The remarkable little critter pictured below is not what it appears to be…indeed, it “hopes” that potential predators are tricked into thinking it’s actually another creature they wouldn’t want to eat. What Government of Guam Biologist, Dave Burdick, is holding in the palm of his hand is a juvenile Graeffe’s sea cucumber, Pearsonothuria graeffei. This individual was found at a Rapid Ecological Assessment site off the coast of Rota, but this species is found throughout the Marianas, and is relatively common across much of the Indo-Pacific. Juveniles such as this one are rarely seen, but adults are occasionally encountered on reefs across its range.
A juvenile Graeffe's sea cucumber, Pearsonothuria graeffei, found at a Rapid Ecological Assessment (REA) site off the coast of Rota. Photo by Dave Burdick.
As with most other sea cucumbers, known as balate in Chamorro, this elongated echinoderm uses tube feet to slowly move across the reef in search of food (mostly decaying organic matter), which it picks up using an array of oral tentacles rimming its mouth. This colorful youngster appears strikingly different from adults of this species – the yellow-spotted, black and white juveniles eventually trade their bright coloration for a more muted, brown and cream-mottled look (see photo below).
An adult Graeffe's sea cucumber, with its less conspicuous coloration, crawling across a reef on Guam. Photo by Dave Burdick www.guamreeflife.com.
But why would a young sea creature be more colorful than the larger, less vulnerable adults? Wouldn’t this attract attention to would-be predators? The strategy employed by this sea cucumber is known as Batesian mimicry, a defense against predators that involves a harmless species mimicking a toxic species that is brightly colored to denote the foul-tasting, or potentially deadly chemical compounds it possesses. In this case, the juvenile Graeffe’s sea cucumber is likely mimicking the toxic sea slug, Phyllida varicosa (see photo below).
A sea slug, Phyllidia varicosa, in the Batangas, The Philippines. This noxious nudibranch is presumed to be the species that juvenile Graeffe's sea cucumbers mimic to gain protection from predators. Photo by Dave Burdick.
Interestingly, adult Graeffe’s sea cucumbers actually do possess toxins that make them distasteful to potential predators, but the young individuals are not yet equipped with these chemical defenses and have to rely on mimicry to survive long enough to grow into their chemically-defended adult form.
The development of this marvelous adaptation occurs through natural selection, an elegantly simple mechanism of evolution first described by Charles Darwin in 1859 (and nearly simultaneously by the lesser -known Alfred Wallace), all unbeknownst to these beautiful and fascinating sea cucumbers inhabiting the reefs of the Mariana Islands. Who knew balate could be so cool?!
