About the Cover
Cover
Riftia pachyptila, a species of vestimentiferan tubeworm, is shown here in an aggregation on Tubeworm Pillar, a large hydrothermal chimney on the East Pacific Rise. Vestimentiferans-those odd gutless worms found at hydrothermal vents and cold seeps in the deep ocean-have strange helical spermatozoa that are bound together in bundles; they also have external ciliated grooves that connect to the openings of the gonoducts. These traits suggest the possibility of sperm transfer and internal fertilization. Indeed, direct transfer of large sperm masses (spermatozeugmata) has been described in two species. It is therefore puzzling that neither mature eggs nor developing embryos have ever been discovered in the oviducts; oocytes removed by dissection always have large germinal vesicles. In estimates of the dispersal time of tubeworm larvae, it had been assumed that fertilization is external and that the dispersal period includes the entire embryonic and larval period. If this assumption is incorrect (e.g., if early embryos are brooded), then dispersal time and distance could be overestimated by as much as 60%.
Using inverted plankton nets floating above clumps of Riftia pachyptila at hydrothermal vents in the Eastern Pacific, Hilario, Young, and Tyler (page 20, this issue) show that eggs released by females are still in the primary oocyte stage, an observation that was initially interpreted as evidence for free spawning and external fertilization. However, they also show that oocytes dissected carefully from the oviducts of seep tubeworms (Lamellibrachia luymesi) mature and undergo embryogenesis without the addition of spermatozoa, suggesting that oocytes are somehow inseminated internally even before egg maturation is complete.
Histological sections through the trunks of five vestimentiferan species from both seeps and vents revealed a previously unknown sperm storage structure at the far posterior end of the female reproductive tract. Here, in the portion of the tract lying between the ovary and the oviduct, sperm are stored in a spermatheca that consists of distinct outpocketings of the oviduct epithelium. The inset on the cover shows a histological section through the spermatheca of R. pachyptila, stained with hematoxylin and eosin. Pink oocytes near the bottom of the image are in the ovary; the central oocytes lie in the lumen of the spermatheca, surrounded by pockets containing purple-stained sperm. Oocytes are presumably inseminated when they pass from the ovary through the spermatheca to the oviduct, where they are stored in the primary oocyte stage until release. Thus, despite the earlier erroneous assumption of external fertilization, the dispersal estimates remain valid.
Credits: Photograph of R. pachyptila aggregation: taken by Alvin pilot Blee Williams, while diving with co-authors Craig Young and Paul Tyler. Inset: micrograph by author Ana Hilario. Cover design: Beth Liles (Marine Biological Laboratory).
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Copyright © 2005 by the Marine Biological Laboratory.
