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Crab invasion 47507813


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Crab invasion 47507813

  1. 1. 76 The Crab Invasion of 2006 Karl A. Aiken and Anita R. Pal WHAT ‘INVASION’? The appearance of many hundreds of tiny crabs, first described in the printed press in Jamaica as “mangrove- type” crabs,1 on the eastern and north- eastern shores of Jamaica during the period 11–15 May 2006, was rated as newsworthy because of their large numbers and their unusual intrusion inside people’s homes located near the shoreline. Persons living in coastal communities in Nine Miles, Bull Bay, St Thomas and parts of Portland near Port Antonio reported that they noticed the little crabs in the night “when they fell in pairs from the ceiling” and crawled over their bodies as well. Many residents were reported by the press to have been “traumatised” by the “millions” of tiny crabs.2 One tiny coastal settlement called Beach Road near Nine Miles, St Thomas, had a particularly large number of crabs invade the community from the sea in the early hours of 13 May. Many persons interviewed by the press believed there might have been some religious significance to their sudden appearance and prolific numbers, comparing them with biblical plagues. Others imagined that they might have foretold of some forthcoming disaster that was to befall the community. In places, the crabs were said to be so numerous as to cause the sand and rocks near the shore to have a pink colour. Many persons enquired of the University of the West Indies, Mona campus as to their identity and source. This article therefore identifies the tiny crabs that were involved in the incident in the Bull Bay, St Thomas area in May 2006 in Jamaica as the black or purple land crab, Gecarcinus ruricola (Linnaeus, 1758). This species is from the family Gecarcinidae, all of which are terrestrial, and from the order Grapsoidea which are all decapod crustaceans. ECOLOGICAL ASPECTS So, what do we know about these land crabs? Land crabs are known in Florida and throughout the Caribbean as seasonal sources of protein in coastal communities. The land crabs of genera G. ruricola and Cardiosoma guanhumi are a part of the cultural fabric of the island of Jamaica. Each rainy season (April to May and October to November annually), they are collected islandwide in relatively small quantities, and sold at several locations around Jamaica. There are a few locations, such as near Savannah- la-mar, Westmoreland, and Jackson’s Bay, Clarendon, where they appear to be collected in greater numbers for sale elsewhere. In the 1960s, some data were published by Hartnoll3 on the grapsid crabs of Jamaica and by Warner4 on the ecology of mangrove crabs. Up to the time of writing there has been no recently published study on land crabs in Jamaica. The family Gecarcinidae, to which they belong, is found in tropical and sub-tropical America, West Africa, and the Indo-Pacific area. Gecarcinus, along with Cardiosoma, lives in coastal fields as far north as Texas, in southern Florida, tropical America and the West Indies.5 G. ruricola is found in coastal areas where there is damp soil, and is thought to be, at present, relatively widely distributed around Jamaica, but restricted to small zones near coastal wetlands and mangroves (but see section on conservation aspects below). In a 1918 study by Rathbun, they were reported to live in the low and marshy ground of the savannahs of the West Indies, not far from the coast.6 Like most other land crabs, they tend to live in burrows7 or even beneath stones. Rathbun noted that they hollowed out their burrows, which were inclined obliquely, and intersected each other in all directions. He observed that they were not very mobile except at night, when they roamed in order to feed, and that they were primarily vegetarians and scavengers.8 In daylight hours they were known to stand like sentinels at the edge of the openings, and at the slightest noise, they would run rapidly into these for refuge.9 In times of heavy seasonal rainfall they were said to distribute themselves more widely. sc ien ce and technology
