Sperm Storage, Internal Fertilization, and Embryonic Dispersal in Vent and Seep Tubeworms (Polychaeta: Siboglinidae: Vestimentifera)

2005·4.1 MB·

by  Ana Hilario

Reference: Bwl. Bull. 208: 20-28. (February 2005)
2005 Marine Biological Laboratory
Sperm Storage, Internal Fertilization, and Embryonic
Dispersal in Vent and Seep Tubeworms (Polychaeta:
Siboglinidae: Vestimentifera)
ANA HILARIO1. CRAIG M. YOUNG2'*, AND PAUL A. TYLER1
1 School ofOcean and Earth Science. University ofSouthampton. SOC. European Way, Southampton
SO14 3ZH UK: and 2 Oregon Institute ofMarine Biology. University ofOregon. P.O. Bo.\ 5389,
Charleston, Oregon 97420
Abstract. Vestimentiferan tuheworms are ecologically polychaetes (Young etal.. 1996; McHugh, 1997; Rouse and
important members of deep-sea chemosynthetic communi- Fauchald, 1997) currently classified within the polychaete
ties, including hydrothermal vents and cold seeps. Some are family Siboglinidae (Rouse and Fauchald. 1997), which
community dominants and others are primary colonists of includes the frenulate, vestimentiferan, and moniliferan
new vent sites; they include some of the longest living and Pogonophora of previous authors.
fastest growing marine invertebrates. Their mechanisms of Vestimentiferan tubeworms came to symbolize hydro-
propagation, dispersal, and genetic exchange have been thermal vents in the Pacific, and with theirdiscovery atcold
widely discussed. Direct sperm transfer from males to seeps in the GulfofMexico and elsewhere, became the icon
females has been documented in one species, Ridgeia of communities driven by chemosynthetic primary produc-
piscesae, but others are known to discharge what are ap- tion. Species ofvestimentiferans are known tobe among the
parently primary oocytes. Brooding of embryos has never first colonizers ofnewly formed vents (Hessler et al.. 1988;
been observed in any vestimentiferan. These observations Shank et al.. 1997), some of the fastest growing marine
have led to the supposition that fertilization might be exter- invertebrates (Lutz et al.. 1994), and the longest lived non-
nal in most species. Here we report sperm storage at the clonal animals known (Fisheretal.. 1997). Because oftheir
posterior end ofthe oviduct in five species, including tube- ecological importance at vents and seeps, there has been
worms from both vents and seeps. We show experimentally considerable interest in their reproductive and dispersal
that most eggs are inseminated internally, that fertilization biology.
rate is typically lower than 100%, that meiosis is completed In vestimentiferan tubeworms, small, yolky, and slightly
after eggs are released from the female, and that the dis- buoyant eggs develop into nonfeeding trochophore larvae
persal phase includes the entire embryonic period. that are thought to disperse in the plankton forup to several
weeks (Young et al.. 1996; Marsh et al., 2001). facilitating
Introduction genetic exchange and colonization of newly available seep
and vent habitats. It has been estimated that larvae of the
The discovery ofhydrothermal vents in 1977 heralded the vent species Riftia pachyptila can disperse more than 100
description of large vestimentiferan tubeworms that were km overa 5-week period (Marsh etal.. 2001). This estimate
initially placed as a class in the phylum Pogonophora is based on the assumption that passive dispersal occurs
(Jones, 1981 ) and then elevated to the phylum level (Jones, during a 3-week embryonic period as well as a 2-week
1985). Genetic and embryological evidence now shows period when larvae are capable of independent ciliary
convincingly that these gutless, deep-sea tubeworms are movement (Marsh et al., 2001). However, because the site
of fertilization and the location of embryogenesis remain
unresolved for all species of vestimentiferans, these dis-
R*eTcoeivwehdo5mOccotrorbeesrpo2n0d0e4n;ceacscheoputledd 3b0e Naoddvreemssbeedr.2E0-0m4a.il: cmyoung persal estimates could be as much as 60% too high. Thus,
uoregon.edu the site offertilization in vestimentiferans is ofgreat interest
20
INTERNAL FERTILIZATION IN TUBEWORMS 21
and has been the subject of considerable debate. Internal ciliary tracts (Fig. 1A. B) leading away from the male
fertilization and brooding ofembryos appear to be the norm gonopores on the dorsal surface of the vestimentum
for frenulate siboglinids (Bakke, 1974: reviewed by South- (Gardiner and Jones. 1993). However, massive spermato-
ward. 1999). zeugmata are known only in Ridgeia piscesae (Southward
Free sperm have been observed in the oviduct of Riftin and Coates, 1989; MacDonald et /.. 2002) and Tevnia
pachyptila (Gardiner and Jones, 1985). a species that ap- jerichonana (Southward. 1999); in other species, sperm
parently spawns its eggs (Van Dover. 1994). Moreover, appear to be released in smaller flame-shaped bundles (Fig.
