Abstract
Ultrastructure of gametes (sperm and eggs) of vestimentiferan tubeworms and external-internal insemination by means of spermatozeugmata in Riftia pachyptila were described. The spermatozoa of Riftia are threadlike, about 130 μm long, and have a diameter of about 0.7 μm, narrowing to 0.2 μm in the apical portion of the macrodome, and pointed at the end of the tail. Oocytes are produced by the ovaries at the first meiotic prophase stage. The early oocytes are small, hardly exceeding 10 μm in diameter, spherical cells with a poorly differentiated cytoplasm and large nuclei with a nucleolus. Completely formed oocytes reaching up to 130 μm in diameter leave the ovary, their germinal vesicle is unresorbed and has a nucleolus. They are coated by a yolk membrane of 1.2 μm. The eggs enter the oviduct, move along, and accumulate in its expanded anterior portion, the ovisack. The sperm is released in seawater as sperm packages, each having the shape of a torch. Then sperm moves to females and sperm packages at the posterior end of the oviduct surrounding of eggs. Inside the female tube, spermatozoa and, possibly, yet unsplit sperm packages, invade the oviducts through genital openings, where the unfertilized eggs are already present in the terminal portion of the eggsack.
Highlights
External-internal insemination by means of the spermatozeugmata occurs in nature enough often as in vertebrates [1,2] as in invertebrates [3,4,5]
Various aspects of the reproduction and development of Riftia have earlier been studied on specimens sampled in vent sites of the Galapagos Rift [17] and the East Pacific Rise [18,21,22]
Our observations of the gamete morphology and insemination of Riftia mostly agree with the data of other authors who studied the gametes of Riftia [17,18,21,22]
Summary
External-internal insemination by means of the spermatozeugmata occurs in nature enough often as in vertebrates [1,2] as in invertebrates [3,4,5]. Pogonophorans and vestimentiferans, or Siboglinids (Annelida) in modern taxonomy [9,10,11]—deep-sea worms that have chitinous tubes and inhabit reducing biotopes such as hydrocarbon seeps, hydrothermal vents, etc.— play an important role in many marine ecosystems. They have long attracted zoologists, with particular attention to their feeding and reproduction. An electron microscopic study of sperm has been carried out on the pogonophoran Siboglinum ermani [16] and on the vestimentiferans Riftia pachyptila [17,18], Lamellibrachia luymesi, and Ridgeia piscesae [18,19]
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