  2. 2. 77 Rathbun described the crabs as being so abundant at these periods that the countryside appeared “all red” – reminding one of the report in the Daily Gleaner of beach sand and rocks in parts of St Thomas having a pink colour.10 REPRODUCTIVE BIOLOGY AND THE INVASION As adults, G. ruricola are terrestrial, as are other species of Gecarcinidae, and are coastal in distribution, since the females need to return to the sea to release their spawn, usually in the rainy season in May. Rathbun reported that the crabs came down from the hills in vast multitudes, clambering over any obstacles in their way, in their march towards the sea.11 The females entered the sea to wash off the eggs which they carried on abdominal appendages, allowing the young stages to hatch out. Then after this was completed, they returned to their burrows. What was recorded next by Rathbun is of relevance to the events in the eastern part of Jamaica in May 2006. Afterwards, he reported, the adults were followed by the young, which having passed through their larval stages in the sea, left the water, and were found in thousands clinging to the rocks on the shore.12 This is much like what we observed, and what was reported by the public, in parts of St Thomas. We conclude from nearly all the reports of the crab invasion of 2006 that the juvenile crabs came ashore only in the pre-dawn hours (that is, in the dark). No person observed them emerging after dawn, which in mid-May in Jamaica would be at approximately 6:00 a.m. In bright direct sunlight they were observed to fall off of vertical surfaces. As mentioned previously, they usually sought shelter after sunrise, suggesting that they were sensitive to heat and strong light. It appears that the tiny juvenile crabs would not have been normally observed emerging from the ocean, as this occurs in pre-dawn hours. So, whereas mature adult females were seen in 1918 washing fertilised eggs off their abdominal appendages, the emergence some days later of the successfully hatched first terrestrial stages would not have been easily observed. Megalops crabs (a late or advanced larval stage) were identified by us to be the smallest crabs that came ashore in St Thomas. In the megalops stage, the abdomen is not fully tucked under the carapace. Immediately after arrival on land, this stage rapidly metamorphoses to the next fully terrestrial stage, which in essence is a miniature adult. In this stage, the abdominal flap is tucked under the carapace. We observed and also collected a few specimens directly from the water’s edge on 13 May, that on later laboratory examination were confirmed as the megalops stage of Gecarcinus. This was further confirmed in 2007 by Dr R.G. Hartnoll, a leading authority on crabs.13 Many of the other specimens collected from the same location were just- moulted first crabs. The tiny crabs appeared to have been killed by the heat of direct sunlight, according to many eyewitnesses. Many crabs sought shelter under rocks and in crevices. It is not known if all of the tiny crabs were killed by the sunlight. We speculate that mortality would have been very high. On the afternoon of 13 May, relatively few specimens of the crabs were seen despite thorough searching by the authors at Nine Miles, Bull Bay. By 15 May, only one site at Prospect (the western arm of Port Morant, St Thomas) still had a report of tiny crabs present.14 The May migration reported in the 1918 publication matches the appearance of the tiny larval stages that were observed coming from the sea in St Thomas and Portland parishes in May 2006. Again, a large- scale mass return to land was reported opposite page An adult male black land crab, Ge- carcinus ruricola (Linnaeus, 1758), boiled and ready for consumption. Notice the crab’s much larger left claw, showing that it is a mature male. Fertilised eggs from adult females produced the young stages that ‘invaded’ the shore. this page, top Remnants of the many tiny live and dead megalops (larval stage) and just-moulted first- terrestrial-stage land crabs (Gecarcinus ruricola) from the ‘invasion’, found among the pebbles on the shoreline of the beach at Bull Bay, St Thomas on the evening of 13 May 2006. Size of a typical crab in this photograph is approximately 2–3 mm maxi- mum carapace width. bottom Some of the tiny invading crabs along the beach at Copacabana beach, near Bull Bay, St Thomas, seeking shelter under small rocks just out- side the splash zone of this pebbly beach.