transfer of large white spermatozeugmata (sperm masses) 1C, D) that are capable of swimming as coordinated units
from male to female plumes has been documented in and that eventually break down in seawater (Gardiner and
Ridgeiapiscesae (Southward and Coates. 1989; MacDonald Jones, 1985; Gary et ai, 1989). The observations of modi-
et<;/.. 2002) and Tevniajerichonana (Southward, 1999). On fied sperm anddirect spermtransferare difficulttoreconcile
the basis of these observations. Southward (1999) has pro- with numerous field and laboratory observations ofapparent
posed that eggs are fertilized either in the ovisacjust before free spawning in both Riftiapachyptila and Lamellibrachia
spawning or externally as the eggs are released. Internal lityntesi (Gary et al.. 1989; Van Dover, 1994; Young, un-
fertilization is consistent with the unusual sperm morphol- publ. obs.). However, the clouds of presumed gametes ob-
ogy (sperm are elongate and have spiral mitochondria and served in these "spawning events" could consist ofzygotes,
nuclei) in all known species (Gardiner and Jones, 1985; embryos, or even larvae if fertilization is internal.
Gary et al.. 1989), and also with the presence of external Using histology, we have examined the female reproduc-
Figure 1. (A) Dorsal viewofthe vestimentum ofa Lamellibrachia luymesimale, showing genital grooves
leading away frompairedgonopores and diverging nearthe anteriorend(scalebar: 5 mm). (B) Dorsal view of
thevestimentuminaRiftiapachyptilamale,showinggenitalgroovesconverging(scalebar: 1 cm). (C)Actively
swimming sperm bundlesofLamellibrachia luymesidissected from theterminal portion ofthe male gonoduct.
Some ofthe bundles have broken apart upon contact with seawater to release individual sperm (scale bar: 25
fim). (D) Sperm bundles of Tevniajerichonana (scale bar: 25 ;u,m). Gg. genital groove; Gp, gonopores; S,
spermatozoa; SB, sperm bundles; V. vestimentum.
A. HILARIO ET AL
tive system offive vestimentiferan siboglinid species: Riftia studied and on many occasions to assess the gametogenic
pachyptila, Ridgeia piscesae, and Tevniajerichonana from stage just prior to spawning. Immediately after they were
Pacific hydrothermal vents, andLamellibrachia luymesiand removed, eggs were examined with a light microscope for
Seepiophilajonesifromcold seeps in the GulfofMexico. In the presence of visible germinal vesicles.
all five species we found a sperm storage region at the far Oocytes were collected from the ovisacs ofseven female
posterior end of the female reproductive tract. We refer to specimens ofLamellibrachia Invinesi and incubated without
We
this region as the spermatheca. also report in vitro adding sperm to determine whether they had already been
fertilization and field experiments showing that insemina- inseminated internally. New pipettes were used, taking spe-
tion is internal, that meiosis is completed after the eggs are cial care to eliminate any possible contamination from ex-
released from the female, and that the dispersal phase of traneous sperm. These primary oocytes were incubated in
vestimentiferans includes the entire embryonic period. 0.45-/u,m-filtered seawater for 24 h at 6 C, after which at
least 100 were examined from each individual.
