I've been spending some time this week creating smaller sizes of sea urchin beads from polymer clay. I'm also offering the molds as well so that anyone can craft realistic pieces for their summer or other craft.
For the sea urchins the sizes measure from a scant 18mm (shown) to 35mm. Sizes are approximate since one can slightly distort the beads smaller or larger from the mold. The small one here is a little smaller than a penny.(CDN and US pennies are the same size)
Different starfish molds are also available. Sizes are 30mm to 65mm. I've listed these in my ArtFire.