No. 31–Tales of the Horseshoe Crab, Part II

Sam and I had left the big boats in our wake on the dash to the mouth of Parris Creek that runs the length of the western shore of Parris Island. It is not actually a creek in the true sense of the word but rather a narrow waterway separated from the Beaufort River by marsh. It was a good place to drag because the shrimp, if there, would be confined in a space the width of our nets.

From the start of our drag we could see shrimp popping before the net in large numbers. Sam popped the tops on two tall can Old Milwaukee’s, an onboard staple, and we settled in for the drag with great anticipation. Every few minutes Sam would increase power to the Merc to maintain trawling speed as the nets grew heavier. We were on shrimp, by golly, and there would be pay at the end of the day.

The winch groaned the stern dipped low and Sam did a little impromptu shuffle dance as the first of the two nets was pulled aboard and a full bag broke surface off Parris Point. It was a struggle getting it and the second bag on board but in the end I was standing in shrimp to the tops of my boots. It was a good day to be a shrimper.

The next day began with disappointment and ended in near disaster. When our drag of Parris Creek yielded less than ten pounds of shrimp, Sam reckoned they must be on the other side of the river off Land’s End Beach. It turned out to be reckoning.

The Land’s End drag began with a few hoppers and then a few more along the way but nothing to get excited about. We angled from the beach to deeper water where, as the day before, Sam began to increase power as the nets grew heavier and heavier and still heavier till we were barely moving as we angled back toward shore. We had convinced ourselves that we had seen no shrimp popping because of the deeper water. Wrong…

The winch groaned and the stern dipped low but there was no dancing as the net was to heavy to bring aboard. Something was terribly wrong but what? We sat for a moment in the bottom of the boat pondering our plight before Sam rose to get us a couple of tall ones. “Holy —-” he exclaimed and dove for the winch brake releasing the net just in time to avoid our being swamped from astern by the wake of a large cruiser. We drank a beer.

The Merc roared as Sam struggled to drag the unyielding nets into shallower water. Finally, as striker, it was my duty to go over the side to determine the cause and extent of the problem. With zero visibility in the turbid water I reached my arms and upper body into the net opening and with a start, by touch, first beheld the strangest creatures this Oklahoma boy had ever encountered.

The American horseshoe crab, Limulus polyphemus, is one of four remaining species of an ancient line of arthropods dating back to the Silurian period of 440 to 410 mya.* Though Sam and I may have cursed them as we exhausted ourselves wrestling them from the nets that day long ago, they have since captured my attention and curiosity.

The horseshoe shaped portion of the creature is a combined head and thorax. The readily observable primitive compound eyes are able to detect motion but thought unable to distinguish form. A hinge like structure attaches the abdomen to the cephalothorax.

When viewed, the under side presents an ordered jumble of appendages and set of external gills resembling the pages of a book which are, not surprisingly, called book gills. The familiar tail spike is of neither defensive or offensive value but is useful in helping the animal right itself if overturned.

Arthropods, as do crustaceans, have exoskeletons which requires the animal to molt as it grows. Empty horseshoe molt shells may be found on local beaches during the warmer months. The creatures move off shore for the winter as estuarine and near shore waters cool. Take care to pick a live animal up for observation with hands midway on both sides of the cephalothorax. Pinched fingers may be the result of grasping too near the abdominal hinge should the creature fold itself to protect its poorly armored underside. Picking one up by its tail spine may well injure the creature.

Horseshoe crabs inhabit marine and estuarine zones with soft bottoms where they bulldoze the substrate in search of small clams, worms and other invertebrates. They will also scavenge feed and eat algae. The convoluted mechanics by which they get that food into their mouths is a story all it’s own. That plus mating and reproduction and other interesting tidbits I have come across while researching this column will appear with next month’s issue.

*Ruppert, Fox & Brown – Invertebrate Zoology, Seventh Edition