Gustav Paulay; John Starmer

“ I suspect like most island crabs (e.g. many gecarcinids and Birgus) , they are vulnerable to predators, native or introduced at some stage of their life. I believe in many cases, it is the young which are very vulnerable to things like rats, civets, squirrels, monitors etc. and the pressure of these predators on the juveniles forces them to small islands where such predators are absent.” Similarly, in their massive treatise on Chinese crabs, Dai & Yang [28] record Geograpsus grayi only from Taiwan, and Geograpsus crinipes only from Xisha Island (Paracel Islands) , in China. ”

“ Unlike other Greater Antilles, Jamaica is thought to have undergone partial or complete submergence, cleansing the island of most if not all continental taxa [86] . This example demonstrates importance of islands in the origin of terrestriality in crabs. Presumably because of the imperfection of their macroevolutionary transition onto land, land crabs are ecologically important mostly on, and many species are limited to, oceanic islands, where continental predators and competitors (e.g. mammals, large reptiles) are absent. ”

“ Extinction
The occurrence and loss of Hawaiian land crabs has been noted, but the identity of the species previously remained unresolved [37] , [40] . All fossils examined were attributable to Geograpsus, most to G. severnsi, a few to G. crinipes. None of the fossils were attributable to other land crabs putatively recorded from the Hawaiian Islands. However the most deposits from which material was examined are located further inland that the habitat range of other potential species. ”

“ Although oceanic land crabs remain common on many Pacific islands, they appear to be frequently in decline, from a variety of causes. Some are sought after for food, with the very large and long-lived coconut crab most impacted by human predation [76] , [77] . Gecarcinids are also often heavily targeted, but at least some species appear to sustain considerable harvesting pressure (pers. obs.) . Geograpsus species do not appear to be as favored for food as these other terrestrial decapods, and appear to be less commonly harvested [63] (pers. obs.) ”

“ Considerable indigenous knowledge accumulates about land crabs on Pacific islands, with detailed understanding of their migration patterns, life cycles, habitat use, etc., and this information is used for harvesting these animals [81] .
Polynesian colonists also introduced numerous animals and plants, including pigs, dogs, Pacific rats, jungle fowls, various lizards, insects, and land snails [82] . Rats in particular are known to be important predators on island life [33] , although only R. exulans arrived with native Hawaiians, and this species coexists with other Geograpsus (see above) . ”

“ In contrast the coastal G. crinipes, much less common in the fossil record than G. severnsi, is known from multiple historical specimens, although it appears to be quite rare today. The survival and even occurrence of G. stormi is the most poorly constrained.
Abundant Geograpsus fossils have been taken from two excavations with well-dated stratigraphic context, which were studied to document changes in the Hawaiian biota and landscape following human colonization: the Pu'u Naio lava tube on Maui [69] and the Māhā'ulepū Caves and Sinkhole on Kaua'i [40] . ”

“ The Sesarmidae (Brachyura) , a diverse group with a broad range of terrestrial adaptations, has repeatedly invaded land; the three terrestrial species known in Oceania appear to be absent in continental Australia, although others occur there. Among the Grapsidae (Brachyura) only some Geograpsus qualify as land crabs (as defined here) ; this predominantly predatory genus is also absent from the Australian mainland. ”

“ In this paper we define land crabs as species that live inland of the supratidal fringe, in coastal vegetation, not including mangroves. We exclude ghost crabs (Ocypode) , as they are primarily beach inhabitants, although some species enter vegetated zones at beach margins. We include both anomuran and brachyuran crabs. Most land crab species in remote Oceania (Polynesia, Micronesia, and island Melanesia) have wide ranges (all have marine larvae) , yet are restricted to islands. ”

“ Oceanic restriction of many land crabs suggests that they are especially vulnerable to continental predators. Although island land crabs remain understudied, they do appear to be more resilient than many other terrestrial taxa. Marine larval stages, wide dispersal capacity, and wide ranges provide rescue mechanisms against population extinction. In contrast, G. severnsi was a Hawaiian endemic, although its life history is not known. ”

