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A Silent Invasion Threatens to Overrun Pristine Black Coral Beds and Alter Hawaii's Deep Reef Community.

December 14, 2005

Written by Sam Kahng, University of Hawaii

Like a sneak attack under the cover of darkness, a voracious invader is spreading through Hawaii's deep reefs. Because it avoids direct sunlight, snowflake coral appears quite benign to the casual shallow-water diver. However, lurking inside caves, under shaded ledges and piers, snowflake coral is steadily advancing, smothering native sea life with a carpet of white billowy polyps (Figure 1).

Black Corals in Peril

Figure 1. Snowflake coral saturating the roof of a ledge with millions of polyps extended while feeding at 40 feet off Pupukea. Photo courtesy of Sam Kahng.
Figure 1. Snowflake coral saturating the roof of a ledge with millions of polyps extended while feeding at 40 feet off Pupukea. Photo courtesy of Sam Kahng.

Formally known as Carijoa riisei, this alien species is already sweeping through the Au'au Channel between the islands of Maui and Lanai like a silent blizzard. In 2001-2004, deepwater surveys in the Au'au Channel using the Pisces V submersible supported by the Hawaii Undersea Research Laboratory (HURL) determined that >50% of all black coral colonies below 230 feet had snowflake coral overgrowth with most being completely smothered and dead (Figure 2). Less than 15% of the sunlight penetrates these depths so the deep reef is permanently shaded. Scientists worry that black coral, Hawaii 's state gemstone which accounts for several million dollars worth of jewelry sales statewide each year, could suffer large scale mortality throughout the Hawaiian Archipelago.

The problem with snowflake coral is that it can grow very quickly (up to an inch every two weeks) and smothers other slower growing organisms. Black corals grow to over 10 feet tall, however, they grow much slower – only 2.5 inches per year – leaving them susceptible to overgrowth by snowflake coral. Because snowflake coral is an alien species, there are no native competitors and predators capable of moderating its explosive growth. Snowflake coral commonly carpets hard reef surfaces and excludes native species like surf oysters, lace bryozoans, cup corals, and deep water plate corals. A good example of this effect can be seen by local divers on the YO-257 shipwreck off Waikiki where shaded portions of the structure are dominated by snowflake coral (Figure 3).

Figure 2. A black coral colony in the process of being overgrown with snowflake coral. Photo taken ~300 feet in the Au'au Channel, courtesy of Hawaii Undersea Research Laboratory (HURL).
Figure 2. A black coral colony in the process of being overgrown with snowflake coral. Photo taken ~300 feet in the Au'au Channel, courtesy of Hawaii Undersea Research Laboratory (HURL).

Because the large scale black coral mortality lies below SCUBA diving depths, few people actually see its devastation. Black coral divers who harvest their crop above 230 feet do not see the threat because snowflake coral is largely restricted to the undersides of ledges at these depths. However, it may only be a matter of time until snowflake coral takes its toll on the harvest. With depth as a refuge from harvesting, pristine black coral populations below 230 feet are dominated by large colonies with high reproductive output. Traditionally, this deep water population was an important source of larvae seeding the shallower depths subject to harvesting pressure. Because it takes black corals 12 years to reach reproductively maturity and another 4-5 years to reach harvesting size, by the time an impact on harvesting is noticed, it may be too late.

What is Snowflake Coral?

Figure 3. Snowflake coral polyps carpeting a sunken ship (YO-257) in 90 feet. Photo courtesy of Sam Kahng
Figure 3. Snowflake coral polyps carpeting a sunken ship (YO-257) in 90 feet. Photo courtesy of Sam Kahng.

Snowflake coral (also called “branched pipe coral”) is a soft coral with a flexible branching structure. Under the magnifying glass, each soft coral polyp has eight pinnate tentacles (Figure 4), unlike stony coral polyps which have multiples of six tentacles. Snowflake coral was first discovered in Pearl Harbor in 1972, and is believed to be native to the Caribbean or Western Pacific. It likely came to Hawaii by hitching a ride on a large commercial ship or barge spreading from harbor to harbor along the way. It lives exclusively on a diet of zooplankton (tiny marine animals and larvae) which it filters from the water.

