The Horseshoe Crab Diaries: Pt. 1 in Which Paul Asks an Expert About Their Eyes

Two nights ago I had a chance, along with several colleagues, to go out to dinner with Bill Sargent who was speaking that night at the Aquarium's Lowell Lecture series. Mr. Sargent is currently a consultant for NOVA, an author and has been the director of the Baltimore Aquarium, a professor and a researcher at Wood's Hole. He was talking about how climate change has affected the community on Cape Cod and so most of our conversation revolved around climate change and how to communicate these issues to the public.

About halfway through dinner, though, one of the senior educators who was there with us turned to me and said "Oh, by the way, Bill is also an expert on horseshoe crabs...so if you have any questions..." As I have alluded (read: expressed firmly and directly) I love horseshoe crabs. I also spend a lot of time with them at work. And the one thing that has continued to irk me is that no one can seem to give me a straight answer about how many eyes they have. So I asked: "Can you tell me how many eyes they have?"

Apparently the true number is 9. Most sources I have read fall into two camps: that they have ten and that they have some unknown number higher than ten. But Bill seemed sure: they have nine. So they would be: two compound eyes, the most recognizable eyes, lateralized on their shells, three "simple" eyes towards the front of their shell (children often think this is the HSC's nose...of course these animals have no nose at all...but they do taste with their feet like many other arthropods), one "eyespot" on the end of the telson (tail) and three "vestigial eyes" on the ventral side (bottom) of their shells. These final eyes were the ones I was the most skeptical about. Really? Why would they need eyes on the bottom of their shell when that part of the body is facing into sediment most of the time?

Well, he had an answer for that too: Allegedly the ancestors to the HSC would spend a lot of time swimming upside down. They used their lateralized compound eyes for directing themselves and orienting towards the bottom but they needed eyes on the bottom because that is where their mouths are and they were hunting surface-dwelling insects. Now, after millions of years of evolution, they are barely functional...but I guess they still get counted.

Sources tell me that they use the three simple eyes to detect levels of UV light which helps them stay in tune with the phases of the moon. They mate and lay eggs at the biggest full moon of the year in late may or early June and so they need to know when that is. Mr. Sargent, though, believes that they actually have a full year long internal biological clock with which they keep time. Because they spend so much time deep under the waves and even buried under the muck they really don't pick up most of the changes of the moon. That actually makes a lot of sense. Perhaps the simple eyes are there for back-up or resetting the clock should it falter.

I was so excited to talk horseshoe crabs with someone who knows so much about them. Hopefully I'll inadvertently stumble into more meetings with people like Bill.