Int. Revue ges. Hydrobiol.

78

1993

4

521-534

STEFAN N E H R I N G

Institut fürMeereskunde an derUniversität Kiel, Düsternbrooker Weg 20,D-24105 Kiel,FRG

Tube-dwelling Meiofauna inMarine Sediments key words:meiofauna, mucus, tubes, detritus, sediment-water interface Abstract Tube-dwelling hasbeen recognized previously asalife-style for several meiobenthic species,but behavioural observation of living specimens hasrarely been reported. Theextent to which tubebuildingandtube-dwelling occurswithinmeiofauna, andhowtheyhaveinfluenced evolutionary and ecologicalprocesses aswellasmorphology withinthese organisms,isrelativelyunknown but potentially ofgreat significance. In addition to direct observation oftube-building andtheoccurenceof tubes innatural habitats, theinternal anatomy associated with tube-building intwo nematode species(Ptycholaimellusjacobi,P.ponticus) and oneharpacticoid copepod species (Stenheliapalustris)is the focus ofthisstudy. Specialattention isgiventothe secretory products,glands,andtheir association with secretory pores. Also, therole ofmeiobenthic tube-dwelling activities inrelationshipto their environment is extensive discussed.

Contents 1. Introduction 2. Materials andMethods 2.1. Study sites 2.2. Sample treatment 2.3. Behaviour experiments 2.4. Histochemical treatments 2.5. Treatments for observations offixedmaterial 3. Results 3.1. Tube construction bythenematodes Ptycholaimellusjacobi andP.ponticus 3.2. Tube position ofPtycholaimellusponticus in the sediment 3.3. Tube construction bytheharpacticoid copepod Stenhelia palustris 4. Discussion 5. Summary 6. References

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1. I n t r o d u c t i o n Free-living aquatic nematodes andharpacticoid copepods arefound inthesea bottom, where they usually represent t h emost abundant groups ofmetazoans (REMANE 1933, PLATT and WARWICK 1980, HICKS and COULL 1983). However, almost nothing is known

about their mode ofexistence between sand grains and m u d particles. Ithas long been recognized that meiofauna hasahigh potential for modifying their sedimentary microenvironments in t h e sea bed ( P E T R 1976, ANDERSON and MEADOWS 1978, PLATT and W A R WICK 1980, HICKS and C O U L L 1983, MEADOWS and TUFAIL 1986), b u t t h e behaviour of

meiofaunal species has rarely been reported. Tube-dwelling is c o m m o n among macroinvertebrates in contrast to meiofauna, in which this life style is notwell known. Hence, the observations onthe tube-construction

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and behaviour of the nematodes Ptycholaimellusjacobi and P.ponticus aswelltheharpacticoid copepod Stenhelia palustris should beviewed asasignificant extension toourknowledge on tube-dwelling nematodes (NEHRING et al. 1990) and meiobenthic copepods (LORENZEN 1969, CHANDLER and FLEEGER 1984) and their consequences on biotic and

abiotic conditions. Little isknown about gland cells of and mucus secretions bymeiofauna. Forthe ventral gland cell ofnematodes, an excretory function hasbeen assumed (MAGGENTI 1981),but NEHRING etal. (1990) have proved that itisavery active secretory cell inwhich the sticky mucus fortube construction isproduced. Consequently special attention was given tothe ultrastructure of theventral gland cell, to t h e secretory products of hypodermal glands, and their association with secretory pores.

2. Materials and Methods 2.1. Study sites Sediment samples with Ptycholaimellusponticus (FILIPJEV, 1922) and Stenhelia (Delavalia)palustrisBRADY, 1862were collected from thetidal creeks atWesterhever lighthouse off the Eiderstedt peninsula (54°22.0'N,8°38.4'E), andfrom atidalflat atthe northern tipoftheisland ofSylt(Königshafen-List, 54°51.7'N, 8°3.4'E) intheNorth Frisian Wadden Sea(North Seacoast ofFRG). Ptycholaimellusjacobi JENSEN and NEHRING, 1992wassampled inthe vicinity ofthe Hirsholmene islands (57°29.5'N, 10°38.1'E),north-eastern part ofthe Kattegat closetoFrederikshavn, Denmark.

2.2. Sample treatment During low tide, qualitative sediment samples were taken with aspoon inWesterhever andSylt habitats,whileadredgewasusedatHirsholmene.The quantitativeverticaldistributionofthe nematodesinWesterhever sediments was determined byimmediate partitioning ofvertical cores (1.2cm internal diameter X8cmlong)into0.25cmintervals.The number ofnematodes between each intervalwasthen adjusted tonumbers per 10cm2.Fivecoresweretaken andthe subsamples were preserved in borax-buffered formalin. Animals were extracted inthelaboratory bydécantation using a sieve with 45p.m mesh size. Preparations ofundisturbed sediment samples from Westerhever were modified after ALTEMÜLLER (1956, 1962), WERNER (1966) and WATLING (1988). Briefly, I used small Teflon-coated metal forms of 1 cmx 5cm area and penetrating the sediment about 4cm. Samples were immediately deep-frozen in an insulated boxwith liquid nitrogen and transported to Kief' The samples were released from the metal forms in the laboratory and dry-frozen for 24h at ca.6.67Pa (Leybold HeraeusGT2).Thesampleswereembedded inAralditeduringthefollowing 3days,after which they were sectioned, glued toglass slides and polished with Korund toathickness of50pm. These sediment blocks and slices were analysed for tube structures employing microscopic inspection and radiography. Ididnotobserve distortions ofsediments with the liquid-nitrogen freezing technique, in contrast to RUTLEDGE andFLEEGER (1988),whofavoured aslower freezing technique for their samples. Radiographicobservationsweremadewithsandwich-aquaria and sediment resinblocks,and were placed on radiographic films andirradiated with X-rays (30kv/5mA/10-30min).

