Wednesday, July 11, 2012

A Closer Look at a "Living Fossil"

If you've read my previous blogs, you know I'm fascinated with "close-up" photography--I love the small details of nature!  So, can you guess what animal this is?  Its species is considered a "living fossil" and it's on display at our ever-popular Touch Tank.  Here's one of its compound eyes--similar to an insect's eye:
In addition to two compound eyes, it has various "eyes" that help sense different types of light--but they don't see images like our eyeballs!  In the photo below, can you spot a triangle of dots?  Located on the top part of the animal between the compound eyes, the center one is called the "endoparietal eye" and the two on either side of it are the "median eyes"--all play a role in sensing ultraviolet light:
From fossil evidence, we know this kind of animal has been around for about 360 million years.  Today, there are only 4 species worldwide.  The species we have in our Touch Tank, Limulus polyphemus, is found on the eastern coast of North and Central America--the other three species live in the Indo-Pacific region.  Since they are marine animals, they have gills to get oxygen from the water.  A closer look at the gills:
Under the top hard parts, there are thin flat parts like pages of a book--that's why they're sometimes called "book gills."   Not only do the gills help the animal breathe, they help it swim by flapping up and down.  By the way, this animal swims upside down in the water!  Here's what the gills look like when they're closed:
This animal has the word "crab" in its common name, but it's more closely related to today's spiders as well as extinct trilobites rather than crabs or other crustaceans.  In fact, this animal is classified in its very own class, "Merostomata," which means "legs attached to the mouth."  You can see why in this photo--see the bristly mouth and some of its legs?:
There are a total of 10 walking legs and at the end of each leg is a claw.  The claws help move food to its mouth--it eats stuff like worms and molluscs.  If the crab's first pair of claws have a rounded "boxing glove" shape, the crab is a boy, like in the photo below.  (A girl's first pair of claws would look like the two claws on the right in the photo below):
This animal has a tail--don't worry, no stinger!--that helps it steer while swimming and helps flip itself over (remember--it swims upside down!)  The second photo shows the tail where it joins to the body.  Technically the tail is called a "telson":
The two main body parts (cephalothorax and abdomen) have sort of a "hinge" that allows the animal to flex its body:
Here's the side view of both main body parts:
Have you figured out what it is?....Yep, a horseshoe crab!:
If you want to know more about horseshoe crabs, there are two websites I recommend.  The first is "The Horseshoe Crab" run by the Delaware-based Ecological Research & Development Group, a non-profit organization dedicated to horseshoe crab conservation.  It has a detailed description of horseshoe crab anatomy and an explanation of how LAL (Limulus amebocyte lysate) a derivative of horseshoe crab blood is used to test drugs, vaccines and medical products for bacterial toxins thereby saving human lives.  The Maryland Department of Natural Resources has an excellent overview of the life history of horseshoe crabs.

Many thanks to Touch Tank volunteers Dawson Connell, Tatum Conner, Carly Cook, Erin Copsey, Poppy Crawshaw, Megan Holley and Kristen Penrose who helped me with all these photos--very patient folks!  More off the beaten path in two weeks....

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