Hydrobiologia 383: 41–47, 1998.E. Schockaert, N. Watson & J.-L. Justine (eds), Biology of the Turbellaria.©1998Kluwer Academic Publishers. Printed in the Netherlands.

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Spatial and temporal patterns of platyhelminth assemblages in intertidalsediments of northeast Australia

Sabine DittmannForschungszentrum Terramare, Schleusenstr. 1, 26382 Wilhelmshaven, Germany

Key words:meiofauna, distribution, Platyhelminthes, tropical tidal flat, interaction, sediment composition

Abstract

The distribution of Platyhelminthes was surveyed along two transects in tidal flats of the northeast coast of Queens-land, Australia, in May 1990 and September 1991. Species numbers and densities as well as individual abundancesand diversity increased from the high towards the low intertidal in relation to sediment composition. Along thisgradient, platyhelminth assemblages of low intertidal sandflats, muddy sandflats and mudflats were distinguishedwith multivariate analyses. The distribution pattern was affected by the sediment parameter, and also by interactionswith macrofauna. Sampling and analysis of the platyhelminth assemblage in tidal flats of Hinchinbrook Channelover time (November 1988 to October 1991) revealed annual and interannual variation, but no seasonality.

Introduction

Worldwide, species of Platyhelminthes contribute tothe marine meiofauna (Martens & Schockaert, 1986),yet knowledge on distribution patterns of meiofaunafrom tropical marine sediments is scarce. Platy-helminths have been recorded from intertidal sedi-ments of the coast of Northeast Australia with con-tradictory findings regarding their contribution to themeiofaunal community (58–67%: Alongi, 1987a; 3–11%: Dittmann, 1991). While Alongi (1987a,b,c,1990) concentrated on meiofauna distributions (witha focus on nematodes) in mangrove forests along theQueensland coast, I concentrated on the tidal flats.Here, a considerable degree of similarity with tem-perate tidal flats was found in the composition anddistribution of platyhelminths (Dittmann, 1991; Reise,1988). Following that first survey, this paper presentsspatial and temporal patterns of platyhelminths invarious tidal flat habitats of the North Queenslandcoast.

The distribution of platyhelminths and other meio-fauna on sandy beaches, tidal flats, salt marshesand mangroves is often related to sediment properties(Alongi, 1987a, 1990; Armonies & Hellwig, 1987;Jouk et al., 1988; Reise, 1984, 1988). Furthermore,meiofaunal distribution patterns can result from inter-actions with other fauna. Meiofaunal organisms can

be subject to predation, can compete for food withother fauna, or make use of the structural complex-ity provided by biogenic structures in soft-sediments(Dittmann, 1993, 1996; Reise, 1984, 1985; Schrijverset al., 1995). Here, the distribution of platyhelminthsin the two studied tropical tidal flats is discussed bothin relation to sediment composition and to interactionswith macrobenthos.

Material and methods

The platyhelminth fauna in tidal flats of the north-east coast of Australia (Figure 1) was studied between1988 and 1991. Tidal ranges along the coast are 2–2.5 m. Dittmann (1995) gave a detailed account of theinvestigated areas.

To assess the spatial pattern, two transects weresurveyed. In May 1990, a transect of about 0.5 kmlength was sampled in Hinchinbrook Channel. Sixreplicate samples were taken at each of three sites fromthe high to the low intertidal. Corers used had a surfacearea of 10 cm2 and samples were taken to a depth of5 cm. Platyhelminths were extracted alive from thesediment by repeated shaking and decanting througha set of sieves (125-80-62µm), following a narcotiza-tion with MgCl2. The second transect was sampled inthe Haughton River estuary in September 1991. The

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Figure 1. Location of the two studied tidal flats on the northeastcoast of Australia.

length of the transect was 1 km, and 5 sites were sam-pled with 5 replicate samples each. Here, corers of 5cm2 surface area were used and samples taken to adepth of 5 cm. At the mudflat site, a syringe of 1.2 cm2

surface area had to be used to get the mud in and outof the corer. Treatment of samples was as describedabove. Given the finer sediment size in the estuaryand the higher number of replicates for this survey,the smaller sample volume facilitated the extraction ofplatyhelminths and thus ensured a rapid treatment ofsamples. Further sediment samples were taken at eachsite of the studied transects for analysis of grain sizeand organic matter.

