MARINE ECOLOGY PROGRESS SERIESMar Ecol Prog Ser

Vol. 335: 133–141, 2007 Published April 16

INTRODUCTION

The question of how a community of ciliate species canremain in an environment that is mixed and replacedwith water from the open sea twice a day was posed morethan half a century ago by Fauré-Fremiet (1948), follow-ing his study of the ciliate community in a rocky intertidalbasin close to his laboratory in Concarneau (France). Asfar as we are aware, this is the only investigation of itskind recorded in the scientific literature. Fauré-Fremiet(1948) found only a handful of ciliate species, but made 2discoveries—that the community composition remainsrelatively constant over time, although the water is re-newed with every tide, and (in the case of the ciliateStrombidium oculatum), the existence of a ‘tidal rhythm’.The latter has been investigated in more detail (Jonsson1994, Montagnes et al. 2002), but the study of communityassembly, ciliate species richness in rocky intertidal pools,and the adaptations that allow them to thrive there,largely remain a ‘closed book’. Here we report some ofour recent observations and discoveries concerning theecology and natural history of ciliates in rock pools.

MATERIALS AND METHODS

Sampling sites. We studied intertidal rock pools atpristine sites on the Isles of Scilly (also known as ScillyIslands or ‘Scillies’), an archipelago of about 200 is-lands and rocks, of which only 5 are inhabited, located45 km off Land’s End (the most southerly point of theUnited Kingdom mainland) (Fig. 1). The islands arelow lying and mainly composed of flat topped granite.We selected 2 islands to investigate the ciliates livingin intertidal rock pools, i.e. pools that are submerged athigh tide and also subjected to the force of Atlantic‘breakers’. The islands were Bryher, whose west coastfaces the full force of the ‘breakers’, and St. Agnes.Three rock-pools were investigated: 1 on a beach atSt. Agnes (Fig. 2A,B), and 2 at Bryher—one on thebeach, and the other amongst an area of large rocks(Fig. 2C,D). We collected the samples on 23 and 25October 2005, respectively, and examined them dailyover a period of 5 d. We aimed to retrieve and identifyas many ciliate species as possible, and to do this, wecollected and subsequently handled the samples in a

© Inter-Research 2007 · www.int-res.com*Email: gent@ceh.ac.uk

Exceptional species richness of ciliated Protozoa inpristine intertidal rock pools

Genoveva F. Esteban*, Bland J. Finlay

School of Biological & Chemical Sciences, Queen Mary, University of London, London E1 4NS, UK

ABSTRACT: Marine intertidal rock pools can be found almost worldwide but little is known about themicroscopic life forms they support, or the significance of regular tidal inundation. With regard tociliated Protozoa in rock pools, little progress has been achieved since 1948, when the question wasfirst posed as to how a community of ciliates can remain relatively constant in a rock pool that isflushed twice a day. Here we show that local ciliate species richness is very high and that it may per-sist over time. This elevated species richness can be attributed to ciliate immigration with the in-coming tide, and also to the resident ciliate community that withstands tidal flushing. A 15 cm deepintertidal rock-pool no bigger than 1 m2 on the island of St. Agnes (Scilly Islands, UK) contained atleast 85 ciliate species, while a 20 cm-deep rock pool with roughly the same area on Bryher (also inthe Scilly Islands) yielded 75 species. More than 20% of the global number of described marine inter-stitial ciliate species was recorded from these rock pools. We explore the paradox of a persisting com-munity assembly living in an ecosystem that is physically highly dynamic.

