Argopecten nucleus vs A. gibbus, How to tell them apart?

Posted: Jan 8 2007 on LTS forum by Pecten:

Question :
How can I separate Argopecten nucleus from a (small) Argopecten gibbus?

Marlo: As to your question, when you find out, tell me.

Since Argopecten nucleus is fairly consistent in color (chestnut mottlings on cream, sometimes with yellow), you know you don't have one if your specimen displays the bright reds, oranges, purples, etc. of A. gibbus. But, if it displays the usual colors of A. nucleus, which can also be found in A. gibbus, then I throw my hands up in frustration.

Who out there can help us?

Here're an images of A. nucleus courtesy of Jaxshells.

Biscayne Bay, Miami-Dade Co., Florida (about 25 mm.):

Bret posted:

Hello Pecten,

As you have already found out, these two species are difficult to separate. Most of the literature out there is poor regarding the differences. And no one including myself have the distribution correctly delineated. However, all is not lost...

I rarely use color when trying to separate species. Our eyes have a tendency to fool us. I used to use color and rib count to separate nucleus, but was found to be mistaken several times. I have included B&W images of both species. I have found that there is one distinct feature which appears very consistent. When looking at the profiles of the two species note how nucleus is quite flattened in the middle, while gibbus is more rounded. I have found this characteristic to be consistent regardless of size.

Marlo: With you concurrence I created a slide from your post directly presenting the specifics of your comparison to help with my identifications. I thought I'd post a copy that others may wish to use.