Materials and Methods To determine if tubeworms release eggs, zygotes, or
embryos under natural conditions, we deployed inverted
Samples of RiftHI pachvptila, Ridgeia piscesae, and plankton nets over aggregations of Riftia pachyptila adults
Tevniajerichonana were collected from a depth of 2500 m in the field (Fig. 2). Nets were 0.5 m in diameter and had
at hydrothermal vents on the East Pacific Rise (950'N), 100-/j,m mesh. Disks of syntactic foam attached to the cod
using the deep submergence vehicle Alvin. Lamellibrachia ends floated the nets upward, and polypropylene funnels at
liivniesi and Seepiophila jonesi were collected with Johnson- the mouths ofthe cod ends prevented escape ofthe buoyant
Sea-Link submersibles from various cold methane seeps eggs during recovery. Nets were deployed for various pe-
m
between depths of 600 and 800 on the Louisiana Slope. riods of time ranging from 3 to 10 days. They were recov-
Major sites included GC-234 (2744.7'N, 9113.4'W) and ered by using the submersible's manipulatorandtransported
Bush Hill (2746.9'N. 9130.5'W). to the surface in closed plastic boxes to maintain tempera-
For histological studies, the worms were removed from ture. Immediately after the dive, eggs and embryos were
their natural tubes, preserved in 5% seawater formalin for concentrated with nylon mesh filters and examined with a
48 h, and subsequently transferred to 70% ethanol. The compound microscope to assess stages ofgametogenesis or
trunk ofeach individual was divided into 10 equal regions, emhryogenesis.
mm
and a single section 1 in length was randomly chosen In broadcast spawners with external fertilization, fertili-
from each region. The segments were slowly dehydrated by zation rates virtually always decline as a function of dis-
transfer to 90% propan-2-ol overnight followed by a period tance from spawning males (e.g., Pennington, 1985; Levi-
of 9 h in 100% propan-2-ol, with change of solution every tan, 1991; Levitan and Young, 1995; Metaxas et ai. 2002).
3 h. Before being impregnated with paraffin wax at 70 C We conducted a field experiment with Lamellibrachia
for 12 to 24 h, the segments were cleared with 100% xylene hivniesi eggs to determine whether distance from a dense
for 6 to 12 h, depending on the size of the segment. The aggregation ("bush") of tubeworms had any effect on the
impregnated tissue was then embedded in wax, sectioned at fertilization rate of oocytes removed from the ovisacs of
5 jam, and stained with Mayer's hematoxylin and eosin. females. Eggs were held 25 cm above the bottom in small
For electron microscopy, pieces of gonadal tissue dis- plastic tubes (2-cm diameter, 2-cm length) capped on both
sected from female Lamellibrachia Iimiiexi were immersed ends with 50-/nm nylon mesh. Three were deployed by
for 1.5 h at room temperature in 2.5% glutaraldehyde buff- submersible in a large bush of tubeworms, and the others
M
ered with Millonig's 0.4 phosphate bufferat pH 7.4. Due were deployed in a straight line with three replicates each at
to exigencieMs of ship scheduling, tissueMwas stored in Mil- 1.5 m, 3.0 m, 4.5 m, and 6.0 m downstream. The cultures
lonig's0.4 phosphate bufferwith 0.6 NaCl for2 weeks were left to incubate for 24 h. As in the laboratory experi-
prior to post-fixation and embedding. Tissue was post-fixed ments mentioned above, we took special care to sterilize all
Mfor 1 h at room temperature in 1% osmium tetroxide in 0.1 containers, including the acrylic plastic box in which eggs
phosphate buffer plus 1% NaCl at pH 7.2. Tissue was were transported to the bottom, to eliminate spurious fertil-
then dehydrated in ascending concentrations of ethanol to ization events.
100%, immersed in acetonitrile for 10 min, left overnight in
a 50%' solution of acetonitrile in resin, and embedded in Results
epoxy resin. Thin sections were stained with uranyl acetate
and lead citrate and examined with a Hitachi H7000 trans- General anatom\ ofthefemale reproductive trad
mission electron microscope. The reproductive system of female vestimentiferans
Eggs of some vestimentiferans are stored in expanded opens at the anterior end of the trunk, between the vesti-
distal ovisacs prior to release. We collected eggs from the menlal wings. A pair of meandering oviducts extend
ovisac or the distal region of the oviduct in all species through the trunk, surrounded by trophosome tissue.