Temporal patterns were followed in two tidal flatsof Hinchinbrook Channel on 5 occasions between No-vember 1988 and October 1991. Sampling took place

at the onset and at the end of the wet season. The are-as lay at the landward side towards the northern endof the Channel, with site A being the furthest north.Site B was the study site for the transect mentionedabove. This survey was part of a monitoring of faunalchanges at the two sites and therefore the samples weretaken throughout the tidal flats at random. Sampleswere taken with a corer of 5 cm2 surface area to 5 cmsediment depth. The numbers of replicates were: Nov.1988n = 9, May 1989n = 6, Oct. 1989 and May 1990n = 8, Oct. 1991n = 10. Treatment of samples was asdescribed above.

The composition and structure of platyhelminth as-semblages were assessed with multivariate analysesusing the PRIMER software package from PlymouthMarine Laboratory (U.K.). A double square root datatransformation was carried out before cluster analy-sis (Bray-Curtis index, group-average linkage) andnon-metric multidimensional scaling (MDS). The dis-crimination of assemblages along the studied transectswas tested with one-way ANOSIM (Clarke, 1993).

Taxonomic descriptions of the species of Platy-helminthes are still under way.

Results

Spatial patterns

At both study sites, sediment grain size increased fromthe high towards the low tide line, resulting in a se-quence from mud- to sandflats across the transects.The sediments were generally richer in organic matterat the muddy sites, but the highest value was recordedin the sandflat near the low tide line of the HaughtonRiver estuary (Tables 1 and 2).

The distribution of platyhelminths in the tidal flatsfollowed the large scale zonation of abiotic factors.Species numbers and densities increased from themudflat and muddy sandflat in the high and mid-intertidal area of the transect towards the lower-lyingsandflat (Table 1). Here, 18 platyhelminth speciesoccurred out of a total of 32 recorded at this inter-tidal area in May 1990 and diversity was higher. Themean individual numbers did not differ along the tran-sect. The platyhelminths of the lower lying sandflatformed a distinct assemblage, but platyhelminth as-semblages from the two muddier sites in the high andmid-intertidal could not be distinguished by multivari-ate analyses (Figure 2) (ANOSIM:R = 0.285,p =0.2%). At the latter two sites, predatory species of

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Figure 2. Dendrogram of a cluster analysis on the platyhelminth recordings at three sites of a transect in a tidal flat of Hinchinbrook Channel,May 1990. H = high intertidal mudflat, M = mid intertidal muddy sandflat, L = lower lying sand flat. 1–6 = replicates at each site.

Table 1. Sediment parameter and Platyhelminth assemblages along a transect in a tidal flat ofHinchinbrook Channel, May 1990. Species density and individual abundances given are meanvalues (n=6) with standard deviation. Platyhelminth species taking the first 3 ranks of abundance(comprising 48–73% of total numbers) are listed.H ′=Shannon-Weaver diversity index.

Mudflat Muddy sandflat Sandflat

Sediment parameter

Grain size

Median (mm) 0.36 0.41 0.53

Sorting coefficient 1.55 1.66 1.48

Organic matter (%AFDW) 3.82 1.98 1.21

Platyhelminthes

species numbers 12 12 18

species density 10 cm2 3 (2) 3 (1) 5 (1)

abundances 10 cm2 6.67 (4.37) 7.76 (7.71) 7.33 (3.33)

% Grazer 2.5 11 48

% Predators 97.5 89 52

H ′ 1.84 1.87 2.55

Abundant species Proseriataindet. Proseriataindet. Myozonariasp.

