How do you make babies, give them a decent start in life, and send them off to perpetuate the species from a lightless, toxic environment in the bowels of the ocean? Scientists studying deep-sea tubeworms have unmasked one secret to their success: Females fertilize their eggs internally and release the resulting embryos at an early developmental stage, optimizing their chances for survival and long-distance dispersal.
Ever since the discovery of gutless vestimentiferan worms in 1977, researchers have puzzled over how the worms colonize highly ephemeral, unstable hydrothermal vent habitats spaced hundreds of kilometers apart. The prevailing theory holds that, like other marine invertebrates, tubeworms release masses of eggs and sperm in a hit-or-miss manner, called broadcast spawning, into the surrounding waters.
But the new study indicates that this isn't the case. Biology graduate student Ana Hilário of the University of Southampton in the United Kingdom, and her colleagues used a deep-sea submersible to collect tubeworms of five species from Pacific vents and cold seeps in the Gulf of Mexico. In thin slices of the females' reproductive tracts, Hilário found a hook-shaped region, called the spermatheca, where sperm are stashed and eggs undergo fertilization before spawning. At the study sites, fertilized eggs released naturally and collected over a period of days proved to be in very early stages of development, indicating that fertilization occurs internally rather than outside the worms.
This may protect the delicate sperm and eggs from ambient environmental poisons, says collaborating biologist Craig Young of the University of Oregon. While many other animals fertilize and store sperm internally, they all "brood" embryos up to some level of development. Young believes the tubeworms' strategy of arresting development soon after the embryo forms "is absolutely unique in the animal kingdom." Furthermore, it maximizes the potential for dispersal, since no time is wasted while the embryos are retained by the mother, he says. The team reports its findings in the February issue of The Biological Bulletin.
Deep-sea biologist Charles Fisher, of Pennsylvania State University, University Park, says researchers have witnessed isolated spawning events among tubeworms, but never the kind of synchronized mass releases associated with animals that reproduce by external fertilization in response to some environmental cue. "This work explains why," he says. The findings also give marine ecologists a much better understanding of tubeworm embryonic development and dispersal potential, he adds.