Limulus polyphemus: sand artist and one of nature's wonders

 

So OK, Jim Denevan is in a different league, but back in May, when the tides were high and the first full moon of the month was rising over Queens, New York, Larry Deemer sent me this picture of a visiting artist on his beach.

This horseshoe crab, Limulus ployphemus (which is not, as we shall see, a crab at all), is probably a male, and appears to have been either drunkenly or creatively celebrating a night of mating - for when the May moon is full and the tides are high, that is the time for such things in the world of the horseshoe crab. Either way, he was out of sorts, as Larry described him before helping his return to the sea.

Horseshoe crabs are Arthropods, animals having articulated bodies and limbs, and as such are more closely related to spiders than they are to crabs. And they are really ancient - their fossil ancestors would seem to be trilobites and they themselves have existed, essentially unchanged, for hundreds of millions of years; the mass extinction that did for the dinosaurs had little or no effect on them. Around the world, there are four species, Limulus ployphemus occupying the eastern seaboard of North and Central America, and the others the western Pacific rim. All these species are built and operate in pretty much the same way, one particular characteristic being that the males are smaller than the females; the females are built big in part simply to be able to carry their huge burden of eggs - up to 80,000 a year. Another key difference between the males and the females reflects the process of fertilising all those eggs - the female's
front two appendages, out of a total of six pairs, are pincers; but the males have quite different claws, sometimes referred to as "boxing gloves," and specially designed for holding on to a female - sometimes for very long periods of time (months?). For a useful guide to horseshoe crab-sexing, see The Horseshoe Crab website, where there is a vast amount of fascinating information. So, in Queens in May, like many beaches up and down the east coast, the sands are littered with stuck-together pairs of horseshoe crabs - and lone males. It seems that, as the female digs a shallow hole in the sand to lay her eggs, they are fertilised not just by the hanger-on, but also by what are referred to as "satellite males" - the voyeurs of the horseshoe crab world.

The eggs are pearl-like - and provide a fabulous buffet for a variety of birds:

 

Here, however, is the problem. Horseshoe crabs have long been used as a source of bait for various types of fishing, and their numbers have plummeted. It's not that long ago that a full moon in May would have resulted in beaches literally covered in horseshoe crabs, often several deep, but no longer. And the fall in their numbers is disastrous not just for them but for the migratory birds that rely on this food supply. Intensive conservation efforts are now in place, but success has yet to be demonstrated.

But horseshoe crabs are extraordinary and wonderful not just because of their dramatic spawning rituals. Consider the following:

• unlike the giant one-eyed son of Poseidon, after whom they were given their name, they actually have two compound eyes. But they also have five additional eyes on the top of their shell, and two more on the underside. This visual system is unique in nature and, understandably, the subject of much research. The 1967 Nobel Prize for medicine was awarded for "discoveries concerning the primary physiological and chemical visual processes in the eye" - research based significantly on the visual system of the horseshoe crab.  
• while vertebrates use haemoglobin, an iron-based protein, for oxygen transport in their blood, horseshoe crabs use a copper-based protein - when their blood is exposed to the air, it's blue. (Truly blue-blooded, unlike the British aristocracy).
• but the most extraordinary thing is that any of us that has had an injection, a vaccination, or an intravenous drip should be deeply grateful to Limulus ployphemus. As described on the site linked above:

An extract of the horseshoe crab's blood is used by the pharmaceutical and medical device industries to ensure that their products, e.g., intravenous drugs, vaccines, and medical devices, are free of bacterial contamination. No other test works as easily or reliably for this purpose.

In the 1960's, Dr. Frederik Bang, a Johns Hopkins researcher working at the Marine Biological Laboratory in Woods Hole, Massachusetts, found that when common marine bacteria were injected into the bloodstream of the North American horseshoe crab, Limulus polyphemus, massive clotting occurred. Later, with the collaboration of Dr. Jack Levin, the MBL team showed that the clotting was due to endotoxin, a component of the marine bacteria originally used by Dr. Bang. In addition, these investigators were able to localize the clotting phenomenon to the blood cells, amebocytes, of the horseshoe crab, and, more importantly, to demonstrate the clotting reaction in a test tube. The cell-free reagent that resulted was named Limulus amebocyte lysate, or LAL. The name LAL is extremely descriptive: Limulus is the generic name of the horseshoe crab, amebocyte is the blood cell that contains the active components of the reagent, and lysate describes the original process used by Levin and Bang to obtain these components. In Levin and Bang's process, amebocytes, after being separated from the blue-colored plasma (hemolymph), were suspended in distilled water where they lysed (ruptured) due to the high concentration of salt contained in the amobocytes versus the absence of salt in the distilled water. Surprisingly, this same process with some minor modifications is still used today to produce LAL.

• and, furthermore:

Besides LAL, a number of reagents and medically useful compounds have been discovered in the blood of the horseshoe crab. These include:

· A new test for fungal infections (G-Test) which is already in use in Japan and is expected to be licensed in the US next year

· An endotoxin-neutralizing protein which has potential as an antibiotic as well as an alternative endotoxin assay. This protein, ENP, can be made synthetically, which would eliminate the use of live horseshoe crabs for the LAL reagent.

· A number of other proteins that show anti-viral and anti-cancer activity.

Sometimes - often - I set out on the track of a simple-sounding post and end up discovering extraordinary things - this is a classic example. Oh, and Larry tells me that the penis of a male horseshoe crab is bifurcated - I haven't been able to confirm this, but will readily admit that I haven't pursued this topic in detail.

 

[photos from http://www.ahherald.com/oaktrail/2006/oot060608_more_horseshoe.htm, http://citybirder.blogspot.com/2006_06_01_archive.html, and http://www.bbc.co.uk/radio4/worldonthemove/reports/red-knots-of-delaware-bay/; also see a NY Times article by Natalie Angier, the Project Limulus website, and numerous sources on Wikipedia.]

Comments

[F]

Very cool.

I remember seeing, many years ago in Discover magazine, a photo of an horseshoe crab being "tapped" for blood as one would tap a maple tree for sap. Incredible critters, really. It is sad that their numbers are so low.

...their fossil ancestors would seem to be trilobites...

Really? I've never heard this. It would be very interesting to me if true. I've always had a thing for trilobites.

Ah, after poking around a bit, I can still only find references to horseshoe crabs and trilobites being relatives (most certainly!). Hm, one mention that the entire subphylum are trilobite descendants. No matter, a lot of arthropoda are really neat.

[Sandglass]

Fair comment! Since horseshoe crabs were around at the same time as trilobites, I should clearly not describe the latter as "ancestors" of the former. "Close relatives" would be a better term. There's an interesting piece on the oldest horseshoe crab fossils - 455 million years old - at http://www.sciencedaily.com/releases/2008/02/080207135801.htm.

I haven't seen an evolutionary tree for them (only references to the need to construct a detailed one), so how these relationships worked is unclear.

Thanks for picking up on this - all such comments much appreciated!

But I was just thinking that word usage is such a thorny subject - I coexisted with my grandparents, who could reasonably be described as my "ancestors".......

[Evelyn Wangari]

Great creatures.