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Mus. Reg. Sci. Nat. Torino, 2011 IX Colloquium Crustacea Mediterranea TOrino, September 2-6, 2008: pp 375-385

Richard G. HARTNOLL'

Ascension : contrasting of land and rock

ABSTRACT

The geographical distribution is described for two common crabs on Ascension Island in (he South Atlantic: the Jand 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 . 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 Seychellum alluaudi (A. Milne Edwards & Bouvier, 1893) on the : 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). Johngarthia malpilensis and J planatus occur on is- lands off the Pacific coast of Central America, J lagostoma is found on four Atlantic islands as described above, and J. ,veileri on west African islands in the Gulf of Guinea (Tiirkay 1970, 1973). The parsimonious conclusion is that the initial separation of Gecarcinus and Johngarthia occmTed in the Central Amer- ican region. Johngarthia subsequently dispcrsed wcstward into the Pacific is- lands as J malpilensis and J planatus, and eastwards across the Atlantic. The concept ofa single eastward migration is difficult to sustain - Ascension Island is geologically recent, and its population of J lagostoma appears conspecific with the West Atlantic populations. There are no genetic differences between the Ascension Island population and that on Fernando de Noronha (A. Sole Cava, pel's. comm.). However, J weileri has diverged fmiher, suggesting an ear- lier eastward migration, presumably predating the emergence of Ascension Is- land, but possibly involving some mid-Atlantic islands now lost (Wilson, 1963). A later eastward migration colonised Ascension Island. Garth (1967) provided a convincing argument for the eastward transport of crab larvae in the Pacific, counter to the obvious current systems: his arguments are cqually relevant to the Atlantic.

Origin and afllnities olGrapsus adscensionis The morphological differences between G. adscensionis and G. grapsus are small (see Manning & Chace, 1990), the major distinction being colour pattern. Genetic studies are needed to clarify the taxonomic status of the various popu- lations. Nevertheless, on the basis of these characters, the affinity of the Ascen- sion Island population is clearly \vith the East Atlantic crabs, and they differ consistently from the West Atlantic G. grapsus. Grapsus could have reached Ascension by rafting, being well equipped for this as an active prehensile crab, capable of indefinite immersion in sea water. I! could equally have arrived by larval dispersal. Only the first zoea of G. ad- scensionis has been described (Cuesta et al., 1997), but G. strigosus (Herbst) has five zoeal stages similar to gecarcinids (Gohar & AI-Kholy, 1957), and pre- sumablya similar larval duration. Given the affinity of the Ascension crabs with those in the Eastern Atlantic, the simple and parsimonious conclusion is that they reached Ascension (by either of the above mechanisms) via the westerly flowing northern limb of the South Atlantic Gyre (Fig. 1), which would facili- tate transport from the African side. This would also account for their presence on St Helena.

Divergent origins The two crabs under study appear to have reached Ascension Island, on the basis of the distribution of their near relatives, from different sides of the At- 382

lantic. The land crab Johngarthia lagostoma colonised from the American side, most probably by larval dispersal. The rock crab Grapsus adscensionis arrived from the African side by either larval dispersal or rafting. Cun-ent patterns could explain both pathways. Why these different origins? One possibility is that the wind and cun'ent patterns differentially favour rafting (wind driven) and larval dispersal (current driven). Data to evaluate this hypothesis are not adequate. An examination of the geographic origin of other Ascension Island shallow water species is relevant (Manning & Chace, 1990, Tab. 4). The partitioning of West Atlantic and East Atlantic species is respec- tively 19:14 for decapods, 21 :5 for fish (based on Lubbock, 1980),6:4 for echin- oderms (based on Pawson, 1978) and 21 :24 for molluscs (based on Rosewater, 1975). All of these taxa have colonised Ascension from both sides of the At- lantic, with a slight bias towards a West Atlantic origin, suggesting a surprisingly strong influence ofthe Equatorial Countercun-ent. Given this divergent pattern, it is perhaps not surprising that the two crabs considered in this study have orig- inated from different sides of the Atlantic.

Implications/or conservation There are three factors to consider here for each species: is the species en- demic to Ascension Island, how widespread is it elsewhere, and what is the cur- rent status of the Ascension Island population? The land crab Johngarthia lagostoma does not appear to be endemic to As- cension Island. Preliminary genetic studies have not found ditferences from the Fernando de Noronha population (A. Sole Cava, pel's. eomm.), with which it also appears to be morphologically identical. Genetic comparisons with the Trindade and Atol das Rocas populations have not been possible. Nevertheless, the global distribution of the species is limited to these four small islands, of which Ascension Island is by far the largest in area (Ascension, ~ 100 km2; 2 Trindade, ~ 10 km ; Fernando de Noronha, ~20 km2; Atol das Rocas,

ACKNOWLEDGEMENTS

I was sUPPOltedin this work by a Leverhulme Emerinls Fellowship - many thanks to the Leverhulme Trust. I am grateful to the Ascension Island Conservation Otllce for help and en- couragement during the tleld work. Dr Anronio Sole Cava, Universidade Federal do Rio de Janeiro. generously provided the provisional results of his genetic comparison of Johngarrhia material.

RIASSUNTO

E' stata desclitta la distribuzionc gcografica di due granchi comuni sull'lsola di Ascen- slone nel Sud Atlantico: il granchio [elTestreJohngarthia lagostoma c 11granchio di roccia Grap- sus adscensionis. 11primo scmbra avcr avuto originc dall'Atlantico occidentale, dove si trovano popolazioni di conspecifici. 11secondo, al contrario, G. adscensionis, sembra essen~una specie dell' Atlantico OI;cn[alea cui conispondc una specie ditferente nell'Atlantico occidentale. Questa diversita e discussa in relazione a ditferenze nei potenziali mezzi di dispersione, alia distribu- zione delle specie imparentate e alroriginc degli al[ri taxa di acquc basse sull'lsola di Ascen- sione.

Parole chiave: Asccnsion Island, biogeografia, granchi, .!ohngarlhia. Grapsus.

Richard G. HARTMJLi. School olBiological Sciences UniversiZv ojLiveJpool. Liverpool, u.K., and Marine Biological Associarion. The LahoralOry. Ciladel Hill. Plrmouth. u.K. Postal address: ret/la Vcg. Lime Stred. Port SI Man', Isle olMan Pvf95ED, UK E-mair harlnoll(i~advsrs.co.uk

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