Abstract We studied the sclerobiont community associ-ated with organogenic and lithic cobbles from soft bottomsin the Khao Lak coastal area (Andaman Sea) that was dam-aged by the 2004 tsunami. The 15 cobbles examined origi-nate from grab and hand sampling carried out in the years2006 and 2007 in the depth range of 4.6–15.2 m. A richendo- and epibenthos was identiWed, mainly consisting ofalgae, foraminifers, sponges, cnidarians, polychaetes, bry-ozoans and bivalves. Associations on each examined cob-ble show similarities in composition and structure beingcharacterized by a few dominant groups. DiVerences werenoted in the degree and pattern of colonization, distinguish-ing for each cobble an upward and a downward side at thetime of sampling. The mean total coverage is 15.09% beinghigher on the upper sides (17.4%) compared to the lowersides (12.8%). Calcareous algae, bivalves and spongesprevail on upper sides, while bryozoans prevail on lowersides. The sclerobionts distribution allowed to infer the
orientation of cobbles on the seaXoor during colonization.Major colonization values, exceeding 30% coverage, wereobserved on organogenic cobbles located in the proximityof reefs or collected from below 12 m of water depth. Con-versely, cobbles from the shallowest stations result poorlycolonized, independently of their composition. The waterturbidity and wave motion as a possible cause of theobserved distributions were discussed. The Khao Lak cob-ble community seems to be largely unaVected by the tsu-nami event, as suggested by the estimated biodiversity,abundance and coverage of sclerobionts.
A number of ecological and paleoecological studies docu-ment diversity, life histories and interactions of organismscolonizing hard substrata (sclerobionts sensu Taylor andWilson 2002) scattered on bottom sediments in diVerentenvironmental settings (Taylor and Wilson 2003). Literaturedata mainly refer to cold-to-temperate, recent and fossilenvironments. Records from tropical regions are scant andare mainly restricted to the Caribbean Sea, documenting thepattern of recruitment of coral rubble-related sessile organ-isms (Choi and Ginsburg 1983; Choi 1984; Neal et al. 1988;Meesters et al. 1991; Gischler and Ginsburg 1996; Gischler1997). Further literature concerns the sclerobiont coloniza-tion pattern on shells from the northern Red Sea (Zuschinet al. 2001; Zuschin and Baal 2007) and the role of encrus-ters in binding bottom clasts (Rasser and Riegl 2002).Nevertheless, the colonization of small hard substrata in
R. SanWlippo (&) · A. Rosso · I. Di Geronimo · R. Di GeronimoDepartment of Geological Science, Catania University, Corso Italia, 57, 95129 Catania, Italye-mail: [email protected]
D. Basso · E. RobbaDepartment of Geological Sciences and Geotechnologies, Milano-Bicocca University, Piazza della Scienza, 4, 20126 Milan, Italy
D. ViolantiDepartment of Earth Science, Torino University, Via Valperga Caluso, 35, 10125 Turin, Italy
F. BenzoniDepartment of Biotechnology and Biosciences, Milano-Bicocca University, Piazza della Scienza, 3, 20126 Milan, Italy
soft-bottoms is still poorly known, mostly focusing on coralreef ecosystems and reef-dwelling sessile organisms.
The present study represents the Wrst contribution to theknowledge of the sessile cobble-dwellers from the southwest-ern coast of Thailand (Andaman Sea), not investigated so far.Faunistic data from the area mainly consists of check-lists ofbivalves and gastropods (Tantanasiriwong 1978, 1979), taxo-nomic monographs (Natheewathana and Hylleberg 1991), anddescriptive macrofaunal accounts (Boonlert 1992; Dexter1996; Barrio Frojan et al. 2005, 2006). Likewise, knowledgeof other benthic macro- and microfauna derives from papersdealing with the Indian and PaciWc Oceans.
The Khao Lak coast (Fig. 1), located north of Phuket, isone of the Thailand coastal areas most severely damaged bythe 2004 tsunami (Yeemin et al. 2006; Choowong et al.2007; Di Geronimo et al. 2009; Kendall et al. 2006, 2009).The major destructive eVects were produced inland withinundation and deposition of a large amount of marine sandsand pebbles eroded by the tsunami from shallow depths.Sediments were later displaced by the return Xow and redis-tributed along the coast by the littoral drift producingtemporary high turbidity and local muddy deposition in low-hydrodynamic areas (Di Geronimo et al. 2009). The studiedbiogenic and lithic blocks represent hard substrata suitablefor colonization by sessile organisms (mainly algae,sponges, polychaetes and bryozoans), which usually formtypical hard-substrate communities. The distribution patternof these organisms on such substrata depends on their eco-logical requirement and preferences for exposed upper sur-faces of the fragments or, conversely, for shadowed andsheltered parts like crevices and bottom-facing sides.
