Popular Science Monthly/Volume 34/February 1889/The Dance of the Lady Crab
By T. H. MORGAN.
ABOUT the 12th of September, 1888, there was brought into the laboratory of the United States Fish Commission a male specimen of the lady crab (Platyonychus ocellatus), which was placed in an aquarium with a female crab of the same species. During the evening of the 13th, while sketching some hermit-crabs which had previously been placed in the same tank, I was attracted by the movements of the male Platyonychus. Without apparent cause he was seen to rise upon the third and fourth pairs of legs; his large chelæ were thrown above his head with the claws open and their points touching in the middle line; his fifth pair of feet were held horizontally behind, and his body perpendicular to the floor of the aquarium, or at right angles to the normal position, as shown in the accompanying figure. The posture was ludicrous, and, when in this position he began slowly to gyrate, his movements and attitude were the cause of much merriment upon the part of the spectators. At times he balanced on two legs of one side, again on two legs of opposite sides. Now he advances slowly and majestically, and now he wheels in circles in the sand on the floor of the aquarium, and now for a few moments he stands as if transfixed in this unnatural position. An electric light hung above and to one side of the water, which suggested the possibility that it might be the exciting cause. It was turned out, and still the dance went on, and the joy was unconfined. At last, from sheer exhaustion, lie sinks down to the sand in his usual attitude.
But now the female, who has all this time remained tucked away in the sand, comes forth and begins to move about the aquarium; soon she comes near to the male crab, when instantly he rises to his feet and begins to dance. Again and again the performance
Dance of the Lady Crab (drawn from life by T. H. M.).
is repeated, and each time the approach of the female is the signal for the male to rear high upon his hind-feet, and to reel about the aquarium as if intoxicated.
At times, when the female approached as he danced, he was seen to make attempts to inclose her in his great chelate arms, not with any violence, for the claws never snapped nor closed violently; but she was coy, however, and refused to be won by his advances, for the dance may have been nothing new to the lady crab, nor half as interesting as it was to the two spectators outside the water. Later, he too buried himself in the sand, and the performance came to an end.
The next day, and the day following it, the two crabs were watched, but without anything unusual taking place. The colors and markings of the male and female were much the same, though it seemed that the male had slightly more brilliant tints. To determine whether or not there is any marked sexual difference, a greater number of both sexes will have to be examined, and this at the time when the males woo the females under perfectly normal conditions.
Performances such as these are by no means uncommon among the vertebrates, especially with male birds in their endeavors to attract the female; but I believe there are few, if any, performances of this kind on record below the vertebrates.
To any one who has watched the crabs in their natural environments, the complex psychological development which may here be brought into play will not be surprising; yet, if the instinct which leads the male to dance is the same that we see in male birds, and if the female shows any discrimination between the dancers, the mental development must be considerable. Darwin has, in his "Sexual Selection," recorded among Crustacea many instances of difference in structure, and a few cases in which the color of the two sexes is slightly different, but does not mention any performance comparable to the dance of the Platyonychus.