DISTRIBUTION AND POPULATION DYNAMICS OF THREE POPULATIONS OF SIPHONARIA ON ROCKY INTERTIDAL SHORES IN HONG KONG

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  • /. Moll. Stud. (1994), 60, 431-443 The Malacological Society of London 1994

    DISTRIBUTION AND POPULATION DYNAMICS OF THREEPOPULATIONS OF SIPHONARIA ON ROCKY INTERTIDAL

    SHORES IN HONG KONG

    J .H. LIUThe Swire Marine Laboratory, The University of Hong Kong, Hong Kong

    (Received 17 September 1993; accepted 16 June 1994)

    ABSTRACT

    The pulmonate limpets Siphonaria japonica andSiphonana sirius occur over a wide range of localhabitat types in terms of exposure to wave action andsalinity. This is a study of these two species on threedifferent shore types in Hong Kong, ranging from anextremely exposed, high salinity (32-35%o) shore atCape d'Aguilar to a sheltered, low salinity (1633%o)shore at Tai Lam Chung. Both species are restrictedto the low shore, year round. 5. japonica is a winterbreeder and recruitment occurred between Octoberand January. The recruitment of 5. sirius could not berecognised from the size-frequency histograms. Thealgal standing crop at Tai Lam Chung was higher thanat Wu Kwai Sha during the winter period, i.e., be-tween October and April. Seasonal fluctuations ingrowth rate were recorded for both Siphonariaspecies with the time of fastest growth occurring inwinter. S. japonica grew faster at Tai Lam Chungthan at Wu Kwai Sha. Food availability is thought tobe an important factor affecting growth.

    INTRODUCTION

    Although much work has been done on thepopulation biology of prosobranch limpets (seereview by Branch, 1981), most studies onSiphonaria have focused on activity rhythmsand homing behaviour (Cook, 1969, 1971,1976; Thomas, 1973; Cook & Cook, 1975,1978,1981; Garrity & Levings, 1983; Verderber etal., 1983; Branch & Cherry; 1985; Branch,1988) and competition (Creese & Underwood,1982; Ortega, 1985; Sutherland, 1986; Lasiak &White, 1993). Ortega (1987) studied habitatsegregation and seasonal changes in density ofS. gigas Sowerby and S. maura (Sowerby).Johnson and Black (1982) studied the variationin four polymorphic enzymes of an undescribedspecies of Siphonaria, from a rocky shore atRottnest Island, Western Australia. Little in-formation is, however, available on the popula-tion biology of species of Siphonaria and this ismainly from temperate shores. For example,

    Quinn (1988a, b) has studied the ecology of theintertidal pulmonate limpet S. diemenensisQuoy & Gaimard in terms of population dy-namics and food availability and reproductivepatterns and energetics. The population dy-namics of S. denticulata and S. virgulata havealso been studied by Creese (1981).

    Siphonarid limpets are common on the rockyintertidal shores of Hong Kong but informationon them is restricted to a general study of theiroccurrence and vertical distribution (Morton &Morton, 1983). Three species occur, i.e.,Siphonaria japonica (Donovan, 1834), Sipho-naria sirius (Pilsbry, 1894) and Siphonaria atraQuoy & Gaimard, 1833 (Christiaens, 1980,1982). The present study investigates the distri-bution, abundance and population dynamics ofthe three Siphonaria populations on three HongKong shores at Cape d'Aguilar, Wu Kwai Shaand Tai Lam Chung (Figure 1). Such shoresexperience a variety of hydrographical con-ditions and this study aimed at determining howeach species responded to such differences interms of patterns of recruitment, growth andpopulation dynamics.

    MATERIALS AND METHODS

    Stations and species studied

    Cape d'Aguilar is located on the south-eastern tip ofHong Kong Island. As predominantly easterly windsprevail throughout the year, the shore experiencesheavy wave action and is characterized by near-normal salinities (32-35%o). Wu Kwai Sha, located inthe embayment of Tolo Harbour, is protected fromheavy wave action and experiences salinities of be-tween 28-33%o. Tai Lam Chung is a moderatelysheltered shore located in the western estuarine re-gion of Hong Kong. Fresh water issuing from thePearl River affects salinity, causing it to vary over awide range (16-33%o).

    The substratum at Cape d'Aguilar is a coarse darkgrey tuff. It forms thick massive, beds with no inter-nal stratification (Allen & Stephens, 1971). The

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  • 432 J.H. LIU

    LAW UA BLAND

    D NIHEPtO0

    ETAOLOLAR

    @* WMLAII ISLAND

    POTtKOBOUP

    Figure 1 A map of Hong Kong showing the three study sites.

    shore, in this area, falls gently to the sea formingpools and crevices. The shore substratum whichSiphonaria inhabits at Wu Kwai Sha is granitic rocks(Allen & Stephens, 1971). The shore falls steeply tothe sea and comprises broken granitic rocks withmany crevices. The Sung Kong Granite of Tai LamChung is uniformly coarse-grained (Allen andStephens, 1971) and the shore falls smoothly andgently to the sea.