The present study points: (1) to describe the live sessilebiota of the organogenic cobbles; (2) to investigate the rela-tionship between the colonizers and the substratum orienta-tion; and (3) to infer the tsunami eVects on the cobblecommunity after some months or a few years from theevent, on the base of the composition and structure of scle-robionts and their distribution on the cobbles.
Materials and methods
The study area comprises the Khao Lak coast in the prov-ince of Phang Nga (Andaman Sea, SW Thailand), extend-ing approximately 25 km in the North–South direction andfor 6 km seaward, down to the depth of 15 m (Fig. 1). Thecoastal morphology is heterogeneous, characterized byembayed areas alternating with rocky headlands. Opensandy beaches occur to the North, while sheltered bays arepresent in the half southern part. Rocky coasts consist ofdead fringing reefs at Laem Pakarang headland and LaemAo Kham Peninsula, and of granite rocks that crop out at
Fig. 1 Map of the Khao Lak coastal area showing the location of sta-tions containing examined cobbles
the Laem Hin Chang headland and discontinuously in thenearest coastline.
The seaXoor also displays a varied morphology. The bot-tom is gently sloping down to 15 m, except for rocky zonessouth of Laem Hin Chang and restricted areas oV fringingreefs. Nevertheless, at depths shallower than approximately10 m, the topography is locally extremely irregular due tothe presence of granite outcrops and blocks (Fig. 1). Rubbleof biohermal limestones (Fig. 2a–c), from a few centime-ters to more than 2 m in diameter, and granite boulders andcobbles, are scattered on the bottom mostly near dead reefsand outcrops (Di Geronimo et al. 2009).
Sediments are heterogeneous in composition and tex-ture, mainly consisting of sandy gravels and gravelly sands(Di Geronimo et al. 2008). According to Di Geronimo et al.(2009), gravels are exclusively present close to rocky out-crops while sediments with a more or less conspicuousmuddy component locally occur in large pockets. The bio-clastic fraction mostly consists of corals, molluscs, bryozo-ans and foraminifers, whereas the lithic component derivesfrom igneous and sedimentary rocks.
Examined material originates from grab and hand sam-pling carried out in May 2006, December 2006 andNovember 2007, in the depth range of 4.6–15.2 m(Table 1).
Fifteen cobbles (reaching a maximum intermediatediameter of 250 mm) were examined taken from 11 stationsto check for living sclerobionts. Cobbles consist of nineslabs of hermatypic corals, four lithic clasts, and two largeoyster shells. Cobble orientation at time of sampling wasassessed visually during scuba diving for hand-sampledcobbles. Grab-samples orientation was established on thebasis of undisturbed surface of grab samples. Cobbles werephotographed at 1:1 scale and the projection of the totalarea (mm2) was measured for each side. The colonizationby soft-bodied macro-organisms was evaluated throughobservations and photo documentation of the wet surfaceson board (Fig. 3a, b); the dry surfaces of calcifying biotawere analyzed in the laboratory (Fig. 3c–h).
Organisms (Xeshy and calcareous macroalgae, foramini-fers, sponges, cnidarians, bivalves, polychaetes, bryozoans,brachiopods, barnacles and ascidians) were mapped for
Fig. 2 a View of the sea bottom from where grab sample LP54 wastaken (Laem Pakarang, ¡8 m), showing boulders and cobbles scat-tered within coarse sediments, heavily colonized by evident calcareousand green algae. b Large colonies of brittle reteporiform bryozoans
encrusting lateral sides of a cobble from Laem Pakarang (¡14 m).Scale bar 3 cm. c Halimeda sp. and erect octocoral colonies growingon the upper convex surface of a boulder (Laem Pakarang, ¡11 m).Scale bar 5 cm
each cobble on both sides. IdentiWcation was performed athigh taxonomic level for some groups and at genus or spe-cies level for the others (Table 1).
