Why Can Clams and Oysters Sometimes Be Poisonous?
― The Poison Is Not Made by the Shellfish, but by the Sea Water They Filter
Clams from the same beach can be declared "unsafe to eat" in one particular spring. The culprit is plankton so small you cannot see it. Shellfish strain it out of tens of litres of seawater as food, and they store the poison in their bodies along with it.
You are about to go clam digging on a spring day. Then the news says, "Shellfish poison has been detected on this coast. Do not eat any shellfish collected here."
Last year you dug plenty on the same beach and made miso soup with no trouble. The shellfish look the same and smell the same as always.
You might think that cooking them well would make them safe. But this poison does not go away, however long you boil or grill them.
Just two reasons
Clams and oysters live by drawing in water and straining out food. When plankton that carry poison increase, the toxin builds up steadily in the shellfish.
Some of the toxins that build up in shellfish are not destroyed by heating. If a person eats them, the nerves can no longer send signals, causing numbness and paralysis.
In other words, the shellfish do not make the poison themselves. Whether the same kind of shellfish is safe depends on when and where poisonous plankton bloomed in the sea. Let's look at it step by step.
Shellfish are "living filters"
Put a clam in salt water and it stretches two tubes out through a gap in its shell. It draws water in through one tube, strains out tiny plankton with its gills, and pushes the water out through the other.
One clam is said to filter roughly 0.5 to 1 litre of water per hour. Its body, the soft flesh, is only a few millilitres. A tiny body passes tens of thousands of times its own volume of water.
The sea holds tiny plankton called dinoflagellates. Some kinds of them make strong poisons. Shellfish strain them out as food without telling them apart, so the poison builds up in their flesh too. See Figure 1.
When poisonous plankton multiply so much that they turn the sea red, an event called a "red tide", you can see it. But shellfish are said to carry poison even when plankton are so few that the sea does not change colour. Even a small amount adds up when it is gathered from tens of thousands of times more water.
Boiling or grilling does not remove the poison
The one that Japan warns about most is "paralytic shellfish poison". Its main toxin is a substance called saxitoxin, which is known to be very heat-resistant. Making it into miso soup, grilling it, or freezing it barely reduces it.
Our nerves pass on electrical signals by opening and closing tiny "gates" on the surface of their cells. This poison plugs those gates. Because signals no longer arrive, the mouth and tongue first tingle and go numb.
In severe cases, the arms and legs become hard to move. Further on, signals stop reaching the muscles used for breathing, and a person can stop being able to breathe. Symptoms are said to sometimes start about 30 minutes after eating.
There is said to be no drug at present that can neutralise this poison right away. The poison does leave the body over time, though. Being helped to breathe in hospital while the poison clears is the most important thing for saving a life.
"Diarrhetic shellfish poison" is made by a different plankton. Instead of numbness, it is said to cause severe diarrhoea and nausea within a few hours of eating. Heating does not remove this one either.
Many bivalves are said to suffer little harm from storing the poison. In one North American clam, the shape of the nerve "gate" was found to be slightly different, so the poison has trouble plugging it. The more resistant a shellfish is to the poison, the more of it it can store.
So what should we do?
- Check the "shellfish poison information" for that sea before you go diggingPrefectures and fishery cooperatives publish test results and collection warnings for each sea area.
- You cannot tell from looks, smell or how well it is cookedHeating and freezing do not remove it, so do not eat shellfish from a sea where a warning is out.
- If your mouth or tongue goes numb, call 119 at onceIf you still have some of the shellfish you ate, keep it and tell the doctor.
Shellfish sold in shops are tested before shipping, and shipments from sea areas over the limit are stopped. Shellfish you collect yourself have not gone through this testing. In sea areas with a warning, please refrain from collecting them. If you feel numbness around the mouth or in your hands and feet, dizziness or difficulty breathing after eating, do not wait and see. Call 119 immediately.
Summary
The poison in shellfish was not made by the shellfish. It was made by tiny plankton in the sea. Shellfish filter tens of thousands of times their own volume of water to collect food, so they concentrate and store the sea's poison. That poison resists heat and stops nerve signals.
Shellfish are the sea water's "summing-up".
What you cannot tell by looking at a shellfish, information about the sea can tell you.
Tiny living things in water making poison is not only a story of the sea. For the colour change of ponds in summer, see "Why does a summer pond turn green like paint? Is it safe to touch?".
- Prepare two clear glasses and put salt water about as salty as seawater (roughly 30 g of salt per litre of water) in each.
- Add 2 to 3 drops of milk to both so the water turns faintly cloudy. Put 5 to 6 supermarket clams sold for sand removal into just one of them.
- Put them somewhere dark and quiet and compare them every 30 minutes. Only the glass with clams should clear quickly.
The clams are straining out the tiny milk particles just as they strain out plankton. In the sea, this same ability also gathers up poison. Do not eat the clams used in this experiment; throw them away.
For those who want to know more ― terms, formulas and links to textbooksShows which level each part belongs to, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Biology"
- HS+Advanced high school content, or textbook sidebar material
- UniversityUniversity specialist subjects not taught in high school (neurophysiology, fisheries food hygiene)
- ResearchNot taught even at university as settled fact; researchers are still studying it
Junior highTerms: this phenomenon has names
- Shellfish poisoning (kaidoku): poison a shellfish stores in its body by eating plankton. The two main kinds are paralytic shellfish poison, which causes numbness, and diarrhetic shellfish poison, which causes diarrhoea.