Cheliplanasp. Typhloplanoidasp.5 cf.Cylindrostomide

Gyratrix Acoelasp.2 Schizochilus sp. 1

hermaphroditus G. hermaphroditus

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Figure 3. Dendrogram of a cluster analysis on the platyhelminth recordings at five sites of a transect in the Haughton River estuary, Septem-ber 1991. M = high intertidal mudflat, A =Avicennia marinastand, C = mid intertidal muddy sandflat with callianassid shrimps (Trypaeaaustraliensis), S = sandflat, L = lower lying sand flat. 1–5 = replicates at each site.

Figure 4. Multidimensional scaling of the platyhelminth assem-blages at the muddy sandflat and sandflat sites of the Haughton Riverestuary. See legend of Figure 3 for details. Stress = 0.162.

Platyhelminthes prevailed, whereas the assemblage inthe lower sandflat had equal shares of the two trophictypes (Table 1).

A transect through the estuary of the HaughtonRiver confirmed the increase of species numbers andspecies densities from the mudflat in the high intertidaltowards the muddy sand and sandflat sites in the lowerintertidal (Table 2). Note, however, that the speciesvalues given for the mudflat site are based on a smallersample size and may thus be underestimated. In this

estuary, abundances increased along the gradient ofsediment types and reached highest values in the sand-flat sites, where diversity was highest as well. Theplatyhelminth assemblages were quite distinct alongthe transect (Figure 3). Platyhelminths occurred inconsiderable numbers only in sediments of the mid-and low intertidal sites. A multidimensional ordination(Figure 4) of these three sites confirmed the separa-tion of the assemblages following this environmentalgradient (ANOSIM:R = 0.52,p < 0,1%). Here, too,the trophic group composition of the platyhelminth as-semblage changed from a dominance of predators inthe muddy sites to more equal proportions of predatorsand grazers at the sandflat sites.

Temporal patterns

Species numbers and abundances of platyhelminthsvaried between the 5 sampling dates in HinchinbrookChannel, but no pattern could be observed between theOct./Nov. values (at the beginning of the wet season)and the May values (end of wet season) (Table 3).Abundances were lowest in October 1989 and stayedlow at site A, whereas at site B densities recordedin Oct. 1991 were similar to those three years be-

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Table 2. Sediment parameter and Platyhelminth assemblages along a transect in the Haughton River estuary, September 1991. Speciesdensities and individual abundances given are mean values (n=5) per site with standard deviation. At the mudflat site, a smaller corer was usedand species values are given per 1.2 cm2 (marked with asterisks). Platyhelminth species taking the first 3 ranks of abundance (comprising50–75% of total numbers) are listed.H ′=Shannon-Weaver diversity index.

Mudflat Avicennia marina Trypaea australiensisSandflat Lower sandflat

stand flat

Sediment parameterGrain size

Median (mm) 0.11 0.11 0.15 0.14 0.22

Sorting coefficient 1.51 1.22 1.28 1.24 1.37

Organic matter (%AFDW) 2.4 1.7 1.4 1.2 2.7

PlatyhelminthesSpecies numbers 3∗ 4 16 16 18

Species density 5 cm2 1 (1)∗ 1 (1) 6 (4) 8 (2) 8 (3)

Abundances 5 cm2 4.07 (4.07) 1.6 (1.95) 12.40 (10.06) 16.60 (7.86) 13.20 (5.54)

% grazer 67 17 6.5 21 40

% predators 33 83 93.5 79 60

H ′ 1.33 1.39 1.99 2.08 2.47

Abundant species Macrostomumsp. Cheliplanasp. Cheliplanasp. Cheliplanasp. Myozonariasp.

Dalyellioide indet. Acoelasp. Proseriataindet. Myozonariasp. Cheliplanasp.

