Japanese Scientist
 Hiroaki Sugita

Mr. Sugita is a Japanese scientist at the Institute of Biological Sciences at the University of Tsukuba. He has studied horseshoe crabs for a long time. He wrote to tell us about the Japanese horseshoe crab (Tachypleus tridentatus) and what he is studying. He reported the following:

        1.  The horseshoe crabs come to the beach to lay eggs in the sand (10-15cm from the surface) on the spring tide.  In Japan, only the pairs of the horseshoe crabs come to the beach, while many single males of the American horseshoe crab (Limulus polyphemus) come in with one female in Delaware.

        2.  The eggs are in the sand where seawater does not come over them almost all day long. Furthermore, the rain washes over them. They develop to the larval stage (until hatching out) in a risky place.  Please see the photo of the birds watching the beach in the photo below, and note that the developing horseshoe crabs are under the sand and not the seawater.
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        3.  You can also see developing horseshoe crabs in the picture below: two eggs covered with chorion one of which was stained by red pigments (about 3mm in diameter), two embryos after the second embryonic molting in semitransparent membrane, that is, the inner egg membrane, with the broken chorion, three embryos after the third embryonic molting, and one embryo after the fourth embryonic molting (immediately before hatching; about 7mm in diameter) on the right side of the picture. Please notice that the inner egg membrane becomes bigger as the development goes on and the membranes never shrink and puncture when they are washed by rain.

        4.   I studied the swelling mechanism of the inner egg membrane.  Na, Cl and Ca ions pass freely through the inner egg membrane.  Therefore, when the outer solution is thinner than the solution in the inner egg membrane after it rains, the ions go out from the inner egg membrane so that water does not come in the membrane (please remember the osmotic pressure). The opposite occurs when the eggs are covered with seawater after it rains, the ions come into the inner egg membrane.

        5.  However, high molecular weight substances such as protein, amino acids and sugar do not pass through the inner egg membrane, so the inner egg membrane can swell constantly during the embryonic development because of the osmotic pressures caused by such substances which are constantly secreted from the egg surface.


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