Fiddler crabs are-fast running crabs that spend a great deal of their time in air, although they must keep their gills moist. They tend to occur in the upper part of the shore in quiet sandy and muddy sediments. You can often see their distinct burrows, which are as much as 2-3 cm in width. If you look down as you walk along, you may never see one crab! They have stalked eyes with an excellent ability to see vertical motion. If they see you approach they will scurry to their burrows. So how do you see them? Go to a place where there are lots of burrows, stand very still and wait. After a few minutes they will come out and start feeding.They will ignore you, so long as you stay absolutely still. In the peak of summer, the upper quiet sandy shore will be packed with males. Just look at them and what do you see?

This a male sand fiddler crab, Leptuca (formerly Uca) pugilator. It is waving its left claw, which is about one third of its total body weight. This crab and his claw is one of West Meadow’s great wonders, for literally thousands of fiddlers will energetically wave their claws in a very distinct waving pattern. But why? For the ladies, silly! Males wave their large claws in order to attract females to the burrow. As a female moves near a group of males, the waving quickens and appears almost agitated! Females have two small claws, both of which are used for feeding on the sand and scraping organic matter. The poor males only have one feeding claw, because they have sacrificed the other claw as a display and combat device. They suffer because they can feed at a slower rate. Something like overspending your budget to buy one of those big pickup trucks, eh?

When a female enters the arena of males, she is interested in a mate. But how to choose one? This is a bit controversial. Some evidence shows that females choose males with the largest major claw. A study by Ann Pratt demonstrated that males with proportionally larger major claws got the most mates! Others believe that the largest males get the best burrow locations because they are good fighters (see below). So this point of view argues that big males get the most females because they can fight their way to the best burrow locations. Roving males often challenge residents, but the residents usually win. You can watch the fighting! Females then simply choose the best burrows and mate with the males that happen to be there, but they clearly are also selecting a good burrow within which to raise their embryos. A good burrow is high in the intertidal and is well aerated. This is important because the female will spend two weeks in the burrow.

When a female chooses a mate she approaches the male, whose waving continues. Then they establish contact and the female enters the male’s burrow. They copulate and the female then stays in the male’s burrow for two weeks or so, while her newly inseminated eggs are developing into embryos and then larvae that will eventually hatch into the water. During this development time the male covers the female with a nice moist bed of sand so she can stay wet and cool. It seems likely that the male leaves before the female is ready for hatching but another male finds the empty burrow and occupies it.

The male’s giant claw is not just for show. Males also fight each other. If you continue to stand absolutely still you will occasionally see a male moving around near the entrances of males who already have burrows. A fight will ensue. Sometimes the roving male will try to grab the resident male from his burrow and they will then grapple. You can almost hear them shouting “En garde, mon ami!” The claws will grapple and sometimes you can see one grabbing the other and tossing him down. Being thrown on your back is not good if there are predators nearby, so the loser scurries away. It is believed that the successors in these fights win out in the evolutionary race, causing the evolution of the large claws. All sand fiddler crabs have one large claw used in display and combat, while the other is small and used for feeding. It resembles the feeding claws of the female, but the male can obviously not gather food as fast.

Here’s a homework assignment for you: are the major claws of male sand fiddlers right-handed or left-handed? Count the claws of about 20 crabs and see what you find.

Once in a while, things go wrong for these males and a developmental error occurs. Instead of one major and one feeding claw, a male may rarely have two full-sized major claws. How these guys get along is not understood, but those that are found seem to be alive and well. What do you think they are doing?

 

There are about 105 species of fiddler crabs throughout the world, mainly in the tropics. At West Meadow we only have two species:

1.Sand Fiddler, Leptuca pugilator: Males have a large major claw, back of carapace in summer is a light blue with often complex patterns, resembling painted blue china.

2.Mud Fiddler, Minuca pugnax: Males have a distinct ridge on the “palm” of the inner part of the major claw. The back of the carapace is usually a dull dark brown-green.

Want to know more about all of the world’s fiddler crabs? See Dr. Michael Rosenberg’s fiddler crab web page.

J. Levinton August 2023