KEY WORDS: Marine ciliates · Marine biodiversity · Intertidal rock pools

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Mar Ecol Prog Ser 335: 133–141, 2007

variety of ways. For sample collection we used (1) ster-ile, 50 ml centrifuge polypropylene tubes, (2) Nalgenebottles (250 ml), and (3) sterile 25 cm2 polystyrene cellculture flasks containing 2 ml of soil extract medium(Catalogue of UK National Culture Collection, UKNCC2001). Rock pools containing a layer of fine sand weresampled with corers (internal diameter 2 cm), and thesamples placed in sampling tubes. We used sterile,single-use pipettes to remove sediment and other‘material’ from crevices, rock surfaces, and the floor ofeach rock pool. This material was a combination ofmainly fine sand, algae, and pieces of decomposingmacroalgae. Some was thoroughly bonded with therock surface and could be removed only with a smallspatula. Thereafter it was placed in sterile samplingtubes, leaving a head space of a few cm. A variety ofmarine macro-algae was also collected for examinationof sessile Protozoa. The granulometry of the sand in therock pools ranged from very fine to slightly coarse. Thedepth of the rock pools varied from 10 to 20 cm andthey were generally up to 2 m in length with a maxi-mum area of up to 1 m2 (Fig. 2B,D). All samples wereinvestigated in a small laboratory on shore as soon aspossible after sampling. We also set up manipulationexperiments (e.g. Finlay et al. 1996, Fenchel et al.1997) based on a variety of culture media, oxygen andredox gradients (to encourage growth of anaerobic cil-iates) and various culture vessels. For each ‘manipula-tion’, we used 2 ml subsamples of the original samples.

Cultures for incubation were preparedimmediately after collection by placing10 ml subsamples in culture vessels with2 ml of soil extract medium, to encouragerare and encysted forms (if any) to growout and be recorded. All samples andcultures were kept in a 20°C incubatorthroughout the investigation. Fresh sam-ples were assessed every day for previ-ously unrecorded ciliate species during ourstay on the Scillies, and twice weeklythereafter. The assessment of species rich-ness was based on 1 ml subsamples of theoriginal samples and cultures, and pickingout individual specimens. We followed theice extraction method of Uhlig (1966) toextract the larger ciliates without damag-ing them.

Ciliate species were identified afterimpregnating specimens with silver (Fer-nández-Galiano 1994, Esteban et al. 1998)when sufficient cells were available. Iden-tification of species represented by fewindividuals (e.g. inadequate for confirma-tion with silver impregnation) were identi-fied to putative species of the living form

with recourse to our extensive bibliography of previ-ously published marine material (e.g. see Esteban &Finlay 2004) and other well-established identificationguides, e.g. Kahl (1935). Single specimens of ciliatesobserved once only were identified to genus level.

Electron microscopy. In order to investigate thebacteria upon which the rock-pool ciliates might feed,subsamples from the Bryher open-ocean rock poolwere used to make shadow-cast direct preparations fortransmission electron microscopy (TEM). Preparation,handling, and examination of samples for TEM wascarried out by K. J. Clarke (Freshwater BiologicalAssociation, UK). Replicates were washed with dis-tilled water and centrifugation to remove the salts. Thewashed material (a 3 to 4 µl-sized drop) was placeddirectly on Formvar-coated specimen grids, then fixedwith OsO4 vapour, air-dried at room temperature, andfinally shadowed with chromium. Examinations weremade using a JEOL 100CX TEMSCAN.

Data handling. In order to assimilate and comparedata sets, we calculated size-frequency distributions(SFDs). The main advantage of using SFDs with a loga-rithmic body size axis is that the shape of the distribu-tion can immediately be compared with other distribu-tions to determine (e.g.) where the modal size class isand whether the distribution is skewed towards large orsmall species. We compared the SFDs for the rock-poolciliates with the SFDs of (1) the 168 species of ciliateswe recorded from the Isles of Scilly, (2) 170 ciliate spe-

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Fig. 1. Location of the Isles of Scilly (UK). d: sampling sites on St. Agnes and Bryher

