wells, 1986.distribution of molluscs across a pneumatophore, australia

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J. Moll. Stud. 1986), 52, 83-90.

D I S T R IB U T I O N O F M O L LU S CS A C R O SS A P N E U M ATO P H O R EBOUNDARY IN A SMALL BAY IN NORTHWESTERN AUSTRALIA

FRED E. WELLSWestern ustralian Museum , Perth 6000, Australia

(Received 18 June 1985)

ABSTRACT

The number of species, total density and total biomassof molluscs were examined for 50 m either side of anAvicennia pneumatophore boundary in the Bay ofRest, North West Cape, Western Australia (22°18'S;114° 08'E). Two mollusc assemblages were found, sep-arated almost exactly at the pneumatophore bound-

ary. The mudflat association had more species ofmolluscs but a lower total density and total biomassthan the assemblage in the Avicennia zone. Stationsamong pneumatophores on the seaward fringe of theAuicennia zone were more diverse and had a greaterdensity and biomass than stations among the trees.Possible reasons for the pneumatophore area havinga higher density and biomass of molluscs than theadjacent mudflat and tree zones are discussed.

INTRODUCTION

Mangroves fringe the shoreline of coastal baysand estuaries across northern Australia and inmany other tropical and subtropical areas of theworld. There has recently been an increasingawareness of the role of mangrove trees in pro-viding a major so urce of primary production notonly in the forest, but also in adjacent marineareas. Few animals consume the plants directly;instead they are broken down into detritus by acombination of biological and physical factorsto form the basis of a detrital foodweb (Day,1975; Evink, 1975). Invertebrates in the man-groves are thought to be an important link inenergy flow to higher order predators such asbirds and fish (Odum and Heald, 1972), butthere are few quantitative data on the diversity,density and biomass of invertebrates in man-groves (Milward, 1982; Redfield, 1982).

As one step in documenting the importanceof invertebrates in mangrove systems I recently(Wells, 1983; 1984) examined th e marine invert-ebrate fauna of a small bay, the Bay of Rest, atNorth West Cape, Western Australia (22° 18'S;114°08'E). The bay was divided into four

distinct intertidal habitats: seaward mudflat,landward backflat, and the intermediate Avicen-nia marina and Rhizophora stylosa tree zones.Ther e was a steady decline in diversity of speciesand total invertebrate density with the mudflatstations being richest and the Avicennia, Rhi-zophora and backflat stations being progress-

ively less diverse and having lower inverteb ratedensities. Dry, decalcified biomass followedessentially the same progression except that itwas higher in Avicennia than on the mudflat,due entirely to the gastropod Terebralia sulcata,which constituted 55.4 of the biomass of allmarine invertebrates in the Avicennia zone.

Within each habitat stations to be sampledwere selected randomly; transects were notused. Despite this there was an apparentlygreater density and biomass of marine invert-ebrates among the pneumatophores at the sea-ward edge of the Avicennia zone than eitheramong the trees themselves or on the adjacentmudflat. The present paper examines the num-ber of species, density and biomass of molluscsin the transitional zone between the openmudflat and the centre of the mangrove forest.Molluscs were selected for investigation becausethey were the most important group in terms ofnumbers of species and biomass in both themudflat and Avicennia stations sampled, andwere also important in terms of density (Wells,1983; 1984).

MATERIALS AND METHODS

Eight transects 100 m long were established in May-June 1983 to cover the major areas of the bay. Eachtransect was centred on the junction of the mudflat

with the Avicennia pneumatophores, at a midtidallevel of about 1.4 m (Wells, 1980). When the stationswere sampled they we re marked with surveyor's tape.Shore heights relative to the pneumatophore bound-ary were measured with a metre stick on the sub-sequent high tide. At each station the following physi-



84 F.E. WELLS

cal measurements were made: sediment grain size,pH, salinity and sediment surface temperature. Thesame techniques were used as described by Wells(1983). Salinity measurements were incomplete dueto an equipment malfunction on an early transect.More comprehensive information on physical con-ditions in the Bay of Rest is contained in W ells (1980;1983; 1984).