    Siphonaria japonica occurs mainly on shelteredshores but a few were found at Cape d'Aguilar.Siphonaria sirius tends to be a species of near-normalsalinities, common at both Cape d'Aguilar and WuKwai Sha but, at the latter, not available in sufficientnumbers to be sampled and only a few were found atTai Lam Chung. Siphonaria atra occurs on bothexposed and sheltered shores, i.e., Cape d'Aguilarand Wu Kwai Sha but was present in insufficientnumbers to be sampled. S. atra and 5. sirius from WuKwai Sha were not studied.

    Distribution and abundance

    To investigate the distribution of the limpets, fieldwork was undertaken at Cape d'Aguilar, Wu KwaiSha and Tai Lam Chung (Figure 1) between August1987 and December 1988. The vertical distributionand limpet abundance were assessed using 0.25 m2

    quadrats placed at 1 m intervals down transect lines

    set from Extreme High Water Spring Tide (EHWST)to the waters edge (ELWST). Height (m) of thequadrats above Extreme Low Water Spring Tide(ELWST) were calculated from the profiles obtained.The distribution and abundance of limpets were re-corded monthly in terms of the numbers of limpets.0.25 m~2 along the transect.

    Chlorophyll analysis was used to estimate theabundance of the algal assemblage present at eachsite, i.e., encrusting algae, microalgae and filamen-tous algae. Samples were obtained every threemonths. Usually, six randomly-chosen rock samples,each of about 10 cm2, were chopped from the rockbelow mid tide level (MTL) where the limpets occur-red. Samples were analysed within 24 hours of collec-tion, usually after overnight storage in an air-tightand light-sealed container placed in a refrigerator(4 C). Exposure to strong light and high tempera-tures were also avoided in transit. Chlorophyll wasextracted from each sample using the acetone extrac-tion technique of Parsons et al. (1984).

    Size-frequency histograms

    Limpets were sampled monthly, i.e., from August1987 to March 1989 for Siphonaria sirius at Caped'Aguilar; from August 1987 to December 1988 forSiphonaria japonica at Wu Kwai Sha and fromNovember 1987 to December 1988 for S. japonica at

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  • POPULATION BIOLOGY OF SIPHONARIA 433

    3 20

    10

    15 J_CD

    15 1 0 -szCO

    5-

    0

    jjn

    19871988

    N D J F M1987 1988

    N D

    Figure 9 The estimated mean shell length of each cohort of the population of Siphonaria japonica at Tai LamChung. The number represents the year of birth of each cohort.

    Table 1. The chlorophyll content of rock samplestaken from the three sites, sampled betweenMarch 1988 and March 1989: mean SD chloro-phyll u.. cm"2; n = 6.

    Sites Mar. Jun. Sept. Dec. Mar.1988 1989

    Cape d'Aguilar 3.54 1.56 1.70 2.53 3.87(1.22) (0.68) (0.55) (1.22) (1.56)

    Wu Kwai Sha 3.24 1.30 1.48 2.57 3.16(1.05) (0.65) (0.54) (1.04) (1.07)

    Tai Lam Chung 5.5 0.86 1.08 6.17 6.57(1.19) (0.24) (0.24) (1.38) (1.02)

    The population dynamics of a species can beexpected to differ at sites where different bioticand abiotic environments are encountered. Thegrowth and population dynamics of the samespecies of limpets may also be expected to varyamong different habitats (Sutherland, 1970,1972; Branch, 1975, 1976; Creese, 1980; Work-man, 1983; Fletcher, 1984). Lewis and Bowman

    (1975) stressed the importance of biologicalhabitat upon growth and pointed out thatchanges in the surrounding species compositionwould produce corresponding changes in thelimpet populations. Estimates of growth bySiphonaria japonica indicate that the popula-tion at Tai Lam Chung grew faster than that atWu Kwai Sha (Figure 10).

    On the shore at Wu Kwai Sha, the mostabundant herbivores were Patelloida pygmaea(Dunker), Siphonaria japonica and S. sirius.These three species characterize the middle andlower shore. Other grazing gastropods such asMonodonta labio (Linnaeus) and Nerita albi-cilla Linnaeus were also common on the middleand lower shore. The shore at Tai Lam Chungwas occupied only by 5. japonica, from Novem-ber to April, with other herbivores occurring inlow numbers. Liu (1992) has shown that thesiphonariid radula comprises numerous fineteeth of uniform size, capable of rasping macro-algae but not of scraping into rock. Competi-tion for food may lead to different growth rates

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  • 440 J.H. LIU

    Table 2. Mean air temperature and relativehumidity of Hong Kong between 1987-1989 (Datafrom: Monthly Weather Summary 1987-1989,Royal Observatory, Hong Kong).

    Jan.Feb.Mar.Apr.MayJun.Jul.Aug.Sep.Oct.Nov.Dec.