Esteban & Finlay: Ciliates in intertidal rock pools 135

Fig. 2. (A,B) St. Agnes: (A) beach with rocks and sand, (B) close-up of rock pool investigated. (C,D) Bryher: (C) open-ocean rocky-coast of Bryher (arrow shows pool sampled), (D) close-up of rock pool. (E–G) Live specimens of Aspidisca magna (lightmicroscopy): (E) conspicuous ribbed dorsal surface (arrowheads); (F) ventral surface of cell (phase-contrast), top arrowheadindicates tight group of frontal membranelles with cilia directed forward, middle arrowhead frontoventral cirri (not all are infocus on this field plane) and bottom arrowhead transverse cirri; (G) ventral surface (Nomarski interference contrast). (H) Dio-phrys scutum (brown specimen) trying to ingest large A. magna. (I) Coleps pulcher (Nomarski interference contrast). (J,K) Speci-mens of Aspidisca pulcherrima impregnated with silver-carbonate; in (J) arrowheads indicate cytoplasmic spines (hazy in back-ground) that characterise this species (Ma = horseshoe-shaped macronucleus); in (K) ventral surface shows arrangement

(arrowheads) of frontoventral cirri, frontal membranelles and transverse cirri. All scale bars = 50 µm

Mar Ecol Prog Ser 335: 133–141, 2007

cies from Nivå (a brackish, estuary-like habitat north ofCopenhagen, Denmark, that is permanently coveredby the waters of the Øresund; Finlay & Fenchel2004a,b), and (3) the global list of interstitial marineciliate species taken from Hartwig (1980) and Carey(1992). We selected (2) and (3) because they are theonly data sets available that include a full checklist ofspecies and the body length of each species. The Excelfile for Nivå includes data for 170 ciliate species foundin interstitial sandy sediment, and is available on-line(Finlay & Fenchel 2004b). We compiled a further Excelfile for the recorded global list of interstitial marineciliate species according to Carey (1992). The prefaceof Carey (1992) remarks that ‘the book is the first identi-fication guide to include all ciliate species recordedfrom marine sands’. Although a few new species havebeen discovered since the book was published, theirnumber is not great enough to significantly alter thesize-frequency distributions. We used the 826 speciesfrom Carey (1992) after removing those species whoseusual habitat is not marine (e.g. Loxodes spp.) We as-signed a geometric mean size to each species basedmainly on body sizes given in Carey (1992), and thedata set thus prepared can be used as a global inven-tory of interstitial ciliates species. We then classified thedata of body length into 11 logarithmic size-classes, 4size classes per log cycle ranging from 7.5 to 2370 µm.Ciliates were allocated to their appropriate size classes.

RESULTS AND DISCUSSION

Having investigated sandy sediment and water sam-ples from 3 rock pools in the Scillies, we identified 168marine ciliates to genus and/or species level. The sam-ples from St. Agnes and from 1 of the 2 rock poolson Bryher yielded 85 ciliate species, and 75 species,respectively (Table 1) whereas the samples from Bry-her’s beach yielded 50 species (Table 1). The commu-nity in the rock pools is interesting because only 10nominal species were recorded simultaneously fromall 3 rock pools: Cyclidium glaucoma, Diophrys appen-diculata, D. scutum, Euplotes elegans, Litonotus duplo-striatus, Plagiopyla frontata, Pleuronema coronatum,Trachelostyla pediculiformis, Uronychia transfuga andUropedalium pyriforme. This suggests that the remain-ing species richness in rock-pools is fairly hetero-geneous. The ciliate Aspidisca magna (Fig. 2E–G) wascommon to both rock pools on Bryher but we did notobserve it in rock pool samples from St. Agnes. The cil-iates in the rock pool on the beach of Bryher were basi-cally a mixed community of the species found in therock pools of St. Agnes and Bryher. For the purpose ofthis investigation, we focus on these rock pools at St.Agnes and the open-ocean rock pool at Bryher be-

cause they presented the highest species richness.The size-frequency distributions (SFDs, Fig. 3) for bothrock pools are similar, even though that on St. Agneshad a greater number of larger ciliate species (e.g.tracheloraphids, geleids).

