Genetic Relationships (RAPD-PCR) Between Geographically Separated Populations of the "Cosmopolitan" Interstitial Polychaete Hesionides gohari (Hesionidae) and the Evolutionary Origin of the Freshwater Species Hesionides riegerorum

11 Pages·1999·4.4 MB·English

by  H Schmidt

Reference: Binl. Bull. 196: 216-226. (April 1999)
Genetic (RAPD-PCR) Between
Relationships
Geographically Separated Populations of the
"Cosmopolitan" Interstitial Polychaete Hesionides
gohari (Hesionidae) and the Evolutionary Origin of the
Freshwater Hesionides
Species riegerorum
HARTMUT SCHMIDT AND WILFRIED WESTHEIDE*
Spezielle Zoologie, Biology/Chemistry, University ofOsnahriick. 49069 Osnabriick, Germany
Abstract. In an analysis of the population genetics ofthe Introduction
tiny meiofaunal polychaete Hesionides gohari. the RAPD-
PCR method was applied to 49 specimens from 7 collecting The many species of Hesionides Friedrich, 1937, are
sites far apart on three continents: French Atlantic coast, highly characteristic meiofaunal polychaetes with a great
Mediterranean (Majorca, Giglio, Crete), Red Sea, Indian variety of morphological, reproductive, and behavioral ad-
Ocean (Phuket), and U.S. Atlantic coast (Florida). In the aptations to the habitat of interstitial crevices in marine
band patterns produced with 14 arbitrary decamer primers, surf-beaten sand beaches (Westheide, 1967, 1971, 1984).
496 genetic characters were detected. Genetic distances Several of the Hesionides species are known to be charac-
between the H. gohari populations vary between 0.55 and terized by another pecularity of the marine interstitial
0.70. The data were evaluated by three cluster programs; in fauna a broad geographic distribution, comprising inter-
the almost congruent phenograms, three clades were found tidal localities throughout the world (Westheide, 1971: Ster-
rer. 1973; Giere, 1993). Within the genus, H. arenaria
with high bootstrap values: ( 1 ) European Atlantic-Mediter- Friedrich, 1937, and H. gohari Hartmann-Schroder, 1960,
ranean-Red Sea. (2) Indian Ocean, (3) Western Atlantic. In
appear to have the most cosmopolitan distribution (West-
all clusteranalyses. Hesionides riegerorum from aU.S. east heide, 1977); they occuron allcontinents except Antarctica.
coast river system is shown as genetically nearest to the H. gohari wasdiscovered by AdolfRemane in littoral sands
Florida specimens of H. gohari, making it most probable near Ghardaqa (today: Hurghada) in the Red Sea (Remane
that this freshwater species of the genus originated from a and Schulz, 1964); it was described by Hartmann-Schroder
Western Atlantic H. gohari population. The genetic dis- (1960). Westheide (1970, 1972a, 1972b) investigated the
tances detected between the H. gohari specimens from the morphology, reproductive biology, and local distribution
threecontinents are almost identical to those found between pattern of populations occurring in beaches of northern
morphologically similarinterstitial polychaete species pairs. Tunisia (Mediterranean Sea).
Thus, the degree ofgenetic consistency is considered not to This tiny species (max. length 1.5 mm) is typical ofwarm
be high enough to corroborate the notion ofa cosmopolitan seas and has a range extending as far north as Arcachon, on
distribution pattern, but rather suggests that the three clades the Atlantic coast of France (Westheide. 1972/73). New
represent different species. localities for the species are still being recorded worldwide
(Hartmann-Schroder, 1991; Westheide. 1992). Its most
characteristic diagnostic features are considered to be its
length, the proportions ofthe head appendages, the position
Received 9 September 1998; accepted 29 January 1999. of penis papillae, the dentation of notopodial chaetae, and
*To whom correspondence should he addressed. E-mail: westheide(S' especially the shape of the anal appendages. Among the
mail.biologie.uni-osnabrueck.de other Hesionides species of comparable size, one inhabits
216
POPULATION GENETICS OF A MEIOFAUNAL POLYCHAETE 217
Figure 1. Records (opencirclesl and sampling sites (solidcircles)forHesionidesgohariand sampling site
for H. riegemrum (triangle).
