Christmas tree worms (of the genus Spirobranchus) are small segmented worms that build a tube (their home) into the hard skeleton of corals. Once settled—often on a parrotfish’s feeding scar on Porites coral—the worms stay fixed in their tube. They grow their tube at a rate similar to the coral’s, so the worm hole is never overgrown.
The worms extend their feather-like gills (radioles, actually) into the water column for respiration, and for filter-feeding on bacteria, particulate organic matter, and phytoplankton.
Christmas tree worms are a common sight on tropical coral reefs, and come in a myriad of colours. The tiny animals are highly sensitive to vibration and fast changes in light (signs that a predator may be passing overhead), to which they react by rapidly retracting into the safety of their tube homes.
Since Christmas tree worms aren’t mobile, they reproduce by broadcast spawning into the water column, as seen in this video. The event usually occurs at dusk, and is synchronised with neighbouring Christmas tree worms to maximize fertilization—just as with corals and other sessile invertebrates.
While photographing a Christmas tree worm using a 105mm macro lens + a diopter, we noticed these tiny critters in the lower right-hand corner of the image. We’ve never seen them in Christmas tree worms before. They may be a regular occurrence, but rarely noticed because of their tiny size. We asked our invert guru, Dr. Steve Webster, and he asked his friends… and they tell us the critters may be copepods—possibly females with eggs. Copepods are minuscule crustaceans, usually planktonic, but often parasitic—which may be the case here.
—Josh & Liz