The St. Agnes rock pool (Fig. 2A,B) is located on abeach that is partly rock and partly sand. In contrast,the rock pool on Bryher faces the open ocean and thefull force of the breakers (Fig. 2C,D). At St. Agnes, tidalwaves regularly wash the sand on the beach surround-ing the rock pool, and this sand is probably the sourceof most of the ciliate species in the pool. As the rockpool also has a well-developed layer of fine sand thathas accumulated amongst the rocks and pebbles, thesand was assumed to offer a niche for immigrant ben-thic and interstitial ciliates. To test this, we carried outa general screening of the ciliate community from sandsamples immediately above and around the rock pool,and found that 62% of those in the surrounding sandwere also present in the pool. This value is almost cer-

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Fig. 3. Size frequency distribution of ciliate species recordedin 2 intertidal rock pools on Isles of Scilly. (A) St. Agnes;(B) Bryher (open-ocean rocks). Body size is geometric mean

for each size class

Esteban & Finlay: Ciliates in intertidal rock pools 137

Table 1. Species of ciliated Protozoa retrieved from 3 pristine intertidal rock pools on Isles of Scilly (UK): SA: St. Agnes, rock poolon beach; BO: Bryher, open-ocean rock pool; BB: Bryher, rock pool on beach; x: present

Ciliate species SA BO BB Ciliate species SA BO BB

Acineria sp. xAcineta tuberosa xActinotricha saltans xActinotricha thiophaga x xAnigsteinia clarissima xAspidisca dentata xAspidisca magna x xAspidisca major x xAspidisca cf. polypoda x xAspidisca pulcherima xAspidisca robusta xAspidisca cf. turrita (50 µm) xAspidisca sp. x xBryophyllum sp. (70 µm) xCardiostomatella vermiforme x xChaenea clavata xChilodonella sp. xChlamydodon sp. xCohnilembus cf. longivelatus xCohnilembus stichotricha xColeps pulcher xColeps tesselatus x xCondylostoma arenarium x xCondylostoma magnum xCondylostoma sp. xCothurnia sp. on Gammarus sp. xCristigera cirrifera x xCristigera media x xCristigera phoenix xCristigera setosa x xCristigera vestita x xCryptopharynx setigerus x xCyclidium citrullus xCyclidium glaucoma x x xDexiotricha sp. xDiophrys appendiculata x x xDiophrys histrix xDiophrys scutum x x xDiscotricha papillifera xDysteria appendiculata xDysteria cf. calkinsi xDysteria monostyla (200 µm) x xDysteria pusilla xDysteria recta xDysteria sp. xEnchelyodon elongatus xEpistylis sp. xEuplotes cf. trisulcatus xEuplotes charon xEuplotes crassus var. minor xEuplotes elegans x x xEuplotes eurystomus xEuplotes harpa xEuplotes moebiusi x xEuplotes vannus xEuplotes vannus var. balticus xEuplotes sp. (with green algal xendosymbionts?)

Euplotes sp. x xFrontonia marina x xFrontonia sp. xGeleia orbis xGlauconema trihymene xGruberia lanceolata xGruberia uninucleata xHartmannula acrobates xHelicoprorodon sp. xHippocomus loricatus xKentrophoros fasciolata xKentrophoros rubra xKentrophoros sp. (100 µm) x xLacrymaria coronata xLacrymaria delamarei xLacrymaria olor xLacrymaria sp. (35 µm) xLitonotus duplostriatus x x xLoxophyllum helus x