The percentage coverage was calculated considering theattached surface for the encrusters and the hole surface forthe borers (Table 1). In order to test for the possible eVectof sample size on colonization, a Pearson’s correlation
analysis was performed between coverage and abundance(number of specimens) of all detected groups of sclero-bionts and the corresponding upper and lower surfaces(mm2) of cobbles. K-dominance curves (softwarePRIMER, Plymouth Marine Laboratory, UK) based oncoverage data have been used to validate the samplingintensity.
Fig. 3 a Convex upper side of the cobble from stn. LP50. Elevated Xe-shy algae mainly belonging to Padina sp. are evident. b Concave bot-tom facing side of a coral slab from stn. LP49 heavily colonized bybryozoans and serpulids. c Upper side of a Xat lithic cobble from stn.BN24. Dead barnacles, calcareous algae, and foraminifers are evident.d Lower side of a Xat lithic cobble from stn. BN24 heavily encrustedby serpulids and bryozoans. e Box-shaped lithic cobble from stn.BTL6 encrusted by dominant calcareous algae on upper side. An adult
specimen of the encrusting bivalve Chama brassica Reeve, 1846 isevident. f Lower side of the cobble from stn. BTL6 encrusted by cal-careous algae along its peripheral edges. g An ostreid valve from stn.BTL6 colonized on its convex upper side by two specimens of Plica-tula australis Lamarck, 1819. h Living and dead serpulids and bryozo-ans are dominant on the concave lower side of the valve from stn.BTL6. Scale bar 1 cm
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Low magniWcation photos of colonizers were acquiredwith a Zeiss Discovery V8A stereomicroscope equippedwith an Axiocam MRC and Axiovision acquisition system.
Results
Lithic cobbles are slab- to box-shaped and possess a rathersmooth surface (Fig. 3c, d). Encrusters are relatively scarceon their surfaces and boring organisms are quite rare orabsent. In contrast, coral cobbles and shells are more or lessconcave-convex in shape (Fig. 3a, b, g, h). Coral cobblespossess rough surfaces characterized by several mm- to cm-sized cavities like calices and inter-corallite spaces. Muddysediment may locally coat surfaces, Wlling crevices andcavities. Sessile organisms are generally quite abundant anddiversiWed on these cobbles.
All the examined cobbles are colonized on both sides.Associations are quite diversiWed and show similarities incomposition and structure, seemingly irrespective of thesampling period (see Table 1). The K-dominance curvesbased on coverage data (Fig. 4) are asymptotic, thus con-Wrming that the sampling intensity was adequate. In partic-ular, the upper side of cobbles are colonized by a morediverse association compared to lower sides. Lower sideshave comparably few, highly dominant taxa, as shown bythe Wrst values and the slope of the k-dominance curve.
As a general feature, elevated Xeshy algae like Halimedaand Padina together with erect Xexible animals like penna-tulaceans and erect hydrozoans (Fig. 2c), are exclusivelypresent on the upward exposed surfaces (Figs. 2a, 3a).Large agglutinant sabellariids (Fig. 5a), solitary corals(Caryophyllia sp.), and other scleractinians belonging tothe families Rhizangiidae and Agariciidae, among whichCulicia sp. (Fig. 5c), hydrozoans and sabellids mainlyoccur on the upper sides. Analogously, encrusting bivalves,
such as Plicatula australis Lamarck, 1819, Chama brassicaReeve, 1846, Chama sp. A and Ostreidae sp. A, preferen-tially colonize the upper sides (Fig. 3e, g), as do Wlamen-tous green algae and calcareous algae of the generaLithophyllum, Hydrolithon and Sporolithon (Fig. 3e, f).Calcareous algae may extend laterally on some cobbles toencrust as far as the peripheral edge of lower sides (Fig. 3f).Bioerosion is a dominant feature of the upper sides and ismainly due to clionaid sponges, acrothoracic cirripeds andlithophagine bivalves mainly belonging to Pholadidae andGastrochaenidae (Fig. 5d–f). Sponges and foraminifersoccur on both sides of the cobbles, but single speciesexhibit selective preferences for exposed or cryptic sides.Among foraminifers (Fig. 5g–i) Planorbulinella larvata(Parker and Jones 1865) and other Planorbulinidae speciesare dominant. They seem to prefer the exposed upper sides,as does Sagenina frondescens (Brady 1879). Other com-mon species are Carpenteria utricularis (Carter 1876),almost exclusively detected on clast undersides, and Dyoci-bicides biserialis Cushman and Valentine 1930, both typi-cal of reef environments.