Proseriataindet. Proseriataindet. Typhloplanoidasp.A Typhloplanoidasp.A Typhloplanoidasp.A

Gyratrix herma-

phroditus

Table 3. Platyhelminth assemblages over time at two tidal flats in Hinchinbrook Channel. The two sites layabout 6 km apart. Species densities and individual abundances given are mean values per site with standarddeviation. In May 1989, a smaller corer was used and for this date species number and density are given per1 cm2 (marked with an asterisk), at all other dates per 5 cm2. H ′=Shannon-Weaver diversity index. Site Bwas not sampled in May 1989.

Nov 1988 May 1989 Oct. 1989 May 1990 Oct. 1991

Site ASpecies numbers 32 9∗ 9 17 12

Species density 5 cm2 10 (3) 2 (2)∗ 3 (1) 5 (2) 2 (3)

Abundances 5 cm2 20.00 (6.82) 12.50 (12.94) 3.13 (1.89) 7.50 (1.60) 5.30 (11.93)

% grazer 49 73 56 30 40

% predators 51 27 44 70 60

H ′ 2.93 2.12 2.05 2.35 1.85

Site BSpecies numbers 24 6 19 24

Species density 5 cm2 7 (2) 2 (2) 6 (2) 5 (3)

Abundances 5 cm2 13.22 (3.46) 2.50 (0.93) 12.25 (8.75) 12.50 (14.68)

% grazer 45 60 47 29

% predators 55 40 53 71

H ′ 2.69 1.27 2.30 2.36

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fore. At most sampling dates, grazing and carnivorousplatyhelminths occurred in equal proportions.

Discussion

Worldwide, a wide variety of abundances of Platy-helminthes has been recorded from various habitats(Martens & Schockaert, 1986). Compared to recordsfrom intertidal locations in temperate areas (Armonies& Hellwig, 1987; Reise, 1984, 1988; Schockaert, pers.comm.), species numbers, diversity and abundanceswere lower in the tropical tidal flats investigated inthis study. Care should be taken to interpret this asa general trend, as the investigation of tropical tidalflats is still in the beginning compared to decades ofresearch activities in their temperate counterparts. Inthe tropics, the species stock has not yet been fullyassessed and taxonomic endeavors have to be under-taken. The study presented here also showed a highvariability in platyhelminth numbers. Together withthe low species frequency (Dittmann, 1991) this callsfor high numbers of replicates for ecological studies.

The distribution of sediment types in the studiedtropical tidal flats showed a shoreward fining trend,following a decreasing energy gradient as known fromtemperate tidal flats (Flemming & Nyandwi, 1994).Distinct benthos communities accompany the zona-tion of mud, muddy sand and sandflats from thehigh towards the low tide line (Dittmann, 1995). Inthis paper, the distribution of free-living meiobenthicplatyhelminths associated with these sediment types isdescribed.

In the tidal flats studied on the coast of NorthQueensland, meiobenthic platyhelminths formed dis-tinct assemblages along the intertidal gradient of in-creasing sediment grain size towards the subtidal.These patterns were similar between the two studysites. Alongi (1987c) found inter-estuary variationwhen comparing the nematode fauna of five mangrovesystems along the northeast coast of Australia. He alsorecorded an intertidal zonation of nematodes with in-creasing densities towards the low intertidal position.In my study, there was no increase in abundance inHinchinbrook Channel, but platyhelminth abundancesin the Haughton River estuary increased towards thelower intertidal. Alongi (1987a) related the zona-tion of meiofauna in tropical shores to environmentalcues such as temperature and sediment properties. Asin the study presented here, Armonies & Hellwig(1987) found highest meiofaunal abundances in shel-

tered sand of a temperate tidal bay. They recorded thehighest per surface area abundance of platyhelminthsin sand, which could be explained by the increasedinterstitial space in sandy substrates. Longer exposureto higher temperatures near the high tide line couldreduce the platyhelminth assemblage in the mudflats,where vertical migration is restricted. The low speciesnumber in the mudflat of the Haughton estuary re-ported here could, however, be due to a smallersampling device used at that site. In general, the zona-tion of platyhelminths reflects the living conditionsprovided by the different sediment compositions.