Loxophyllum sp. 1 x x xLoxophyllum sp. 2 (60 µm) x x xLoxophyllum sp. 3 (250 µm) xMesodinium pulex xMesodinium rubrum xMetacystis elongata xMetanophrys sp. xMonodinium sp. xNassulid-brownish (70–80 µm) xOxytricha sp. xParaspathidium fuscum xParaspathidium sp. xPeritromus faurei x xPeritromus gigas (350–400 µm) xPeritromus ovalis xPhilasterides/Metanophrys sp. xPlacus sociale xPlagiocampa marina x x xPlagiopogon loricatus xPlagiopyla frontata x x xPleuronema coronatum x x xPleuronema marinum x xPodophrya sp. xPorpostoma/Philasterides sp. xProrodon discolor xProrodon marinus x xProrodon morgani xProrodon teres xProtocruzia pigerrima xPrototrachelocerca caudata x xP. cf. fasciolata xPseudokeronopsis rubra x xRemanella margaritifera x xRemanella rugosa x xR. rugosa var. unicorpusculata xRimostrombidium cf. conicum xSonderia cyclostoma x xSonderia mira xSonderia vestita xSpathidium cf. extensum xSpathidium sp. 1 xSpathidium sp. 2 xStrombidium oculatum xStrombidium sulcatum/Pelagostrombidium simile x x

Strombidium sp. 1 x xStrombidium sp. 2 (35–45 µm) xStrombidium sp. 3 xStrombidium sp. 4 xTachysoma pellionella x xTrachelocerca cf. arenicola xTrachelocerca sp. (100 µm) xTrachelophyllum cf. brachypharynx xTracheloraphis cf. longicollis xTracheloraphis cf. margaritata xTracheloraphis cf. oligostriata xTracheloraphis phoenicopterus xTracheloraphis sp. xTrachelostyla caudata xTrachelostyla pediculiformis x x xTrimyema marinum xTrimyema minutum xTrochilia sigmoides x xTrochilia sp. xTrochilioides recta x xUronema elegans xUronema marinum xUronema nigricans xUronychia transfuga x x xUropedalium pyriforme x x xVaginicola cf. amphora x xVorticella elegans xVorticella cf. marina xVorticella sp. 1 x xVorticella sp. 2 (30 µm) x xWoodruffia rostrata xTotal number of species 85 75 50

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tainly an underestimation, since more trachelocercidspecies (i.e. typical vermiform, narrow and elongatedciliates adapted to live amongst sand particles) wereretrieved from the rock pool samples than fromthe neighbouring sand. In contrast, only 38% of thespecies recorded in the sands of St. Agnes were alsorecorded in the open-ocean rock pool of Bryher (wherethere is no surrounding beach).

Fig. 3 shows the SFDs for ciliates from the St. Agnesand Bryher rock pools. In both rock pools there was ahigher proportion of ciliates in the 56 to 100 µm sizeclass (mean body size 75 µm). We compared both SFDswith the SFD of (1) all ciliate species recorded from allour Isles of Scilly samples (i.e. 168 species), (2) the SFDof ciliate species from Nivå (170 ciliate species; Finlay &Fenchel 2004b), and (3) the SFD of the global listof world marine interstitial ciliates (826 species; see‘Materials and methods’).The results are shown in Fig. 4.

The SFDs for the ‘local diversity’ data sets, i.e. foreach separate rock pool, for Nivå Bay, and the Isles ofScilly, indicate a higher proportion of smaller ciliatesthan does the SFD for the ‘global diversity’ data (SFDmodes at 42 and 75 µm, respectively, i.e. 1 or 2 sizeclasses smaller; see Figs. 3 & 4), indicating a lack oflarge species or a predominance of medium-sizedspecies. Apart from Geleia orbis, no other geleiids(a genus with about 20 currently known species[Dragesco 1999]) were observed in the samples fromthe rock-pools investigated, but the results also seemto suggest that local habitats such as rock pools mayhave a limited number of available niches, especiallyfor very large ciliates. It is also important to bear inmind that the data set we obtained from Carey (1992)is 5 times bigger than that from the study at Nivå(Finlay & Fenchel 2004b) and our own data set fromthe Scillies. A larger data set from the latter regionwould almost certainly produce a bell-shaped distribu-tion, as in the Carey data set.