Molluscs were sampled by placing a 0.75 m diametersteel ring on the sediment surface, and sieving theupper 5 cm of sediment through a 2 mm mesh. Thedeeper sediment was searched by hand for largerspecies. Four samples were made at each station andpooled . Molluscs were preserved in 10 formalin,buffered with borax and transferred to 70 alcoholon return to the laboratory, where specimens weresorted, identified and counted. Voucher specimens ofeach species were deposited in the Western AustralianMuseum where they have catalogue numbers WAM4310-83 to 4351-83. Dry weight determinations weremade as described previously (Wells, 1983).

Overlaps between stations were calculated usingJaccard's coefficient, c /a + b -c where a is the numberof species in sample A, b is the number of species insample B, and c the number of species in common atstations A and B (Popham and Ellis, 1971). Thedendrogram was constructed using the weighted pairgroup method with average linkage (Sokal andSneath, 1963). The primary species of each assem-blage were determined using the ranking method out-lined by Sanders (1960). A rarefraction technique(Simberloff, 1978) was used to de termine the num berof species which could be expected to occur in a sampleof a given size from each habitat.

RESULTS

A total of 38 species was collected: 17 occurredonly on the mudflat; 7 only in Avicennia; and14 species in both habitats (Table 1). The totaldensity of molluscs in the mudflat stations washalf that in the A vicennia and the biomass wasonly 14 as grea t. The mudflat was not domi-nated by a single species or genus. Cerithideacingulata was the m ost num erous species, with36 of the total, followed by Marginella sp .with 8 . In terms of biomass the most importantmolluscs on the mudflat were Saccostrea sp. with38 of the total and Rhinoclavis vertagus with19 . In Avicennia, Terebralia sulcata formed50 of total num bers and 8 5 of total biomass. palustris was relatively minor with 1 of totalnum bers and 4 of total biomass.

In the Avicennia stations the mean numberof total mollusc species was greatest near thepneumatophore boundary and declined after10 m shoreward (Table 2). Total mollusc densityand biomass, largely T. sulcata, were greatest at5 m upshore from the boundary. The Avicennia

habitat can be subdivided into two areas: sta-tions in the first 10 m upshore from the pne-umatophore boundary, which are among pne-umatophores but seaward of the actual treetrunks, and stations 15 m or m ore from theboundary, which are among the trees (Table 2).Considered in this manner the pneumatophorestations were more diverse (mean numberof species per station is 3.29 versus 1.81),had a greater total density of molluscs (115.8 vs41.9/m2) and a greater total biomass (21.6 vs8.4 g/m-) than stations am ong the tree zone.All of these differences are statistically signifi-cant (t-test, p < 0.05). In a ddition, all of thespecies found in both mudflat and Avicenniastations occurred in the pneumatophorestations.

In the mudflat stations the total number ofmolluscan species at each level was large, rang-ing from 10 to 18. The mean numbe r of speciesper station, total density and total biomass, were

all greatest or second greatest at the station 1 mdownshore from the pneum atophore boundary.The m ean number of species per station declined2 m downshore and remained low until the lasttwo stations. Total density declined to its lowestlevel at the 5 m down shore from the p neu-matophore boundary then increased consist-ently. Total biomass was lowest at the 5 m down-shore station then fluctuated with increasingdistance from the pneumatophore boundary.

Analysis of affinity levels between stationsshows two distinct clusters, with an affinity ofonly 0.12 between them (Fig. 1). Mangrovestations form one cluster at an affinity level of0.36. The downshore pneumatophore station,one metre from the pneumatophore boundary,fits into the mudflat cluster. The remainingAvicennia stations form a second cluster. W ithinthis grouping the pneumatophore stations (2 m,5 m , 10 m upshore from the bounda ry) had thegreatest affinity, 0.43 or more. Stations amongthe actual tree zone (15 m upsh ore) had pro-gressively lower overlaps with the pneum-atophore stations. The rarefraction analysis(Simberloff, 1978) prov ided addition al evidencethat there were two separable mollusc assem-blages. If the sample size was 250 individualsfrom each habitat there would be little dif-ference in the num ber of species expected in theAvicennia stations, with 12.2 species among thetrees and 13.5 species among thepneumatophores. A combined sampling of 250individuals in Avicennia would be expected toyield 14.3 species, about half of the 25.1 whichwould be expected in the mudflat. The Sanders(1960) ranking method showed that the mudflat



Table 1. Molluscs collected at stations sampled across a pneumatophore boundary in the Bay of Rest in northwestern Austral ia.