“ Although the importance of land crabs is obvious on remote and relatively undisturbed islands, it is less apparent on others, likely because they are decimated by humans and introduced biota. The loss of Geograpsus and potentially other land crabs likely had profound consequences for Hawaiian ecosystems. ”

“ Oceanic land crabs are similarly under-represented in Southeast Asia, although the ranges of all but one of the 18 species listed straddle the area. Peter Ng (in litt. 25 March 1998) noted that Geograpsus is absent “in SE Asia per se. It occurs only on islands or peninsulas in areas washed by more oceanic waters. I have caught these guys in southern Taiwan and adjacent islands though! They are apparently affected badly by human activities, especially pests. In one island in Taiwan, their population is decreasing, possibly because of human activities. ”

“ Low starting diversity and absence of various major taxa generally lead to ecosystems with fewer types of biotic interactions than on continents. This in turn provides ecological release for some colonizers, while preventing the establishment of others. Some strong ecological interactors, such as mammals and ants, are characteristically underrepresented on or absent from many oceanic islands. ”

“ However a consequence of the evolution of fully terrestrial/freshwater life histories, as in this Jamaican sesarmid radiation, is that it may restrict crabs to the island they evolved on, rendering them evolutionary dead ends from a global perspective.
Adaptations for semi-terrestrial existence may also make it difficult for crabs to return to subtidal habitats; alternatively, intertidal habitats may be a safe place for lineages that have not faired well in more diverse, and biologically intense, subtidal settings. ”

“ The ridge along the upper border of the buccal cavern are closer to typical G. lividus than G. stormi, as are the teeth along the anterodistal angle of the second walking legs. The carapace of the larger specimen diverges posteriorly, while that of the smaller is subparallel. As the Hawaiian biota includes some East Pacific elements, the possibility that Garrett's specimens are G. lividus remains. However G. stormi is known from the adjacent Line Islands (UF 10621) . Although distributed across the Indo-West Pacific, G. stormi is rather cryptic, typically uncommon, and rarely collected. ”

“ Thus either some unusual combination of factors made pressure on land crabs greater in the Hawaiian Islands than elsewhere, and/or G. severnsi was especially vulnerable.
Macroevolutionary transitions to land
Macroevolutionary transitions between marine and non-marine habitats are uncommon and rarely lead to long-term success in the invaded habitat [83] . Overall, decapod crustaceans have not been particularly successful on land. ”

“ Several species of land crabs have been implicated as predators on bird chicks and eggs, and Atkinson [33] argues that land crabs may have made birds on tropical islands behaviorally more resilient to rats, than birds were on temperate islands, where land crabs are absent. ”

“ The smallest fossil G. severnsi sternites are 9.5 mm/20 mm (TW/BW) wide, with two clearly female sternites 10.8 and 10.9 mm (TW) wide. Thus it is likely that all fossil sternites (TW = 12.4+/−1.9, range: 9.5–17, N = 28; TW = 23.9+/−3.0, range: 20–29.5 mm, N = 18) are from mature individuals and thus can be used to sex the crabs. Furthermore, all sternites complete enough to allow measurement of BW and S (N = 17) had male shapes and BW/S ratios appropriate for male Geograpsus species (Table 3) . ”

“ Lower, but still large numbers of crabs are known from other islands that harbor some introduced mammals. On Christmas Island (Indian Ocean) , the large (500 grams) , endemic Gecarcoidea natalis occurs at densities of >1/m2, for an estimated population size of 100 million crabs [91] . They consume most accessible plant matter, including seeds, seedlings, and leaf litter, keeping the forest floor unusually clean and clear [91] , [92] , [93] , [94] , [95] . ”

“ The terrestriality of G. severnsi further supports its relation to G. grayi, the most terrestrial living grapsid.
It is unlikely that G. severnsi had the highly male-biased (8∶1) sex ratio) observed among 4th thoracic sternites (N = 18) ; rather this suggests preservation bias. It is also unlikely that female 4th thoracic sternites would be less likely to be preserved under the same conditions as male sternites. Rather, the biased sex ratio suggests a difference in where males vs. females were most likely to molt or die. ”