All ciliate species recorded from the second rockpool on Bryher (i.e. the pool on the beach) (Table 1) arebenthic ciliates commonly found in the interstices ofsandy sediments (see Fenchel 1964, Carey 1992 andreferences therein, Al-Rasheid 1996). ‘Interstitial cili-ates’ is a broad term. Strictly speaking, it refers toflattened and ribbon-shaped ciliates adapted to glideover and within the interstitial habitat, and associatedwith the contour of each sand grain (Carey 1992). Mostinterstitial ciliates (e.g. the trachelocercids) are thig-motactic and contractile. The largest free-living ciliatesever described have been found gliding amongst sandgrains, but many other ciliates thrive in this and othermarine habitats (e.g. species of Cristigera, Cyclidium,Pleuronema, Strombidium, Uronema, and Uropedalium)and, as a result, the term ‘interstitial ciliates’ is fairlycomprehensive.

From the pools on the Bryher open-ocean rocks andSt. Agnes beach, we also retrieved stalked ciliates, i.e.the peritrich genera Vorticella and Epistylis and thesuctorian genera Acineta and Podophyra. Althoughnot considered to occur in marine sands (Carey 1992),we consistently observed these genera after sedimentfrom the pool had been allowed to settle for 24 h.Sessile ciliates have been documented from marinesandy sediments before (Finlay & Fenchel 2004b).Both groups of sessile ciliates have a free-swimminglarval stage that enables them to move through sedi-ment interstices similar to other benthic ciliates. Strom-bidium spp. and Rimostrombidium sp. (ciliate generacommon in marine plankton) are also frequently ob-served in benthic interstitial sediments (Fenchel 1964,

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Fig. 4. Size frequency distributions for (A) all marine intersti-tial ciliate species (following Carey 1992; see ‘Materials andmethods’), (B) ciliate species recorded for rock pools on theScillies, and (C) interstitial ciliate species recorded in NivåBay, Denmark (data from Finlay & Fenchel 2004b). Body size

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Esteban & Finlay: Ciliates in intertidal rock pools

Carey 1992 and references therein, Al-Rasheid 1996,Fenchel et al. 1997). We too retrieved them from thesandy sediment of the rock-pools in our study. How-ever, we did not record any truly planktonic ciliates,e.g. tintinnids. Our finding of 3 species of Peritromus(P. faurei, P. ovalis, and P. gigas) is also interesting.These 3 species were present in the rock pool onBryher, but we failed to find any in the St. Agnes rockpool despite intensive search. Their apparent absenceis most probably the result of inadequate sampling.

We recorded around 80 species of ciliates in eachrock pool examined, so why did Fauré-Fremiet (1948)record so few ciliates in similar habitats? He found 15species, along with a few others, small in number andsize, which were not listed. There is no record of thetime of year when his pools were sampled, although hedid refer to extreme daily variations in temperatureand salinity within the pools as a reason for low speciesrichness, suggesting that the pools were examined inwarm weather, and were small and shallow. Neverthe-less, there may be other explanations. The rock poolsmay have lacked a layer of sandy sediment; howeverFauré-Fremiet (1948) did observe Trachelocerca (Tra-cheloraphis) phoenicopterus, which is a typical inhabi-tant of sands. Alternatively, the sandy layer may nothave been thick enough to support a greater ciliatediversity.

One of the main objectives of our investigation wasto retrieve as many ciliate species as possible, includ-ing cryptic species or those present in such low num-bers that they would normally escape detection,whereas Fauré-Fremiet wished to discover how a pro-tozoan community can remain within the limits of arock-pool environment that is flushed twice a day bythe open sea. Thus, our sampling strategy included theuse of a large range of vessels, tissue flasks, tubes, ice-extraction etc., as well as enrichment techniques (see‘Materials and methods’)—probably a very differentapproach from that of Fauré-Fremiet (1948). It is alsopossible that pristine and unspoiled environments suchas the Isles of Scilly (Faubel & Warwick 2005) supporta richer protist diversity than more impacted areas.