Species

l tA\rtt nn

IVI

TotalIndiv-iduals

219212—

3938—

4333362524

2181412—6662

54—433—21

—111111

Density no./

m2)

0.113.6

0.9

2.82.7—3.02.32.61.81.70.11.31.00.9

0.40.40.40.1

0.40.3—0.30.20.2

0.10.1

0.10.10.10.10.10.1

Biomass g/m 2 )

>0.00.3

>0.0

0.80.2

—>0.0

0.1>0.0>0.0

0.2>0.0

0.4>0.0>0.0

>0.0>0.0>0.0>0.0

>0.0>0.0—

>0.00.1

>0.0

>0.0>0.0

>0.0>0.0>0.0>0.0>0.0>0.0

PneumatophoreTotalIndiv-iduals

388101161

59262326

2

1

13

2—

——261

11

——3

—1

—

——

Density no./m 2 )

54.914.322.8

8.43.73.33.7

0.3

0.1

2.1

0.3—

0.30.80.1

0.10.1

—0.4

0.1

—

—

Zone

Biomass g/m 2 )

19.00.30.80.21.10.2

>0.0

>0.0

>0.0

>0.0

>0.0—

>0.0>0.0>0.0

>0.0>0.0

>0.0

>0.0

—

Avicennia stations

TotalIndiv-iduals

1696—

73—2

20

—

1

5

——

11

2

141

—

—2—

Tree Zone

Density no./m 2 )

23.90.8—

10.3—

0.32.8

0.1

0.7

—1.6

0.3

0.10.60.1

—0.3—

Biomass g/m 2 )

6.7>0.0—0.3

—>0.0>0.0

——

>0.0

>0.0

——1.3

>0.0

>0.00.1

>0.0

—

—>0.0—

TotalIndiv-iduals

557107161132262546—2

2

18

2—

11

461152

—3

12

—

Overall

Density no./m 2 )

39.47.6

11.49.31.81.83.3—0.1

0.1

1.3

0.1—0.8

0.30.40.10.10.40.1

—0.2

0.10.1—

Biomass g/m 2 )

12.80.20.40.30.60.1

>0.0—

>0.0—

>0.0—

>0.0

>0.0—0.6———

>0.0>0.0>0.0>0.0

0.1>0.0

——

>0.0—

>0.0>0.0

—

——

zoow2or —

Terebralia sulcata (Born, 1778)Cerithidea cingulata (Gmelin, 1791)Clypeomorus sp.Cerithium coralium Kiener, 1843Saccostrea sp.Cominella acutinodosa (Reeve, 1846)Modiolus auriculatus Krauss, 1848Marginalia sp.Nassarius livescens (Philippi, 1849)Dentallum sp.Laternula creccina (Reeve, 1860)Anomalocardia squamosa (Linnaeus, 1758)Marinula sp.Rhinoclavis vertagus (Linnaeus, 1767)AtyidCirce sulcata Gray, 1838Terebralia palustris (Linnaeus, 1767)Polinices conicus (Lamarck, 1822)Placamen gravescens (Menke, 1843)Salinator sp .Acanthopleura gem mata (Biainville, 1825)Nerita chamaeleon (Linnaeus, 1758)Ischnochiton sp.Septifer bilocularis (Linnaeus, 1758)Cerithidea largellierti (Philippi, 1836)Onchidium sp.Placamen callophylla (Philippi, 1836)Morula margariticola (Broderip, 1832)Patelloida mimula (Iredale, 1924)Katelysia hiantina (Lamarck, 1818)Thais sp.Neritina crepidularia (Lesson, 1831)Trochus hanleyanus (Reeve, 1843)Vexillum vulpecula jukesii (A. Adams, 1851)Venerid sp. juvenile)Nassarius dorsatus (Roding, 1798) rea sp. juvenile) sognomon sp. juvenile)

TOTALS 538 38.3 2.1 8 7 115.8 21.6 97 41.9 8.4 1114 78.8 15.1

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Table 2. Biological characteristics for all mollusc species and physical parameters of stations sampled across a pneu ma tophore boundary in theBay of Rest in northwestern Australia.