Bryozoans and serpulids (Fig. 6a–i) are subordinate onthe upper sides where bryozoan colonies occur in crevicesand microcavities, whereas large-sized serpulids like Pom-atostegus stellatus (Abildgaard 1789) and Spirobranchussp., and encrusting bryozoans as Mucropetraliella sp. Aand Rhynchozoon sp. C colonize exposed surfaces withlarge-sized specimens. The most abundant serpulid speciesHydroides albiceps (Grube 1870) indiVerently encrustsboth sides. Most bryozoans, such as Metacleidochasmaovale Soule, Soule and Chaney 1991 and Bryopesanserspp. behave similarly, but several, as Robertsonidra prae-cipua Hayward and Ryland 1995, Parasmittina spp.,Celleporaria sp. A, reach larger sizes on the lower sur-faces, whereas a few ones form larger colonies on theupper sides as do Reptadeonella sp.. Conversely, on lowersides, serpulids and mostly bryozoans are decidedly domi-nant, showing a high number of species and specimenswhich are low-proWled and small-sized like the serpulidsVermiliopsis sp. A and Serpula hartmanae Reish, 1968and the bryozoans Trypostega henrychaney Tilbrook,2006, Crepidachantha crinispina Levinsen 1909 and Lifu-ella sp. A. Erect Xexible species, like Vasignyella sp.,colonize crevices (Fig. 6f).
Lower surfaces are also preferentially colonized byencrusting stolonar octocorals and didemniids. Brachiopodsare all over extremely rare.
Summarizing, at high taxonomic rank, data points to thefollowing colonization pattern. Soft algae, hydrozoans,octocorals, sabellids and sabellariids are mainly distributedin the central part of upper sides. Some laminar encrustingand reteporiform bryozoan species, together with softsponges, corals and encrusting bivalves occur all over the
Fig. 4 k-dominance curve for upper sides (green line) and lower sides(red line) of cobbles. The taxa are ranked in order of importance interms of coverage on the x-axis (log scale) with percentage dominanceon the y-axis (cumulative scale). Note that the cumulative dominancefor lower sides starts at 40% with the three most important taxaexceeding 70%
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upper sides. Calcareous algae and, to a lesser extent, agglu-tinant worms and corals, seem to be homogeneously andwidely distributed on the upper sides, also encrusting thelateral parts of the cobbles and the peripheral edge of theundersides. Small byssate bivalves (arcids and mytilids)and erect Xexible bryozoans indiVerently colonize bothsides of clasts but they selectively occur in sheltered cre-vices, often in contiguity with silty sediment. The centralsurface of the undersides hosts the greatest part of the ser-pulid and bryozoan species and also some sponge and fora-minifer species.
The average total coverage is 15.1%, and particularly17.4% (from 0.6 to 33.7%, SD 10.9) on the upper sides and12.8% (from 1.9 to 30.1%, SD 8.2) on the lower sides(Table 2, Fig. 7). As a result, interactions between livingtaxa are uncommon, almost exclusively occurring betweenborers and encrusters.
Major coverage values are those of calcareous red algae(mean total coverage 5.7%), bivalves (total coverage 2.0%)and sponges (total coverage 4.6%), which prevail on theupper sides, and those of bryozoans (total coverage 5.4%),which prevail on the lower sides (Figs. 7, 8).