INTERNAL FKRTILI/.ATION IN TUBEWORMS 23
Figure 2. (A) Two egg traps deployed in a large aggregation ofRiftitt pachyptila at 950'N on the East
Pacific Rise. Depth is 2500 m. The downward-facing net openings are 50cm in diameter. (B) Close-up ofan
eggtrapdeployedoveraclumpofR.pachyptila(scalebar: 10cm). SF,syntacticfoam;F,positionofthefunnel
on the inside ofthecodend. (C) Light micrograph ofa primary oocyte removed from the female ovisac ofR.
pach\ptilaandartificially inseminatedwithfreesperm.Severalfilament-likespermheadsareattachedtotheegg
surface(scalebar: 25/am).GV.germinal vesicle;S,sperm.(D)TypicaluncleavedR.pachyptila/ygotecaptured
in an egg trap (scale bar: 25 /am).
Whereas in Riftin pach\ptila, Ridgeia pisceaxe, and Tevniu vealed no openings that would permit the passage of oo-
jericlioncuui the gonad runs through the whole length ofthe cytes from the gonocoels to the oviducts. Thus, all eggs
trunk, in Lawellihrucliici luymesi and Seepiophilu joncsi it must pass through the posterior ends of their respective
can only be found in the anterior two-thirds of the trunk. gonocoels before entering the oviducts.
The paired gonoiroels, which contain the ovaries, run par-
allel and dorsal to the oviducts. At the posterior end. the The spermatheca
gonocoels bend and pass anteriorly as paired oviducts. In
Riftici pcichyptilu, Riilgeia piscesae, and Lamellibrachia Through most of its length, the wall of the oviduct is
luymesi. the terminal portion ofeach oviduct is enlarged as composed ofaciliated cuboidal epithelium surrounded by a
an egg storage compartment known as the ovisac. thin layer of circular muscle. However, in all five species
A sheet ofconnective tissue separates the gonocoels from examined, the oviducal epithelium is folded into a series of
each otherand from the ventral blood vessel. A narrow strip loops and sacs at the far posterior end of the reproductive
of germinal epithelium grows from this connective tissue tract, where the ovarian gonocoeljoins the oviduct (Fig. 3).
sheet into each gonocoel, filling the gonocoel cavity with In every female examined, these small sacs contained clus-
rows ofdeveloping eggs. Webb (1977) described structures ters of spermatozoa, with the heads all aligned towards the
called "transverses," situated at irregular intervals along the wall of the sac (Fig. 3A-D). These sperm are no longer
length ofthe trunk, where the gonocoel, the oviduct, and the packaged in the discrete flame-shaped bundles released by
We
ventral blood vessel cross from right to left, and where the the males (GardinerandJones, 1985; Gary etat., 1989).
oviduct opens into the gonocoel by a nonciliated funnel. refer to this region ofsperm storage as the spermatheca. In
These structures were also reported by Malakhov ct ul. Laniellihrachia luymesi and Seepiophilu jonesi the sper-
(1996) in Riilgeia piscesae. Although we have observed matheca appears as a white, hook-shaped structure easily
transverses in this same species, histological sections re- visible with the naked eye through the body wall of the
24 A. HILARIO ET AL
Figure 3. (A-D) Light micrographsofhistologicai sections through the spermathecaregionsoffemalesof
fourspecies: RiftinI'lichvpliUi (A), Tevninienchoiunui (B),Laincllihnichin limui'si(C). andSeepiophilajonesi
(D).TheinsertinAshowsdetailofthespermathecaofRiftiu/yuchy/itilnwithclustersofspermato/oa. Scalebars
forA-D: 200 jj.m. (E) SpermathecaofLuiiii-llihruchin liiynu^i as seen through the wall ofthe body (scale bar:
I mini. (I-) Squash preparation of the spermatheca from a Lamellibrachia luymesi female, showing isolated
sperm in the lumen lyingclose to primary oocytes. Scalebar: 75 ,um. (G)Transmission electron micrographof
thespermathecaofLaiiielli/i/'cichiciluvmesi. showinganoocyteandseveral spermato/.oainproximity(scalebar:
0.5 jum). EE. egg envelope; G. gonad; Gc. gonocoel; Gr. granule with filamentous glycocalyx; Mv, microvilli;
PO, primary oocyte; S, clusters ofspermatozoa, Sp. sperm: St. spermatheca; Tr. trophosome; Y, yolk granule.