fresh water, H. riegerorum Westheide, 1979: it can be uals in the seawatercolumn along shores has been observed
distinguished from H. gohari by various morphological and demonstrated experimentally (Hagerman and Rieger,
features. 1981; Palmer, 1988; Armonies. 1989). These observations
The demonstrated almost cosmopolitan distribution of ledtothe proposal that the cosmopolitan distribution pattern
these Hesionides species, together with that of many other is ascribable not togeologically ancient processesbut rather
species of various meiofaunal taxa, gave rise to a still- to occasional contemporary events (Sterrer, 1973: Gerlach.
controversial hypothesis of Sterrer (1973) (see also Rao, 1977) involving long-range dispersal by birds, on drifting
1972) based on the notion that speciation of these forms is material, or in ballast sand or tanks in ships.
extraordinarily slow. It is postulated that they were already Lately, arguments for one or the other view have been
present on an old supercontinent and were distributed over supported by genetic investigations. Todaro el al. (1996)
their present vast range by the drifting apart of the conti- expressed the conviction, based on their restriction-frag-
nental plates. This explanation could apply only if the ment length polymorphism analyses, that the proposed cos-
genetic changes in the separated populations were relatively mopolitan species Xenotrichula intermedia (Gastrotricha),
small overlong periods oftime, and mostly had noeffect on apparently present both in the Mediterranean Sea and along
the phenotype. Such minimal genetic variability can be the coast of North America (Ruppert, 1977), consists of
explained by assuming a complete constancy ofthe ecolog- more than one taxon. Similarly, with RAPD-PCR (random
ical factors in the habitat ofthese animals, the sand beaches. applied polymorphic DNA-polymerase chain reaction)
Sterrer's hypothesis appeared necessary because long- Soosten et til. (1998) found genetic differences between
range, transoceanic dispersal ofthese meiofaunal organisms specimens of the polychaete Petitia amphophthalma from
seemed inconceivable: they are not capable ofactive swim- Europe and North America, although these differences are
ming and, with few exceptions, have no larval dispersal considerably smaller than those between certain morpho-
stages. logically distinct species ofother taxa. In the present study
However, it gradually became evident that, even without of Hesionides gohari collected from seven intertidal local-
dispersal stages, many of these species do succeed in colo- ities on three continents (Europe, North America, Southeast
nizing geologically young islands farfrom any coast (West- Asia; Fig. 1), the RAPD-PCR method was likewise chosen,
heide. 1991): furthermore, dispersal of meiofaunal individ- to evaluate the results obtained for P. amphophthalma with
218 H. SCHMIDT AND W. WESTHEIDE
Table I
Collection informationforH. gohari and H. riegerorum
Species
POPULATION GENETICS OF A MEIOFALINAL POLYCHAETE 219
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Figure 2. RAPD fingerprinting ofHesionidesgohari specimens from Phuket (2. 31, Florida (8-11), Crete
(12-151.Giglio(16-18).Arcachon(19-23).andH. riegerorumspecimensfromNorthCarolina(4-7); 1 and24;
100 bp marker. Primer OPB 6.
Technologies, Inc., Berlin, Germany, or Pharmacia Biotech, and Sokal. 1973), and neighbor joining (Saitou and Nei,
Uppsala, Sweden). The reproducibility of the data was 1987) including bootstrap proportions (Felsenstein. 1985).
checked at regular intervals on different levels throughout
all three parallel experiment lines. First, at the beginning of Results
each series the optimal DNA concentration for PCR was
determined for the individual animal, by testing the PCR For the seven sampled populations ofH. gohari and one
with three DNA concentrations per animal (cci. 1 ng, 3 ng, population of H. riegerorum, with 14 different primers a
and 10 ng per 25 jul reaction volume). In pilot experiments total of 496 different DNA fragments were detected, rang-
with larger annelids, it became clear that the results were ing in length from 150 bp to 1900 bp (e.g., Fig. 2).