The Silent Underwater Invasion

Unfortunately, marine alien invasions rarely capture the same amount of attention as terrestrial alien invasions. Many people do not recognize alien marine species when they see them or visit the underwater realm enough to notice that it is quickly changing with many new faces and fewer old ones. Because the effects are often hidden from view, many people have difficulty relating to the marine alien species issue or understanding their consequences. Complicating this issue is the seemingly logical, but false notion, that all marine ecosystems are connected via the ocean. In reality, many marine organisms are restricted to coastal areas and unable to disperse across large, open ocean distances – that is without the aid of humans. The coastal marine ecosystem surrounding Hawaii has evolved largely in isolation, and it is possible to have alien species invade and wreak havoc here.

Alien species spread via human activities. Alien marine species hitchhike on ships, stowaway in aquaculture product shipments, escape from private aquariums, or are even intentionally released by misguided pet owners. These activities all contribute to alien invasions which are steadily displacing our native species in Hawaii.

branches of snowflake coral
Figure 4. Close-up of snowflake coral polyps showing their eight pinnate tentacles. Photo coutesy of Sam Kahng.

A Tropical Snowstorm

Unlike physical pollution that often degrades and dissipates with time, biological pollution can be self-perpetuating, self-dispersing, and grow with time. In many cases, an alien species can persist for years at a low level, slowly gaining critical mass until their numbers explode. Snowflake coral is a good example of this phenomenon.

When snowflake coral was first discovered in 1972, it captured little attention. By the late 1980's, snowflake coral was reported at select sites in Molokai, Maui, and the Big Island . However, it was originally thought to be harmless; after all, it only grows in the shade. Now, evidence shows that snowflake coral is accelerating its spread and exploding in abundance. Fortunately, snowflake coral has not been reported from the surveys of the Northwestern Hawaiian Islands (NWHI).

It is possible that snowflake coral spread from Oahu to the other islands naturally. However, its abundant, dense populations in most of Hawaii's harbors suggest that maritime traffic aids its dispersal. Unlike most stony corals, many soft corals are known to produce larvae that sink, crawl, and do not travel far from parent colonies. With grant support from the Hawaii Coral Reef Initiative Research Program, a CSCOR-funded research program, and the Hawaii Invasive Species Council, Sam Kahng, a Ph.D. student at the University of Hawaii, is researching snowflake coral's biology, ecology, and dispersal capability to understand how it spreads and what can be done to minimize its ecological impact.

Snowflake Emergency in Kauai

Figure 5. A healthy black coral colony with snowflake coral encroaching on the surrounding reef at approximately 270 feet in the Au'au Channel. Photo courtesy of Hawaii Undersea Research Laboratory (HURL).
Figure 5. A healthy black coral colony with snowflake coral encroaching on the surrounding reef at approximately 270 feet in the Au'au Channel. Photo courtesy of Hawaii Undersea Research Laboratory (HURL).

In 2002, snowflake coral was discovered in Kauai at Port Allen by Steve Coles of the Bishop Museum. This new discovery is worrisome because Kauai is home to large, pristine beds of Hawaiian black corals – the second largest in the Archipelago next to the Au'au Channel. Recent coastal surveys by the Bishop Museum, University of Hawaii, and Hawaii Division of Aquatic Resources suggest that the snowflake coral invasion is in an early stage of population growth with only a few confirmed shallow-water locations. A recent HURL deep-water submersible survey of south Kauai did not find any snowflake coral among the dense black coral colonies on the deep reef.

Unlike the other main islands, Kauai appears to be a favorable candidate for snowflake coral eradication. Because early population growth can accelerate with time, the opportunity to protect Kauai and its pristine black coral beds diminishes with time. If snowflake coral can be eradicated before it is able to widely disperse or establish a stable, deep water population, Kauai 's black coral population can be protected. Kauai's location as the gateway to the NWHI is also strategic for managing alien marine species. If the dispersal of snowflake coral can be stopped in Kauai, an actively managed zone of control can be established as a line of defense.

The strategic opportunity to eradicate snowflake coral on Kauai is time limited and must be addressed before it proliferates to unmanageable levels. Without immediate and decisive action, the opportunity to save Kauai may be lost regardless of future efforts.