INTERNAL FERTILIZATION IN TUBEWORMS 25
Table 1
Percentage ofRiftia pachyptila eggs, zygotes, andembryos thathadnot
cleavedby the lime they were recoveredin invertedplankton nets (egg
traps) deployedoveradultaggregations
26 A. HILARIO ET A/-
Table 3
Percentage of Lamellibrachia lumesi f/H/vw.v undergoing cleavage at
1 atm in the laboratory and in mesh-covered vials outplantedto thefield
ut a depth of700 m
Embryos Cleaving
INTERNAL FERTILIZATION IN TUBEWORMS 27
gonoduct. In eggs of other polychaetes. granulated glyco- Cathy Allen. Shawn Arellano. Adam Marsh. Alison Green,
calyces similar to the one found in oocytes collected from Doug Pace and many others helped develop tubeworm
the spermatheca of Lamellibrachia luvmesi are known to culture techniques and assisted with field work on various
We
possess sperm receptors (Eckelbarger, 1992). hypothe- cruises.
size that as primary oocytes pass through the spermatheca,
sperm bind to the primary oocytes. which are arrested at the Literature Cited
first meiotic prophase until they are released by the female. Bakke. T. 1974. Settling of the larvae of Siboglinum (jordicum Webb
This hypothesis is supported by electron micrographs (Fig. IPogonophora) in the laboratory. Sarxia 56: 57-70.
3G) showing sperm heads in direct contact with the glyco- Gary,S.C.,H. Felbeck,and N.D. Holland. 1989. Observationson the
calyces of primary oocytes in the spermatheca. Westheide reproductivebiologyofthehydrothermal venttubewormRiftiapachy-
(1988) has categorized the sperm storage organs of ptila. Mar. Ecol. Pm.>>. Ser. 52: 89-94.
polychaetes into three major types. The spermathecae of Eckeclrboascrogpeirc,AKn.atJ.om1y99o2f.InvPeorlteybcrhaateetsa,:VOoolg.en7e.sFi.s.W.PpH.ar1r0i9s-on12a7ndinS.MiL-.
vestimentiferans, which are outpocketings of the oviduct, Gardiner, eds. Wiley-Liss. New York.
fall clearly into his Type 2a classification. Fisher,C.R..I.A.Urcuyo,M.A.Simpkins,andE.Nix. 1997. Lifein
The breeding strategy ofinternal fertilization followed by the slow lane: growth and longevity of cold-seep vestimentiferans.
zygote release rather than brooding assures a high level of Mar. Ecol. 18: 83-94.
fertilization without sacrificing dispersal potential. More- Gardesiinserin,tSh.eLv.e,stainmdenMti.feLr.anJotnuebse.w1o9r8m5.RifUiliatrpaascthruycpttuirleao(fPosgpoenrompihoogrean:-
over, sperm storage is an ideal strategy in an environment Obturata). Trans. Am. Microsc. SM: 104: 19-44.
where periodic cues for gametogenesis and spawning syn- Gardiner,S.L.,andM.L.Jones. 1993. Vestimentifera.Pp.371-460in
chrony are limited (Young, 1999). Indeed, hydrothermal Microscopic Anatomy ofInvertebrates. Vol. 12. F. W. Harrison and
vent polychaetes from many different families are now M. E. Rice, eds. Wiley-Liss. New York.
known to possess specialized organs for sperm storage (Zal Gardoibsneerrv,atSi.onLs.,oSn.oEo.geSnhersaidserin,tahnedtuMb.e Lw.orJomneRsi.fti1a99p2a.chypPtriellaimJionnaersy
et at.. 1995; Jollivet et al.. 2000). (Vestimentifera). Am. Zool. 32: 124A.
