Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Rock Barnacles

July 7, 2011

It is not easy being a barnacle.
This creature eaks out an existence on our seashore’s most inhospitable environment - the rocky coast. Think for a moment what life is like on the rocky coast. Twice a day, it is exposed to the deadly air. A hot summer afternoon can reach well over 100 degrees while the winter temperature can drop below zero degrees. A rainstorm at low tide can expose it to toxic fresh water and winter waves are constantly trying to knock it loose. No, life is not easy for the barnacle. So why does it make its home in this most inhospitable place? Perhaps because it can, while most other animals can't. The barnacle is specially designed to withstand the rigors of the rocky coast where few of its competitors and predators are found.

If life is hard for the barnacle, it is especially so in the winter. Barnacles are stationary creatures. To avoid getting washed out to sea by the waves that constantly pound them, they must glue themselves to the rocks. Pounding waves seldom bother the barnacle. What does cause them problems is winter ice. The only place our oceans freeze solid is the intertidal area, that area that is home to the barnacle. During a very cold winter the ice that forms along the shoreline is moved over the rocks by the rising and falling tides. This moving ice scours the rocks, knocking loose anything in its path, including the barnacle. As you might imagine, a loose barnacle is a dead barnacle.
A look at the intertidal rocks the next summer indicates the severity of the past winter. If the rocks show large barnacles covering them, the winter was mild. If the barnacles are all tiny - first year barnacles, then the winter was mighty cold. The only large barnacles you will find will be located in the cracks and crevasses where the ice couldn't reach.

barnaclesThe barnacle has got to be one of our seashore's most unusual creatures. At one time, they were thought to be a type of mollusk or seashell. Not until 1829 did scientists recognize it as a type of arthropod, closely related to shrimp and crabs. This mistake is understandable since the barnacle spends its entire adult life locked in its shell stuck in one place. But take a close look inside and you will see a rather strange animal. The barnacle looks like a tiny shrimp living in a limestone house. The creature is truly housebound - its head is glued to the floor. To eat, it must stick its feet out the door and kick food into its mouth.
If you have never seen barnacles feed, you have missed one of nature’s truly beautiful sights. However, this is easily remedied. Next tine you are at the beach at low tide, find a barnacle covered rock that is above the tide line. These barnacles are probably hungry. Carry the rock down to the water, place it in a sheltered tide pool and watch. Soon the barnacles will open their doors and wave their feathery feet through the water, kicking the tiny plankton into their mouths. When they are all feeding, it reminds me of a miniature underwater ballet. I have often wondered if their rhythm is affected by the temperature of the water, much like the chirps of the snowy tree cricket is dependent on the air temperature.

Being stuck in one spot for life creates certain problems. It means you can't seek out food. Barnacles must wait till high tide brings food to them. It also causes reproductive problems. You can't go searching for a suitable mate. Perhaps to ease the task of finding a mate, barnacles don't have to worry about finding a member of the opposite sex. Each and every barnacle is the opposite sex. They are hermaphroditic - both male and female. This means that any barnacle can mate with any other barnacle. Or to be more precise, any barnacle can mate with any barnacle adjacent to it. Remember, they are stuck to the rock and stuck with the choices next to them.

Many non-mobile sea creatures mate by spawning. Both sperm and eggs are released into the water at the same time. Hopefully, the sperm will bump into an egg and fertilize it. This tactic, however is not for the barnacle. Internal fertilization is the rule. The barnacle extends its sperm tube through the doors of an adjacent barnacle where the eggs are fertilized. After a short time, the egg develops into a "nauplius" larva. This larva is set free to wander the oceans.
The nauplius larva has six legs and one eye. It wanders the oceans as part of the plankton. After a few weeks, it changes into a "cyprius" larva. It now has two eyes and a shell on its back. It is time for the young barnacle to settle down. With any luck it will settle down on a rock in the intertidal area. Without any luck, it won't, in which case it will die.
If you look carefully with a good magnifying glass in late May or early June, you might see these cypriid larvae on the rocks among the adult barnacles. Soon they will glue themselves to the rocks by their antenna, change form once again and begin a long, arduous life (three to five years) as an adult barnacle.
That is, assuming the winters are mild.