Fig. 5 a Large agglutinant tube of a sabellariid worm on upper side ofthe cobble from stn. LP 48. Scale bar 1 cm. b The solitary coral Culiciasp. (Rhizangiidae) growing on the upper side of the cobble from stn.LP 46. Scale bar 200 �m. c Young specimen of an agariciid coral onupper side of the cobble from stn. LP 46. Scale bar 100 �m. d Clionaidsponges boring bryozoan colonies from the upper side of the cobblefrom stn. LP64. Scale bar 2 mm. e Eight-shaped borehole of a gastro-chaenid bivalve from the upper side of the cobble from stn. BTL6.Scale bar 1 mm. f Acrothoracic barnacles bores into a living coralline
algae growing on the upper side of the cobble from stn. KL28. Scalebar 1 mm. g The foraminifer Planorbulinella larvata (Parker andJones 1865) colonizing the exposed upper side of the cobble from stn.BTL23. Scale bar 1 mm. h The foraminifer Dyocibicides biserialisCushman and Valentine 1930, from the upper surface of the cobblefrom stn. LP48. Scale bar 1 mm. i The foraminifer Carpenteria utric-ularis (Carter 1876), almost exclusively detected on the lower sides ofclasts. Stn. LP54. Scale bar 2 mm
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DiVerences were observed in the degree of colonizationamong cobbles of diVerent composition. The total meancoverage is higher on organogenic cobbles (17.8 and
16.3%, respectively, for shells and coral cobbles), while itis lower (11.0%) on lithic cobbles (Table 2). Furthermore,coverage is higher on cobbles originating from sites close
Fig. 6 a Large tube of the serpulid Pomatostegus stellatus (Abildg-aard 1789) on the upper side of the cobble from stn. BTL2. Scale bar1 mm. b The bryozoan Rhynchozoon sp. Stn. BTL6. Scale bar 500 �m.c Distal part of the tube of Hydroides albiceps (Grube 1870). Stn.BTL2. Scale bar 1 mm. d The bryozoan Lifuella sp. A mainly encrust-ing the lower sides. Stn. LP48. Scale bar 500 �m. e A small colony ofthe bryozoan Fenestrulina sp. A. Stn. BTL6. Scale bar 500 �m. f Anerect Xexible colony of the bryozoan Vasygniella ovicellata Vieira,
Gordon and Coreica, 2007. Stn. LP49. Scale bar 1 mm. g The bryo-zoan Metacleidochasma ovale Soule, Soule and Chamey, 1991, com-monly present on both sides. Stn. BTL2. Scale bar 1 mm. h Theserpulid Vermiliopsis sp. A exclusively recorded on lower sides. Stn.LP50. Scale bar 1 mm. i The bryozoan Crepidahcantha cfr. crinispinaLevinsen, 1909, encrusting the lower side of cobble b from stn. BTL2.Scale bar 500 �m
Table 2 Mean total coverage of sclerobionts on cobbles. Values are compared in distinguishing cobbles with diVerent composition
to the coral reefs and from deeper bottoms. However, it isworth noting that for each compositional set of cobbles,those from the shallowest stations are comparably less col-onized (Table 1, Fig. 8). Interestingly, no relevant diVer-ence was envisaged between cobbles sampled insubsequent sampling surveys from April 2006 to November2007, thus exposed to colonization for diVerent time spans,after the tsunami event (December 2004). Only erect algaetend to be more abundant in samples collected during theseasonal monsoons from inland (December 2006 andNovember 2007).
Pearson’s correlation coeYcient reveals that a certaincorrelation exists between cobble size and coverage andabundance. In particular, for the lower sides of cobbles,coverage and abundance show values of 0.040 and 0.626,respectively, revealing a weak to strong positive correla-tion.
Discussion
Acquired data represents a Wrst insight on cobble-dwellersfrom shallow-water bottoms from the Andaman Sea. Thesclerobiont community consists of a large number of spe-cies within several high taxonomic groups (including algaeand invertebrates, almost entirely consisting of Wlter-feed-ers), nearly all already reported from coral rubble commu-nities from other tropical areas. Nevertheless, only a fewgroups, namely calcareous algae, sponges and cheilostomebryozoans dominate, as expected from information avail-able for tropical coral and coral-rubble sclerobiont commu-nities (Jackson and Winston 1982; Nebelsick et al. 1997;
Richter et al. 2001; Zuschin and Baal 2007). However, it isdiYcult to compare our data with the available literature, asmost papers document occurrence and distribution of scle-robionts, exclusively or mostly, from the undersides of cob-bles (Choi and Ginsburg 1983; Choi 1984; Neal et al. 1988;Meesters et al. 1991; Gischler and Ginsburg 1996; Gischler1997). Comparison is even more diYcult at the specieslevel, due to the geographical replacement of species andthe strong spatial heterogeneity, even at a small scale (Jack-son 1984).
The occurrence, distribution pattern and coverage ofsclerobionts vary between cobbles. Such variations havebeen considered as strictly related to the substratum nature,size, and shape and largely inXuenced by environmentalfactors, mainly hydrodynamics and turbidity, as well aslight in shallow-water environments (McGiunness 1987a,b; Martindale 1992; Fabricius and De’ath 2001; Caragnanoet al. 2009).
