Crassostrea and TBT – a very thick shell

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I been looking out some specimens of oyster shells that I collected from modern oyster beds on the south coast of England in the early 1980s. Way back in the early to mid 20th century, the hulls of boats were often coated with anti-fouling paint containing Tri-butyl tin (TBT). It was discovered that seawater contaminated with this chemical had an affect on some marine invertebrates. One of the noticeable phenomena was the way its presence caused the shell layers and chambers of the imported Pacific oyster (Crassosrea gigas) to wildly proliferate. Shells do in fact thicken naturally with age anyway – but in shells affected by TBT the rate of thickening is abnormal. Ultimately this unusual thickening prevented the oyster from opening up its two shell valves to feed and thus caused death of the organism.

A lot of research has been done regarding the affects of TBT on oysters. Laws have been introduced banning the substance in anti-fouling paints. The specimen illustrated in this post (with the images showing the same shell from various perspectives) is probably a rare survivor from the era when such abnormalities were fairly common. Nowadays, proliferation in shell growth of this type is more likely to be caused naturally and not chemically induced.

14 Replies to “Crassostrea and TBT – a very thick shell”

  1. What a strange and slightly disturbing thing to happen – proof that we affect our environment in ways we barely notice, let alone understand. The result is eerily beautiful though – like old paper, as one reader says.


  2. Are they trying to get rid of the poison by producing more layers of shell and depositing it inside, or is the poison just confusing the shell-producing metabolism?


  3. Such detailed photos, you can really see the fragility of the layers. I’ve never heard of this before, thanks for raising awareness of something I would have thought was natural.


  4. Thank you, Jo. Yes, we can affect our environment in a bad way without noticing what is going on. The problem illustrated in this post of the effects of TBT may have first been brought to notice because of the bizarre response of the Crassostrea oysters. Then it was observed that it was affecting other marine organisms in a more significant way (such as causing sex change in dog whelks). Subsequently, this species of oyster was used as a bio-indicator for the chemical pollutant. I am not up to date on the latest research so I am not absolutely positive but I believe legislation against the use of TBT in antifouling paints has mostly eliminated the problem these days.

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  5. Hello, Nannus. I think the latter – the chemical was affecting the shell-producing process itself. I will need to do a bit more reading the understand the cause and effect of it all. There is a lot of information on the internet. I haven’t thought about the issue since I was first given the shell 30 years ago when I was doing my PhD, and I was concentrating more on the flat oyster (Ostrea edulis) which was not affected this way by TBT.

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  6. Thank you, Emma Jane. Oyster shells can be extremely variable in size, shape, and thickness anyway and you can get some very old and thick ones but within a sort of normal range. Flat oyster shells (O. edulis) are not affected by TBT in the same way as Pacific oysters (C. gigas). When contaminated by TBT, even young Pacific oyster shells abnormally proliferated both the growth layers and the number of chambers contained within them.


  7. Very interesting! ๐Ÿ™‚ We have here in Sweden, Freshwater pearl mussel who can be more the 100 years old! The oldest one found was 256 years old!


  8. That is amazing, John. I have heard about a type of marine bivalve that can live hundreds of years but not about a freshwater one. The oyster in these pictures looks old but is in fact only about four years old due to the effects of the contaminant in the water.


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