Sunday, June 5, 2011

Crab Census

Don't have much of anything prepared for this week, I'm afraid: for the last four days or so I've been kept out of town and away from my writing desk by various obligations, not the least of which was the annual Delaware Bay horseshoe crab census at the extreme southern end of New Jersey (far south enough that the locals can be heard saying "y'all" and "youse"). My friends and I volunteered last year and had so much fun that we decided to help out with the 2011 effort.

The program and procedure are both pretty simple. From the mid 1800s through the 1960s, horseshoe crabs were harvested en masse from the Delaware Bay area to be ground up and sold as fertilizer. Between this, the increased human presence on the bay shores, and increasing demand for the crabs as eel bait and biomedical test subjects, the horseshoe crab population has seen dramatic decreases throughout and beyond the latter half of the 20th century. Delaware, Maryland, New Hampshire, New Jersey, and Virginia have imposed stricter regulations on horseshoe crab harvesting, while South Carolina (oddly enough) has imposed a permanent moratorium on the commercial fishing of horseshoe crabs for all industries except medical research.

Beginning in 1990, the University of Delaware Sea Grant College Program and the National Oceanic and Atmospheric Administration have led and sponsored an annual series of crab counts at different sites through the Delaware Bay area during May and June (the peak of the species' mating season).

The census is a straightforward affair: a group of about four to eight people (usually students, biologists, and local eccentrics) don their galoshes and hit the bayside beaches of Delaware and south Jersey at high tide during the days of, before, and after the new and full moons. They walk along the waterline with a pair of 1 x 1m PVC pipe squares (or "quadrants," if we want to sound more like the rigorous empirical investigators we are), laying them down at twenty-meter intervals and counting the number of horseshoe crabs inside each quadrant. (My job was carrying the clipboard, running back and forth between the two groups, and jotting down their findings.)

And this brings us to the second, and probably more immediate question: horseshoe crabs? Why should anyone care?!

The boxed buzzphrase most frequently used whenever someone finds himself compelled to write about horseshoe crabs is "living fossil." The term verges on cliché, but is none the less appropriate for it. Fossils belonging to members of the horseshoe crab family (Limulidae) have been found dating back to 430,000,000 BCE, and the animal hasn't changed much since then. (It should be mentioned, however, that the species in the Delaware Bay -- Limulus polyphemus -- seems to be a much more recent variation of the genus Limulus, which dates back "only" 20,000,000 years or so.) Horseshoe crabs shared the Pangean seabeds with the trilobites, scuttled in the passing shadows of plesiosaurs, survived two ice ages, and greeted Homo sapiens when he first emerged from the African jungles some 195,000 years ago. (Just to put our families' timelines in perspective: the earliest known Hominidae fossil has been dated a paltry seven million years old. I'll allow you to cross-multiply 430,000,000 and 7,000,000 and come up with a ratio on your own.)

That the horseshoe crab evolved and proliferated under vastly different circumstances than most animals we see in our everyday experiences is obvious on a glance. The terrestrial life machine simply doesn't build them like this anymore. The horseshoe crab is the organic echo of an Earth that no longer exists; a classic model that keeps chugging along with the sleeker, more modern makes.

Here's what it looks like from above.

And now, underneath:

Say what you will. I think it's cute.

Note: none of the photographs in this entry belong to me or anyone in the census group. We all forgot to bring our cameras.

Quick observational facts: the pair of larger pincers in the front -- one is partially cropped at the image's bottom -- tell us that this one is a male (it uses these to latch onto the female during mating). That fuzzy orifice toward the center is its mouth. Yes, those white things toward the front are barnacles; you'll also sometimes find mussels and other stationary sea critters hitching rides on horseshoe crabs' shells.

While talking with the folks in charge of conducting the census, I learned a few interesting things about the horseshoe crab's eyes. Let's look at an excerpt on the matter from

A quick glance at the horseshoe will show the crab's two compound lateral eyes -- unusual because no other living Chelicerate possesses compound eyes. These are used primarily for finding mates, but the horseshoe has many more light-sensing organs.

There are 5 additional eyes on the top of its shell (two median eyes, one endoparietal eye and two rudimentary lateral eyes). The median eyes have cells sensitive to visible light and others to the ultraviolet range. The rudimentary lateral eyes are photoreceptors that become functional just before the embryo hatches. A clock in the anterior part of the brain sends out signals that control the sensitivity of the lateral and median eyes.

The tail also has a series of light sensors along the top and side that keeps its brain synchronized with cycles of light and dark. Additional signals from the small median eyes enhance the degree of adaptation to darkness, according to the amount of ultraviolet light those eyes receive from the sky at night.

On the underside of the crab, there are two ventral eyes, located near the mouth, which may help orient the animal when swimming.

So, if we count the photoreceptors on the tail as one eye, the horseshoe crab has a total of ten eyes across its body. This is remarkable enough in itself, not even speaking of the peculiar instance of compound eyes or its ability to "see" EMR wavelengths in the ultraviolet.

If (and I suppose this is a rather large "if") you're interested in reading further about horseshoe crabs, you can find plenty of more detailed and authoritative information elsewhere on the Internet without much trouble. For my part, I would like to close by sharing a fervent hope of mine: that someday, some obnoxious undergraduate couple headed for the Parkway after an evening of boozing takes a bayside detour to skinny dip or screw on a dark, secluded part of the shore, dives towards the water, and finds themselves unexpectedly balls-and-snatch deep in a shorelong heap of antediluvian arthropods in the middle of their own mating frenzy. Only through continued efforts to preserve the species can we ever hope to see my dream fulfilled.

Last note: perhaps you're wondering about that image up top. Well, you might be interested to know that the Japanese term for horseshoe crab is "Kabutogani." As for the fossil-type's in-game transformation into a sleek fighter with blades for arms, my guess is it has something to do with the old Japanese belief that horseshoe crabs were the reincarnations of valiant samurai warriors who sacrificed themselves in battle. Why the horseshoe crab has never received any such noble designations in the American poetic imagination is beyond me.


  1. A likely related fact: "Kabuto" is the name of the samurais' helmet. (Yeah, the one Darth Vader's one was based on.)

  2. Ohhhhhhhhhhhh. Did not know that!

  3. I have friends that use to do something similar to this, but for them it was the gooey duck...which I conclude is decidedly less cute.

  4. After a quick Google image search, I can think of at least two things wrong with the name "gooey duck."

  5. I can't believe you've almost got me convinced the underside of a horseshoe crab is cute.

  6. Why almost? What's not to like? Don't you just want to tickle it?