“ If we assume this relationship also held for G. severnsi, then we can estimate carapace width from size of sternites. Sternites of these fossils slightly exceed the width of even the largest G. crinipes (Fig. 11) , even though only 18 G. severnsi sternites were available, while the sample size for whole G. crinipes was much greater and included the largest specimens we have seen. The largest G. crinipes measured (UF 2383: Pitcairn I, CW = 63 mm, CL = 55 mm) were close to the maximum recorded size for the species (CL = 61 mm [38] ) . ”

“ The fourth sternites of males have a more strongly demarcated and narrower depression than those of mature females for accommodating the telson. This is also expressed in the meristic BW/S ratio, which is >2 in all male and <2 in all female Geograpsus species (Table 3) . Fossil sternites of G. severnsi clearly show this sexual dimorphism and provide a means of sexing the population. Immature females have sternites similar to that of males. In G. grayi, females develop mature sternite and abdomen morphology at ca. ”

“ Fourth sternite trapezoidal in shape, lacking anterior constriction of Grapsus species and G. stormi; with convex anterolateral, and straight anterior margins; densely punctate, thus densely pubescent as punctures hold setae in life; punctae densest in paired tracks that diverge anteriorad around middle of sternite and also abundant along anterolateral margin; punctae generally aligned in short, oblique, postero-lateral to antero-medial, rows. Third sternite typical of genus (Fig. 1) .
Endostome (Fig. 5) : Upper buccal ridge tuberculate, straight, close to anterior margin of endostome. ”

“ H James, J Earle. 1 male 4th sternite, 2 claws, 2 cheliped carpi, leg fragments; same crab? USNM 539,751: Hawai'i, new cave system, ca. 12m south of and parallel to Owl Cave system, below 213m asl. 3.III.1993. H James, Lopez, Cooper, Griffen, Cobb, B Schaefer. 1 claw, 2 chelar carpi, leg and chelar meri. USNM 539,752: Makalawena cave, 646 m, upslope of first entrance, near third entrance. 2.VI.1991. H James, J Earl. 4 claws, 2 partial chelar meri, 1 dactylus, 1 partial chelar carpus. USNM: 539,753: Kau Kaaluau-Waiohinu Road Cave, 4.5 mi. ”

“ Taxonomy, distribution and biomass (Decapoda) ., vol. 61, pp. 125–140 ↑ Tesch, JJ (1917) , Synopsis of the genera Sesarma, Metasesarma, Sarmatium and Clistocoeloma, with a key to the determination of Indo-Pacific species., vol. 3, pp. 127–260 ↑ Holthuis, LB (1953) , Enumeration of the decapod and stomatopod Crustacea from Pacific coral islands., vol. 24, pp. 1–66, Maps 61–62 1 2 3 4 5 Yaldwyn, JC & Wodzicki, K. (1979) , Systematics and ecology of the land crabs (Decapoda: Coenobitidae, Grapsidae and Gecarcinidae) of the Tokelau Islands, central Pacific., vol. ”

“ Pu'u Naio. 1984. M Severns. 1 claw dactyl. UF IP 168,826: Maui: Pu'u Naio. 1984. M Severns. Carapace pieces.
Hawai'i: USNM 539,749: Point caves, cave on road towards/into subdivision (near Mark Twain Tree) ca. 5 miles near main highway (Makai) , ca. 4.4 mi. NE of S Point Road on left side of dirt road. Crabs from deep in cave, many present. E Paxinos, J Wilcox, R Basona, J Lilly. leg carpus, endostome with associated mandible, carapace fragment, 2 entire claws, 4 cheliped carpi, 2 cheliped meri, 8 ambulatory leg meri, 4 ambulatory leg carpi, 2 male 4th sternites, fragments. ”

“ Christmas Island Naturas History Association 1 2 3 4 5 6 Alexander, HGL (1979) , A preliminary assessment of the rôle of the terrestrial decapod crustaceans in the Aldabran ecosystem., vol. 286B, pp. 241–246 1 2 Etpison, M (2004) , Palau: Natural History, Koror, Palau: Tkel Corporation 1 2 3 4 5 Ng, PKL ”