Venus' Flower Basket (Euplectella aspergillum) is a beautifully ornate sponge that resides at the bottom of the ocean. Its impressive exoskeleton is made from silica, the same material that makes up common glass.
Although not many have seen the living organism, newlyweds in Asia give the dried skeletons of these sponges to their loved ones. Named after the Roman goddess of love and beauty, it makes sense that they would be used as a symbol of love—as long as you consider love as being locked in a room with someone forever.
When tiny female and male abyssal shrimps meet each other and fall in (the shrimp version of) love, they make a very serious commitment. When they decide to be together, they move into a Venus' Flower Basket, where they will spend the rest of their lives. As the tiny twosome grow, they become too big to leave the glass palace again, and are thus trapped with their mate until death do they part. When they have offspring, the tiny baby shrimp have no problem escaping the confinement by swimming through one of the many small holes.
Luckily, plenty of food drifts through the latticework of the structure, so the shrimps don't go hungry. In exchange for free rent, the shrimps keep the sponge tidy. Despite the constant scraping from the little shrimps' pincers, the glass prison does not break. The crustaceans are doomed to a lifetime of marital bliss. [PDF]