Besides the sediment composition, further fac-tors have to be considered. At higher intertidal sites,tannins derived from mangrove roots and leaves caninhibit and regulate meiofaunal assemblages (Alongi,1987b,c). Comparably, the low numbers and diversityof platyhelminths in the mudflat and in the vicinity ofAvicennia marinamangroves may be attributable to anegative effect of tannins. Whether such a mechanismdoes exist should be tested experimentally.

Furthermore, interactions with macrofauna can af-fect the distribution patterns. Reise (1984) showed thatsandflats are a structurally complex and variable envi-ronment for platyhelminths, where lugworm (Areni-cola marina) burrows were inhabited by a diverseassemblage. Such biogenic structures contribute to thespecies richness and patchy distribution of infaunain soft-sediments (Meyers et al., 1987; Reise, 1984,1987). As Dittmann (1996) showed, similar mecha-nisms of sediment amelioration function in tropicaltidal flats and could explain the higher species diver-sity and abundances in the muddy sand and sandflatsof tropical tidal flats (Dittmann, 1991 and this study).Burrows of the callianassid shrimpTrypaea aus-traliensisare inhabited by significantly higher platy-helminth numbers than adjacent sediment (Dittmann,1996). This accomodation was species specific andpredatory platyhelminths were more abundant in theshrimp burrows. Further biogenic structures are pro-vided by holothurids (Paracaudinasp.) and entero-pneusts in the sandflat. To determine the obligation ofpromotive interactions for the meiobenthic species intropical tidal flats, more studies are required.

Whereas Martens & Schockaert (1986) report lowpredation by macrofauna on platyhelminths in theirreview, the platyhelminths in the studied tropical tidalflats were subject to predation. In the mid-intertidalarea, soldier crabs (Mictyris longicarpus) frequentlygo on feeding treks, thereby reducing platyhelminthnumbers up to 100% (Dittmann, 1993). Abundances

47

and species numbers increased significantly in an ex-periment excludingM. longicarpus. The exclusion ofepibenthic detritivores in a mangrove of East Africa(Schrijvers et al., 1995) led to increased meiofaunanumbers. Here, the mechanism of interaction wasidentified as competition for food. Thus, various in-teractions with macrofauna can affect the spatial dis-tribution of meiofauna in tropical intertidal sediments.

The feeding mode of platyhelminths can oftenbe directly observed under the microscope or byrapid-freezing of sediment samples (Kennedy, 1994).Preliminary classifications of the feeding modes pre-sented here revealed a dominance of carnivorousspecies of Platyhelminthes in muddy sediments. Thisis in contrast to the distribution of feeding modesin a temperate tidal flat (Reise, 1988). At the lowersandflat sites, diatom-feeders and carnivores were al-most equally abundant. The trophic composition of theplatyhelminth assemblages in Hinchinbrook Channelchanged little over time and was not related to season.Further studies should address the food web of tropi-cal benthos communities, and data are needed on theabundance of diatoms as possible food sources.

The temporal variation of the platyhelminth assem-blages at the studied sites showed annual as well asinter-annual differences, but did not reveal any distinctseasonal pattern. Alongi (1990) reported fluctuationsof nematode numbers over time in tropical mangroveand sandflat sites, possibly due to seasonal changesin sediment temperature. The platyhelminth recordspresented here are snapshots over time and point to therelevance of temporal repetition of investigations. Thisis especially required in the tropics, where seasonaldynamics of infaunal species and their abundances arenot yet understood.

Acknowledgements

This study was financially supported by the DeutscheForschungsgemeinschaft (IIIO2-Di396/1-2). The Aus-tralian Institute of Marine Science provided excellentresearch facilities. I thank the crews of the ‘HarryMessel’ and ‘Pegasus’ as well as all the volunteerswho joined my field trips. Ulrike Siebeck helped withthe grain size analysis. Leigh Winsor and Lester Can-non gave moral support for the study of worms downunder. Comments by an unknown reviewer and E.Schockaert improved the manuscript.

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