on Fernando de Noronha, Atol das Rocas, St Paul Rocks, throughout the Caribbean islands, and Bermuda. On the East Amer- ican mainland it occurs from Southern Florida to Pcrnambuco, Brazil (Chace & Hobbs, J 969). 0° VJ -..J 'Xl • J. lagostoma o •• . 0" o J. weileri Bioco 0 Atol das Rocas. o 0° •• Fernando de Noronha. I • Ascension. Annobon 0 o St Helena •Trinidade • Fig. 2 - The geographical distributions of.Johngurlhia lagosroma and J wei/cri. 0° G. adscensionis D ••• o 0.: DO' •- G. grapsus Atol das Rocas. · 8t Paul. 0° o • • • Fernando de Noronha. I ~•• • Ascension D Annobon • D .••- •• o 8t HelenaD • • ?? • •Trinidade • •• •• •• •• ,,,;.) --J Fig, 3 - The geographical distributions or (;/'''f'''(f\ "l!scCIISiOllis and G. gmpsl/s, \0 380 Origin and affinities ofJolmgarthia lagostoma The distances from Ascension Island to possible sources of land crab re- cruitment are all substantial. There are several potential mechanisms of disper- sal, but one can be discounted - that they arrived on Ascension when the Atlantic was much narrower. A similar explanation has been offered for the occurrence of the freshwater crab Seychellum alluaudi (A. Milne Edwards & Bouvier, 1893) on the Seychelles: that it is a relict of the time when the Seychelles were joined to the African/Asian land mass some 35 my a (Ng et al., 1995). However, As- cension Island arose only I - 1.5 my a (Atkins et al., 1964), when the Atlantic Ocean was at roughly its current dimensions. So three possibilities remain. Colonisation from West Africa, colonisation from the East American region, or colonisation from some now disappeared central Atlantic islands. They could have arrived by pelagic larval dispersal, or by rafting. The latter is a feasible proposition for marine crabs, but from what we know of the physiology of gecarcinid crabs, it would seem for them an un- likely method: the congeneric 1. planatlls survives only some 12 hours ofim- mersion in sea water (Ehrhardt 1968; Niaussat & Ehrhardt, 1971). So larval dispersal is the probable mechanism. Manning & Chace (1990) discuss the ori- gin of the decapod fauna of Ascension Island in some detail, and make two im- portant points. Firstly they outline the complex current patterns, and show how these could facil itate the transport of larvae from either the east or west Atlantic. Ascension is generally considered to lie within the westerly tlowing northern limb of the South Atlantic Gyre (Fig. I), which would facilitate transport from the African side. However, it is periodically washed by the easterly flowing Equatorial Countercurrent providing a route from the American side (Briggs, 1974). Secondly, they conclude that transport from either side to Ascension might be possible within the period of the larval development 01'1. lagosroma. Based on the larval development of the related 1. planatus, this could substan- tially exceed 30 days (Cuesta et al., 2007). Ashmole & Ashmole (2000) suggested "a possible scenario for the origin of the South Atlantic Island populations (ofland crabs) would be colonisation of Ascension from the western coasts of Afi"ica, and of Brazilian islands from As- cension". However, this assumes only a prevailing westerly current, which is not the case. The absence ofland crabs from St Helena (equally within the South Atlantic Gyre) suggests that westwards transport has not prevailed for this group. Furthcnnore, the distribution of related species docs not support such an origin. The concept of a series of now-submerged mid-Atlantic islands maintaining populations from the narrow-Atlantic era was developed by Wilson (1963), and is discussed it'!Chace & Manning (1972). It would facilitate the colonisation of Ascension Island, but does not define the origin of its land crabs. If it was the case, a greater divergence ofthe Ascension populations from related ones would be expected from the time interval involved. The geographical distribution of the most closely related species (Gecarci- 381 nus spp. and Johngarthia spp.) is informative. Gecarcinus is rcstricted to the Pacific and Atlantic coasts of America, and to the Caribbean Islands and Bermuda (Tiirkay, 1970).