Not all oocytes taken from the ovisacs of female vesti- Hessler, R. R., W. M. Smithey, M. A. Boudrias, C. H. Keller, R. A.
mentiferans undergo development, an observation that sug- Lutz, and J. J. Childress. 1988. Temporal change in megafauna at
gests imperfect internal fertilization. However, the number the Rose Garden hydrothermal vent (Galapagos Rift; eastern tropical
of embryos undergoing development is often higher for JolliPvaecitf,icD)...DeAe.p-ESmepahiRse,x.M3.5:C1.6XBIa-kIe7r0.9.S. Hourdez, T. Comtet, C.
cultures reared in the field than in the laboratory (Tables 1, Jouin-Toulmond, D. Debruyeres, and P. A. Tyler. 2000. Repro-
2). This raises the possibility that the percentage of eggs ductive biology, sexual dimorphism, and population structure of the
developing is not the same as the percentage fertilized; thus, deep sea hydrothermal vent scale-worm, Branchipolynoe seepensis
observed rates of development lower than 100% may be (Polychaete: Polynoidae). J. Mar. Biol. Assoc. UK80: 55-68.
caused by culture conditions ratherthan fertilization failure. Jonemse,ntMi.ferLa.n1w9o81r.m fRriofmtiathpeacGhaylpatpialgaosJonRiefst:. oSbcsieernvcaeti2o1n3s:o3n33t-h3e36v.esti-
Previous estimates ofdispersal times and distances based Jones,M.L. 1985. OntheVestimentifera, newphylum: sixnewspecies,
on lipid stores, larval metabolism, and current speeds andothertaxa.fromhydrothermalventsandelsewhere.Bull. Biol.Soc.
(Marsh et al.. 2001 ) have assumed external fertilization and Wash. 6: 117-158.
dispersal during the entire embryonic period. Even though Jones, M. L., and S. L. Gardiner. 1985. Light and scanning electron
the presumed location offertilization was clearly erroneous wmiocrrmosRciofptiicapsatcuhdyipetsiloaf(sPpoegnonnaotpohgoernae:siOsbtuinrattah)e.vTersatnism.enAtmi.feMriancrotsucb.e
in these studies, the dispersal estimates themselves remain Soc. 104: 1-18.
valid, since embryogenesis does not begin until after insem- Jones, M. L., and S. L. Gardiner. 1989. On the early development of
inated oocytes are released into the water column. the vestimentiferan tube worm Ridgeia sp. and observations on the
nervous system and trophosome ofRidgeia sp. and Riftia pachvptila.
Biol. Bull. 177: 254-276.
Acknowledgments Levitan, D. R. 1991. Influence ofbody size and population density on
fertilization success and reproductiveoutput in a free-spawning inver-
This work was supported by NSF grants OCE-9619606 tebrate. Biol. Bull. 181: 261-268.
and OCE-01 18733, NOAA/NURP UNCW grant NA96RU- Levitan, D. R.,and C. M. Young. 1995. Reproductive success in large
0260, FCT grant SFRH/BD/7084/2001, and submersible populations: empirical measures and theoretical predictions of fertili-
funding from the NOAA Office ofOcean Exploration. The 9z0a:tio2n21i-n2t4h1e.sea biscuit Clypeasterrosaceus. J. Exp. Mar. Biol. Ecol.
NSF grant that supported work on Riftia pachyptila was a Lutz. R. A.,T. M. Shank, D. J. Fornari, R. M. Haymon, M. D. Lilley.
collaborative effort that also included grants and cruises led K. L. Von D.I11ini. and D. Desbruyeres. 1994. Rapid growth at
by Donal Manahan and Lauren Mullineaux. Chuck Fisher deep-sea vents. Nature 371: 663-664.
provided samples of Ridgeia. We thank the Biomedical MacDonald,I. R., V.Tunnicliffe,and E.C.Southward. 2002. Detec-
Imaging Unit, University of Southampton, for technical tationEnofdesapveorumrtrSaengsfmeernta,ndJsuyannchdreonoFuuscaferRtiidlgizea.tioCnahin.RiBidogle.iaMpairs.ce4s3a:e
help in transmission electron microscopy. Sandra Brooke, 395-398.
