Crabs

Terrestrial Decapods of the Indo-West-Pacific

Land crabs, together with their close relatives the land-hermit-crabs and robber-crabs, represent a small but ecologically conspicuous subset of the decapod crustaceans that have adapted to life away from the sea. In the Indo-West-Pacific region, these animals range from tiny forest-floor species barely visible to the naked eye to the massive robber-crab, Birgus latro, which may attain a span of three feet across the first pair of walking-legs and a weight of five to six pounds [1, pp. 47–48]. On small oceanic islands such as Christmas Island and Pulau Jarak, one or a few species may occur in immense numbers and constitute the most conspicuous non-flying animals present [1, p. 43]. Their significance extends beyond mere abundance: they serve as scavengers, as a source of protein for island communities, and as agents of soil disturbance, while their physiological and behavioural adaptations to terrestrial life have made them a subject of sustained scientific interest [1, pp. 61–62].

Taxonomic Scope and Distribution

The terrestrial decapods of the region belong to several distinct lineages. Among the anomurans, the family Coenobitidae contains the land-hermit-crabs of the genus Coenobita and the single robber-crab species B. latro [1, p. 45]. Coenobita rugosa is the only species common on the mainland coasts of southern Asia, where it is often abundant at the back of sandy beaches in Malaysia, though it appears confined to a narrow coastal strip usually less than 100 yards in width [1, p. 46]. The robber-crab, by contrast, never occurs naturally on continental mainlands or the coasts of large islands, being restricted to small islands in the Indian and western Pacific Oceans [1, pp. 49–50].

Among the true crabs, the grapsoid families provide the bulk of terrestrial species. The genus Sesarma contains the most thoroughly terrestrial of all crabs, with small species such as S. gracillima recorded at heights of up to 2,500 feet and distances of over 50 miles from the sea, and S. foxi ascending to nearly 4,000 feet on mountain summits [1, pp. 51–52]. The family Gecarcinidae, the “true” land-crabs, includes the genera Cardisoma and Gecarcoidea; the latter’s single species, G. lalandei, is the abundant red crab of Christmas Island and occurs on continental-shelf islands such as Pulau Jarak and Pulau Ular off Malaya [1, pp. 52–53]. River-crabs of the family Potamonidae, though primarily freshwater, include some truly terrestrial species and are of economic importance in South-east Asia where they damage the bunds of rice fields by burrowing into them [1, p. 45].

Physiological and Behavioural Adaptations

Life on land imposes demands for water conservation, aerial respiration, and novel sensory strategies. Land crabs lose water more readily than many insects, and no species is known to survive indefinitely in moderately dry air without access to water; the Atlantic species Gecarcinus lateralis survived 91 hours at 74 per cent humidity, a record that Indo-West-Pacific forms such as Birgus latro may exceed but which has not been demonstrated [1, p. 56]. Respiration is achieved through modified gill-chambers that function as lungs, with the most extreme development in Birgus, whose lung surface forms a complex honeycomb-like structure [1, pp. 56–57].

Reproduction remains tied to the sea for most species: ovigerous females of Coenobita, Birgus, and the Gecarcinidae must return to the water to release their planktonic larvae [1, pp. 46]. At Christmas Island, G. lalandei undergoes spectacular mass spawning migrations in which populations numbering hundreds or thousands move to the sea, and the young return onshore in thousands, forming a moving red carpet that may completely blanket large patches of ground [1, pp. 54–55]. The small Sesarma species of the Geosesarma section, however, appear to have direct development, freeing them entirely from marine larval stages and enabling their colonisation of inland mountain forests [1, pp. 51–52].

Behaviourally, terrestrial crabs display some of the most complex patterns found in non-social invertebrates. The robber-crab is an omnivorous scavenger of considerable flexibility, feeding on coconuts, pandanus fruits, carrion, and even dead conspecifics, and it has been observed to drag food items many yards to a retreat before consuming them [1, pp. 48–49]. Several species possess stridulating organs capable of producing high-pitched sounds, and behavioural studies indicate these serve as a basis for auditory signalling, though no hearing organ has yet been demonstrated in any crustacean [1, pp. 59–60].

Economic and Ecological Roles

The larger land-crabs of coral and oceanic islands are a ready source of first-class protein for atoll dwellers, and over-collecting has apparently exterminated or nearly exterminated the more nutritious species from a number of inhabited islets [1, pp. 60–61]. The robber-crab is much sought after as food throughout much of its range, and human predation may greatly reduce or even eliminate local populations [1, p. 49]. On Christmas Island, Cardisoma hirtipes is favoured as food by the local Chinese community, while G. lalandei, despite its abundance, is not [1, p. 53]. The small terrestrial potamonids and grapsids of mainland areas are of little direct economic importance to humans, though their burrowing activity undermines sawah-bunds in rice-growing regions [1, pp. 45]. As scavengers, the larger island species are generally considered to do more good than harm to food crops [1, p. 62].

Research and Documentation

The study of land crabs in the Indo-West-Pacific has been shaped by the island expeditions of C.A. Gibson-Hill, whose field observations and collections from Christmas Island, Pulau Jarak, and the Cocos-Keeling group formed the empirical foundation for much subsequent work [1, p. 43]. His 1947 paper on the terrestrial crabs of Christmas Island and his 1949 account of the robber-crab remain key references, and his unpublished field notes, held at the Raffles Museum, Singapore, continue to inform synthesis [1, pp. 43]. D.S. Johnson’s 1965 review, published in a memorial volume honouring Gibson-Hill, represents a pre-molecular-systematics consolidation of the scattered taxonomic, physiological, and behavioural literature, bringing together perspectives that had previously been dispersed across specialised journals [1, pp. 43–66]. The article highlights several unresolved questions that continue to frame research: the mechanism of spawning synchronisation in Gecarcoidea, the sensory basis of stridulatory communication, the rarity of large land-crab species on mainland coasts, and the unconfirmed castle-building behaviour of Cardisoma carnifex in the Andaman Islands [1, pp. 53–54].

MBRAS Sources

• D.S. Johnson (1965). Land crabs. JMBRAS 38(2): 43–66

References

1. D.S. Johnson (1965). Land crabs JMBRAS 38(2): 43–66. Read on JSTOR