Here's a set of functions, drawn mostly from here and a portion of this screencast. It is not a perfect answer at this point; the major problem being the small tendrils that link features together. It is possible that some type of filter with a DiskMatrix
mask will lead you to a better result, but for the moment, I suspect there's a lot to wade through here.
seimg = Import["http://i.stack.imgur.com/LWNaK.png"];
i = GaussianFilter[FillingTransform[seimg], 15];
b = Binarize[i];
d = DistanceTransform[b, Padding -> 0];
m = MaxDetect[ImageAdjust[d, 0.2]];
w = WatershedComponents[GradientFilter[b, 3], m, Method -> "Rainfall"];
s = SelectComponents[w, "Count", 20 < # < 10000 &];
ms = ComponentMeasurements[
s, {"Centroid", "EquivalentDiskRadius", "Label"}];
Show[seimg,
Graphics[{Blue, Circle @@ # & /@ (ms[[All, 2, 1 ;; 2]]),
MapThread[Text, {ms[[All, 2, 3]], ms[[All, 2, 1]]}]}]]

Binarize[img, 0]
for a nice surprise! It was there all along! $\endgroup$ – Aisamu Nov 26 '14 at 22:19