Eddy Dieplinger, age 11, of Kingston, Ontario, Canada, for his question:
Where do land crabs go at low tide?
A pale ghost crab spends the day in a burrow high up on the beach. After dark, he scuttles over the sand to the water's edge, high tide or low tide. Some of the land crabs dunk only once in a while. Others spend their entire adult lives on land, often in trees. However, the females return to the sea to lay their eggs and the young larva crabs spend their childhood days in the water.
There are about 4,500 different kinds of crab and we tend to think that all of them belong to the sea. In a way they do, for under their crusty coats they have gills to extract dissolved oxygen from the water. What's more, all of them spend the first stages of their lives in the water, but many of the adult crabs manage to spend long periods on land and return to the sea only to lay their eggs.
The most familiar crabs live along tidal shores, at least during the summer. When the tide goes out, they are often stranded on the beach or marooned in tidal pools. This does not bother them at all, because plenty of oxygen rich moisture is trapped inside their crusty shells. Many species live inside damp burrows and visit the sea only once in a while.
As a general rule, the crabs depend less on the sea than most other shellfish. One of the true land crabs is the robber crab, alias the coconut crab. This crusty character spends his entire adult life high and fairly dry on the beach, scampering up and down coconut palm trees.
There is a reason why this species and other land crabs can survive out of water. True, they have gills as other crabs do. But their gills are rather small and they have help. The gills of the robber crab are tucked inside pockets of spongy flesh. This special flesh is riddled with tiny blood vessels that are able to extract oxygen from moist air.
The fleshy pockets act somewhat like lungs, which is why a land crab does not depend on high and low tidal waters. In fact, it has been proven that certain land crabs die when forcibly kept under water for several days. Apparently their pockets of spongy, lung like tissue are more important than their ancestral gills.
The crusty robber crab lives on beaches around the Indian Ocean and on many Pacific islands. He is one and a half feet wide and weighs six or seven pounds. Some observers say that he climbs palm trees because he likes coconuts. Others suspect that he dines mostly on carrion and berries and may climb aloft because there is dewy moisture among the foliage.
The male robber crab may not return to the sea at all during his adult life. But once a year, the female returns to lay her eggs in the water. The young crablets remain in the sea through their complex larval stages. As mini crabs they spend some time on the shore. Often one of these youngsters makes his home in a deserted snail shell. Later, he leaves the tidal beach for life on the land.