Dive Dry with Dr. Bill

#467: Just Fiddling Around, Crabby

Two years ago, I gave up watching TV... and my life has been greatly enriched as I returned to a passion from my youth, reading (and being able to use my imagination). Recently I had an opportunity to tune in to "The Today Show" during a week of "Where in the World is Matt Lauer?" Hmmm... no TV at home so how did I do that? Nope, I didn't meet the woman of my dreams and spend time at her ocean-front mansion. So, where in the world is Dr. Bill? I'll give you a hint... it's warmer than Catalina, I have a host of mosquito and fire ant bites on my legs, I'm right on the water (but not SCUBA diving... sniff) and I've been filming roseate spoonbills, wood storks, sandhill cranes... and even alligators.

To make a short story long, about two months ago I was going to fly to Atlanta, connect with my mother and fly up to Michigan for my niece's wedding. Most of the family would be joining us for a fun-filled reunion. Then, the day before my flight, Mom called. Her doctor told her she couldn't make the trip. We didn't know why, but suspected after 86 years of good health and a very upbeat personality, Mom was facing something serious. The weekend of the wedding we called her constantly to update her and I filmed everything (well, almost everything) to share with her. After a visit to her doctor that Monday, Mom was scheduled for major cancer surgery in two days. My sisters and I flew down from Michigan to be with her, and I'm staying with her til early January to take care of her.

Of course I brought my high definition camcorder, but not my underwater housing or even my SCUBA gear. Instead of filming fish and marine invertebrates, I've focused my lens on aquatic birds and other critters that can be seen along the shore. Mom lives overlooking a small "yacht" basin in Sarasota, and there are plenty of victims... er, subjects... for me to film here. So this week you will read about a rather interesting one... the fiddler crab. We have our own species, Uca crenulata, in southern California but the pictures in this column are of one of its undetermined Uca relatives here in Florida. There are about 100 species of fiddlers world-wide.

Thirty-two years ago when Mom and Dad bought in Pelican Cove, the "yacht basin" was being created and had an almost entirely boulder bottom. Developer Bob Morris was very sensitive to the native flora and fauna, and let the mangroves grow along the banks. These trees capture sediments and enrich them with their leaves, etc. Over time a rich layer of dark mud evolved around the base of the mangroves and created a perfect habitat for the "billions and billions" of fiddlers that frequent the flats. These crabs live in mud burrows and come out at low tide to forage over the mudflats. During high tide they retreat into their homes and plug up the entrance.

There are several species of fiddler crabs in Florida. Each one is adapted to different sized grains of sediment ranging from coarse sand to fine mud. Their mouths contain hairs whose spacing is adapted to scrape microscopic algae and diatoms off different sized grains. Males use their smaller claw for feeding. The fiddlers separate the munchies from the sediment in the mouth, then roll up the inedibles into small balls of mud. These small balls dot the landscape of the mudflat, and are washed away with the next high tide.

Fiddler crabs are very important in mudflat ecosystems. Their tiny burrows, some as small as a fraction of an inch in diameter, may number up to 125 per square yard. The largest ones may be 1 1/2" in diameter and three feet deep. The burrows aereate the soil so plants grow better, and allow decomposers to break down leaf and other litter to release nutrients. Fiddler crab "poop" also increases "fertilizer" levels. By thus increasing plant growth, they also facilitate the production of the leaf litter that is part of their food. This "interdependence" of species is what healthy ecosystems are all about. Fiddlers also keep the tiny algae from forming a thick carpet on top of the mud.

In turn fiddlers are munched on by a variety of predators. I've found the ones here in Sarasota to be quite skittish, rushing to their burrows as my shadow approaches or the wooden walkway vibrates. Makes it difficult to get up close and personal to film them, even with a telephoto lens. I can't blame them. Birds such as egrets and ibises and racoons attack them from the shore while fish go after them from the sea. In fact, fiddler crabs are used for bait to catch red drum and sheepshead. They may even get munched by other fiddler crabs.

I've saved the best for last. Like so many other species, reproduction by fiddler crabs is very interesting. These critters are sexually dimorphic with the males possessing the much elongated claw or cheliped that gives them their name (my violin-playing.friends may not see any resemblance to their bows). The average female of our species may check out two dozen potential mates, or on occasion more than 100 of them. The males size, especially that of the elongated claw, may be the first "measure" of suitability. Males are known to wave their claws at females at a faster rate when other males are vying for the lady's attention. Males will occasionally fight one another to impress the girls... sound familiar?

If the female likes what she sees, she then inspects his bachelor pad... usually by entering it. She wants it to be the right size to ensure their brood develops quickly enough and escapes from the burrow at high tide to begin their own journey. If she likes it, the two will enter the burrow, plug up its entrance with mud (must be a bit modest) and mate. If all goes well, the eggs will hatch in time for the emerging larvae to be "swept away" by the bi-monthly high tide to seek their fortune. In some species the female will mate, then retreat to her burrow coming out when the eggs are ready to hatch into larvae. I guess she just doesn't trust her guy to take care of the kids.

© 2011 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Mudflat habitat with fiddler crab burrows, round mud pellets near burrows;
male crab with elongated claw, two males fighting for females.

This document maintained by Dr. Bill Bushing.
Material and images © 2011 Star Thrower Educational Multimedia