Those Little Crabs

One of the neatest things to do at the beach is turn over a rock and look for those little crabs. It is so much fun that even adults cannot resist the temptation to flip over a rock or two.

So what are these things? Are they baby crabs that will grow large or are they just little crabs that never get very big. Contrary to what many believe, these are not the young of larger ocean crabs, but are simply small sized species. Under most rocks on Puget Sound shores you can find tiny shore crabs ranging in size from smaller than a fingertip to about the size of a half-dollar.

These small crabs are called purple shore crabs and they can be found on beaches from Alaska to the north to California to the south. They are often found under rocks because they feel safe there. They can range in color from greenish yellows to deep purples. They are certainly fun to watch but be careful; those little claws can pinch your finger.

What do these critters eat? They primarily eat sea lettuce and other green algae. Sometimes, they also eat small dead animals. Scientists classify these crabs as scavengers and, like any scavenger, these crabs are essential in maintaining a stable habitat and a clean beach.

In Puget Sound, females carrying eggs are found from January to mid-July; especially in April. A single female may carry from 400 to 36,000 eggs. Wow, 36,000 eggs – that is a lot of eggs!

If you visit a beach and look for these crabs, be careful not to harm them. Be very careful when you turn over a large rock that you do not crush the crabs. Also, if the rock is large, it is probably better not to try to put it back because you might hurt one of the crabs. Remember that while it is fun to investigate these little creatures, they play an important role in the health of Puget Sound.

I Wonder ...

• These crabs are called scavengers. I wonder what a scavenger is and if there are other animals or birds around Puget Sound that are also classified as scavengers.
• I have also heard of a crab called a hermit crab. I wonder if the hermit crab and the purple shore crab are actually the same creature.
• These crabs seem to be able to walk around on the beach. I wonder how they can breath under the water and also walk around on the beach were they are not under water.

M. Tiller

The Wonderful Sand Dollar

Let’s say you are walking on the beach, looking under rocks for little crabs, and you find a sand dollar. You know, that round, white thing with what looks like a star on its back.

 

What do you do? Do you throw the shell into the water trying to skip it across the waves? Do you put it into your pocket to take home and show others? Do you turn it over and wonder why there is a small hole in the center?

What have you really found? Does an animal live inside the round shell or is the shell part of an animal? Well, if you have found one of these white, round shells, what you have discovered is the remains of what was once a living creature that lives on the beach and in the shallow waters near the beach.

The living sand dollar is not white and it is not smooth. It is a “fuzzy” creature that is “purplish” in color. The “fuzzy” look of the sand dollar is caused by fuzzy spines and tiny hairs called cilia.

On the bottom of the sand dollar, the side with the hole in the center, there are tiny spines and hairs used to move food into the sand dollar’s mouth. Yes, that hole in the middle is the mouth. The sand dollar’s mouth has a jaw with five teeth-like sections to grind up tiny plants and animals. Sometimes a sand dollar “chews” its food for fifteen minutes before swallowing. It can take two days for the food to digest.

On the top side of the sand dollar, the tiny spines serve as gills. Gills are used by the sand dollar to breath (fish also breath using gills). The spines on the underside of the sand dollar are also used to move around the sand (yes, the sand dollar can walk around the sand).

 

In quiet waters, these flattened animals stand on end, partially buried in the sand. When waters are rough, sand dollars hold their ground by lying flat—or burrowing under the sand.

Scientists can determine the age of a sand dollar by counting the growth rings on the plates of the shell. Sand dollars usually live six to 10 years but some have been found that are 13 years old.

Sand dollars are usually crowded together over an area—as many as 625 sand dollars can live in one square yard. Use a ruler or tape measure and mark off a square 36 inches by 36 inches. Imagine 625 living sand dollars in this square – that’s a lot of sand dollars.

If you live in the south part of Puget Sound, you may be able to see a bed of living sand dollars. During the summer, you need to visit a sandy beach when the tide is low. If you are ever lucky enough to see a bed of living sands dollars, be very careful and do not walk on them. They are easily crushed and, like most things found on the beach, they play an important role in the health of Puget Sound.

I Wonder ...

• It says that sand dollars eat small plant and animals. I wonder what kinds of plants and animals they eat.
• I know that there are different species of dogs, like poodles, spaniels, and collies. I wonder if there are different species of sand dollars.
• The story says that I have to look for live sand dollars when the tide is low. I wonder why live sand dollars don't live higher up on the beach so I could find them when the tide is high.

M. Tiller