By Alyssa Dykman and Emily Lu
Our dive in Apra Harbor, Guam, marked our first scuba dive in Micronesia. We first dove at Western Shoals, a reef full of vibrant marine life. The 84-degree tropical water was a welcomed change compared to the 60-degree water of Santa Catalina Island, where we had been practicing our diving skills for the past three months. Every previous dive had been a buildup of anticipation to the moment we would enter the water with our four-pound weight belt without a 7mm wetsuit. Soon after our descent to 50 feet, the elephant ear sponge, Ianthella basta, caught our attention. The massive blue sponge is only found in Apra Harbor, and nowhere else on the island.
While not many people would consider seeing a sponge the highlight of their dive, most do not appreciate a sponge’s true aesthetic and ecological value. Sponges come in an unbelievable number of shapes, textures, sizes and colors, offering a diverse variety of phenotypes in contrast to the common sponge that people recognize in everyday use. Though sponges are among the oldest known animals, they are also considered some of the most primitive organisms in existence.
Sponges are sessile and have no tissues or organs as most animals do, but play a crucial role in aquatic and marine ecosystems. They are filter feeders that pass water through their pores, extracting particles, such as bacteria, out of the water. Some sponges have been shown to filter as much as 1200 times their body volume in a single day (Branches of the Tree of Life: Sponges). Sponges are also a vital part of the marine food chain. They recycle nutrients for other organisms to use, and serve as the primary food source for some types of endangered sea turtles.
Guam is home to the most diverse reefs under U.S. jurisdiction (Paulay, Kirkendale, Lambert, & Meyer, 2002). The elephant ear sponge adds to the livelihood of Apra Harbor’s reef, and its population appears to be spreading (Kelly, Hooper, Paul, Paulay, Van Soest, & De Weedert, 2003). Since Ianthella basta is only found in Guam in Apra Harbor, the largest harbor in the western Pacific, scientists wonder whether the sponge is native or introduced. The sponge is found in waters ranging from the Mascarene Islands to Vanuatu, and from the Philippines to Guam, yet not around Palau and the Federated States of Micronesia, which are located between these regions (Kelly et. al, 2003). This geographic gap suggests that the elephant ear sponge may have naturally settled within these waters, or anthropogenically been introduced. By either means, the elephant ear sponge has become a fixed species in Apra Harbor, and is an integral member of the coral reef structure.
Unfortunately, the elephant ear sponge is threatened by potential military buildup in Apra Harbor. The coral reef in the harbor is at risk of being dredged to make room for nuclear aircraft carriers. In addition to the direct impact of removing coral from the site, there will also be various indirect impacts. These impacts include debris and sediment released into the immediate area, affecting the remaining coral and the sponges. For example, sand and silt can easily clog the delicate sponge’s pores, blocking its ability to filter water. Therefore, these sessile organisms have no way of surviving the anthropogenic destruction of their habitat.
Learning about these pressing environmental issues has made us more sensitive to the importance of protecting animals that cannot defend themselves. As we continue on our Micronesian journey to Palau, we look forward to seeing more unique marine creatures that we have only thus far learned about in the classroom. Through this course, we have come to realize that even a simple organism, such as a primordial sponge, contributes much more to the ecosystem than what meets the eye.
Branches of the Tree of Life: Sponges. (2011). In BioMedia Associates. Retrieved from http://www.ebiomedia.com/prod/BOsponges.html
Kelly, M., Hooper, J., Paul, V., Paulay, G., Van Soest, R., & De Weedert, W. (2003). Taxonomic Inventory of the Sponges (Porifera) of the Mariana Islands. Micronesica 35-36, 100-120.
Paulay, G., Kirkendale, L., Lambert, G., & Meyer, C. (2002). Anthropogenic Biotic Interchange in a Coral Reef Ecosystem: A Case Study from Guam. Pacific Science, 56(4), 403-419.