The Destructive Feeding Habits of Starfish

When Ecosystems are Devastated by Overabundant Sea Stars

Oct 24, 2009

Some species of starfish wreak havoc on shellfish populations and coral reefs when an abundance of food and shortages of predators fail to keep their numbers in check.

Starfish are marine invertebrates found in all of the world’s oceans, with the most variety inhabiting the northern Pacific. There are 1,800 species of starfish, which are also known as “sea stars” because they aren’t really fish. Some are voracious eaters that can destroy entire shellfish beds or coral reefs. This happens when their ecosystems become unbalanced and their numbers increase.

Northeastern U.S. Shellfish Populations Affected by Starfish

Overabundant populations of the Northern Sea Star have caused problems in New England for decades. A recent Boston Globe article by Beth Daley, dated June 15, 2009, entitled “Seeing sea stars—and lots of them: Shellfish predator’s population surging,” shed light on starfish infestations in the spring of 2009.

Divers in Buzzards Bay, MA, found “carpets” of starfish stretching 100 yards along the sea floor. Fishermen and dredgers from Rhode Island to Massachusetts were reporting an increase in the number of starfish. The article cites a 1990 U.S. Fish Commission report that described how one starfish ate 50 young clams in six days and increased in size by 300 percent.

A starfish’s size, therefore, has more to do with its feeding habits than its age. The Northern Sea Star can live three to four years in deep or shallow waters, but the shallow waters have the variety of shellfish they prefer as prey. This is particularly unhelpful to fishermen who rely on sufficient stocks of mussels, clams, oysters and scallops for their livelihood.

Starfish Infestations a Problem Throughout 20th Century

The Rhode Island Restoration Portal’s information about sea stars reveals that 10 million sea stars were removed from 11,000 acres of oyster beds in Narragansett Bay in 1929, a time when oystering in the area was big business. In the Depression era, Massachusetts and Rhode Island even placed a bounty on starfish, and farmers used bushels of them to fertilize their land.

A March 27, 1958 release from the U.S. Fish and Wildlife Service announced a bumper crop of starfish ten times their normal size in the Long Island area, which destroyed 60 to 90 percent of the oyster crop in some beds. Officials considered establishing refuges in certain Long Island Sound estuaries where oysters could be placed during such invasions, because starfish cannot tolerate brackish water.

This same report mentioned that oyster beds had been similarly affected eight years earlier, but along the Connecticut coastline, placing that event around 1950. There seems to be an natural cycle to such outbreaks in New England. Daley reported in the Boston Globe article that “the last starfish boom was eight to 10 years ago,” which would have been between 1999 and 2001.

The Crown-of-Thorns Starfish Devours Coral Reefs

There is another species of starfish whose destructive feeding habits are noteworthy. The Crown-of-Thorns starfish has destroyed coral reefs in the Indo-Pacific region and areas of the Great Barrier Reef off Australia. There have been three major outbreaks of this species since the 1960s, according to the article “Starfish Outbreak Destroying Coral Reefs in Asia,” by Tan Ee Lyn, filed by Reuters on January 16, 2008.

The article calls attention to the then-current outbreak in the Coral Triangle—the area between the Philippines, Malaysia, Indonesia, Papua New Guinea and the Solomon Islands. A 6.2-mile circle of reef was reportedly wiped out. This species of starfish has up to 21 arms, which are covered with venomous spines. It typically feeds on one species of coral, in one part of a coral colony. But when populations explode, the competition among the starfish expands their appetite and they will eat other types of coral and practically destroy entire colonies.

If the Crown-of-Thorns kill most of the coral in a part of reef, it can take 10 to 15 years for the reef to restore itself. Since the adult starfish have few predators and don’t really succumb to any diseases, once there is a proliferation of Crown-of-Thorns, they are very difficult to eradicate.

So What Causes Starfish Population Explosions?

There are many theories why starfish overpopulation occurs. Some researchers attribute it to natural cycles, water temperature or weather patterns. In the case of Northern Sea Star, some relate it to shellfish population trends or a decline in sea star predators like spider crabs. With Crown-of-Thorns, perhaps it’s linked to overfishing, the timing of El Niño, or a shortage of their predators, like the giant triton snail.

One fact generally agreed upon is that human activity contributes to the cause. Pollution created by agricultural fertilizers and sewage runoff results in a buildup of nutrients in the waterways, especially in times of heavy rain. The nutrients allow algae to grow and the starfish larvae feed on the algae. More food available to the starfish enables more of them to survive to adulthood, causing their numbers to increase.

Until scientists find more answers, cleaning up the oceans would not only help restore balance in the delicate ecosystems, its beneficial effect on all ocean life would be amplified. Read more about the starfish that destroys coral in the article “Managing the Crown-of-Thorns Starfish: How Australia Deals With Population Outbreaks.”

The copyright of the article The Destructive Feeding Habits of Starfish in Ecosystem Preservation is owned by Cheryl Kraynak. Permission to republish The Destructive Feeding Habits of Starfish in print or online must be granted by the author in writing.