  3. 3. 78 in the Colombian archipelago of the San Andres Islands 250 km east of Nicaragua in June 2004, from the island of Providencia.15 The invasive stage was confirmed to be the megalops larval stage of the crab.16 This is virtually identical to what was observed in Jamaica in May 2006 where vast numbers of tiny crabs (with 2.9–3.4 mm maximum carapace width) emerged from the sea over a three- to four-day period. As in the San Andres Islands, the crabs in St Thomas rapidly headed inland. Rathbun had reported in 1918 that the young left the sea having passed through their larval stages, and emerged on rocks by the seashore in their thousands.17 This is almost certainly what was observed as the ‘invasion’ by the general public and by the authors in May 2006. A very few reports from Portland parish (northeastern coast) indicated that the scale of the event there was smaller than that in St Thomas. What was similar to both eastern areas, however, was the rarity of the event and the relatively high numbers of tiny crabs involved. Another similar feature was that the tiny crabs appeared near to mangrove areas. There was no rainfall before or during the event in Jamaica, unlike San Andres. What appeared to kill them in large numbers locally was bright direct sunlight, whereas in San Andres mortality was partly due to standing freshwater accumulations and flowing water in drainage ditches. SOURCES OF THE LARVAL INVASION Of considerable interest is the fact that in Jamaica no seaward migration of ovigerous (egg-bearing) adult females was observed. First returning tiny late-larval-stage crabs (megalops) were reported on Providencia island about eighteen days after the seaward migration of ovigerous females.18 This duration is similar to the nineteen days noted from hatching to the megalops stage in a related species, Gecarcinus lateralis,19 and the twenty-two days recorded for Cardisoma guanhumi.20 We speculate that in Jamaica a large migration of spawning females must have taken place unnoticed in the largely uninhabited, mangrove- dominated St Thomas wetlands, some time previous to the emergence of the megalops. In the San Andres Islands, higher densities of this species were found in areas adjacent to coastlines where egg-bearing females enter the sea and where returning larvae recruit.21 There is no close-lying island immediately upcurrent (eastward) of Jamaica, thus these crabs are most likely to be from a local source. We suggest that genetic studies be undertaken in order to resolve this issue. RARITY OF THE EVENT AND LUNAR LINKAGE One striking aspect of the May 2006 event was the relationship to the full moon phase: a full moon occurred on the night of 13 May, suggesting a link to the emergence of the tiny juvenile crabs from the sea. Many invertebrates have a link to the lunar phases, with some preferring full moon and other new moon phases to mate. By seventy-two hours after full moon in May 2006, we found that the event was over. Another feature was that this event was apparently extremely rare. Reports by older coastal residents indicated that even persons seventy years old had no similar recollection. When interviewed, one resident of Prospect, St Thomas, reported that in his thirty years in that area, no previous occurrence could be recalled. This matches, to some degree, the fact of the megalops returning in large enough numbers to be noticed in the San Andres Islands. This was not an annual event, occurring instead at roughly six-year intervals between 1992 and 2004.22 In Gecarcinus natalis on Christmas Island, there was a mass recruitment event roughly every five years.23 The coconut crab Birgus latro this page Two specimens of the megalops or last larval crab stage of Gecarcinus ruricola, collected from the shoreline near Bull Bay, St Thomas, May 2006 (photographed in a laboratory at the Life Sciences Department, University of the West Indies, Mona). The large eyes are typical of the larval stage, as is the small narrow posterior abdomen. These specimens (L–R) are of 3.1 and 2.3 mm maximum carapace width, respectively. (The dark area is an artefact of the photographic image.) opposite page An example of the newly moulted first crab stage of G. ruricola, collected from the splash zone, photographed at the Bull Bay beach, St Thomas, 13 May 2006. This photograph shows the relatively small size of the stage (2–3 mm).
