These Christmas Tree Worms Add a Festive Spirit to Underwater Life
.jpg)
Of all the countries in the world, there is no other country as enthusiastic about Christmas as the Philippines. Starting in late August, Christmas songs play in malls, kids go caroling from house to house, and the check-out ladies in the supermarkets wear cute Santa Claus hats. So it’s just fitting that the archipelago’s coral reefs are covered in tiny Christmas trees as well.
Embedded in encrusting corals, particularly in shallow reefs, a multitude of tiny pine-tree-like appendages stick out. They come in many colors, from orange, brown, blue to yellow. When a diver or snorkeler is swimming too close to them, they quickly disappear in their small holes below the coral surface.
.jpg)
What are these reef-Christmas trees?
ALSO READ
See Glimpses of History in the Shipwrecks of Subic Bay
The Mighty Sardines of Moalboal Are a Sight to Behold
They are Christmas tree worms, or marine relatives of the earthworms known to every gardener. The worms are parasites of hard corals, and drill tubes into the coral colonies. When a potential threat approaches, the worms retract into the safety of their tubes. A few Christmas tree worms on a coral colony are not a problem for their hosts, but too many of them can break up the structure of the corals and impede their growth.
The similarity with European pine trees (and their plastic copies in Philippine malls) which have come to symbolize Christmas is, of course, a coincidence, and the worms developed these Christmas-tree looking appendages over the course of evolution. They are mouth parts—not biting or chewing jaws, but filter-feeding mouths. The ocean is full of plankton, tiny organisms which float through the water. The Christmas tree worms filter the plankton and feed on it.
.jpg)
.jpg)
Swimming by the Christmas tree worms and waving your hand close to them so that they retract is a popular trick by dive masters to entertain their diving clients. Generally, irking wildlife is something a responsible diver should avoid, but this case of wildlife interaction seems rather harmless. The worms generally re-emerge after a few minutes.
While worms are stereotypically slow animals, this is not the case for the Christmas tree worms: their retraction is stunningly fast, a fraction of a second, on par with the striking speed of the fish which would like to snack on them. It takes the special high-frame rate settings of a camera meant for sports or wildlife videography to film the retraction behavior. Only with such a fast camera is it possible to get footage which can be slowed down to produce videos of the worm retraction which look smooth to the eye of the viewer.
An interesting question is how the Christmas tree worms even see the approaching hungry fish or playful diver. Remember, the “Christmas trees” are the worms’ jaws, and they are the only part of the worms which stick out from the coral.
Worm anatomy
Are there eyes on the worms’ jaws? It turns out there are. Eyes have developed in the evolution of animals many times independently. As an example, our mammalian eyes and the eyes of insects became complex organs independently. The common ancestor of humans and insects, which was a simple worm eons ago, did not have complex eyes. Only many millions of years of natural selection brought eyes about independently in insects and in us humans. The anatomy of an invertebrate (insect or worm) eye is very different from the anatomy of the human eye, but the function is the same: to sense light. Evolution took different paths to solve the same problem: how to see.
.jpg)
The amazing fact about the Christmas tree worms is that their eyes evolved twice in one animal. The worms have the same simple eyes like all other segmented worms, located on the front part of their bodies. But once they started drilling into corals, and once they started developing their unique Christmas tree jaws, these eyes were stuck in the dark and hence useless (and they are only present small and reduced now). As a consequence, a second set of eyes developed in worm evolution, placed on their ring of filter-feeding jaws. With these, the worms can see approaching fish when their mouth parts are extended out of the coral colony.
If the worms have eyes on their jaws, why can’t you see a ring of eyeballs sticking out from the little Christmas trees? The reason is that the worm eyes are much smaller and simpler than the eyes of a mammal, like a human. The worms can see if a predator or scuba diver is approaching them, but they can’t resolve fine visual detail, like a human can. They couldn’t tell Santa Claus from an elf.
.jpg)
.jpg)
So, Merry Christmas to all of you, and if you have a friend who is an underwater photographer, you will probably see images of the Christmas tree worms on social media. Often the underwater photographers copy-paste a Santa Claus with a sled on top of the underwater photograph showing the Christmas tree worms. It’s a bit tacky, but it lights up the Christmas spirits, and it might get the audience to read up on these fascinating marine animals.
Travel tip
You will be able to see Christmas tree worms all over the Philippines, wherever there is a healthy coral reef. A lot of the images, and the video from this article are from Dauin, just south of Dumaguete on Negros Island. The coral reef in Poblacion/Dauin has the advantages that it’s 1. in a marine protected area, where fishing and coastal development are prohibited, so it’s a very healthy reef, and 2. The reef is very shallow, so you can see the worms in only a meter or two of water. You can observe these curious animals on scuba or on snorkel, since you generally don’t have to go deep to see them.
.jpg)
.jpg)
.jpg)
As usual, we are guests in the ocean, which is the home of the Christmas tree worms, corals, and the more than a thousand fish species living on the Philippines’ coral reefs. Please act like a good guest, and don’t touch or break anything. Particularly, avoid standing on corals, which can do damage which takes months or years to grow back.
