Intertidal Organisms

First you will explore the intertidal area. This is the habitat between the high tide mark (you will see a line of items washed up on the shore like dead seagrass, bits of wood, seaweed, etc) and the low tide mark (the farthest area uncovered by the water during the day. These can change with season and organisms will move accordingly. For today, you should use the measuring rope or tape to find out the size of the intertidal. Within the intertidal, the area where water is moving back and forth is called the swash zone. Note whether the water is moving gently over the swash area or whether it is coming and going rapidly. This will tell you whether the intertidal is low energy or high energy (do you think that different animals live in high energy and low energy areas? Do you think that the size of the sand grains differ on high and low energy beaches? Explain). To learn about waves and longshore currents, you can try a simple experiment. Longshore currents generally form when waves approach the shore at an angle and break unevenly along the wave front.

Do you see any vegetation in or near the water? If the vegetation is in the water, it is likely seagrass, a submerged Florida native plant. All of the seagrasses are similar to grasses found on land. They are flowering plants that have a true root system, stems, and blades and live under water. They have special adaptations for living in this habitat. Remarkably, they can pollinate under water, or they can grow vegetatively by the rhizomous roots that they have. A big problem for any plant that is submerged is how to keep from being waterlogged. Seagrasses have special cells on their surfaces that allow them to control the amount of water that enters and leaves the plant tissues. The process of controlling water content is called osmoregulation. Another difference between these plants and their terrestrial counterparts is that their blades do not have openings to perform gas exchange. All of the gas exchange for seagrasses occurs across the surface of the blade. Take a small pinch of seagrass blade and look at it with your hand lens. What does the surface look like? Seagrasses around Sarasota Bay occur both intertidal and subtidal. Those that grow in shallow areas can be exposed for several hours at low tide, especially during the winter months. The depth at which they can grow is dependent on light penetration and water quality. Think about how humans might affect where seagrass can grow [For the teacher: this is a good place to stop and talk to students about the effects of hardened surfaces, fertilizers, and other factors that contribute to pollution in the Bay.] Much of the Bay is shallow, so there is potential for many areas to contain seagrass beds. Some seagrass can be found in small patches while other seagrass occurs in “meadows”. [teacher note: Globe Videos has several short videos that will show how different physical measurements of nutrients in water are taken. There are different formats for the videos that should be useful.]

Seagrasses have several important functions in the bay. You can find information on how to identify the various seagrasses that occur in Florida here; the three most common species that occur off of Sarasota and Manatee Counties are Thalassia testudinum, Syringodium filiforme and Halodoule wrightii. Ruppia maritima is another type of seagrass that tends to dominate in areas where rivers or tidal creeks empty into the bays; they tolerate a wide range of salinities and are called euryhaline. Functions include providing a habitat for many invertebrates (crabs, shrimps, barnacles, amphipods, isopods) as well as vertebrates (fishes, sea turtles). For some animals, the seagrasses are critical resources for only a part of their life cycles. For example, some fish larvae and juveniles shelter and feed in seagrasses before moving to deeper areas of the bay or into open ocean. Seagrasses also have epiphytic algae associated with them; algae that grow on the seagrass blades. These algae are important in both releasing and fixing nutrients in the water such as nitrogen as well as in providing food for grazing animals. Like grasses on land help to prevent erosion, seagrasses help to hold sediment in an area. The seagrass blades slow down the water movement over their surfaces because of friction. This “dampening” effect has two important purposes. First, finer sediments become trapped in the seagrass as the water slows down. When these types of particles are removed, the water seems clearer or less turbid (density of particles in the water is lower). You can do an experiment to see how removing these particles can clear the water. Second, small animals can shelter in the grasses from the full effects of water movement around them. Third, the blades also hold moisture and provide protection for organisms when the grassbeds are exposed at low tide. Seagrasses themselves can also be major food sources for marine animals. The sea urchin, Lytechinus variegatus, found in Sarasota Bay and along the Gulf coast can consume large amounts of grass daily. Large organisms like manatees (Trichechus manatus latirostris) and hawksbill (Eretmochelys imbricata) and green (Chelonia mydas) sea turtles graze on seagrasses. [teacher note: there is an excellent set of manatee activities for different grade levels at the Manatee Education Center website.

In the shallow waters and on the shore you may see both living and dead horseshoe crabs (Limulus polyphemus). These are not true crabs, but they are arthropods. The blood of these animals is used in cancer research and the shell (chitin) is used to coat sutures. They also have several other uses in medicine. So many have been collected along the eastern coast of the US for research that conservationists are concerned about populations of seabirds that are dependent on the hatching larvae of the horseshoe crabs for food. Horseshoe crabs lay their eggs in pits on the beach that they cover.

Next: Shoreline plants