egg
蜑 centers on the egg, specifically as a reproductive body produced by certain animals. The meaning is narrow and concrete.
蜑 combines 虫 (insect/reptile) with 延, which likely contributes a phonetic element. The exact historical development is uncertain, but the structure suggests an egg-laying creature.
The bottom 虫 (insect) sits under the top 延 (prolong). Picture an insect prolonging its species by laying an egg.
For タン, imagine a tan-colored egg laid by the insect: tan -> タン, and the egg is a warm tan shade.
ox-palate nerite (Nerita albicilla); blotched nerite