Flexopecten Sacco 1897 in Mediterranean Sea:
flexuosus vs. glaber
what kind of shell is this ?
Most of times, the two larger species of Flexopecten which occur in Mediterranean sea are easy to distinguish, but it can happen that a specimen shows a mixture of features belonging to both species, at least at first sight, and then often at second sight. What could we do ? The two ranges overlap in western Mediterranean, so that if you've got a subadult and pale shell trawled off Malaga, with the pattern of a flexuosus and the shape of a glaber, what do you decide ? I assure you, these shells do exist. If you have not the right glasses, if you're tired and have too many books related to Pectinidae, these shells begin to appear more and more. A friend of mine had this problem and asked the question: how to distinguish ?
  What say the books ?

Flexopecten flexuosus (Poli, 1795) is thicker than Flexopecten hyalinus (Poli, 1795) and less rounded.
Fl. hyalinus has flatter ribs, is translucent and is smaller.
The valves of flexuosus show a different shape than glaber's ones.
Good! If I have a glaber somewhere, I will be able to compare.
Fl. flexuosus has five to six ribs. The left valve can be paler than the right one.
Fl. glaber (Linnaeus, 1758) has ten flat and large ribs. The right valve can be paler than the left one.
Great ! Here's a nice difference. Left valve in flexuosus, right valve in glaber.
More: flexuosus has crenulated margins. Glaber's ones show growth lines, sometimes.
So, here again, a distinctive feature. We are lucky. So what's the shell pictured above ?

  What says my brain ?

Oops! All my flexuosus have a right valve paler than the left, as if all were glaber. But they aren't, sure.
And the glaber... have their right valves paler than the left ones, that's right.
So, this difference in colouration of the valves cannot be used as a distinctive feature, at least in my drawers. The margins of flexuosus are not always fully crenulated, but often; the best crenulations occuring on the form pyxoideus Locard, 1888.

10, 7, 6, 5, 7, 10, 6, 8, 7 and 6... That's the number of the ribs of the first ten glaber I find.
More than five... Around 7, sometimes 10.
5, 5, 5, 7, 5, 5, 6, 6, 7 and 8... The number of the ribs of the first ten flexuosus I find.
Less than ten... Around 6, often 5. But what are exactly these ribs I count ?

In order to raise a bit the level of interest in these pages, let's show some pictures. Of course, everybody knows how these two species are built, but I suppose there's no harm done by looking at shells, and maybe we will discover something.

Flexopecten flexuosus (Poli, 1795)

  Average shells. Click on each pic to load a larger view
1- The ribs are often strong, always smooth, rounded and separated by deep furrows.
2- They are three in the center of the left valve, with one on each side. Total 5.
3- The furrows are radially striated and sometimes shelter one secondary rib, often obsolete.
4- These secondary ribs are clearly seen in the 4th shell. Counting them increases the total to 8 or more, as I did above. Not a good idea.
5- The margin can be less or more crenulated, at least in the prolongation of the furrows.
6- The anterior auricle, on the right valve, shows a kind of flat trench which separates the undulated part of the auricle from the shell. This trench is large and leads to a well defined byssal notch (e.g. 8th shell).

Flexopecten glaber (Linnaeus, 1758)

  Average shells. Click on each pic to load a larger view
1- The ribs are flat on both valves. You will notice their poor quantity: generally they are five to seven on the left valve, as you can see in shells 4, 5 and 6. We are far from the ten mentionned in litterature.
2- If you thought that the comparison of rib-numbers was a good distinctive feature, 5 for flexuosus and around 10 for glaber, then you will be lost as soon as you will receive some of these nine shells pictured here, especially those that lack the typical glaber pattern.

Something special which didn't appear in flexuosus is the array of radial striations which cover the whole valve, ribs included. And the margins are not crenulated, but all the shells show a less or more visible group of concentric lines, sometimes less growth lines than flat scales. These scales are obvious in shells 2 and 4, while the growth lines are better seen in shell 8 and even in the young 9th.

Auricular trench: it is not as obviously pronounced as in flexuosus, maybe for two reasons.
Aut, aut!... Aut the anterior auricle is not as striated, folded and scaled as in flexuosus, aut the trench itself is striated, as in shells 2, 8 etc.
  External distinctive features

Flexopecten flexuosus:

1- Ribs generally strong, smooth, separated by deep furrowsthat are radially striated, and which sometimes bear a secondary rib.
2- Crenulations can appear on the margins.
3- Anterior auricle looks like those of Mimachlamys varia, with well defined trench and byssal notch.

Flexopecten glaber:

1- Ribs flat. The whole valve is radially striated and covered by a concentric array of growth lines or scales.
2- No secondary ribs, no crenulations on margins.
3- Anterior auricle looks like those of Aequipecten opercularis. Trench and byssal notch less pronounced.

  More features ?
  Why not ? Let's try to discover some on next page.