Questions My Mother Asks Me: Shellfish edition

seafoodAs a native New Englander, I firmly believe that it wouldn’t be Memorial Day Weekend without flags flying, veterans handing out poppies, and a seafood picnic.

And while family and friends chow down of copious amounts of various marine invertebrates (yup, that’s the scientist in me coming out), I invariably field questions about the life cycle of lobsters, the difference between a mussel and a clam, and what, exactly, is red tide?

Well, in honor of the shellfish buffet I hope you enjoyed yesterday, this week Kraken & Friends is going to take on one of these important questions:

What is red tide?

red tideAt some point, we’ve all heard warnings of red tide. That mysterious crimson nuisance that means seafood is going to be hard to come by for a couple weeks. But what is it?

Microscopic image of a red tide-causing dinoflagellate called Alexandrium fundyense. Image credit: NOAA

Red tide is caused by a type of phytoplankton – tiny plants suspended in the water column – called a dinoflagellate. These single-celled plants contain a pigment that can vary in color from a deep red to brown and when environmental conditions allow, aggregations of dinoflgellates on the ocean surface can make the water appear red. Or brown. Or white.

In the scientific community, we call these aggregations harmful algal blooms.

But why are they harmful?

Depending upon the species causing it, harmful algal blooms can cause a host of problems within the ecosystem. Anytime there is a massive increase in the concentration of a species, one of the most pressing concerns is a rapid depletion of resources.

Let’s think about that for a minute. If you were a farmer with 10 acres of pasture, it might be reasonable to maintain 5 cows on that land. Now, if you were to suddenly increase the number of cows to, say, 500, what would happen? Well, among other things, you’d probably run out of food and water pretty quickly.

This is essentially what happens during a harmful algal bloom. Frequently, we witness something called anoxia associated with any type of algal bloom – this means that the plants grew so quickly they drained all of the oxygen out of the water column. And this is bad, not only for the plants, but also for the mammals, birds, and fish using that area.

In addition to anoxia, red tide is often associated with neurotoxins. Many species of dinoflagellates produce a variety of toxins, which are then ingested by countless marine species, particularly filter-feeding shellfish.

Toxins?!? But why don’t the shellfish die?

Mussels and shrimp photographed on a seamount. Image credit: NOAA

In most shellfish, the toxins associated with red tide are stored in the digestive tract and other tissues. Depending on the species, these toxins are generally expelled from their body shortly after ingesting them. This means that the animals themselves are frequently unaffected by the toxins.

For the species feeding on them, however, it becomes a problem. When we eat steamed mussels or clams, our body absorbs their nutrients –including the toxins – and assimilates them into our tissues. And this is true of other marine predators, as well.


The good news is…

Because red tide poses a substantial public health risk, shellfish fisheries are generally closed when a harmful algal bloom is identified. But because the shellfish are not actually absorbing toxins into their bodies, these fishery closures are temporary – after the bloom dissipates, the fisheries may be reopened.

But beware.

Just because shellfish fisheries have reopened following a red tide, doesn’t mean all is well. In many instances, eating ingesting trace amounts of a toxin will be okay. But there are some things to avoid. For example, the tomalley of a lobster – that green bit that looks like wasabi – functions as its liver, which filters out and accumulates toxins.  Yeah, don’t eat that.

3 thoughts on “Questions My Mother Asks Me: Shellfish edition

  1. It’s the best time to make some plans for the future and it’s time to be happy.
    I’ve read this post and if I could I want to suggest you some interesting
    things or suggestions. Maybe you could write next articles referring to this article.
    I desire to read more things about it!


  2. Hi Kristen,
    Thanks for your comment. We’re glad that you enjoyed this post and I’m sure we’ll be writing several follow-ups to this one. If there’s anything in particular you’d like to learn more about, send us an email through the Contact Us page and we’ll try to tackle as many of your questions as we’re able to.
    – The Kraken & Friends Team


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