  4. 4. 79 has a recruitment that has been spasmodic and unpredictable, taking place every five to ten years.24 COMMERCIAL VALUE Traditionally, G. ruricola has been known to be a source of food for coastal villages around the Caribbean and south Florida.25 Early writers mentioned it was of superlative quality and delicacy when “fat and in a perfect state”.26 Customarily, in many locations, these crabs are boiled and served whole. In the early part of the twentieth century they were said to be also frequently stewed before serving. In the city of Kingston at the time of writing, two of the more well-known crab-selling locations are in Kingston Gardens and in Liguanea. One female vendor interviewed at the latter site reported that in the rainy seasons each year, these crabs are brought by road from Westmoreland in western Jamaica. The public purchased about six dozen large crabs from this vendor every two days in the primary rainy season, and in the secondary rainy season (May), approximately two hundred adult crabs weekly. With a retail value of approximately US$2 per adult, the market value in the primary rainy season is equal to approximately US$500 per week or US$2,000 per month at this one location. CONSERVATION ASPECTS Formerly widely distributed, G. ruricola is threatened in Jamaica by rapidly growing human population numbers and concomitant draining and development of coastal wetland areas for housing and related uses. The number of adults of this species in Jamaica at the time of writing is unknown, but generally, is likely to be in the hundreds of thousands. Their future is unknown, however, and a proper comprehensive study is needed of the land crabs of Jamaica. ACKNOWLEDGEMENTS We thank Lt Cdr Michael Rodriguez for the very first report from Nine Miles, Bull Bay, St Thomas; Mr Marlon Hibbert, former Scientific Officer of the Port Royal Lab, for information on the event from Portland; Mr E. Lewis of East Prospect, for similar information from St Thomas; and also Professor Ivan Goodbody, Professor Emeritus of Zoology, University of the West Indies, Mona, for his assistance with this paper. 1. Daily Gleaner, 12 May 2006, 1. 2. Ibid. 3. R.G. Hartnoll, “The Freshwater Grapsid Crabs of Jamaica”, Proceedings Linnaean Society of London 175 (1964): 145–69. 4. G.F. Warner, “The Life History of the Mangrove Tree Crab, Aratus pisoni”, Journal of Zoology 153 (1967): 321–35; G.F. Warner, “The Occurrence and Distribution of Crabs in a Jamaican Mangrove Swamp”, Journal of Animal Ecology 38, no. 2 (June 1969): 379–89. 5. M.J. Rathbun, The Grapsoid Crabs of America, Smithsonian Institution, United States National Museum, bulletin no. 97 (Washington DC: Smithsonian Institution, 1918). 6. Ibid. 7. R.D. Barnes, Invertebrate Zoology (New York: Saunders Publishing, 1980). 8. Rathbun, Grapsoid Crabs. 9. Ibid. 10. Daily Gleaner, 12 May 2006, 1. 11. Rathbun, Grapsoid Crabs. 12. Ibid. 13. R.G. Hartnoll, personal communication, 2007. 14. E. Lewis, personal communication, May 2006. 15. R.G. Hartnoll and P.F. Clark, “A Mass Recruitment Event in the Land Crab Gecarcinus ruricola (Linnaeus, 1758) (Brachyura: Grapsoidea: Gecarcinidae), and a Description of the Megalop”, Zoological Journal of the Linnaean Society 146, no. 2 (2006): 149–64. 16. Ibid. 17. Rathbun, Grapsoid Crabs. 18. Ibid. 19. K.A. Willems, “Larval Development of the Land Crab Gecarcinus lateralis lateralis (Freminville, 1835) (Brachyura: Gecarcinidae) Reared in the Laboratory”, Journal of Crustacean Biology 2 (1982): 180–201. 20. J.D. Costlow and C.G. Bookhout, “The Complete Larval Development of the Land Crab, Cardiosoma guanhumi Latrielle in the Laboratory (Brachyura, Gecarcinidae)”, Crustaceana, supp. 2 (1968): 259–70. 21. R.G. Hartnoll, M.S.P. Baine, Y. Grandas, J. James and H. Atkin, “Population Biology of the Black Land Crab, Gecarcinus ruricola, in the San Andres Archipelago, Western Caribbean”, Journal of Crustacean Biology 26, no. 3 (2006): 316–25. 22. Hartnoll and Clark, “Mass Recruitment Event”. 23. J.W. Hicks, H. Rumpff and H. Yorkston, “Christmas Crabs, Christmas Island, Indian Ocean” (Christmas Island Natural History Association, 1984). 24. I.W. Brown and D.R Fielder (eds.), “The Coconut Crab: Aspects of the Biology And Ecology of Birgus latro in the Republic of Vanuatu”, ACIAR Monograph 8 (1991): 1–136. 25. Barnes, Invertebrate Zoology. 26. Rathbun, Grapsoid Crabs. All photos ©Karl Aiken. NOTES
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