Distancefrompneumatophoreboundary m )

M 1M 2M 5M1 0M1 5M2 0M3 0M5 0Means

A1A2A5A1 0A1 5A2 0A3 0A5 0Means

TotalSpecies

1812121012131817

14.0 ± 3.2

1288

126

1083

8.4 ± 3.0

Biological

Mean spp./station±1S.D.

4.50 ± 2.072.13 ±1.551.75 ±2.192.25 ± 2.052.50 ±1.412.88 ±1.0 04.25 ± 2.554.63 ± 2.923.11 ± 1.17

3.38 ± 1.693.38 ± 0.743.13 ±1.643.25 ± 2.381.63 ±1.512.63 ±1.602.00 ± 1.931.00 ±0.932.55 ± 0.90

Mean density No./m2 ± 1S.D.)

Mean biomass g/m2±1S.D.)

Mudflat S tations8.89 ±11.133.29 ± 3.321.75 ± 2.024.01 ± 3.064.48+ 3.454.27+ 4.266.65+ 6.665.74 ± 5.374.89+ 2.19

0.4 ± 0.60.3 ± 0.70.1 ±0.10.4 ± 0.80.1 ±0.10.2 ± 0.20.2 ± 0.20.5 ±1.10.3 ± 0.2

vicennia Stations10.50+11.3113.02 ± 9.6022.82 ± 24.2410.92 ± 11.304.62+ 5.727.14+ 3.816.58+ 8.372.52 ± 3.339.76+ 6.31

1.9 ±1.72.1 ±2.15.0 ± 3.82.4 ± 2.11.5 ±1.80.9 ± 0.81.3 ±1.30.7 ±1.32.0 + 1.3

Grain size 0±1S.D.)

2.68 ± 0.222.59 + 0.222.78 ± 0.282.68 ± 0.262.70 ± 0.252.69 ± 0.252.64 ± 0.292.70 ± 0.262.68 ± 0.05

2.78 ± 0.232.74 ± 0.282.85 ±0.172.86 ±0.122.88 ±0.133.00 ±0.1 42.83 ± 0.212.84 ±0. 142.85 ± 0.08

Shore cm ±

height1S.D.)

-3.3 ± 3.3-6.6-9.6

-12.1-13.6-16.3-17.4-24.5-12.9

+4.0+9.1

+ 18.6+30.8+37.6+46.0+ 50.8+57.9+31.8

±5.4±8.1±9.5±9.8+ 8.4±7.5±9.7±6.7

± 2.1± 3.4± 8.6± 11.4± 7.6± 13.2±10.2±11.8±19.8

pH ±1S.D.)

7.0 ± 0.57.0 ± 0.47.1 ± 0.46.9 ± 0.47.0 ±0.57.1 ±0.57.2 ± 0.57.4 ± 0.67.1 ±0.1

6.9 ± 0.47.0 ± 0.37.0 ± 0.36.9 ± 0.47.1 ±0.57.2 ± 0.57.2 ± 0.57.1 ±0.67.1 +0.1

Physical

Salinity( o)

454342424342424042

45

4645444244434344

Temperature °C± 1S.D.)

22.3 ± 3.322.6 ± 3.023.0 ±3 .122.8 ± 3.522.5 ± 3.123.0 ± 3.522.9 ± 3.122.8 + 3.722.7 ± 0.3

21.6 ±3.621.4 ±3.621.3 ±4 .021.2 ±3.021.6 ±3 .621.2 ±3.521.0 ±3.921.7 ±3 .721.4 ±0 .2

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MANGROVE MOLLUSCS

STATION87

Mudfla t Avicennia

T 0.5-

Fig. 1. Dendrogram of mollusc associations calculated by the weighted pair group method with averagelinkage in stations sampled across a pneumatophore boundary in the Bay of Rest in northwesternAustralia.

mollusc assemblage can be c haracterised by Cer-•ithidea cingulata and Marginella sp., while theAvicennia grouping can be characterised by Ter-ebralia sulcala and Cerithium coralium.