To gain some idea of cryptic species richness in the‘seed bank’, we plotted the cumulative number of spe-cies recorded in the 3 rock-pools over a 60 d period. Wefound more than 168 ciliate species (Fig. 5) and, ofthese, 135 were observed during our 5 d field investi-gation on the Scillies. The remaining species (>30)were retrieved during the subsequent 2 mo periodusing enrichment cultures and other experimentalmanipulations that had previously proved successful inencouraging excystment and growth of undetectedciliate species (Finlay et al. 1996, Fenchel et al. 1997).The retrieval of 30 species over 2 mo seems fairly low,but this may be an artefact peculiar to marine samples,

which tend to decay more quickly than those fromfresh waters.

The present study has shown that (1) intertidal rockpools can maintain diverse ciliate communities, (2) arelatively small-scale investigation of ciliates in rockpools yields a high local:global ratio of species, e.g. ashallow intertidal pool with a length <1 m supportedmore than 20% of the global number of describedmarine interstitial ciliates. High local:global ratiosare characteristic of small-sized organisms (Finlay &Fenchel 2004a), and there are several examples in therecent literature to support this. Agamaliev (1983) andAlekperov & Asadullaeva (1996) documented 351 spe-cies in the Caspian Sea, 318 ciliate species have beenrecorded in the Baltic (Agamaliev 1983), 450 benthicciliate species have been documented from the BlackSea (Azovsky & Mazei 2003), and Mazei & Burkovskii(2005) reported 273 benthic ciliate species in the estu-ary of Chernaya River in the White Sea (Russia) overa 2 yr period.

Ciliates and other protists living in variable environ-ments such as intertidal rock pools need to toleratevariable salinity, a degree of desiccation, and thepounding action of ocean waves. The heterogeneousciliate communities in the rock pools of St. Agnes andBryher share at least 2 characteristics. First, the specieshave all been recorded previously in other marine orsaline environments worldwide (see e.g. Kahl 1935,Dragesco & Dragesco-Kernéis 1986, Fenchel et al.1995, Al-Rasheid 1996, Mazei & Burkovsky 2005);second (as pointed out by Fauré-Fremiet 1948) theyare able to sustain a diverse community despite regu-lar wave action.

Ciliates in intertidal rock pools are closely associatedwith the surfaces of rocks and stones, creeping andsliding or attaching by means of a stalk. How do theyescape removal by tidal waves? Many small organisms(e.g. Protozoa) are small enough to live within the ben-

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Mar Ecol Prog Ser 335: 133–141, 2007

thic boundary layer, where they are subject tominimal drag. Moreover, typically ‘flat’ Protozoasuch as the amoeba Vannella spp., the ciliateChilodonella spp. and other organisms that lieadpressed to the substrate or creep over it, usu-ally project no more than about 10 µm above thesurface, so that laminar flow is rarely affected.The same mechanism probably operates in run-ning waters (Sylvester & Sleigh 1985, Fenchel1987). Baldock (1980) found a strong negativecorrelation between current velocity and thepresence of peritrich ciliates. She also mentions ariver in spate in which velocity increased to 70 cms–1 and washed off most Vorticella spp., diatomsand algae. To ameliorate drag, it is probablyadvantageous for an organism to extend its bodyparallel to the substratum, as increased drag ismore than compensated for by the increased con-tact area for adhesion (Sylvester & Sleigh 1985,Fenchel 1987), especially in the case of organismson an organic substrate, which offers muchgreater adhesive strength. As micro-organismsthat live or creep on submerged surfaces do notproject further than a few microns above the sur-face, and as the velocity of the water flow overthe surfaces is virtually zero, the drag on micro-organisms is probably negligible (Sylvester &Sleigh 1985).