reproducible with up to 25 ng/25 /u,l reaction volume. With The calculated degree of polymorphism is similarly high
DNA concentrations as high as ca. 50 ng/25 /u.1 reaction in all the populations tested: for Arcachon, 96%; Giglio,
volume, reproducibility was distinctly worse. A further in- 94%; Majorca, 95%; Crete, 94%; Hurghada, 89%; Phuket,
crease in DNA concentration (to over 100 ng/25 ju.1 reaction 89%; Florida, 97%; and for H. riegerorum of North Caro-
volume) brought the PCR to a complete halt, so that no lina, 86%, the lowest value. For none of the European
specific DNA fragments could be detected. Reproducibility populations, nor for the population at the Red Sea, could
DNA
was tested furtherthroughouteach test series by running the diagnostic fragments be detected. However, if the
same experimenttwice in parallel reactions. In addition, for European and Egyptian animals are considered as a single
each series one reaction was carried out with a "blind" group, its members are found to exhibit 33 common poly-
sample lacking DNA, to check the possibility of contami- morphic characters that are absent from the individuals
nation with foreign DNA. We also looked fordifferences in living in Thailand and Florida. With few exceptions, char-
the amplification patterns that might derive from the use of acters present in the animals from Thailand are the same as
different thermocyclers (Biischer et al., 1993; He et al.. those found in the European and Egyptian individuals.
DNA
1994) or preservation methods (drying or deep-freezing), Three diagnostic and five polymorphic fragments
but found none. were observed in the Phuket population only. Only one
The detected amplification product patterns were exam- characteristic polymorphic character was detectable in the
ined visually for monomorphic and polymorphic markers animals from Florida, and none of the primers revealed
(Hadrys et al.. 1992). The degrees of polymorphism are diagnostic DNA fragments. A surprising result was ob-
given in percentages. The banding patterns were then trans- tained by comparing the population of H. riegerorum with
lated into a 0/1-matrix (0 for absence, 1 for presence of a those ofH. gohari. Although this freshwater species can be
specific DNA marker) and fed into the cluster analysis distinguished from the H. gohari populations by 12 diag-
program TREECON 1.2 (Peer and Wachter, 1994), which nostic DNA fragments and 9 polymorphic characters, all
alsotransformed thedataintodistance values in percentages other DNA bands are common to the two groups. There is
(Nei and Li, 1979). Cluster analyses were carried out by an especially close match with the animals of the Florida
UPGMA (unweighted pair-group method using arithmetic population: of the 191 shared characters, 39 polymorphic
averages; Sneath and Sokal, 1973), single linkage (Sneath DNA fragments are present only in these individuals. The
220 H. SCHMIDT AND W. WESTHEIDE
Table III
Comparison ofgeneticdistancesgeneratedfrom RAPDdata (afterNeiandLi, 19791 within andbetiveen the seven Hesionides goharipopulationsand
thf solepopulation ofHesionides riegerorum
Locality
POPULATION GENETICS OF A MEIOFAUNAL POLYCHAETE 221
0.1
98
222 H. SCHMIDT AND W. WESTHEIDE
0.1
94_
94
\
POPULATION GENETICS OF A MEIOFAUNAL POLYCHAETE 223
0.1
224 H. SCHMIDT AND W. WESTHEIDE
(Bochert, 1997), whereas H. gohari is thought to undergo although considerably less than that between the latter and
direct development (Westheide, 1970). Along-shore migra- the gohari individuals from Thailand or Europe. But
tion is presumably a general feature ofthe latter species, but whereas the freshwater species is also clearly distinguish-
the rate ofgeographic expansion achieved by this means is able morphologically (e.g.. bythe different notopodial chae-
unknown. tae. penis papillae, and anal lobes; Westheide, 1979), no
H. gohari is considered to be one of the many cosmo- such diagnostic characters have yet been identified for the
politan interstitial polychaete species (Westheide, 1971; populations classified as H. gohari. The small geographic
Sterrer. 1973: Westheide and Rao. 1977; Riser. 1981; Giere. distance between the new freshwater species and its marine
1993; Soosten et at., 1998). As yet no morphological dis- population of origin has evidently produced morphological
tinctions have been found among individuals from the most character shifts, some of which may act as isolating mech-
diverse ofthe earth's marine regions (see Fig. 1), so that the anisms for instance, the differently positioned and shaped
species must temporarily be regarded as distributed world- penis papillae on the prostomium (see also Westheide,
wide on the coasts of warm seas (Westheide. 1977; Hart- 1984, fig. 8). In contrast, the large geographic distance
mann-Schroder, 1991) provided that a morphological spe- separating the European/Egyptian, Thai, and North Ameri-
cies concept is applied. In the present genetic study, all the can individuals from one another is correlated with genetic
cluster analyses employed show clear differences between distinctions but not with any morphological ones that can
the animals in the regions (1 ) European Atlantic/Mediter- readily be discerned, perhaps because ofthe stability ofthis
ranean/Red Sea. (2) Indian Ocean, and (3) Western Atlantic marine habitat.
