The fascinating life of “beach spiders”

The spring mating season is here for animals, birds and reptiles that inhabit Southwest Florida. There are hundreds of miles of shoreline and shallow areas that are ideal for these activities to take place.

Many of these areas are extremely secluded and may not have had human intrusion for decades. Other areas are frequented by boaters, fisherfolk and beachgoers.

If you are like me, you love to explore the new. After 40-plus years in this area, there are still places I run across that I haven’t been to before. (I keep thinking I am going to be famous someday by discovering a new species of insect, bird, animal or fish in these unexplored places. With all the talk of climate change and such, this thought may not be too unrealistic.)

There are strange sightings by quite a few people in this area, but not many people go so far as to research more about what they see. In the days of hardback encyclopedias on your home library shelf, this might’ve been so. Today’s websites and electronic devices make instant identification very easy.

Visitors to our area always seem to be amazed at both the amount and the variety of wildlife we have. Many of us locals often take these sightings for granted. Pelicans, gulls, terns, jumping fish, bobcats, panthers, alligators, snakes — all these and more are as common as robins, blue jays and squirrels up north.

One of the less common creatures we are now seeing on our local beaches are something we call “beach spiders.” Yes, spiders.

They are large, ugly prehistoric shells with spines and spikes and legs for crawling. Most of us refer to them as horseshoe crabs, but, actually, they are a species of ancient spider descended from the sea scorpion Eurypterid.

For beachgoers and boaters, sighting some of these may seem rare, but these species have been around for more than 400 million years.

Horseshoe crabs got their name because of the shape of the underside of the hard shell — which, of course, looks like the shape of a horseshoe. The term “crab,” though, is actually a misnomer, popular as it might be. These creatures have an ancient ancestry more closely related to spiders and scorpions.

The good news is: These light brown, armored, scary looking, creepy crawly critters are harmless, even with that long, sharp tail. Although the tail might look dangerous, it is actually used as a rudder and a tool for the animal to turn itself over. Horseshoe crabs do not attack people and will slowly move away toward deeper water if disturbed. They can range in size from an inch or smaller after hatching, to more than 18 inches long as adults. The ones we see on the beaches are adults, as juveniles live in tidal waters well below tide lines.

They have several compound eyes and can see ultraviolet light. The three major divisions of the body are the tail, the abdomen and the large shell, or cephalothorax. These creatures predate man, flying insects and dinosaurs. They are closely related to trilobites, which existed more than 500 million years ago.

The range of the horseshoe crab is from Mexico, all around the Gulf and on up the Eastern Seaboard as far up as Maine. They migrate yearly to Southwest Florida beaches to invade our shallow water to mate and lay eggs in the sand and mud. Up north, the invasion of these crabs usually takes place in May and June — their peak spawning season — along sandy beaches protected from high surf. Tens of thousands can cover the beaches and mud flats. Unfortunately for the horseshoe crab, its eggs are a main food source for shore birds, fish, sea turtles and invertebrates.

Horseshoe crabs feed on fish, clams, worms and other invertebrates, algae and crustaceans as they move along the bottom, using their legs and feeder claws. Remarkably, they can go a year without eating.

But there is more to the horseshoe crab than meets the eye.

In the 1960s, Johns Hopkins researcher and marine biologist Dr. Frederick Bang discovered the clotting tendency of the horseshoe crab. Collaborating with Dr. Jack Levin, Dr. Bang discovered that this clotting tendency could aid in surgical procedures in which blood flow was excessive. Collecting the blood does not harm the crab, and it is released back into the wild.

Today, more than half a million are harvested yearly for medical research. Scientists will drain about a third of the crabs’ blood, which is blue in color and has a unique chemical called limulus amoebocyte lysate that activates when a disease-causing organism enters the crab’s system through a cut, broken shell or contaminated food source. The chemical will surround the invading microorganism and thicken on contact, becoming a physical barrier to keep it from spreading to other internal parts of the crab.

Some factors that reduce the population of these creatures are red tide, bluegreen algae and human pollution — including fertilizer runoff from waterfront homes and other water flow areas. (It has always been a belief of mine that chemical spraying for weed control and aquatic vegetation should be stopped immediately. The sprays are poison, and the natural flow of water carries it right out into our oceans.)

We all need to take care of our environment and be active watchdogs for detrimental activities. Nature is an amazing, living, dynamic organism, and being observant in our daily lives will allow us to appreciate what it has to offer for a long time. We should not only enjoy our great outdoors, but also protect it for future generations to enjoy, too.

— Capt. Dennis Kirk and his wife, Nancy, are avid mariners and outdoor enthusiasts currently living in North Port. Since the 1970s, their love of nature in Southwest Florida has allowed them to experience the dream of writing about their travels and adventures of sailing, fishing and flying.

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