Mercury levels in Tuna

[Richard]

I noticed that Tuna is on the "SAFE FOODS" list. I am a bit puzzled by this because, Tuna contains mercury levels that are so high, they are considered harmful for young children and pregnant woman. I don't think occasionally feeding Tuna will kill or harm the crabs. Just like occasionally feeding a non organic banana probably wont hurt them either. But if Tuna is used as a fixed part of their diet, it might cause problems. Why would anybody ever feed their crabs fish with high levels of mercury, when they can feed them other fish that does not contain these high levels ?


The mercury levels are high in all fish that grow old and are on the top of the food chain. Like sharks, King Mackerel and so on. The safe food lists says

"Fish, all species^"

I assume this would also include sharks, swordfish and other fish that contains undesirable levels of mercury. I do realize that nobody is gonna feed shark, or swordfish as a fixed part of the hermit crab diet. Tuna on the other hand, might be

Considering this Is Tuna really a safe food ?

[aussieJJDude]

Truth to be told, everything is deadly in excess. Feeding foods containing loads of sugar is probably just as damaging to their bodies as feeding large quantities of tuna.

After a quick wiki (totally trustworthy i know) the most abundant form of mercury is methylmercury. This can easily enter the human bloodstream - which you know of already - but has a half life of 50 days in thr human blood. Which i imagine similar would happen in hermit crabs...

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[Richard]

I agree that pretty much everything is deadly in excess. At the same time I thought the point of the "SAFE FOODS LIST" is to provide a list of foods that can safely be fed to hermit crabs. You could argue that non organic fruit is safe. As long as you don't feed it in excess amounts, and maybe remove the outer layer. But it is recommended to feed the crabs organic foods, because they don't contain the same levels of pesticides. I looked up the wiki page for Methylmercury. It's says the half-time is "about 72 days in aquatic organisms". So the crabs will be able cut the total level mercury in half after 72 days. But still contain a higher than normal level mercury for a very long time after that. Imagine if you feed your crabs Tuna once a week (because it's safe). The accumulation of mercury over time would be very high. And while some of the effects are unknown. I hope we can agree that Mercury is not gonna help the crabs, stay healthy. So unlike the other items on the "SAFE FOODS LIST" this should probably only be given as a rare (mercury filled) treat rather than a fixed part of their diet.

I don't think anybody would want to feed their crabs something, that could potentially, be bad for them in the first place ? Smaller fish don't have this problem to the same extend. So if you want to feed them fish, you could feed them smaller fish, that does not contain the same high levels of mercury (and other heavy metals)

[aussieJJDude]

This may sound rude, but i do think your over thinking this slightly. Many things we eat/give to our plants is considered toxic to one life form or another... likewise, hermit crabs are nature's scavengers and would be exposed to mercury in their diet from dead fish washing up on the shoreline from time to time.

As caregivers for our crabs, we need to feed them a varied diet... so if anything, we wouldn't be feeding them at most tuna every week or so. They eat in small quantities - if they decide to eat it at all - so i imagine that the level of mercury in their 'blood' would be small enough not to do serious harm.

I do understand your concerns, but i feel their is no reason to justify the removal of fish such as tuna from the safe food list. For example, sardines contain high levels of PCB - a fire retardant - which is toxic to humans in high concentrations... does this mean we also shouldn't feed our crabs sardines?

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[Richard]

I might be overthinking the whole thing. Or at least I misunderstood the purpose of the lists. I had a good long look at the UNSAFE / UNPOPULAR foods list. Many of the plants on this list are deadly to humans and will probably kill crabs too.

I guess the point with the safe and unsafe lists are that : If the crabs will take the food, and it does not cause acute poisoning, it goes on the SAFE LIST.

If it will probably kill them or they won't take it (the comment on the top of the list mention lemons) it goes on the UNSAFE / UNPOPULAR list.

You would have to be pretty dense, to wake up one morning and decide to mix up a salat for your crabs consisting of Hemlock, Tobacco leafs and Thornapple.

I noticed that the unsafe lists says : "Compost (unless 100% organic)"
Will even small traces of pesticides kill the crabs ? or is this sort of the same thing as with Mercury in the Tuna ?

I fully understand that there is a huge difference between the two. Because fruits and veggies makes up a bigger part of their diet. Most people will probably be feeding fruit, and veggies many times a week. Unlike Tuna that might be given rarely. I still don't think that foods with high levels Mercury and other heavy metal is going to help the crabs.But luckily nobody is forcing me to feed my crabs Tuna or shark

[wodesorel]

Pesticides are considered dangerous to crabs, although some are unavoidable in small amounts due to the way fruit and veggies are grown. Certified organic is always a safer choice, but being smart (peeling and washing) with feeding regular doesn't seem to affect them in practice as far as we can tell. Composting tends to concentrate pesticides even more since everything is broken down so much which is why organic there is recommended. It also in theory prevents any kind of chemical contamination which is common in commercial soils.

I really view the unsafe/unpopular list as just the "they won't eat it so don't bother list". To our knowledge no crab has ever died from something it ate. Crabs are not humans, or even mammals. Things that hurt us will not hurt them, and vice versa. Insects can eat tobacco and nightshade and hemlock and a ton of other plants that would easily kill us, and crabs are much closer in relation to them. We can eat cinnamon which most insects avoid like the plague. We list the plants with known natural pesticides that take specialty eaters (tobacco), things crabs don't seem to like (unsweeted citrus), and we have even removed some items that seemed like it might be bad or unwanted after learning crabbers fed them without having ever looked at the lists and did not experience any deaths from it (cherry wood and lavender).

