This document summarizes the "Crab Invasion of 2006" that occurred along the coasts of Jamaica. Hundreds of tiny crabs appeared in coastal communities in eastern and northeastern Jamaica in May 2006. The crabs were identified as the black or purple land crab (Gecarcinus ruricola). Their sudden appearance in large numbers was unusual. It is believed the crabs were juvenile crabs emerging from the sea after hatching from eggs laid by female crabs earlier. These types of crabs normally live inland but females return to the sea to release eggs. The 2006 event was likely linked to spawning and hatching around the full moon in May. While a regular source of food, the black land crab

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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. 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. 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. 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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