Fig. 7 Diagrams showing the coverage (%) of each taxonomic groupon upper and lower sides of cobbles (respectively, above and belowzero in the graph)
Fig. 8 Percentage total coverage of benthic groups detected on cob-bles. Coverage values are shown separately for the upper and lowersides of cobbles (respectively, above and below zero in the graph).a Encrusters versus borers. b The association of sclerobionts. Ob-served cnidarians include only hydrozoans, erect and stolonar octoco-rals and scleractinians. Sabell. for sabellarids and sabellids; CCA forcrustose coralline algae
In the present instance, the low taxonomic diversity ofthe sclerobiont community on lithic cobbles (Table 1) couldbe explained by: (1) their smooth outer surface seeminglydiscouraging larval settlement; (2) their small sizes(Fig. 3c, d) which, especially at the shallowest sites, favortheir displacement during storms which periodically occurin such tropical environments; and (3) their platy shapes,which hamper the larval settlement on most of the lowersurfaces constantly in contact with the bottom. In contrast,sessile organisms are more abundant on organogenic cob-bles, which possess uneven surfaces seemingly fosteringlarval settlement and also oVering sheltered microhabitatsfor additional taxa, excluded from lithic cobbles (Taylorand Wilson 2003, inter alias).
Additionally, the tendency raised from correlation analy-sis and K- curves to have relatively high coverage andabundance values on bottom-facing sides of large cobblescan be explained with their mostly concave shape, whichrepresents a preferential habitat for sciaphilic groups likebryozoans and serpulids.
The sclerobiont distribution on cobbles follows a charac-teristic pattern, with species composition diVering betweenexposed upper-facing sides and sheltered lower sides,according to the ecological preferences of each taxon, andreXecting the position of clasts on the sea bottom. Observedpattern is comparable to that of many hard substrata inmodern and ancient marine ecosystems (Taylor and Wilson2003). Particularly, serpulids and some bryozoan speciesare dominant on the lower, concave sides of valves and ofmost cobbles (Fig. 9). This distribution seems related to theprotection oVered by the undersides of clasts acting as ref-uge (Bishop 1989; McKinney 1996; Perry and Hepburn2008). The sharp dominance of Xeshy photophilic algae,and particularly of Padina (Fig. 3a), on the upper surfacesof most cobbles sampled in November and December, maybe the result of high light intensity and water transparencyduring the dry season (Vasuki et al. 2001). In contrast, the
seasonal high turbidity and the consequent reduced lightingcould cause calcareous algae, usually prevailing on theundersides of coral colonies in shallow-water reef frame-works, to regularly colonize the upper sides of cobbles,only sometimes extending to their lateral edges.
A clear pattern is also produced by the distribution oftaxa with diVerent morphologies: low-proWled speciesdominate on the lower surfaces, in order to avoid contactwith the seaXoor. Conversely, erect forms like some cnidar-ians and reteporiform bryozoans (Fig. 2b, c) develop onexposed sides, as also remarked by Wilson (1987).
As a whole, the occurrence of several taxonomic groups,including high competitive late successional taxa, such assabellariids, bryozoans, sponges and didemniids, wouldindicate a climax stage (see Choi 1984; McKinney 1996;Taylor and Wilson 2003), suggesting that the cobble com-munity presumably reached the maturity after relativelylong exposure on the seaXoor. Data concerning ecologicalsuccessions in tropical environments is scant and some-times conXicting but, interestingly, communities develop-ing on both panels and coral rubble from shallow waterreach their mature stages in a 3-year period (Choi 1984;Hirata 1987). Further indication is supplied by single spe-cies. Although most of them probably colonize the studiedcobbles with specimens and colonies pointing to short (sea-sonal to annual) life spans, it could be remarked that someserpulids and bryozoans reach relatively large sizes point-ing to ages probably longer than the time elapsed from thetsunami event to the collecting time. Taking into accountthat a rough relation can be assumed between ages and col-ony sizes (Gordon et al. 2009) and using the few data ongrowth rate available for tropical encrusting (Winston andJackson 1984) and non-tropical erect (Lombardi et al.2008) bryozoan species, it can be hypothesized that someof the largest bryozoans (as the reteporiform colonies inFig. 2b and specimens of the encrusting R. praecipua)could be 2 or more years old. Analogously, some serpulid
Fig. 9 a Diagrams showing changes in bryozoan coverage (%) on up-per and lower sides of cobbles (respectively, above and below zero inthe graph). b Diagrams showing changes in serpulid coverage (%) on
upper and lower sides of cobbles (respectively, above and below zeroin the graph). Sample depths are given in Table 1
specimens belonging to the genus Spirobranchus (Kupriya-nova et al. 2001), including some long-living species, andreaching relatively large sizes, could be more than 2 or3 years old. Furthermore, some adult bivalve specimens ofChama brassica Reeve, 1846 (Fig. 3e) and Plicatula aus-tralis Lamarck, 1819 (Fig. 3g) have been found, surelyexceeding such an age. Interestingly, these large-sizedspecimens have been found in one of the deepest sites, fromabout 9 m.
