Light and Scanning Electron Microscopic Studies of Spermatogenesis in the Vestimentiferan Tube Worm Riftia pachyptila (Pogonophora: Obturata)

Abstract

The process of spermatogenesis was studied in the obturate pogonophoran Riftia pachyptila from hydrothermal vents on the Galapagos Rift. Early divisions of presumed spermatogonia lead to the formation of curious nuclei that are found in the incipient cytophoric mass. An increase in numbers of these nuclei may be due to unobserved nuclear divisions, to nuclear migration into the cytophoric mass, or to deposition of these nuclei in the cytophore. Subsequent to a peripheral disposition of the crescentic nuclei in the cytophore, a partial cytokinesis results in the formation of grapelike clusters of apparent spermatocytes, connected centrally by cytoplasmic bridges to the cytophore. An increase in numbers of connected spermatocytes culminates in as many as 1,000-2,000 slender, helically coiled spermatids per cytophore. Later development into mature spermatozoa is accompanied by a loss of the cytophore and the grouping of from 300-700 spermatozoa each in sperm bundles. No spermatophores are formed by Riftia pachyptila. The time and site of the reductional meiotic division has not been determined. Later spermatogenesis in Riftia pachyptila appears to be similar to that of the perviate Pogonophora, but earlier stages appear to be at variance with those of the perviates. Recently, Jones (1981) divided the phylum Pogonophora into two subphyla, the Obturata and Perviata. The subphylum Obturata presently includes three described species, namely Lamellibrachia barhami Webb, 1969, L. luymesi 1We thank L. J. Cullen, of J. C. Harshbarger's Registry of Tumors of Lower Animals, for assistance in certain histological procedures, M. R. Carpenter, Department of Invertebrate Zoology, for assistance in some photographic processing, and W. R. Brown and M. J. Mann, Scanning Microscope Laboratory, for the operation of their equipment, all three of these units being in the National Museum of Natural History; R. M. Rieger, University of North Carolina, for darkroom facilities and supplies (NSF Grant DEB 8119652). We further thank Drs. K. Fauchald and W. D. Hope, National Museum of Natural History, for their stimulating discussions and for their reviews of this manuscript. 2 Contribution No. 39 of the Galapagos Rift Biology Expedition, supported by the National Science Foundation. 3 Present address: Department of Biology, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010,