(North America). Whether the observed genetic distances To continue to regard all these populations as belonging
reflect species differences that is, reproductive barriers to a single species. H. gohari, is problematic. They would
cannot be decided on the basis of current observations. In then constitute something like a paraphylum (Lorenzen,
another cosmopolitan morphospecies, Petitia amphoph- 1976). with one population (Florida) more closely related to
rlmlina Slewing. 1956 (Syllidae). the distances found by the distinct sibling species (H. riegerorum) than to the other
RAPD analyses between North American and European populations. As soon as possible, therefore, at least the
animals correspond to the values found here: 0.60-0.66 North American "//. gohari" should be given the status ofa
(Soosten et ai, 1998). However, similar distance values separate species. However, we prefer not to undertake its
(after Nei and Li, 1979) of geographically separated but description until the animals on the coast of the United
morphologically differentiated pairs of interstitial poly- States have been further examined in a specific search for
chaete species have been revealed by RAPD analyses as morphological diagnostic characters, so that the practical
well: Nerilla antennata Schmidt, 1848-/V. mediterranea taxonomic work will not be hampered by a species analysis
Schlieper, 1925 (Nerillidae): 0.74-0.77 (Schmidt and based exclusively on genetic tests and on the geographic
Westheide. 1998): Microphthalmus carolinensis West- situation.
heide and Rieger, 1987-A/. nahuntensis Westheide and Regarding Sterrer's (1973) hypothesis, it is irrelevant
Rieger, 1987: 0.77 (unpubl. data). Schirmacher et al. whether these genetically derived clades represent distinct
(1998) found a distance value of only 0.17 between two species or merely populations ofa single, and hence indeed
enchytraeid sibling species. Thus the notion ofH. gohari cosmopolitan, species. Testing this question, however, de-
representing acosmopolitan species will have to be aban- mands a larger number of specimens from different sites
doned. separated by oceans and sequencing techniques.
It is remarkable in this connection that the Florida pop-
ulation, identified as H. gohari, in all analyses formed a
sibling cluster with the freshwater species H. ricgeronmi Acknowledgments
Westheide, 1979. These animals are the nearest neighbors
geographically to the latter species, with recorded finds on Special thanks for the provision ofworking facilities are
the East Coast of the United States in Florida (Westheide. due to Mr. Somsak Chullasorn and Dr. Anuwat Natee-
1995) and North Carolina; that is, they live only about 100 wathana (Phuket Marine Biological Center), Dr. Mary Rice
km away from the habitat of the freshwater species, which (Smithsonian Institution at Harbor Branch, Ft. Pierce. Flor-
so farhas been found only at a site in the sandy bank ofthe ida), Dr. Charles H. Petersen and Hal Summerson (Marine
Chowan River in North Carolina. When H. riegerorum was Science Institute. Morehead City. North Carolina), Dr.
first described, mention was made ofits close morphologi- Claude Cazaux (Laboratoire Marine. Institut Universitaire
cal resemblance to H. gohari (Westheide. 1979). There is de Biologic marine de Bordeaux. Arcachon), and Mrs. Mar-
much evidence that the freshwater form separated from the tina Ubel (Institut fur Marine Biologic. Giglio). Among the
neighboring marine Hesionides population on the seacoast. various people who helped in the collection of the speci-
The genetic distance between H. riegerorum and the marine mens, we especially thankCornelia von Soosten, MonikaC.
Hesionides individuals on the Florida coast is appreciable. Miiller, and Alice Westheide. We gratefully acknowledge
POPULATION GENETICS OF A MF.IOFAUNAL POLYCHAETE 225
preparation and typing of the manuscript hy Mrs. Anna Riser, N. W. 1981. The aberrant polychaete Slygocniiitella from some
Stein and Mrs. Andrea Noel. American beaches. Wasnmnn J. Biol. 38: 10-17.
Ruppert, K. E. 1977. Zoogeography and speciation in marine Gastro-
tncha. in TheMeiofaunaSpeciesin TimeandSpace. W. Sterrerand P.
