Mystery of the distribution of an estuarine clam species

There is a fairly large species of obligatorily estuarine clam that lives in the Gulf of Mexico. It is edible, and it was so common there that the shells used to be crushed and used as road gravel. The scientific name of this clam is Rangia cuneata -- the Atlantic Rangia. It is not very beautiful, but it is interesting.

This clam species cannot survive in water of full ocean salinity. Adult clams can survive in freshwater, but they can't reproduce there.

During the Pleistocene period, which ended 10,000 years ago, this species lived in the Gulf of Mexico, but it also occurred in various estuaries on the East Coast of the US, from New Jersey south -- we have plenty of fossil evidence of that fact.

In 1955, an observer found a colony of this clam species living in an estuary on the East Coast. This clam species currently lives in several East Coast estuaries, including the estuary that surrounds the island where I live -- the estuary of the Hudson River. Since last April I have found about 50 valves of this species on the Harlem River beaches of Randall's Island Park, Manhattan, NYC. The salinity right there is most often between 18 and 23, but of course it can vary a lot more than that.

One theory as to how these clams ended up living in several different East Coast estuaries is the idea that they were introduced from the Gulf of Mexico by human agency -- either as larvae in ballast water, or with oysters when new oyster beds were being set up.

The other theory is that there were small relict populations of the clam that had survived in East Coast estuaries since the Pleistocene, and that these populations underwent a resurgence during and after the 1950s, and thus became noticeable where previously they were overlooked.

Currently people favor the first theory over the second one, but both ideas seem a little surprising when you think carefully about them.

For example:

  1. How did the clam first spread from estuary to estuary WITHIN the Gulf of Mexico?
  2. If the clam was introduced to the East Coast by humans, how come it took until 1955 to become established? There was plenty of shipping and ballast water before then.
  3. In the 20th century, shell collecting was not popular as a hobby until a few years after WWII, so 1955 would have been more or less exactly the time when people would start looking for, and finding, interesting shells which might have been overlooked previously. Is this relevant?
  4. How about the effect of global warming? When did ocean temperatures on the East Coast start to rise? Is that relevant?

This is a very interesting clam, and it's a bit of a puzzle too.

Posted by susanhewitt susanhewitt, February 01, 2018 17:46

Observations

Photos / Sounds

What

Atlantic Rangia Rangia cuneata

Observer

susanhewitt

Date

October 30, 2017 01:30 PM EDT

Photos / Sounds

What

Atlantic Rangia Rangia cuneata

Observer

susanhewitt

Date

November 15, 2017 02:00 PM EST

Photos / Sounds

What

Atlantic Rangia Rangia cuneata

Observer

susanhewitt

Date

November 16, 2017 02:00 PM EST

Photos / Sounds

What

Atlantic Rangia Rangia cuneata

Observer

susanhewitt

Date

November 29, 2017 01:57 PM EST

Photos / Sounds

What

Atlantic Rangia Rangia cuneata

Observer

susanhewitt

Date

January 27, 2018 12:31 PM EST

Photos / Sounds

What

Atlantic Rangia Rangia cuneata

Observer

susanhewitt

Date

January 27, 2018 01:29 PM EST

Comments

Biogeography is fascinating.

Posted by mrfish33 over 3 years ago (Flag)

Yes, you are right about that!

Posted by susanhewitt over 3 years ago (Flag)

go NYC clams!

Posted by ginsengandsoon over 3 years ago (Flag)

Neat. Sounds like some genetic work needs to be done. I know where to get samples in FL! :) Cool thing is there is a native american midden full of them a few miles from the spot where I have harvested them.

Posted by rogerbirkhead over 3 years ago (Flag)

I really need to do some thorough literature research, because I have a feeling that someone did try to look at the DNA, and found a difference between the GOM and East Coast populations. If I am correct about that, it would tend to support theory 2.

Posted by susanhewitt over 3 years ago (Flag)

It would be interesting to study whether that extirpation was related to Native American exploitation, similar to the mega-fauna extinctions and extirpations. Clams were certainly one of the most exploited estuary resources.

Posted by mrfish33 over 3 years ago (Flag)

Well @mrfish33, that's an interesting idea that I certainly had not thought of!

I had assumed that the Atlantic Rangia died out on the East Coast because of decreasing water temperatures which followed the last warm interglacial period during the Pleistocene. There were several mollusk species that did live up here back then, but which now can't live north of the Carolinas.

However, that would not explain why the Atlantic Rangia can live here now. Even though we do have global warming, the winters are nonetheless bitterly cold for a Gulf of Mexico species. Although the estuary water does not get nearly as cold as the air, every few winters we do have ice floes floating on top of the estuary waters. That happened this winter during our really bad three-week cold snap.

Posted by susanhewitt over 3 years ago (Flag)

Based on my complete un-expertise, maybe similar to those other post-ice age extinctions, likely a combination of changing climate and human pressure.

Posted by mrfish33 over 3 years ago (Flag)

I guess my preferred theory is that they were not extirpated but lived on as small obscure relict populations which for some reason (maybe global warming?) started growing and expanding in the mid 20th century.

Posted by susanhewitt over 3 years ago (Flag)

Some interesting theories. On a tangential note, I wonder how long it will be until there are DNA test capabilities for amateur naturalists and scientists?

Posted by brewbooks over 3 years ago (Flag)

Yeah... we all fantasize about a little hand-held DNA scanner that we can take into the field, but I suspect it will take a while to become a reality, and I would also predict it would be very expensive.

Posted by susanhewitt over 3 years ago (Flag)

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