Shoreline Animals

One of the animals that you will find on the emerged plants is the periwinkle (Littorina anguiliferaLittorina littorea, or Littorina irrorata are common in our area). These animals are very interesting for their adaptations to living in the intertidal. Notice that the shell is thick. Also, if you pull one from a blade of grass, you should see that they are attached by mucous. These animals not only have an operculum (covers shell opening) but also they secrete a mucous to make sure that moisture remains in the shell. They can stay out of water for many hours. They can also do what is called “trickle cooling” by dribbling small amounts of water from their shell, allowing it to evaporate, and cooling slightly the air in front of the opening. The periwinkles have an interesting behavior of moving up and down associated with the tides. There are many aquatic organisms that will eat them. They climb vegetation like Spartina and mangroves as the water moves inshore. [teacher note: You can work with these animals well in the lab. There are some observations and experiments that you can do readily.] Look for other plants nearby. You may see things like glasswort (Salicornia perrenis), railroad vine (Ipomoea pes-caprae) and sea oats (Uniola paniculata).

Look around in the grasses and in the sand around to see if there is evidence of animals. Students may see fiddler crab burrows (Uca species), ghost crab burrows (Ocypode quadrata), and mammal tracks. Many of the fiddler crab species create “feeding balls” by passing wet sand through their mouthparts and stripping the sand of small plants and organic materials. These animals are called fiddler crabs because the males have one big claw and one small claw. The males will stand outside of their burrows and wave their claws to attract females. [teacher note: has a great deal of information on these crabs including videos of feeding, claw waving, and aggressive displays]. The burrows are very important, serving as shelter from predation. In addition, the fiddler crabs use the burrows to survive high tide. Fiddler crabs breathe in the air as adults (they will drown if submerged too long). So, they use a plug of mud to seal the burrow opening as the tide comes in. This traps a small amount of air in the chamber as the burrow is covered with water. In addition, when the crabs are in the dark of the burrow, their respiration rate decreases, allowing them to breathe the trapped air until the tide recedes. Some species of fiddler crabs will build elaborate entrances to their burrows to aid in attracting a mate. They also use the claw in combat with other males for territory or over mates. The cheliped (claw) is so large, however, that it cannot be used in feeding. If any of the legs are broken off, the crabs can grow a new one after each molt.

Ghost crabs also build burrows. They breathe air like the fiddler crabs. However, they have some interesting adaptations that allow them to survive long periods (several weeks) in their burrows. They can plug up their burrows very tightly so that the opening is nearly invisible. Older crabs tend to burrow farther away from the waterline. They run very fast across the sand. If you get a chance to see these crabs, notice that they run sideways not forward. Usually, they only move forward if they are feeding. Can you think of why it might be easier for these crabs to run sideways? Ghost crabs will scavenge (eat dead or dying animals) along the shoreline, but they are also known to be predators of sea turtle eggs.

[teacher note: This site gives a good introduction to interpreting animal tracks and this book is a good introduction to the tracks of vertebrates in Florida; you can use Plaster of Paris, also called casting plaster, to make a copy of the track; craft stores carry this material generally; it takes about 20 minutes to dry but if you add about a teaspoon of table salt per pint it will dry a little faster. You can give your plaster mold a border by cutting a ring from a quart or half gallon milk jug.]

You can do field observations on the distribution of the fiddler crab burrows. If students see feeding balls nearby, the burrows are “active”. If there are no nearby feeding balls, it is likely that the burrows are “inactive”. As a class project, you can do transects through the area, create a grid for sampling, or toss a hoop. If you are doing transects, decide whether you are going to direct them parallel to the shoreline or perpendicular. If the transects are parallel, there is likely less variation in overall vegetation and slope of the area. If the transects are perpendicular, one might be able to observe the effects of different slope and vegetation. So, depending on what your interest is in making the observation, you can choose how to direct the transect. Typically, the class should do two or more transects so that there is a better representation of the overall area. Once the direction of the transect is decided, you need to determine two other things: how wide will the transect be and how long will it be.

Next: Birds