The Texas High School Coastal Monitoring Program Field Guides
Mustang Island Field Guide - Field Stop 6

The South Jetty at the north end of Mustang Island is a very popular place among fisherman (Fig.1). It's also a great place to watch the sunrise over the Gulf of Mexico and to watch boats and tankers enter and exit Aransas Pass. Aransas Pass runs between Mustang Island and San Jose Island and provides access for deep-draft vessels to Corpus Christi Bay and the city port, to the United States Navy Battleship Group Homeport, and to the Gulf Intracoastal Waterway. The jetty, constructed in the early 1900's of large granite blocks quarried from Marble Falls, Texas, extends approximately mile from the northern tip of Mustang Island into the Gulf of Mexico, providing stabilization to the entrance of the Corpus Christi Ship Channel. To see aerial photos of Aransas Pass jetties, please visit: http://texascoastgeology.com/passes/aransas%20pass.html.

The Port Aransas South Jetty extending into the Gulf of Mexico.
Figure 1. The Port Aransas South Jetty extending into the Gulf of Mexico.

Please be careful as you investigate the jetty. There are numerous opportunities to trip and fall, and don't go on the jetty when the waves are high. The wet surface can be very slippery, and moving water is a powerful force.

Aransas Pass is a natural channel between Mustang Island and San Jose Island that was stabilized by the jetties. It is now dredged to maintain a depth of about 40 feet. Before the two jetties were built, the mouth of Aransas Pass frequently developed a sand bar that was hazardous to ships. With a rising tide, water from the Gulf moves into the bay, and with a falling tide, waters from the bay move into the Gulf. With the passage of tidally driven currents, sand and other sediments are carried into and out of the bay. With a rising tide, flood currents carry sediments into the bay. As the currents spread out in the bay, they slow down and deposit the sediments in a large sheet-like pile called a flood tidal delta. With a falling tide, ebb currents carry sediments into the Gulf. As the currents spread out n the Gulf, they slow down and deposit the sediments in a large submarine deposit at the mouth of the tidal pass called an ebb tidal delta.

So why were the jetties built? Because nature is not predictable. Natural processes created the pass, but natural processes also created deposits of sediment at either end of the pass and caused the pass to migrate. At times the natural channel was too shallow to allow the passage of large ships, and in early attempts to dredge the channel it was quickly refilled by sediments. When the channel was stabilized with the long jetties, some sediments carried by currents along the shore were trapped behind the jetties. Look at the beach adjacent to the jetty as well as the beach a little farther down the beach. You should notice that the beach adjacent to the jetty is much wider.

The Port Aransas Jetty is interesting, and well worth a visit. It's easily accessible, and the organisms are easy to see. You can readily contrast this environment with other more extensive habitats on Mustang Island and see how changes in the physical setting have a big influence on the biologic assemblages. You'll see many of the same organisms on other "hard shores" (piers, boat hulls, breakwaters, and other hard surfaces) on Mustang Island and very similar organisms and a similar zonation of species on hard shores at many other places in the world.

The jetty is a far different environment from the beach. The jetty is a "hard shore" that doesn't shift and change with the passing waves, and this is just what some organisms need. The barnacle you're most likely to see, the so-called fragile barnacle (Chthamalus fragilis) is especially common in about a 1-foot-wide band near the water line. The high energy of the passing waves might seem to make this a poor choice, but this dynamic environment and the plates of its shell provide some protection from the sea urchins, snails, and flatworms that prey on the barnacles. The barnacles themselves eat mostly plankton and tiny bits of detritus. When feeding, the barnacle opens the protective cap and extends its "feet" (cirri) into the water. The barnacle's feet move in the passing water to help divert tasty tidbits to its mouth.

Other organisms seek out this hard shore environment as well. There's no place to burrow, but there are nooks and crevices to hide in. For those critters fond of algae for breakfast, lunch, and dinner, a tasty meal can be found in the green algae attached to the boulders of the jetty. Like barnacles, algae can't survive for long on the beach. Port Aransas Jetty is a well-established neighborhood because the original jetties, one on each side of the channel, were built almost 100 years ago. The community is best developed a couple of hundred yards out from shore, beyond the surf zone, but you can see many organisms anywhere along the jetty.

Look first at the splash zone, the area washed by the passing waves that is not continuously under water. The common animal here is a snail, a gastropod, the lined periwinkle (Littorina sp.). The snail is very common and widespread in the Gulf, from Florida to Mexico. The periwinkles use their radula, a rasplike "foot," to scrape algae from the surface of the boulders. Look at the distribution of the lined periwinkles. Some brave the full force of the waves, but others seek protection from the waves in cracks and crannies or snuggle in among the barnacles at the base of the splash zone.

As you walk along the jetty, keep watch for a 14-legged roachlike animal that skitters across the rocks. These guys have several common names, such as rock louse or rock roach, but don't be put off by the name. These are isopods, another crustacean, and you may not consider them attractive, but they are interesting. For example, they can change color. They slowly get darker or lighter depending on their surroundings by displaying or hiding cells with a dark pigment. Rock roaches tend to be mostly nocturnal but can usually be seen at the beginning and end of the day and on cloudy days. They like to hang out between the large granite blocks on the jetty.

There are other interesting animals-sea anemones and urchins, egrets, and shorebirds-and most fishermen are happy to show you their catch. If the sea is relatively calm and the light is right, you may see fish and other animals, even an octopus (Octopus vulgaris), but they are masters of disguise and readily blend in with their surroundings.