Starfish. Sea star. No matter what you call them, they're pretty dang amazing. They can regenerate, after all! Can you imagine losing an appendage and having it grow back in a year's time? Very cool, right?
Some Interesting Facts:
- Starfish are not fish. The scientific world would like us to call them "sea stars"
- They live in all bodies of salt water, and even in brackish water (low concentration of salt), but never in fresh water
- They are echinoderms, or marine invertebrates
- They have hard, calcified skin which protects them from predators
- Sea stars commonly have 5 arms, but the sun star has 40
- They come in many colors-ranging from bright yellow to dull brown
- They vary in size from tiny (size of your fingernail) to huge (size of a yard stick)
- Some sea stars are poisonous and even deadly to humans, like the spiny sea star
- Their eye spots are sensory cells on the tips of their arms. They can sense light and simple shapes
- In Japan, China, and New Zealand, the starfish is considered a delicacy
- In some species, their stomach can come out their mouth to digests and eat their prey
Dissection:
The white dot, seen in the picture above, is called a madreporite. This sucks in sea water to support its water vascular system.
On its dorsal surface (top) we could see the spines (the white protrusions) and the skeletal structure right under the skin.
The skeleton is made of many rods, plates, spheres, and crosses that are connected with flexible joints. The spines are ossicles (calcified part of the skeleton) that protrude out away from the body. They can be solid or hollow.
Skeletal structure seen under the organs
We could also see all the dermal branchiae which absorb the oxygen out of the water. They are orange in the picture above.
At the base of each spine, there are little pincer-like structures, pedicellariae, that clean the surface of the star. Below is a close-up of what these look like.
SEM pic from Dearborn et al., 1991
On the oral (bottom) surface of the sea star we could see all the feet that allow the starfish to move, and this sea star even had part of its stomach sticking out of its mouth in the very center of its body!
Next, it was time to explore the inside of the sea star.
From the doral surface: opening from the anus to the pyloric stomach, with the cardiac stomach directly beneath
At the center of the star's oral surface is its mouth
When a sea star eats, the food enters its mouth (or stomach if it releases its stomach through its mouth) and then enters the first stomach, called the cardiac stomach. After the food is partially digested, it enters into the pyloric stomach, where it is further digested. The pyloric glands help in this process by releasing digestive enzymes. The glands are the olive green color masses in the pictures and look similar to a fern branch. There are two branches per arm. The anus is in the center of the dorsal surface where waste is released.
Gonads
Under the pyloric, or digestive glands, the gonads are seen. The gonads, or reproductive organs, are large in this sea star because of where it was in its reproductive cycle. In the second specimen we had, they were barely noticeable and only seen close to the central ring.
Once the stomachs were removed, the central disk was exposed.
Water enters the madreporite and travels through the stone canal and into the central disk. From there, the water is distributed down each arm through the radial canal and into the ampulla.
The ampulla work like hydraulics on machinery. As the water is pushed from the ampulla into the tube foot, the foot pushes out from the star. When the ampulla relaxes, the pressure on the foot is released, and the foot retracts. Each pair of tube feet is attached to a pair of ampulla.
The ampulla work like hydraulics on machinery. As the water is pushed from the ampulla into the tube foot, the foot pushes out from the star. When the ampulla relaxes, the pressure on the foot is released, and the foot retracts. Each pair of tube feet is attached to a pair of ampulla.
Twinkle, twinkle little star, now we know what you are!