Spatial and temporal patterns of platyhelminth assemblages in intertidal sediments of northeast Australia

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<ul><li><p>Hydrobiologia 383: 4147, 1998.E. Schockaert, N. Watson &amp; J.-L. Justine (eds), Biology of the Turbellaria.1998 Kluwer Academic Publishers. Printed in the Netherlands.</p><p>41</p><p>Spatial and temporal patterns of platyhelminth assemblages in intertidalsediments of northeast Australia</p><p>Sabine DittmannForschungszentrum Terramare, Schleusenstr. 1, 26382 Wilhelmshaven, Germany</p><p>Key words: meiofauna, distribution, Platyhelminthes, tropical tidal flat, interaction, sediment composition</p><p>Abstract</p><p>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.</p><p>Introduction</p><p>Worldwide, species of Platyhelminthes contribute tothe marine meiofauna (Martens &amp; 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 (5867%: Alongi, 1987a; 311%: 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.</p><p>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 &amp; Hellwig, 1987;Jouk et al., 1988; Reise, 1984, 1988). Furthermore,meiofaunal distribution patterns can result from inter-actions with other fauna. Meiofaunal organisms can</p><p>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.</p><p>Material and methods</p><p>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 22.5 m. Dittmann (1995) gave a detailed account of theinvestigated areas.</p><p>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 (125-80-62m), following a narcotiza-tion with MgCl2. The second transect was sampled inthe Haughton River estuary in September 1991. The</p></li><li><p>42</p><p>Figure 1. Location of the two studied tidal flats on the northeastcoast of Australia.</p><p>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 cm2surface 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.</p><p>Temporal patterns were followed in two tidal flatsof Hinchinbrook Channel on 5 occasions between No-vember 1988 and October 1991. Sampling took place</p><p>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.1988 n = 9, May 1989 n = 6, Oct. 1989 and May 1990n = 8, Oct. 1991 n = 10. Treatment of samples was asdescribed above.</p><p>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).</p><p>Taxonomic descriptions of the species of Platy-helminthes are still under way.</p><p>Results</p><p>Spatial patterns</p><p>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).</p><p>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</p></li><li><p>43</p><p>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. 16 = replicates at each site.</p><p>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 4873% of total numbers) are listed. H =Shannon-Weaver diversity index.</p><p>Mudflat Muddy sandflat Sandflat</p><p>Sediment parameterGrain size</p><p>Median (mm) 0.36 0.41 0.53Sorting coefficient 1.55 1.66 1.48</p><p>Organic matter (%AFDW) 3.82 1.98 1.21</p><p>Platyhelminthesspecies numbers 12 12 18species density 10 cm2 3 (2) 3 (1) 5 (1)abundances 10 cm2 6.67 (4.37) 7.76 (7.71) 7.33 (3.33)</p><p>% Grazer 2.5 11 48% Predators 97.5 89 52</p><p>H 1.84 1.87 2.55Abundant species Proseriata indet. Proseriata indet. Myozonaria sp.</p><p>Cheliplana sp. Typhloplanoida sp.5 cf. CylindrostomideGyratrix Acoela sp.2 Schizochilus sp. 1hermaphroditus G. hermaphroditus</p></li><li><p>44</p><p>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 marina stand, C = mid intertidal muddy sandflat with callianassid shrimps (Trypaeaaustraliensis), S = sandflat, L = lower lying sand flat. 15 = replicates at each site.</p><p>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.</p><p>Platyhelminthes prevailed, whereas the assemblage inthe lower sandflat had equal shares of the two trophictypes (Table 1).</p><p>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</p><p>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 &lt; 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.</p><p>Temporal patterns</p><p>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-</p></li><li><p>45</p><p>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 (comprising5075% of total numbers) are listed. H =Shannon-Weaver diversity index.</p><p>Mudflat Avicennia marina Trypaea australiensis Sandflat Lower sandflatstand flat</p><p>Sediment parameterGrain size</p><p>Median (mm) 0.11 0.11 0.15 0.14 0.22Sorting coefficient 1.51 1.22 1.28 1.24 1.37</p><p>Organic matter (%AFDW) 2.4 1.7 1.4 1.2 2.7</p><p>PlatyhelminthesSpecies numbers 3 4 16 16 18Species 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)</p><p>% grazer 67 17 6.5 21 40% predators 33 83 93.5 79 60</p><p>H 1.33 1.39 1.99 2.08 2.47Abundant species Macrostomum sp. Cheliplana sp. Cheliplana sp. Cheliplana sp. Myozonaria sp.</p><p>Dalyellioide indet. Acoela sp. Proseriata indet. Myozonaria sp. Cheliplana sp.Proseriata indet. Proseriata indet. Typhloplanoida sp.A Typhloplanoida sp.A Typhloplanoida sp.A</p><p>Gyratrix herma-phroditus</p><p>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.</p><p>Nov 1988 May 1989 Oct. 1989 May 1990 Oct. 1991</p><p>Site ASpecies numbers 32 9 9 17 12Species 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)</p><p>% grazer 49 73 56 30 40% predators 51 27 44 70 60</p><p>H 2.93 2.12 2.05 2.35 1.85</p><p>Site BSpecies numbers 24 6 19 24Species 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)</p><p>% grazer 45 60 47 29% predators 55 40 53 71</p><p>H 2.69 1.27 2.30 2.36</p></li><li><p>46</p><p>fore. At most sampling dates, grazing and carnivorousplatyhelminths occurred in equal proportions.</p><p>Discussion</p><p>Worldwide, a wide variety of abundances of Platy-helminthes has been recorded from various habitats(Martens &amp; Schockaert, 1986). Compared to recordsfrom intertidal locations in temperate areas (Armonies&amp; 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.</p><p>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 &amp; 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.</p><p>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 &amp; Hellwig(1987) found highest meiofaunal abundances in shel-</p><p>tered sand of a temperate tidal bay. They recorded thehighest per surface area abundance of platy...</p></li></ul>

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