At first glance looks like like a miniature gastropod (more like a scaphopod, or even a bivalve as a longtudinal semi continuous aperture is visible in the image).
Before I saw the scale, I might have thought it was a diatom. But it is definitely too big for any phytoplankton at what appears to be about 4 mm. Are species documented for the area (i.e. have other photos been identified)? A species list might help narrow it down a bit, but Abhishek Mukherjee is definitely in the right size range.