Land crabs
D.S. Johnson’s “Land Crabs” (1965) is a comprehensive review of terrestrial decapod crustaceans in the Indo-West-Pacific region, published as a contribution to a memorial volume honouring C.A. Gibson-Hill. The article synthesizes scattered taxonomic, behavioural, and physiological literature to present a unified account of how crabs, hermit-crabs, and their allies have adapted to life on land, drawing substantially on Gibson-Hill’s own field observations from Christmas Island, Pulau Jarak, and the Cocos-Keeling group.
Summary
Johnson’s article surveys the full range of terrestrial and semi-terrestrial decapods, from the small forest-floor species of Sesarma to the massive robber-crab Birgus latro, and from the coastal ghost-crabs to the true land-crabs of the family Gecarcinidae. The central problem addressed is one of evolutionary transition: how a group of organisms whose basic body plan is shaped for life on the sea-bottom came to colonize land, and what morphological, physiological, and behavioural modifications made this possible. Johnson argues that crabs had a particular advantage over vertebrates in this transition because their haemocyanin is insensitive to carbon dioxide accumulation, and because their walking locomotion required no fundamental redesign. The article is organized around the major taxonomic groups, with each section addressing distribution, habitat, feeding, reproduction, and the specific adaptations to terrestrial life.
A recurring theme is the tension between terrestrial independence and the persistent need to return to the sea for reproduction. Most land crabs retain pelagic larval stages that require marine release, confining them to coastal zones and enabling them to colonize remote oceanic islands through larval drifting. The exception is the small species of Sesarma (subgenus Geosesarma) and the river-crabs (Potamonidae), which appear to have direct development and are thus fully emancipated from the sea. Johnson notes that these fully terrestrial forms are restricted to continental land masses and have not established themselves on oceanic islands, suggesting that the loss of pelagic dispersal came at a biogeographical cost.
The article also addresses the unsolved problem of why large land-crabs are rare on mainland coasts of south-eastern Asia despite their abundance on nearby small islands. Johnson considers predation by the crab-eating macaque (Macaca irus), increased competition, and the vulnerability of moulting and juvenile individuals as possible explanations, but acknowledges that no single factor is sufficient.
Key Findings
- Birgus latro (robber-crab) reaches an extreme span of 3 feet across the first pair of walking-legs, a carapace width of over 6 inches, and a weight of 5–6 pounds; specimens up to 60 cm in length have been reported (pp. 47–48).
- Coenobita rugosa is confined to a narrow coastal strip usually less than 100 yards in width, and makes nightly excursions of up to 200 yards to the sea to moisten its gill-chamber (p. 46).
- Sesarma gracillima has been recorded at heights of up to 2,500 ft and at distances of over 50 miles from the sea; S. foxi ascends to nearly 4,000 ft (pp. 51–52).
- Gecarcinus lateralis survived 91 hours in air of 74% humidity without access to water, the longest recorded survival for a terrestrial crab under these conditions (p. 56).
- Cardisoma carnifex in the Andaman Islands constructs “castles” ranging from 30 to 50 cm in height, with a vertical burrow dropping from the summit to ground-level, all openings plugged with clods of soil (pp. 53–54).
- Gecarcoidea lalandii at Christmas Island undergoes 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 (pp. 54–55).
Conclusion
Johnson’s definitive takeaway is that terrestrial decapods represent a remarkable but incomplete conquest of the land. Their success on small oceanic islands—where they may become the most conspicuous non-flying animals—is built on a physiological foundation (haemocyanin, pre-adapted gill-chambers) that made the transition feasible, but their continued dependence on marine larval stages for most species has constrained their biogeographical range and left them absent from continental interiors. The few forms that have achieved full terrestrial independence through direct development have paid the price in dispersal ability and remain restricted to continental land masses.
Context
- The article draws on Gibson-Hill’s unpublished field notes and collections held at the Raffles Museum, Singapore, as well as the published literature of the Bulletin of the Raffles Museum, making it a synthesis that gives lasting value to Gibson-Hill’s island expeditions.
- As a 1965 review, it represents a pre-molecular-systematics synthesis of terrestrial decapod biology in the Indo-West-Pacific, bringing together physiological, behavioural, and biogeographical perspectives that had previously been scattered across specialized journals.