Shore height increased consistently across themudflat and into the tree zone (Table 2). Theother physical factors fluctuated erratically with-out demonstrating consistent patterns. Linearcorrelations were performed on all possiblecombinations of the biological factors of totalspecies, species per station, density, and bio-mass using the raw data, not the derived aver-ages on Table 2, and the physical factors ofsediment grain size, shore height, pH, salinity,and sediment surface temperature. Significantrelationships (t-tes t; p < 0.05) w ere found onlybetween the number of species and depth andsediment surface temperature. In both cases ther2 value was below 0.09. The same analyses wereperformed for all combinations of biological andphysical factors separately in each habitat. Forthe mudflat stations significant relationships (t-test; p < 0.05) were obtained for the number ofspecies versus depth, distance from the pne-umatophore boundary interface, and sedimentsurface temperature. The greatest r2 was 0.13for number of species versus distance from the

pneumatophore boundary. For the Avicenniastations significant relationships (t-test;p < 0.05) were obtained for number of species,density and biomass versus both depth and dis-tance from the pneumatophore boundary. Ingeneral these r2 values were higher, but thegreatest was only 0.23 for the number of speciesversus distance from the pneumatophoreboundary.

Epifaunal species dominated on the mudflatin all parameters measured: number of species,density and biomass (T able 3). Infaunal specieswere an impo rtant com ponen t, with 32 of thetotal number of species, 24 of the density and29 of the biomass. Epifaunal species domi-nated in Avicennia stations; there were onlytwo infaunal species which had an insignificantportion of the total density and biomass. Themudwhelk genus Terebralia, with two specieshad a density of 40.7/m2 and a biomass of13 4 g/m2. The remaining eighteen epifaunalspecies together had a density of 38.1/m2 and abiomass of 1.7 g/m2. Table 3 also shows thatepifaunal species were dom inant in both the treezone and the pneum atophore zone; only isolatedinfaunal species occurred in either zone.Arboreal molluscs do occur in low numbers in



88 F.E. WELLS

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the Bay of Rest (Wells, 1983; 1984), but nonewas encountered in the present study.

DISCUSSION

Cantera, Arnaud and Thomassin (1983) ana-lyzed gastropod species reported from man-groves in geographical areas throughout theworld, and concluded that the gastropods foundamong mangroves are primarily a soft bottomfauna and only 20 of the species are restrictedto the tree area. Despite the overlap in specieswith the mudflat, the mollusc assemblage inAvicennia in Bay of Rest is substantially dif-ferent from that which would occur in theabsence of mangroves. While arboreal speciesof molluscs were not encountered in the presentstudy they do occur in low numbers amongAvicennia (Wells, 1984). Teredo are present inan unknown, but presumably low, density in thetrees themselves. The dense root mat in thesediment of the Avicennia zone prevents infau-nal molluscs from living in the area (Wells,1984). This is a general feature of mangroveswhich has been reported by Berry (1963), Mac-nae (1967), Brown (1971), Sasekumar (1975),Wells and Slack-Smith (1981) and Wells (1984).The association of molluscs among Avicenniawas dominated in terms of density and biomassby two species of Terebralia, which occur onlyamong m angroves. Of the two species T . sulcatalives lower down the shore and is particularlyabundant in the seaward pneumatophore zone(Wells, 1980; present study). The dense popu-lations of these snails would attract potentialpredators, particularly fish. Rays are majorpredators of invertebrates in the Bay of Restand the holes they make in digging invertebratesout of the sediment are abundant on the m udflat.The rays seem to be largely unable to feedamong the pneumato phores, and the populationof T. sulcata is thus protected by the presenceof pneumatophores.

The pneumatophore zone in the Bay of Resthas been shown to have a greater density andbiomass of molluscs than either the adjacentmudflat or th e actual tree z one. The n umber ofspecies of molluscs in the pneumatop hore areais.intermediate between the two adjacent areas.There is evidence in the literature that this is ageneral feature of molluscs in mangroves. Pla-zait (1975) working in Fiji and Murty and Rao(1977) working in India both found the densityof molluscs to be greatest near the fringe of theforest and to decrease progressively into thetrees. Richmond and Ackerman (1975) working



