Column: Hunger games on Guam’s reefs

World Reef Awareness Day on Tuesday provides a great opportunity to spotlight the unique natural heritage of Guam’s reefs and the strong cultural connection of the CHamoru people to this valuable resource.

In recent months, juvenile rabbitfish (mañahak) have traveled from various Micronesian islands to Guam and are now quarantining on the island’s reef flats. Much like our own children, these youngsters have an insatiable appetite and can curtail seaweed gardens on reefs where they couch surf.

In Pago Bay, extensive stands of the angel hair seaweed, a species that has been proliferating on Guam’s reefs since 2012, have now been decimated by these rabbitfish. This is a fine example of the biological control of a nuisance species without human assistance.

Bloom of angel hair seaweed in Pago Bay photo courtesy of Tom Schils

Stonefish in seaweed camo

Mañahak are not picky eaters and they happily feast on the diversity of seaweeds that reefs have to offer. The impact of the rabbitfish raid was striking when I was desperately searching for seaweed during a recent field trip in Pago Bay.

When a boulder generously covered with bright green tufts of seaweed caught my eye, I thought I had struck gold. While plucking off these tufts, the boulder suddenly aroused and charged off at a whipping speed. The two eyes on the boulder glanced back and I realized that this huge stonefish had capitalized on the existing food scarcity by using seaweed camouflage to deceive its rabbitfish prey.

Rabbitfish feasting on angel hair seaweed

Mass spawning

Some seaweeds are untouchable by grazing fish. Vivid green clumps of turtleweed (Chlorodesmis fastigiata) stand out on reefs but are not targeted by herbivorous fish because they contain toxins.

Like rabbitfish, turtleweed releases offspring en masse when the conditions for their survival are optimal. Rabbitfish runs have evolved into seasonal events that coincide with the main growing season of seaweeds.

The poisonous turtleweed courtesy of Tom Schils

Turtleweed is more selective in timing its reproduction. Episodes of mass spawning by this seaweed occur when bare patches of reef become available in the aftermath of storm events. At that point, turtleweed is fully committed to reproduction and the whole seaweed is converted into reproductive cells after which the parent plant dies.

Faking a poisonous appearance

The camouflage trick of stonefish is topped by the disguise-by-resemblance (mimicry) strategy of the Piti Bomb Holes seaweed (Rhipilia coppejansii). Guam is home to several species of Rhipilia, which all form distinctive spongy blades.

The Piti Bomb Holes seaweed is only found on Guam and is unique in forming green tufts of loose filaments that resemble turtleweed. The chemical composition of the Piti Bomb Holes seaweed is as of yet unknown, but its apparent similarity to poisonous turtleweed might ensure its survival on reefs where herbivores abound.

Piti Bomb Holes Seaweed, a turtleweed look-a-like photo courtesy of Tom Schils

Cultural traditions

Humans have become part of the natural dynamics on reefs. The mañahak season brings people together while catching, processing or feasting on this seasonal culinary delight. Observant fishermen are the first to witness the changes that are taking place on reefs. They have already adapted by using the overly abundant angel hair seaweed as fishing bait or as a crispy and tasty additive to salads.

Even in this day and age of magnificent nature documentaries and well stocked grocery stores, it remains important to celebrate and perpetuate cultural traditions that are deeply rooted in the island’s natural heritage. After all, such activities allow us to evaluate the health of our ecosystems and find solutions for environmental issues specific to Pacific islands.

Tom Schils is an EPSCoR Researcher and a professor of marine biology at the University of Guam with a research focus on the diversity and ecology of seaweeds in the tropical Pacific. He can be reached at 735-2185 or tschils@triton.uog.edu.

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