The above-discussed results substantiate a relativelylong exposure of the examined cobbles on the bottom, withWrst colonization at least partially predating the tsunamievent. The long-term exposure of clasts on the seaXoor isalso supported by their relatively high coverage with amean total value of 15.1% for living organisms.
Nevertheless, coverage among cobbles is spatially heter-ogeneous and higher on some clasts seemingly inXuencedby their bathymetric location and/or their proximity to hardcoral reef substrata (BTL 6a, b, KL 28 and LP 54), whosecommunities can source larvae of sessile organisms.
Interestingly, some cobbles host superimposed encrusta-tions mostly formed by bryozoan skeletons and emptypolychaete tubes (constituting the thanatocoenoses, record-ing past succeeding sclerobiont communities), locally cov-ered by the newly settled taxa of the present-daycommunity. Noteworthy, composition and distribution pat-tern in the present-day sclerobiont community and theunderling thanatocoenoses appear substantially similar. Thestrong polarization (Table 1), persisting in time, appearsconsistent with a stable exposure of the cobbles above theseaXoor in the same position being, for a presumable longtime-span, not overturned. The hypothesis of stability isalso supported by the occurrence of erect bryozoans colo-nizing exposed cobble surfaces (Fig. 2b) whose brittle, con-siderably large-sized colonies are undamaged. In contrast,absence of colonization polarization is expected to occur onfrequently overturned substrata (Zuschin and Baal 2007) inhigh hydrodynamic conditions.
Conclusions
A long exposure of cobbles on soft bottoms from the KhaoLak coastal area is suggested by the estimated biodiversity,abundance and coverage of sclerobionts.
The colonization pattern, the presumed age of somespecimens, the presence of late successional taxa and thesimilarity between community and thanatocoenoses sug-gest a long-lasting stability of the cobbles on the seaXoor,as also testiWed by the absence of epibionts with abradedand/or broken skeletons. Only a few small-sized Xattenedcobbles from the shallow (less than 6 m deep) samplingsites, which are remarkably less colonized, seemingly
suVered a certain level of physical disturbance and wereprobably overturned and/or displaced, analogously toobservations on cobbles from tropical intertidal bottoms(see McGiunness 1987a, b).
Thus, it is reasonable to hypothesize that the cobble-dwellers community from the Khao Lak coastal area wasnot substantially aVected by the 2004 tsunami event. Onbottoms deeper than 5–6 m, the tsunami presumably dis-placed only Wne (pelite to little pebble-sized) sediments.This result is in agreement with estimation of the tsunamidamage on coastal resources in a neighboring coastal areaby Kendall et al. (2006, 2009), indicating that the mostsevere impact on biota was on mesolittoral (less than 3 m)environments. Accordingly, in situ observations evidencedas colonies of meter-sized horn-branching Acropora at 5 mdepth in the Laem Ao Cham Reef, endured undamaged dur-ing the 2004 tsunami (Di Geronimo et al. 2009). The pres-ence of few meter-sized coral colonies uprooted andchipped by shocking, at about 9 m depth in front of LaemPakarang, points to a non-homogeneous pattern of tsunamiimpact on the Khao Lak coastal area. This can be explainedby very local characteristics of the seaXoor topography,which is the main factor determining the directionality oftsunami energy propagation (Titov et al. 2005).
Acknowledgments This research was funded by MURST grants(PRIN 2005: Programmi di ricerca di Rilevante Interesse Nazionale,2005048829 Project). Catania Palaeontological Research Group: con-tribution number 363.
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