2.3. Behaviour experiments The behaviour andthetube-building ofnematodes andharpacticoid copepods was observed in Boveri dishes through various microscopes. Ptycholaimellus ponticus specimens were also placed into sandwich-aquaria (THUN 1966) and observed through microscope. Thestructures produced were analysed on radiographs. For further explanation seeNEHRING etal.(1990).

Tube-dwelling meiofauna 2.4. Histochemical

treatments

Proteinsinsecretionsfrom P.ponticushypodermalglandcellswerestainedwithCoomassieBrilliantBlueRmodified after PREMACHANDRANetal. (1988).Nematodeswerewashedwithaquabidest andincubated onacleanmicroscopicslideinadropof0.1°/oCoomassieBrilliantBlueRsolutionin 5%aceticacidsurrounded bytheinclosurereagentGlyceel.Glyceelstimulatestheexudateproductionofnematodes.Thereactionzonewassealedwithacoverslip.Nematodeswereobservedwitha compound microscope immediately at room temperature for several hours.

2.5. Treatments for observations offixed material Nematodesfor morphological analysebylightmicroscopywerefixedinhotTAF(i.e.2partstriethanolamin, 7parts 37%formaldehyde and 91parts water) and placed in adroplet of TAF ona microscope slide and imbedded in paraffin. Forscanningelectronmicroscopy(SEM),tubeswereextractedfrom thesedimentsandpreserved in 4% formaldehyde, washed in distilled water, and dehydrated through agraded ethanol series, finally being100%dehydratedinamixtureof2,2di-methoxypropane(DMP)andhydrochloricacid. After 10minthetubeswereairdried.Tubeswerecoatedwith200À ofgoldandexamined onscanning electron microscope. Fortransmissionelectronmicroscopy(TEM),nematodeswereremovedfromtheextractiondish, fixed for24hoursin4%glutaraldehydeat4°C,rinsedfor 1hourin0.1Mphosphatebuffer andpostfixed for2hoursin2%osmiumtetroxideat18°C.Thespecimensweredehydratedinagradedethanolseries andtransferred topropylene oxide.Eponwasusedasthe embedding medium. Sections were cutwith adiamond knive.Ultrathin sections were mounted on standard copper meshgrids, stained for 30minwith 2% uranyl acetate and 5minwith 0.5% lead citrate.

3. Results 3.1. Tube-construction by the nematodes Ptycholaimellus jacobi and P. ponticus The nematode P. ponticus (Fig.1)is regarded an oxybiotic species feeding on diatoms withtypicalbodyshape andabout 1 mmlongandmax.50umthick (JENSEN 1988, NEHRING 1992a).Thehead issetofffrom thebodybyacollarraisedfrom a2-3um deepgroove;the duct of the large ventral gland cell posterior to the oesophagus opens into this groove (Fig.1 b).The examination ofalarge collection of several species ofthe genusPtycholaimellussubstantiated the collar asan important new generic diagnostic character, hitherto overlooked orwrongly interpreted. The borders ofthe upper and lowerpart ofthe groove are optically reflected as two transverse lines. Possibly, they have been confounded with the amphids inprevious descriptions ofseveral species ofthe genusPtycholaimellus (JENSEN and NEHRING 1992). Evidently,P.ponticus inhabitsthetopmostfirstcmofthesediment (Fig.2),since92.7% of its population was concentrated in densities of approximately 1000 individuals per 10cm2 inthislayer.Thefine-scale disintegration ofthislayer showed an almost equal distribution ofthe nematodes. Only79individuals were found below 1cm depth (7.3%)and none below 1.5cm depth. NEHRING etal. (1990)reported tubes constructed byseveralPtycholaimellusspecies and suggestedthattube construction iswidespread inthisgenus.Thetube-building procedure of adults and juveniles of the tube-dwelling species of Ptycholaimellus has four steps (Fig.3):

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a. the nematode attaches its tail to the tube-wall with mucus secreted by the caudal gland cells, b. thenematode stretches thebodyand extends outwiththeanterior endperforming a 3-dimensional wave movement, c. the nematode secretes mucus with its large ventral gland cell opening close to the head end; upon close contact to detritus the mucus on the head catches a small amount of it, d. thenematode retiresintothetubeand attachesthedetritustotheanterior endofthe tube.

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