Journal archives for February 2018

February 01, 2018

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 on February 01, 2018 17:46 by susanhewitt susanhewitt | 6 observations | 11 comments | Leave a comment