Ax. eds. Workshop Symposium, Bermuda. 1975. Mikrofauna Meeres-
Literature Cited
hodcn 61: 231-251.
\tmimics. W. 1989. Meiot'aunal emergence from inlertidal sedimenl Saitou, N., and M. Nei. 1987. The neighbourjoining method. A new
measured in the Held: significant contribution to nocturnal planktonic method forreconstructing phylogenetictrees. Mol. Biol. Evol. 4: 406-
hioiiKiss m shallow waters. Helgol. Meeresunters. 43: 29-43. 425.
Baslrop, R., M. Riihner, C. Sturnihauer, and K. Jiirs.s. 1997. Where Schierwater, B. 1995. Arbitrarily amplified DNA in systematic* and
didMarenzelleria spp. (Polychaeta: Spionidae) in Europe come from? phylogenetics. Electrophoresis 16: 1643-1647.
Ai/uat. Ecol. 31: I 14-136. Schirmacher, A., H. Schmidt, and W. Westheide. 1998. RAPD-PCR
Bochert, R. 1997. Maren:elleria riridis (Polychaeta: Spionidae): a re- investigations on sibling species ofterrestrial Enchyirueiis (Annelida:
Mew ofits reproduction. Ai/uat. Ecol. 31: 163-175. Oligochaeta). Binchem. Syst. Ecol. 26: 27-36.
Biischer, N., E. Zyprian, and R. Blaich. 1993. Identification of Schmidt, H., and VV. Westheide. 1998. RAPD-PCR experiments con-
grapewme cultivars by DNA analyses: pitfalls of random amplified firm the distinction between three morphologically similar species of
polymorphicDNAtechniquesusing lOmerprimers. Vitis32: 187-188. Nerilla (Polychaeta: Nerillidae). Zool. An:. 26: 277-285.
Felsenstein, J. 1985. Confidence limits on phylogenies: an approach Sneath,P. H. A., and R. R. Sokal. 1973. Numerical Taxonomy. W. H.
using the bootstrap. Evolution 39: 7778-7783. Freeman, San Francisco.
Gerlach, S. A. 1977. Means ofmeiofauna dispersal. In The Mcinfmuw Soosten, C. von, H. Schmidt, and W. Westheide. 1998. Genetic vari-
Species in Time and Spare. W. Sterrer and P. Ax. eds. Workshop ability and relationships among geographically widely separated pop-
Symposium. Bermuda. 1975. Mikrofauna Meeresboden 61: 89-103. ulationsofPeliliaamphophthalma(Polychaeta: Syllidae):Resultsfrom
Giere, O. 1993. \leiobenihology. The Microscopic Fauna in Aauaiu RAPD-PCR investigations. Mar. Biol. 131: 659-669.
Sediments. Springer. Berlin. Sterrer, W. 1973. Plate tectonics as a mechanism for dispersal and
Hadrys, H., M. Balick, and B. Schierwater. 1992. Application of speciation in interstitial sand fauna. Neth. J. Sea Res. 1: 200-222.
random amplified polymorphic DNA (RAPD) in molecular ecology. Todaro,M. A.,J. W. Fleeger,Y. P. Hu,A. W. Hrincevich,and D. W.
Mol. Ecol. 1: 55-63. Foltz. 1996. Are meiofaunal species cosmopolitan'.' Morphological
Hagerman, G. M., and R. M. Rieger. 1981. Dispersal of benthic and molecular analysis of Xenolrichula intermedia (Gastrotricha:
meiofaunabywaveandcurrentactioninBogueSound,NorthCarolina. Chaetonotida). Mar. Biol. 125: 735-742.
USA. Puhhl. Srn. Zoo/. Napoli (/. Mar. Ecol.) 2: 245-270. Westheide, W. 1967. Monographic derGattungen Hesionides Friedrich
Hartmann-Schroder. G. 1960. Polychaeten aus dem Roten Meer undMicrophthalmusMecznikow (Polychaeta). Ein Beitrag zuOrgani-
KielerMeeresfurscli. 16: 69-125. sation und Biologic psammobionter Polychaeten. Z. Morph. Tiere 61:
Hartmann-Schriider, G. 1991. Die Polychaeten der subtropisch-tropi- 1-159.
schen bis tropischen Ostkiiste Australiens zwischen Maclean (New Westheide, W. 1970. Zur Organisation, Biologie und Okologie des
South Wales) und Gladstone (Queensland) sowie von Heron Island interstitellen Polychaeten Hesionidesgohari Hartmann-Schroder (He-
(GroBesBarrierRiff).In:G. Hartmann-SchroderandG. Hartmann.Zur sionidae). Mikrofauna Meeresboden 3: 1-37.