In conclusion, wave force in rock pools may notbe sufficient to dislodge ciliates from submergedsurfaces, but may be an obstacle to immigrantsthat do not possess suitable morphology. Thus,we did not find a single truly planktonic ciliate(e.g. tintinnids) in the samples, but we did find astrikingly large number of species with flat mor-phology. In the pool at St. Agnes 85% and at Bry-her 75% of the species were flat or attached to asurface. Three species of Peritromus were foundcoexisting in the rock-pool at Bryher, where wealso consistently observed P. gigas (a >200 µm-long ciliate not reported since the study of Fauré-Fremiet [1924], who found and described it inpelagic samples). Other ciliates were far largerand flatter than generally reported. Most remark-ably, Aspidisca magna, a ciliate described byKahl (1935) as having a body size of up to 135 µm,and with records rarely exceeding 150 µm, wasalmost 500 µm long in our fresh field-collectedsamples. Dysteria appendiculata was twice itstypical size, as were Uronychia transfuga, Dio-phrys scutum and Condylostoma arenarium. We alsoobserved giant forms of D. scutum in laboratorycultures of samples taken from the Bryher rock poolthat were trying to ingest large specimens of Aspidiscamagna (Fig. 2H) and C. arenarium.

The body size and form of rock-pool ciliates, togetherwith their tendency to adhere to surfaces and to migrateto the benthic boundary layer, may enable their contin-uous presence in rock pools. The fact that we also foundsome anaerobic ciliates (Sonderia spp., Plagiopyla

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Fig. 6. Morphological diversity of some bacteria found in open-oceanintertidal rock pools in the Scillies. (TEM, shadow-cast direct prepa-rations). (A) Types of stalked and peritrich bacteria (most commonform of bacteria found in pristine intertidal rock pools investigated).(B) Common flagellated bacterium found in samples. (C,D) Filamen-tous prokaryotes (probably not cyanobacteria) formed by envelope orcapsule wrapped around the cell; (C) capsule is ornamented; (D) rodbacteria separated by septa (arrowheads) form filament protected byan ‘envelope’. (E) Stalked bacterium, main part of cell (central part) isabout 3 µm long; this bacterial shape has also been found in fresh-

water sediments (K. J. Clarke pers. comm.). All scale bars = 1 µm

Esteban & Finlay: Ciliates in intertidal rock pools

frontata, Table 1) suggests that the sediment at the bot-tom of the pools is not regularly suspended during hightides and may represent a relatively stable and undis-turbed habitat. There were no genuine plankton ciliatesin the pools, suggesting that these are flushed out at hightide. Finally, food availability plays a significant role inthe preservation of the resident rock-pool ciliate popula-tions: electron microscopy revealed that subsamplesfrom the Bryher open-ocean rock pool contained a richcommunity of stalked and peritrichous bacteria (Fig. 6),a food supply that withstands tidal flushing.

Ciliates are apparently capable of remaining in rockpools because of their typically flattened morphology,and their affiliation with the benthic boundary layer.Continuous development of attached bacteria mayserve as their principal food source. The ciliates livingin the rock-pools were almost all confined to the surfaceof rocks and the bottom layer of sandy sediment. Thismight explain the existence of a diverse ‘resident’ cili-ate community which underpins the high speciesrichness. Unfortunately there does not seem to be anyother similar study with which we can compare our re-sults. It is strange that one of the most accessible marineenvironments is also one of the least understood.

Acknowledgements. We thank R. M. Warwick and P. J.Somerfield (Plymouth Marine Laboratory, UK) for field sup-port and logistics on the Isles of Scilly; K. J. Clarke (Freshwa-ter Biological Association, UK) for analyses of samples usingtransmission electron microscopy; and the Natural Environ-ment Research Council (UK) for financial support. This workwas funded in part by the Department for Environment Foodand Rural Affairs, UK (ME3109). This article benefited greatlyfrom the comments and suggestions of 2 anonymous reviewers.

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Editorial responsibility: Fereidoun Rassoulzadegan (Contributing Editor), Villefranche-sur-Mer, France

Submitted: August 25, 2006; Accepted: November 3, 2006Proofs received from author(s): March 14, 2007