- Filter feeding (rokashoku): a way of eating in which water is passed through the body and tiny food is strained out. Bivalves such as clams, oysters and scallops do this.
- Concentration through the food chain: through eating and being eaten, a substance builds up in living things at a much higher concentration than in the water.
Junior highHigh schoolChecking with a formula: how many times its flesh volume of water does one clam filter?
Assume all the poison in the water the clam filtered stays in its flesh, and estimate how concentrated it could get. In reality some of it leaves the body, so this is a rough "upper limit".
| In symbols | N = (F × t) ÷ V |
| In words | Concentration factor = (water filtered per hour × hours of filtering) ÷ flesh volume |
| Where it comes from | Conservation of the amount of poison. The poison that enters the flesh is "volume of water filtered × concentration of poison in the water". Dividing that by the flesh volume gives the concentration in the flesh. Its ratio to the concentration in the water is the factor N. |
| Symbol | Meaning and unit |
| N | How many times higher the poison concentration in the flesh is than in seawater (no unit) |
| F | Water filtered per hour (litres per hour) |
| t | Time spent filtering (hours) |
| V | Flesh volume (millilitres) |
| Water one clam filters per hour (a low estimate) | 0.5 litres |
| Time spent filtering (1 week) | 168 hours |
| Flesh volume of one clam | about 3 millilitres |
| Japan's regulatory limit for paralytic shellfish poison (per 1 g of flesh) | 4 mouse units |
| Water filtered in 1 week | 0.5 × 168 = 84 litres |
| Convert to millilitres | 84 × 1000 = 84000 millilitres |
| Divide by flesh volume (concentration factor N) | 84000 ÷ 3 = 28000 times |
| Compare with a home bath (about 200 litres) | 84 ÷ 200 = 0.42 |
| Eating 100 g of flesh right at the limit | 4 × 100 = 400 mouse units |
One small clam passes about 40% of a bath's worth of water in a week. Even if there is only a little poison in the seawater, the calculation says it can reach up to tens of thousands of times that concentration in the flesh. The regulatory limit is said to be set with a margin, so that ordinary eating does not cause symptoms.
High schoolHS+Why do nerve signals stop?
High schoolNerve cells pass on electrical changes (action potentials) one after another when channels for sodium ions on their surface open all at once. If these channels are blocked, the signal cannot move on.
HS+Saxitoxin fits snugly into the entrance of this channel and keeps sodium ions out. The pufferfish poison (tetrodotoxin) blocks almost the same spot, so the symptoms are very similar. The strength of the poison has been expressed in "mouse units", where one unit is the amount that kills a 20 g mouse in 15 minutes.
UniversityVoltage-gated sodium channels and managing shellfish toxicity
The target of saxitoxin is the outer entrance of the voltage-gated sodium channel (near the selectivity filter). It binds tightly there and stops the rising phase of the action potential. The toxin producers are mainly toxic dinoflagellates such as the genus Alexandrium. In Japan, monitoring by sea area, shipping restrictions and regulatory limits are combined to keep toxic shellfish off the market. In recent years, the mouse test has been gradually replaced by instrumental analysis that measures each toxin component individually.
📖 Derivations and further reading: Saxitoxin (Wikipedia) ／ Sodium channel (Wikipedia)
ResearchWhat is still not clear
- When and where do toxic plankton bloom? Changes in water temperature, nutrients and ocean currents are said to be involved, but predicting outbreaks accurately in advance is still said to be difficult.
- Will ocean warming change where shellfish become toxic? Cases have been reported in sea areas that previously had few shellfish toxins, and the cause is being investigated.
- How do shellfish get rid of the poison? The number of days until the poison clears is said to differ by shellfish species and body part, and the mechanism is not well understood.
So the content of this article too is "an explanation of what is known now". That is exactly why testing and information are kept up every year.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: links between living things (food chain) | Shellfish eat plankton and store the poison |
| High school | Biology: excitation and conduction in nerves | Sodium ion channels and action potentials |
| HS+ | Biology, advanced (how neurotoxins work) | Why saxitoxin and pufferfish poison are similar |
| University | Neurophysiology; fisheries food hygiene | Voltage-gated sodium channels; shellfish toxin monitoring and regulatory limits |
| Research | Marine ecology; harmful algae research | Predicting outbreaks, link to warming, toxin elimination |
| ― | Links to daily life | Checking shellfish poison information before digging for clams |
- Ministry of Health, Labour and Welfare (厚生労働省), "Risk profile of natural toxins: bivalves: paralytic shellfish poison" (自然毒のリスクプロファイル：二枚貝：麻痺性貝毒)
- Fisheries Agency (水産庁), "Guidelines for risk management of shellfish toxins in bivalves and other shellfish" (二枚貝等の貝毒のリスク管理に関するガイドライン)
- Wikipedia (ウィキペディア), "Shellfish poisoning" (貝毒)
- Wikipedia (ウィキペディア), "Saxitoxin" (サキシトキシン)
- Bricelj, V. M. et al. (2005) Sodium channel mutation leading to saxitoxin resistance in clams increases risk of PSP. Nature 434, 763–767.
※This article is a general science explanation. The figures given are rough guides to help you understand how things work. For collecting and eating shellfish, follow the latest information and instructions from your prefecture, public health centre and fishery cooperative. If you feel unwell, do not hesitate to see a medical institution.