It seems somehow ironic that I wrote last week about discovering a seaweed species that I happily found out was not invasive, but was merely unfamiliar to me, as I am now writing about a seaweed species that is invasive. When a friend who was visiting pulled up a glistening sample of dead man’s fingers, we both remarked that, while it was both invasive and had a less than sparkling name, Codium fragile is actually quite beautiful. Its thick, furry fronds, the texture of which are likely the reason it is known as dead man’s fingers, are a verdant green and catch the light easily under the water.
Codium has been here since the 1960s and is native to the Pacific, not the Atlantic. It is thought to have been introduced in the ballast water of ships, a common means of introducing species to new locations. Before we put the little sample back into the water, my friend noticed some tiny white dots moving around in its holdfast.
Being the nerdy scientist that I am, I pulled out my hand lens, which lives in my bag at all times, and looked more closely to find that these dots were perfect baby horseshoe crabs. I could see what is known as the prosoma, the front part of the shell that gives the crab its name as it is shaped like a horseshoe, moving on its hinge with the opisthosoma, the back portion of the shell, in a tell-tale motion. On closer look, I could even see the tiny eye spots on either side of the prosoma.
Horseshoe crabs are one of the most amazing creatures that are easy to spot and fairly common. But just because they are large and easy to find doesn’t mean that they aren’t still full of surprises. First, although I could see their eye spots, which look like nondescript whitish crescents, I couldn’t see any of the detail that makes them so impressive.
Each eye has nearly a thousand receptors on it that are super sensitive to light, allowing horseshoe crabs to see incredibly well at night. This is critical to their success in finding a mate, since this happens at night in the late spring and early summer. If you haven’t seen horseshoe crabs mating, it is a rare phenomenon.
They come up onto the shore, sometimes in large groups with several males attached to the same female, all vying for their chance to fertilize her eggs as she lays them in a shallow hole she digs in the soft sand. Horseshoe crab mating often occurs at the full or new moon. They use an additional set of eyes, which look like dark spots along the ridge where the larger eye is, to help them follow the lunar cycle.
These eyes can pick up specialized light like ultraviolet light from the sun as well as reflected light from the moon. By timing mating during the full or new moon, the crabs can guarantee plenty of shoreline to be able to lay eggs, as this is when the tides are very low. While I have seen horseshoe crabs mating many times, I had never seen the tiny crabs.
After the eggs are laid, they develop for the next few weeks, hatching about a month later as the mini crabs that I found. One neat thing that I didn’t know until looking more closely at the real things, as they wiggled on a piece of Codium, was that these larvae don’t have a tail and won’t for up to a year or so after settling on the bottom. Once they do, they will be much more mobile and look like full-grown adults.
But they won’t be old enough to mate until they are around 10 years old, then restarting the whole cycle. To go back again to last week’s column, which was, in addition to finding a new species of seaweed, more broadly about the importance of looking more closely at things you discover in the natural world, this week was another lesson in just how much there is to see. It made me, yet again, grateful for Samuel Scudder’s essay, which I referenced last week, “Take this Fish and Look at It.
” If you haven’t read it yet, it is worth the time. It might just lead you to learn about a new species or find tiny creatures you’ve never seen before. Susan Olcott is director of strategic initiatives for Maine Coast Fishermen’s Association.
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