    1987

    M.A.T.(C)

    17.318.321.321.925.027.528.928.627.325.721.816.8

    RH%

    717886868982828179807960

    1988

    M.A.T.(C)

    17.916.316.820.826.628.629.027.827.624.419.917.7

    R.H.%

    778184788678818577776668

    1989

    M.A.T.(C)

    15.716.618.622.025.127.528.828.928.125.121.517.8

    R.H.%

    807475868683798077736974

    in S. japonica occupying the two shores, asgrowth is affected by food intake (Paine, 1969;Bosman & Hockey, 1988). However, the foodresources available for S. japonica on the twoshores were also different. At Tai Lam Chung,from October to April, green filamentous algaeformed a thick, soft, layer which coveredalmost 100% of the rock surface from the low tomiddle shore. At Wu Kwai Sha, the encrustingalgae Hildenbrandia prototypus and Brachy-trichia maculans formed patches on the shoreand filamentous algae formed a thin film. Algalproduction at Tai Lam Chung was higher thanat Wu Kwai Sha during the winter period, i.e.,between October and April.

    A variety of factors are known to affect thegrowth rate and ultimate size reached by inter-tidal gastropods. Much of the difference ingrowth rate has been attributed to food avail-ability on the shore (Stearns, 1976; Creese andUnderwood, 1982; Underwood, 1984a, b;

    30-i

    25 -

    2 0 -

    15-

    5-

    0

    Wu Kwai Shaa Tai Lam Chung

    --*-'

    i i i i r i i i r

    A S O N D J F M A M J J A S O N D

    1987 1988Figure 10 Growth curves of Siphonaria japonica at Wu Kwai Sha and Tai Lam Chung.

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  • POPULATION BIOLOGY OF SIPHONARIA 441

    Bosman and Hockey, 1988). Paine (1969) sug-gested that individuals of Tegula grew slowly athigh levels on the shore but, after reaching acertain adult size, moved downshore wherewith increased food availability, grew faster.Quinn (1988a) studied the population dynamicsof Siphonaria diemenensis Quoy & Gaimardand found that food availability was a majordeterminant of growth rate, i.e, limpets in thelower zone where food supply was more con-stant were larger and grew faster than those inthe upper zone where food supply showedstrong seasonal variation. It has also beenshown that growth rates of intertidal gastropodsincrease when densities are experimentally re-duced (Frank, 1965; Sutherland, 1970; Haven,1973; Underwood, 1976). Bosman and Hockey(1988) have studied on the influence of primaryproduction rate on the population dynamics ofPatella granularis Linnaeus in South Africa.They found that the most significant correlateof limpet growth was the rate of algal produc-tion. That is, there was more rapid growth bylimpets on nutrient-rich shores which, com-bined with the effects of predation by AfricanBlack Oystercatchers (Haematopus moquiniBonaparte), led to differences in size structureand life-history patterns of the populations in-habiting enriched and unenriched shores.

    Food availability may be the limiting factoraffecting growth as density increases. Intra-specific competition at increased densities hasbeen shown to greatly affect the growth andmortality of species of Cellana (Underwood,1978; Creese and Underwood, 1982). Highdensities of other grazing gastropods have alsobeen shown to affect the growth and mortalityof species of Cellana (Underwood, 1978). Thus,increased intra- and inter-specific competitionmay be a factor responsible for the observedslow pattern of growth of the Siphonaria japon-ica population at Wu Kwai Sha.

    Seasonal fluctuations in the rate of growthwere seen in the Siphonaria japonica popula-tions from both Wu Kwai Sha and Tai LamChung with the times of fastest growth beingrelated to the times when the weather is coolerand the primary production rate is higher, i.e.,the availability of food is higher, between Octo-ber and March.

    In this study, the lifespans of Siphonariajaponica from both Wu Kwai Sha and Tai LamChung were estimated to be approximately oneyear. Quinn (1988a) showed that the averagelongevity of Siphonaria diemenensis from upperzone to be between 18 months and 2 years whilethose from lower zone likely to be at least 3 to 4

    years. In Hong Kong, from October to Aprilwhen rocky intertidal habitats are relativelywet, cool and protected from intense sunlight,fast-growing algae (blue-green algae, diatoms,filamentous and membranous species of red andgreen algae) and macroalgae species coverthem. During the hot, dry and more sun-exposed summer, algae disappear, leaving themid and high shore encrusted with only patches.The radula of Siphonaria consists of numerousfine teeth of uniform size, capable of raspingmacroalgae but not of scraping into the rock(Liu, 1992). It seems that the lack of food insummer for Siphonaria may affect its lifespan.

    Sources of mortality were not directly deter-mined in this study. Associated with harsh sum-mer conditions, temperature and desiccationare considered the most important physical fac-tors causing limpet mortality on the shore.Quinn (1988a) demonstrated that the annualmortality rates of Siphonaria diemenensis(adults) from upper shore were significantlygreater than those from lower shore and theseasons of maximum mortality for those in theupper shore were summer and autumn whilethere was no clear seasonal trends for those inthe lower shore. In summer, air temperatures inHong Kong may reach 36 C and temperaturesof 50 C have been recorded from rock surfacesin August (Liu & Morton, 1994). Many limpetswere observed dead on the bare rocks and inintertidal pools at Cape D'Aguilar during thesummers of 1988, 1989 and 1990.

    Predation is also a source of mortality. Tong(1986) rep...

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