MANGROVE MOLLUSCS 89

in Fiji found that oysters occurred only at theseaward fringe of the forest. Unfortunatelythese studies provide evidence in favour of thedecline in molluscs among the trees being ageneral feature but they did not compare mol-luscs at the seaward fringe with adjacentmudflats. The question of whether the pneu-matophore fringe has a greater density and bio-mass of invertebrates other than molluscs hasnot been answered. Kolehmainen and Hildner(1975) showed that the greatest biomass ofinvertebrates in a Puerto Rican mangroveoccurred on prop-roots of Rhizophora whichhung into the channels and did not touchbottom; biomass declined in the tree zone.While the pattern is the same as shown herefor molluscs the Puerto Rican situation can beconsidered to be a fouling community, anentirely different habitat.

The question of why the pneumatophorefringe has a greater density and biomass of mol-

luscs than the adjacent mudflat or tree zonemust also be considered. Increase d feeding timecould help to explain the greater density andbiomass of molluscs among the pneumatophoresthan in the trees, but if this was the case densityand biomass on the mudflat should be greaterstill, and they are not. If the basic source ofnutrition for marine invertebrates in the Bay ofRest is the breakdown of mangroves intodetritus as proposed earlier (Wells, 1983), itcould be expected that density and biomasswould be greater at source in the Avicenniazone than in the mudflat. The molluscs whichdominated in the Avicennia zone are in factdependent on the trees for their nutrition. Pla-zait (1977) has shown that juvenile Terebraliapalustris feed on microflora on the mangrovesurface but adults consume fallen leaves. Yipp(1980) found vascular plant tissue to be a majorcomponent of gut contents of T. sulcata in asmall mangrove in Hong Kong. V ermeij (1973)has suggested that the adverse physicalconditions, particularly acidity, make survivalin the tree zone difficult for molluscs with cal-careous shells. If, as could be expected, theadverse physical factors in the tree zone areameliorated at the seaward margin, an increaseddensity and biomass of molluscs could beexpected there.

ACKNOWLEDGEMENTSC. Bryce provided considerable assistance in thefieldwork, under difficult conditions, under-taken for this study. Mr and Mrs J. Lefroy,

owners of Exm outh Gulf Station, were generousin providing access to the Bay of Rest. M. Bez-ant treated the sediment samples. Dr R. Blackof the University of Western Australia kindlyperformed the rarefraction analysis. Dr R.Humphreys and Ms D. Jones critically read adraft of the manuscript.

REFERENCES

BERRY, A.J. 1963. Faunal zonation in mangroveswamps. Bulletin of the National Museum ofSingapore, 32, 90-98.

BROWN, D.S. 1971. Ecology of Gastropoda in aSouth African mangrove swamp. Proceedings ofthe Malacological Society of London, 39, 263-279.

CANTERA, J. , A R N A U D , P. THOMASSIN, B.A. 1983.Biogeographic and ecological remarks on molluscandistribution in mangrove biotopes. 1. Gastropods.Journal of Molluscan Studies Supplement), 12A10-26.

DAY, J.H. 1975. The mangrove fauna of Morrumbene

Estuary, Mozambique. In: Proceedings of theInternational Symposium on the Biology and Man-agement of Mangroves, (G. Walsh, S. Snedekar andH. Teas eds.), 2, 415-430.

EVINK, G.L. 1975. Macrobenthos comparisons inmangrove estuaries. In: Proceedings of the Interna-tional Symposium on the Biology and Managementof Mangroves (G. Walsh, S. Snedekar and H. Teaseds.), 1, 256-285.

KOLEHMAINEN, S.E. HILDNER, W.K. 1975.Zonation of organisms in Puerto Rican red man-grove Rhizophora mangle L.) swamps. In: Pro-ceedings of the International Symposium on theBiology an d Management of Mangroves (G . Walsh,S. Snedekar and H. Teas eds.), 1, 357-369.

MACNAE, W. 1967. Zonation within mangrovesassociated with estuaries in North Queensland. In:Estuaries (G.H. Lauffs ed.), Publications of theAmerican Association for the Advancement ofScience No. 83, 432-438.

MILWARD, N.E. 1982. Mangrove-dependent biota.In: Mangrove ecosystems in ustralia (B.F. Cloughed.), Canberra, Australian National UniversityPress, pp. 121-139.

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