Kenntnis des Eulitorals der australischen Kiisten unter besonderer Westheide, W. 1971. Interstitial Polychaeta (excluding Annelida).
Berticksichtigung der Polychaeten und Ostracoden. Teil 16. Min. Smithson. Contrib. Zool. 76: 57-70.
Hambg. ZoolMus. lust. 88: 17-71. Westheide, W. 1972/73. Nouvelles recoltes d'anne'lides interstitielles
He, Q.. M. K. Viljanen, and .1. Mertsola. 1994. Effects ofthermocy- danslesplagessableusesduBassind'Arcachon. VieMilieuSerieA 23:
clers andprimerson the reproducibility ofbanding patterns in random 365-370.
amplified polymorphic DNA analysis. Mol. Cell. Probes 8: 155-160. Westheide, W. 1972a. Raumliche und zeitliche Differenzierungen im
Kocher, T. D., W. K. Thomas, A. Meyer, S. V. Edwards, S. Paabo, Verteilungsmuster der marinen Interstitialfauna. Verh. Dtsch. Zool.
F.X.Villablanca,and A. C. Wilson. 1989. Dynamicsofmitochon- Ges. 1971: 23-32.
dria! DNA evolution in animals amplification and sequencing with Westheide, W. 1972b. La faune des Polychetes et des Archiannelides
conserved primers. Proc. Nail. Acini. Sci. USA 86: 6196-6200. dans les plages sableuse a ressac de la cote mediterraneenne de la
Lorenzen,S. 1976. ZurTheoriederphylogenetischenSystematik. Verh. Tunisie. Bull. lust. Natl. Sci. Tech. Oceanogr. Peclie. Salammbo 2:
Dtsch. Zool. Ges. 1976: 22X 449-46S.
Nei, M.,and W. H. Li. 1979. Mathematical model forstudying genetic- Westheide, W. 1977. The geographic distribution of interstitial
variation in terms of restriction endonucleases. Genetics 76: 5269- polychaetes. In TheMeiofaunaSpecies in TimeandSpace. W. Sterrer
5273. and P. Ax. eds. Workshop Symposium, Bermuda. 1975. Mikrofauna
Palmer, M. A. 1988. Dispersal of marine meiofauna: a review and Meeresboden 61: 287-302.
conceptual model explaining passive transport and active emergence Westheide, W. 1979. Hexionides riegcrorum n. sp.. a new interstitial
with implications forrecruitment. Mar. Ecol. Prog. Ser. 48: 81-91. freshwater polychaete form the United States. Int. Rev. Gesaintcn
Peer,Y.vande,and R.deWachter. 1994. TREECONforWindows: a Hydrohiol. 64: 273-280.
software package for the construction and drawing of evolutionary Westheide, W. 1984. The concept of reproduction in polychaetes with
trees for the Microsoft Windows environment. Compiit. Appl. Biosci. small body size: adaptations in interstitial species. In Polvchaete Re-
10: 569-570. production, A. Fischer and H.-D. Pfannenstiel, eds. Fortsch. Zool.
For,F.D. 1978. Lessepsian Migration. TheInfluxofRedSeaBiota into 29:265-287.
theMediterraneanhy WayoftheSue:.Canal. Springer. Berlin 228pp. Westheide, W. 1990. Polychaetes: Interstitial Families. In Synopsis of
Rao,G.C. 1972. Onthegeographicaldistributionofinterstitial faunaof the British Fauna {New series) No. 44. D. M. Kermack and R. S. K.
marine sand. Proc. Indian Natl. Sci. Acad. 38B: 164-178. Barnes, eds. Universal Book Services/W. Backhuys. Oegstgeest. The
Remane, A., and E. Schulz. 1964. Strand/onen des Roten Meeres und Netherlands.
ihre Tierwelt. KielerMeeresforsch. 20: 5-17. Westheide, W. 1991. The meiofauna of the Galapagos. A review. Pp.