The safe list only gets foods that have been "food trialed" - fed to multiple groups of hermits over a period of months or years - to make sure that the addition of such item has not lead to any deaths AND the crabs actually eat it.

[Richard]

Good point with the crabs being closer related to insects, than humans. And in theory at least, be able to eat a lot of things, that could kill us. The fact that nobody has ever experienced a crab die, from something it ate, might be due to the fact that the crabs, like a lot of animals, avoid eating things that are bad for them. Probably also that, people don't experiment with plat material that are known to be poisonous. On top of that it's hard to determine the cause of death (unless there is a obvious reason ex: flooded substrate while molting).However if they were to eat something that is very poisonous to them, I think you could expect to see mass deaths ?

The thing about stuff like pesticides, mercury, lead and other things we can assume, are (in excess) bad for the crabs is that . It is almost impossible to tell just how bad these things are. Say someone feed their crabs non organic foods all the time. The crabs might live for around 5-10 years and be just fine, until they pass away. The premature deaths might not even be ascribed to their diet. The only way to test this, as you probably already know, is be to have 2 identical tanks, with 2 identical set of crabs. Feed one organic, and the other non organic. Wait for the crabs to pass away, and finally report the results. While it could be very interesting, I doubt anyone is actually gonna do it.

On the topic of poisons/toxins that land hermit crabs can survive. I read that E's have adapted to eat a type of highly toxic exotic eel, that no other crabs are known to take.

[aussieJJDude]

Going with the comment above, you would need to set up multiple tanks and then feed organic or not to make it a plausible experiment. Just two tanks will have a lot of variables - mostly crab behaviour, each tank has a different tank dynamic - and also would be wise if there are certain 'level' of non organic food they could safely handle - like crabs can handle x ug mol of 'this' type of pesticide...

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[soilentgringa]

Going with the comment above, you would need to set up multiple tanks and then feed organic or not to make it a plausible experiment. Just two tanks will have a lot of variables - mostly crab behaviour, each tank has a different tank dynamic - and also would be wise if there are certain 'level' of non organic food they could safely handle - like crabs can handle x ug mol of 'this' type of pesticide...

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Wouldn't there also need to be some way to definitively prove that the toxicity is what killed them?

So many die of PPS (blanket term for unkown causes associated with hermit crabs failure to survive in captivity).

There are soooo many variables.

Like...why did Jon and Kate crab live for over 30 years when everything Carol Ormes did is very different from the suggested care nowadays?

Some folx on the forum here have had absolutely ideal setups and did "all the right things" and still had unexplained deaths.

I can almost be considered negligent by a lot of crabbers with how hands off I am with mine, and have had no deaths in well over a year now.

Crabs are weird.

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[aussieJJDude]

Yeah, and that too SG .

Since we understand very little about them, most of our care is just through trial and error. For example, i believe carol pioneered in letting the crabs moult in a dark place - slate caves - abd letting them consume their old exo.

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[soilentgringa]

Yeah, and that too SG .

Since we understand very little about them, most of our care is just through trial and error. For example, i believe carol pioneered in letting the crabs moult in a dark place - slate caves - abd letting them consume their old exo.

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Yep. She even helped them out of their exos and allowed them to crawl all over the house and kept their tanks stringently clean. (She was some sort of microbiologist IIRC) Living in Florida totally didn't hurt the humidity and temps being safe tho.

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[wodesorel]

New Jersey. She lived in New Jersey for the first 20 something years.

Everyone goes, "Oh, Florida!" like that explains everything. Nope, they had winters and furnace dryness and low humidity for two decades and change.

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[soilentgringa]

New Jersey. She lived in New Jersey for the first 20 something years.

Everyone goes, "Oh, Florida!" like that explains everything. Nope, they had winters and furnace dryness and low humidity for two decades and change.

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I didn't know they lived in New Jersey. I don't know how I missed that!

Average annual humidity in NJ is between like 60% (afternoons) and over 80% (mornings) so it's probably closer to what PP's and Indos have further away from the beach, considering that NJ is by water too.

But definitely the heater would dry out the air some in winter.

I wish I could talk to Carol and pick her brains aside from random online blurbs and such

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[Richard]

I think the two tank thing could work. Obviously, the more tanks and crabs you have. The more precise a result you would get. You could buy 3 crabs extra for each tank, and calculate with having to remove them. Expecting that a certain percentage would die before or during the time of their first molt. You could number the crabs and randomly remove excess crabs from the tank(s) that suffered the least deaths

However any observations in their behavior patterns would be ruled out, because the person feeding them would know what tank would contain the "healthy" crabs.And so start seeing behavior that he/she might think is related to the food when in reality it is not. As Soilentgringa pointed out : would there even be a definitive way to determine that the crabs died from the pesticides ?

We do know how pesticides affects insects. And i assume the reason to believe that they are bad for crabs, is the fear, that pesticides would affect them in a similar way. So I guess that for amatures, the answer to this question would be no. Because we can't open up their brain and start measuring the level and state of "cholinesterase" (never even heard of this before, but it seems to have to do with how pesticides kill insects)

But if pesticides are as bad as for the crabs,as they are made them out to be. The crabs getting fed stuff full of pesticides would probably not last as long, as the ones getting the "good" food.

We probably all heard a story about someones grand father who smoked 40 cigarettes, and drank ½ bottle whiskey every day. And lived till he was 92. But that does not mean that the vast majority of humans will live to be 92 while ingesting these things on a daily basis.

Likewise in the experiment with the pesticides, you might have a couple of crabs getting fed food full of pesticides and not get affected by it, in the same way as the rest of the crab. If indeed these other crabs got affected by it, at all.