USC Dornsife Scientific Diving: Crown-of-Thorns Outbreaks and Anthropogenic Pollution

by Max Martinez

Originally published at ScientificAmerican.com

Coral reefs across the Indo-Pacific Ocean attract thousands of scuba divers from around the globe on an annual basis. However many of these highly diverse and biologically rich ecosystems are at a risk by a seemingly unlikely predator – the crown-of-thorns sea star Acanthaster planci, which makes a living by consuming the polyps of stony corals. Unlike its distant cousin the sun star Pycnopodia helianthoides (commonly found in the waters off Southern California), the surface of A. planci is covered in venomous thorn-like spines, which serve as an effective defense against potential predators (humans included!).

Crown-of-thorns sea star moving across coral rubble in the Ngederrak Marine Conservation Area, Koror State, Palau. (Photo by Jim Haw; May 2011)

Under normal conditions, A. planci can help to improve coral reef biodiversity by eating a variety of fast-growing species of corals, giving the slower-growing ones an opportunity to reach full maturity. However an entire horde of these creatures has ability to devour several acres of reef in a single sitting (Birkeland & Lucas 1990).

Across the Pacific, recent studies suggest that select populations of crown-of-thorns are increasing at an alarming rate (Brodie et al. 2004). For example, more than one thousand A. planci have been found living on a discrete patch of reef at a single time (Keesing & Lucas 1992, Guam DAWR). Considering an individual can eat up to six square meters of living corals per year (Burdick et al. 2008), the potential threat that population outbreaks of crown-of-thorns impose on coral ecosystems is cause for concern.

Although the definitive cause of crown-of-thorns outbreaks is not fully understood there is little debate that blooms of A. planci have increased in both frequency and severity over the last several decades. One of the primary culprits of these outbreaks is attributed to the increasing problem of terrestrial runoff, which includes a concoction of nutrients, sediments, and pollutants that are regularly swept into coastal waters (Fabricius 2005).

Concomitantly, other factors believed to impact crown-of-thorns outbreaks include the removal of their natural predators, which in effect, has allowed them to feed on corals with impunity. On Guam, the most severe outbreaks are believed to arrive in the wake of severe storms such as heavy rains and typhoons (especially those following drought conditions) (Birkekand 1982). With more than 80% of the Guamanian population living along the island’s coastal zone (Burdick et al. 2008) the possibility that anthropogenic pollution can aggravate outbreaks must be addressed.

USC Student holding a Triton's trumpet snail on Guam during the 2010 expedition. (Photo by Jennah Caster)

The two main predators of the crown-of-thorns on Guam’s reefs are the Triton’s trumpet snail (Charonia variegata) and the napoleon wrasse (Chelinus undulatus), both of which are heavily involved in marine trade despite being recognized as species in need of conservation and/or protection (Guam DAWR). “Curiosity kills the wrasse,” or so they say, as this large, slow-swimming fish is known to directly approach divers underwater, making it an easy dinner. The Triton’s trumpet shell is a highly prized commodity amongst the international shell trade – in some cases, fetching prices upwards of $150.

Overall, crown-of-thorns outbreaks are a major threat to the health of coral reef ecosystems and according to research done at the University of Guam Marine Lab, these population booms “have had, and are continuing to have, a severe impact on many Guam’s reefs” (Burdick et al. 2008, Guam DAWR). Urban runoff and overfishing also have the potential to destroy Guam’s reefs, especially considering that little of the island’s terrestrial and marine areas are protected by local and Federal law.

Works Cited:

Birkeland, CE (1982) Marine Biology 69:175

Birkeland, CE, Lucas JS (1990) CRC Press, Boca Raton, 257

Brodie, J et al. (2004) Marine Pollution Bulletin 51:9

Burdick, D et al. (2008) NOAA Technical Report

Brodie, J et al. (2004) Marine Pollution Bulletin 51:9

Burdick, D et al. (2008) NOAA Technical Report

Faabricius, KE (2005) Marine Pollution Bulletin 50:125

Guam DAWR: http://www.guamdawr.org/

Keesing J, Lucas JS (1992) Journal Experimental Marine Biology Ecology 156:89

Photo of author by Jim Haw

About the author:

Max Martinez is a senior in the USC Dana and David Dornsife College of Letters, Arts and Sciences and will graduate this May with a BS in Environmental Studies. Max has a strong interest in marine science and looks forward to pursuing a graduate degree in resource management and conservation.

Editor’s note: Scientific Research Diving at USC Dornsife is offered as part of an experiential summer program offered to undergraduate students of the USC Dana and David Dornsife College of Letters, Arts and Sciences. This course takes place on location at the USC Wrigley Marine Science Center on Catalina Island and throughout Micronesia. Students investigate important environmental issues such as ecologically sustainable development, fisheries management, protected-area planning and assessment, and human health issues. During the course of the program, the student team will dive and collect data to support conservation and management strategies to protect the fragile coral reefs of Guam and Palau in Micronesia.

Instructors for the course include Jim Haw, Director of the Environmental Studies Program in USC Dornsife, Environmental Studies Lecturer Dave Ginsburg, SCUBA instructor and volunteer in the USC Scientific Diving Program Tom Carr and USC Dive Safety Officer Gerry Smith of the USC Wrigley Institute for Environmental Studies.

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