Elsa Vazquez, Ana Metaxas, Tracey Griffin, Jim Welch. Malakhov,V.V.,I.S.Popelyaev,andS.V.Galkin. 1996. Microscopic
28 A. HILARIO ET AL.
anatomyofRidgeiaphaeophialeJones. 1985(Pogonophora,Vestimen- transfer in a vestimentiferan. Ridgeia piscesae Jones, 1985 (Pogono-
tifera) and the problemofthe position ofVestimentifera in the system phora: Obturata). Can. J. Zoo/. 67: 2776-2781.
ofthe Animal Kingdom. IV. Excretory and reproductive systems and Thorson, G. 1950. Reproductive and larval ecology of marine bottom
coelom. Russ. J. Mar. Biol. 22: 249-260. invertebrates. Biol. Rev. 25: 1-45.
Marsh, A. G., L. S. Mullineaux, C. M. Young, and D. T. Manahan. VanderLand,J.,andA.Norrevang. 1977. Structureandrelationships
2001. Larval dispersal potential ofthe tubewormRiftiapachvptilaat ofLamellibranchia(Annelida.Vestimentifera).K.Dan. Vidensk.Selsk.
deep-sea hydrothermal vents. Nature 411: 77-80. Bio!. Skr. 21(3): 1-102.
McHugh, D. 1997. Molecular evidence that echiurans and pogono- Van Dover, C. L. 1994. In sun spawning of hydrothermal vent tube-
phorans are derived annelids. Proc. Null. Acad. Sci. USA 94: 8006- worms (Riftiapachvptila). Biol. Bull. 186: 134-135.
8009.
Metaxas, A., R.E. Scheibling and C.M. Young. 2002. Estimating fer- WebIbI, TMh.e r1e9p7r7o.ducSttiuvdeieosrgaonns.LZaomoe/l.liJbarharbc.hiAanabta.rBhda.mi97:(P4o5g5o-n4o8ph1o.ra)
tt8ir7lo-ip1zia0cta1il.onsseuaccsetsasriOnrmeaarsitneerbreenttihciuc/aitnusv.ertMeabrra.tesE:coal.casPerosgt.udySerw.ith22t6h:e WestthheeiPdoel,ycWha.et1a9.88W..WeGesntihteaildeoragnands.C.POp..H2e6r3m-a2n7s9,eidns.UlGtursatsatrvucFtisucrheero,f
Penntihengctoonns,eqJ.ueTn.ce1s98o5f.spTehrme edicloultoigony,oafdufletrtialgigzraetgioantioofn,ecahnidnosiydncehgrgos:- YouSntgu,ttgCar.t.M. 1999. Synchrony and sociality: breeding strategies in
nous spawning. Biol. Bull. 169: 417-430. constant and variable environments. Pp. 1-4 in Aquatic Life Cycles
Rouse.G.W.,andK.Fauchald. 1997. Cladisticsandpolychaetes.Zoo/. Strategies: Sun'ival in a Variable Environment, M. Whitfield, J. Mat-
Scr. 26: 139-204. thews, and C. Reynolds, eds. Mar. Biol. Assoc.. Plymouth, UK.
Shank, T. M., D. J. Foranri, K. L. Von Damm, M. D. Lilley, R. M. Young, C. M., E. Vasquez, A. Metaxas, and P. A. Tyler. 1996. Em-
Haymon, and R. A. Lutz. 1997. Temporal and spatial patterns of bryologyofvestimentiferantubewormsfromdeep-seamethane/sulfide
biological community development at nascent deep-sea hydrothermal seeps. Nature 381: 514-516.
vents (9 50'N. East Pacific Rise). Deep-Sea Res. II45: 465-515. Zal, F., P. Chevaldonne, and D. Debruyeres. 1995. Reproductive bi-
Southward, E.G. 1999. Development of Perviata and Vestimentifera ologyandpopulationstructureofthedeep-seahydrothermalventworm
(Pogonophora). Hydrobiologia 402: 185-202. Paralvinella grasslei (Polychaeta: Alvinellidae) at 13N on the East
Southward, E. C., and K. A. Coates. 1989. Sperm masses and sperm Pacific Rise. Mar. Biol. 122: 637-648.