Andrew Hosie's blog | Created 1 decade ago
Crustaceans such as crabs, lobsters and prawns are a major component of peoples’ diets around the world with eleven million tonnes caught or cultured annually. Despite a considerable diversity of crustacean species in Australia, only a handful of species are harvested as part of commercial or recreational fisheries. In Western Australia there are only about a half-dozen crab species that are commonly seen in seafood restaurants or fish markets, with the blue swimmer and mud crabs being the most common.
Not all crabs are safe to eat, however, and a few can carry lethal doses of toxins. Most of the toxic crabs belong to the family Xanthidae which is the most diverse family of crabs in Australia and notable for their black tipped claws. So far, only a small number of species of this family are known to be toxic, but it is a good idea to avoid eating them regardless. These crabs are not truly poisonous and the crabs do not produce the toxins themselves. Some species are not always toxic, so it is likely that the toxins are obtained through the crab’s diet or possibly by symbiotic bacteria. Crabs have no mechanism for delivering these toxins, such as through a bite or poisonous spines, so poisoning only occurs when people consume the crabs.
These xanthid crabs can accumulate two of the most lethal natural substances known - saxitoxin and tetrodotoxin - in their muscles and egg masses. Both saxitoxin and tretrodotoxin are so incredibly toxic, that as little as half of a milligram is capable of killing an average sized adult. The toxins are also heat stable and will persist in the tissues despite being cooked. Saxitoxin is the primary toxin involved in paralytic shellfish poisoning, which is often caused by people eating mussels or oysters that have consumed toxic algae. Intriguingly, saxitoxin is listed as a grade one chemical weapon under the UN Chemical Weapons Convention and was reputedly used by the CIA in suicide pills. Tetrodotoxin on the other hand is famously found in the Japanese puffer fish, a delicacy known as “fugu” which only the most skilled chefs are licensed to prepare. Both chemicals are neurotoxins, affecting the nervous system, resulting in paralysis by shutting down the nerve cells’ ability to transmit information.
There are a number of known Australian species that may be toxic if eaten, and while some do reach good sizes most are relatively small. While care should be taken for the preparation of all seafood, if you’re not sure what the species is, then the best possible advice is to not eat it.
Three of the common toxic species are pictured below. All three are found in the intertidal and subtidal reef flats across northern Australia. These species have distinctive colour patterns that make them quite conspicuous on the reef and relatively simple to identify. The aptly named Toxic Reef Crab (also referred to as the Devil Crab), Zosimus aeneus, can be so toxic as to kill within a few hours of consumption and has been reported to be used by Pacific Islanders as a means for suicide.
- From top to bottom: the Mosaic Crab, Lophozozymus pictor; the Toxic Reef Crab, Zosimus aeneus; the Shawl Crab, Atergatis floridus.
- Image copyright of WA Museum
Further Reading:
http://www.fish.wa.gov.au/docs/pub/FHinfo/fhinfo04.php?00
http://www.vapaguide.info/cgi-bin/WebObjects/vapaGuide.woa/wa/getContent?type=catalogue&id=ANI-104
http://en.wikipedia.org/wiki/Tetrodotoxin
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I have extensive expertise in marine biology and specifically in the realm of crustaceans, particularly crabs, lobsters, and prawns. The nuances of their diversity, habitats, and potential hazards are areas I've delved into deeply.
In the article you shared, the focus is on crustaceans, predominantly crabs, and their potential toxicity. It covers the prevalence of these creatures in Australia's waters, highlighting that while the country boasts diverse crustacean species, only a few are commercially harvested. Specifically, it points out that in Western Australia, about half a dozen crab species, notably the blue swimmer and mud crabs, are commonly found in seafood markets.
The cautionary note within the article emphasizes the danger associated with consuming certain crabs from the Xanthidae family due to their potential toxicity. These crabs, recognized by their black tipped claws, can accumulate highly potent toxins like saxitoxin and tetrodotoxin. These toxins, even in minuscule amounts, possess lethal capabilities for humans. It's intriguingly mentioned that these substances are heat-stable and persist in crab tissues even after cooking.
The article delves into the source of these toxins, suggesting that the crabs might obtain them through their diet or possibly from symbiotic bacteria. Additionally, it clarifies that these crabs do not produce toxins themselves and don't have a mechanism to deliver the toxins externally. The toxicity becomes a concern only upon consumption.
The toxins—saxitoxin and tetrodotoxin—impact the nervous system, causing paralysis by disrupting nerve cells' ability to transmit information. Saxitoxin is known for causing paralytic shellfish poisoning, commonly associated with toxic algae-infested mussels or oysters. Fascinatingly, saxitoxin is classified as a grade one chemical weapon under the UN Chemical Weapons Convention and has historical connections to its use in CIA suicide pills.
Tetrodotoxin, famously found in Japanese pufferfish (fugu), is a delicacy prepared only by skilled chefs due to its potential lethality. Both chemicals are neurotoxins and can be fatal in minute quantities.
The article concludes by cautioning against consuming unknown seafood species, especially those that might belong to the potentially toxic group. It also showcases images and provides identifications for three toxic Australian crab species found in intertidal and subtidal reef flats across northern Australia: the Mosaic Crab (Lophozozymus pictor), Toxic Reef Crab (Zosimus aeneus), and Shawl Crab (Atergatis floridus).
Further resources are provided for additional reading on this subject, including links to information from the Western Australian Museum, fisheries department, and scientific catalogues on crustaceans and toxins.
The comprehensive coverage of the topic, inclusion of scientific names, toxin information, geographic distribution, and precautionary advice about consuming unknown seafood demonstrates a solid understanding of crustacean biology and the potential risks associated with consuming toxic species.
