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Killer
Crabs More Toxic Than Cyanide
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The mysterious death of a man in East Timor in 2000 has
highlighted the danger of eating certain tropical crabs.
The man died within several hours of eating the Zosimus
aeneus crabs, which grow up to 10 cm across the carapace
and are found in tropical reefs worldwide.
Human fatalities from crab ingestion have also been
documented from Japan, Philippines, Fiji, Palau Islands,
Mauritius and Vanuatu.
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A crab from the Xanthidae family
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The most common culprits belong to the Xanthidae
family (table 1), and contain toxins which cause the
disease known as Paralytic Shellfish Poisoning (PSP).
After eating the crabs, people who are affected by PSP
experience symptoms within 30 minutes, starting with a burning
or tingling sensation of the lips and face. This effect
extends to upper body extremities and, over time, spreads all
the way to fingers and toes, causing total numbness.
Gradually the body of the victim becomes paralysed with
voluntary movements becoming increasingly difficult. All the
while, the victim can remain conscious throughout the event.
Other minor symptoms include dizziness, headache, salivation,
intense thirst and perspiration, vomiting, diarrhoea and
stomach cramps. A lethal dose can cause death from respiratory
failure within 12 hours . Victims surviving beyond 12 hours
usually make a full recovery.
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There is no known antidote for Paralytic Shellfish Poisoning.
The 'parent' of the Paralytic Shellfish Toxin (PST) family
(see figure 1), called saxitoxin, is one of the most
potent neurotoxins known and, gram for gram, is 1,000 times
more deadly than cyanide.
The toxin works on humans by blocking the entry of sodium
required by cells to make 'action potentials', rendering the
cells and tissue non-functional.
The first source of PST's was identified in 1966 but these
neurotoxins have been found in many microalgae, from both
marine and freshwater environments. In suitable conditions,
the populations of these plants may bloom, creating a health
hazard not just to humans, but also to wildlife and
agricultural livestock.
These so-called harmful algal blooms (HABs) are not a
phenomenon of modern times. HABs have been recorded back as
far as 208 BC when the Greeks named the Red Sea after the
formation of algal blooms (which can have a red appearance).
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Figure 1.
STX and its analogues, which
make-up the paralytic shellfish toxins
| |
R1 |
R2 |
R3 |
R4 |
| STX |
H |
H |
H |
H |
| neo STX |
OH |
H |
H |
H |
| B1 |
H |
H |
H |
SO3- |
| Gonyautoxin 2 |
H |
H |
OSO3- |
H |
| Gonyautoxin 3 |
H |
OSO3- |
H |
H |
| decarbamoyl STX |
* |
H |
H |
H |
* In this derivative, a proton replaces all of the
structure beyond the wavy line, including R4.
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Most toxic crabs are non-filter feeding crustacea like the
xanthids, which are omnivores that feed on macroscopic food
items. The source of toxins in these crabs is the subject of
some speculation. Bacteria and macroscopic algae have been
implicated as sources of toxins; but this is probably only
part of the story because the crabs may carry enormous amounts
of PST's.
An individual Zosimus aeneus crab, like the one
which killed the East Timorese man in 2000, has been reported
to have enough toxin in its body to kill hundreds, if not
thousands of people.
Between 65 and 100% of this particular species can be
toxic.
It is quite likely that crab poisonings have occurred
previously in East Timor, as anecdotal reports to the United
Nations field medical staff revealed that suicides were
occasionally achieved by the intentional consumption of known
toxic crabs, indicating a local knowledge of the potential
toxicity of these crabs.
The likelihood of victims receiving medical aid is low,
considering that death can occur so quickly after ingestion.
In the most recent recorded case, the victim was dead within
several hours of eating the crabs.
Fortunately, a new device being developed by Toxitech, a
spin-off company of the Australian Institute of Marine Science
and James Cook University, will allow portable testing for
saxitoxin in seafood and drinking water.
Distribution
of crabs that have been reported to have
caused deaths ( )
or severe illness ( )
resulting from PST intoxication, or have been demonstrated to
contain PSTs ( ).
Table 1.
|
Family |
Genus species |
|
Crabs |
|
|
Xanthidae |
Actaeodes tomentosus |
|
|
Atergatopsis germaini |
|
|
Atergatus dilitatus |
|
|
Atergatus floridus |
|
|
Atergatus integerrimus |
|
|
Carpilius maculatus |
|
|
Demania alcalai |
|
|
Demania reynaudii |
|
|
Demania toxica |
|
|
Eriphia scabricula |
|
|
Eriphia sebana |
|
|
Etisus rhynchophorous |
|
|
Etisus splendidus |
|
|
Euzanthus exsculptus |
|
|
Leptodius sanguineus |
|
|
Lophozozymus pictor |
|
|
Lophozozymus octodentatus |
|
|
Neoxanthias impressus |
|
|
Pilodius areolatus |
|
|
Phymodius ungulatus |
|
|
Platypodia granulosa |
|
|
Platypodia pseudogranulosa |
|
|
Zosimus aeneus |
|
Portunidae |
Portunus pelagicus |
|
|
Thalamita stimpsoni |
|
|
Thalamita wakensis |
|
|
Thalamita sp. |
|
Majidae |
Schizophrys aspera |
|
|
Chionoecetes bairdi |
|
Grapsidae |
Grapsus albolineatus |
|
|
Metopograpsus frontalis |
|
|
Percnon planissimum |
|
Calappidae |
Calappa calappa |
|
Dromidae |
Dromidiopsis sp. |
|
Cancridae |
Cancer magister |
|
Parthenopidae |
Daldorfia horrida |
|
Pilumnidae |
Pilumnus pulcher |
|
|
Pilumnus vespertilio |
|
Hippidae |
Emertia analoga |
|
|
|
|
Lobster |
|
|
Homeridae |
Homarus americanus |
|
Copepods |
|
|
Harpacticidae |
Tigriopus californicus |
For more information
contact
Dr. Lyndon Llewellyn, Australian
Institute of Marine Science
e-mail: l.lllewellyn@aims.gov.au
phone: 07 4753 4449 fax 07 4753 4285
international: +61 7 4753 4449; fax +61 7 4753 4285
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