The Wayward Children of Worms
Craig M. Young
Oregon Institute of Marine Biology
View a video of tubeworm sperm being released in actively swimming bundles. (QuickTime, 556 kb)
Tubeworms living at cold seeps and hydrothermal vents are completely dependent on hydrogen sulfide, which provides energy for their growth, metabolism and reproduction. However, sulfide is not abundant everywhere in the marine environment. Because suitable habitats for these worms are found only in places with certain geological features, scientists have long wondered how tubeworms survive when their habitats change, and how the animals locate and colonize new habitats.
Only recently have scientists been able to rear the embryos and larvae of seep tubeworms in the laboratory. Eggs and sperm removed from adults are mixed together in glass bowls to produce embryos, which are kept in incubators. Embryos and larvae of the seep tubeworm Lamellibranchia will live in glass dishes as long as the dishes are held at the same cold temperatures found on the sea floor.
The eggs and embryos are filled with a fatty yolk that is lighter than seawater, allowing them to float upward for more than a week before sprouting tiny cilia that propel them back down toward the bottom. The larvae do not feed while drifting. So they must find a new habitat before consuming all of the nutritious yolk in the eggs, a time span of about three weeks. In the typical deep currents found on the Louisiana Slope, larvae probably can drift as much as 60 km during their three-week dispersal period. Although we do not know how many larvae successfully find a new seep habitat, it seems likely that the majority are either eaten by predators or run out of energy before they find a suitable habitat. However, female tubeworms can produce eggs continuously and may live for as long as 200 years. So their chances of producing at least some successful offspring are probably good.Because larvae are microscopic and difficult to study, many questions remain. For instance, how do the larvae of vent animals identify appropriate sites for attachment? Can they "smell" sulfide or methane? Do they respond to the odors of adults? How, when and where do they obtain their symbiotic bacteria? Do they have a mouth in any stage of their development? What animals eat the larvae? How much of the energy in seep eggs makes its way into the planktonic food web?
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