Benthos Research Vol.54,No.2:71-80(1999) BENTHOS RESEARCH

Benthos Research Vol.54,No.2:71-80(1999) BENTHOS RESEARCH The Japanese Association of Benthology Activity Patterns and Habitat Preferences of Two Herbivorous Gastropods (Gibbula umbilicaris and Jujubinus exasperatus)on Leaves of the editerranean Seagrass Posidonia oceanica M Yoshitake Takada1),Giovanni F.Russo2)and Lucia Mazzella1)•õ Laboratory of Benthic Ecology,Stazione Zoologica•eAnton 1)Dohrn•fdi Napoli, Punta S.Pietro,I-80077Ischia(Napoli),Italy 2)Department of Animal Biology ,University of Catania,Sicily,Italy Abstract:Laboratory experiments were carried out to observe the day/night activity patterns and habitat preferences of two gastropods,Gibbula umbilicaris and Jujubinus exasperatus,on leaves of the seagrass Posidonia oceanica.Both G.umbilicaris and J.exasperatus preferred old(apical)parts of leaves to young (basal)ones,and vertically oriented leaves rather than horizontal ones.Four types of behavior(rest, shaking epipedalic tentacles without any other motion,directional movements ,and shell twisting mainly for foraging)were observed.Also,interspecific differences in the day/night activity pattern were recog nized;G.umbilicaris mainly rested during the day and actively moved and foraged at night ,whereas J. exasperatus continued to move about and forage during both day and night,resulting in a less marked day/night pattern.These results partly explain the observations of previous studies ,in which both species were more abundantly sampled on seagrass leaves at night than by day. Key words:day/night activity,epiphytic snail,habitat preference,Posidonia seagrass (Russo et al.1984b;Russo et al.1991).Most of these INTRODUCTION snails,including Gibbula(Tumulus)umbilicaris (Linne1758)and Jujubinus exasperatus(Pennant, Epiphytic snails on seagrass leaves are one of the most 1777),showed variation in abundance on leaves between important components of the structural(Mazzella et day and night observations(Russo et al.1984a).Both al.1989;Edgar1990;Gambi et al.1992)and functional G.umbilicaris and J.exasperatus belong to the family (van Montfrans et al.1982;Orth&van Montfrans Trochidae and are the dominant herbivores on the epi 1984;Klumpp et al.1992;Mazzella et al.1992; phytic algae that grow on P.oceanica.Their abundance Jernakoff et al.1996)organization of the seagrass eco on leaves is higher at night than by day(Russo et al. system.In seagrass beds of Posidonia oceanica(L.) 1984a).Russo et al.(1984a)suggested that this day/ Delile,several studies have been carried out on the spe night variation in abundance was due to the vertical cies composition and distribution of epiphytic snails migration of the snails along the seagrass leaves,but there has been little direct observation on the behavior Received August16,1999:Accepted November12,1999 of these snails(Mazzella&Russo1989).In this study, *Corresponding author .Present address:Ishigaki we hypothesize that the vertical migration of the snails Tropical Station,Seikai National Fisheries Research is related to their day/night feeding on the epiphytic Institute,Fukai-Ota148-446,Ishigaki907-0451,Japan. algae. Epiphytic snails graze mainly on epiphytic algae •õDeceased June 1999 71 Takada et al. both the leaf age and the shoot structure.Older,apical parts of leaves carry a diverse and well-grown cover of algae,while younger,basal parts have only a primary cover.This spatial variation in algal cover may induce spatial variation in the feeding activity of herbivorous snails,because the snails may encounter different types of food while grazing.Spatial variation in abundance of some species of snails,including Gibbula umbilicaris and Jujubinus exasperatus,indicates their preference for certain types of epiphytic algae(Spada1971; Kitting 1985;Peduzzi1987;Mazzella&Russo1989). However,it is necessary to ascertain the snails'prefer ence by a selection experiment between leaf fragments with different kinds of epiphytic algae.Otherwise,al Fig.1.Experimental aquarium(12cm diameter and6cm depth).Each aquarium contained one snail(Gibbula ternative explanations for this phenomenon,such as umbilicaris or Jujubinus exasperatus).Filtered seawater geotaxis and/or phototaxis,may be confounded with was flowing into the bottom of the aquarium from a preference for any particular type of epiphytic algae.In siphonal tube fixed by a clothes peg.Four fragments of addition,G.umbilicaris and J.exasperatus are rarely Posidonia oceanica leaves were placed vertically in the found on dead leaves that accumulate horizontally on center of the aquarium in the first experiment. the sea bottom(Russo unpubi.).It should be tested whether this is due to the snails'lower preference for epiphytic algae on dead leaves,for horizontally posi (=periphyton,i.e.microalgae and macroalgae growing tioned leaves,or both. on leaves of seagrasses(van Montfrans et al.1982; The present study included laboratory experiments Peduzzi1987;Mazzella&Russo1989;Klumpp et al. to evaluate the effect of epiphytic algae and the spatial 1992;Mazzella et al.1992)).Seagrass shoots carry sev arrangement(vertical or horizontal)of Posidonia eral kinds of epiphytic algal communities(Jacobs et al. oceanica leaves on the day/night feeding activity of 1983;Mazzella&Russo1989;Mazzella&Spinoccia Gibbula umbilicaris and Jujubinus exasperatus.The re 1992;Mazzella et al.1995),primarily depending on sults are discussed in relation to the vertical variation of algal cover and the day/night migration patterns of the snails. Table1.Source and epiphyte cover of the five types of leaf fragments.Four types(A,B,C,and D)of leaf fragments were used in the first experiment,three types(C,D,and MATERIALS AND METHODS E)in the second experiment. Snails(Gibbula umbilicaris and Jujubinus exasperatus) LeaffragmentSourceEpiphytes and seagrass(Posidonia oceanica)were collected in No ABasalpartoftheNomacroscopic vember1995from two localities,Lacco Ameno and youngestleafalgae Castello(Island of Ischia,Gulf of Naples;Colantoni et BIntermediatepartNomacroscopic al.1982),from a depth of3m.Seagrass shoots were ofthesecondandalgae kept in an aquarium with flowing seawater after re thirdyoungest leaves moving all the mobile animals from the leaves.One CApicalpartofthePrimarycover snail was placed in each small(12cm diameter and6cm secondandthirdofmacroscopic depth)aquarium with flowing seawater(Fig.1).After youngestleavesencrustingalgae 2-5days of acclimation,snails did not escape from DApicalpartofDiverseand their aquariums and their behavior was observed.The olderleaveswellgrowncover ofmacroscopic quariums were maintained under natural a thermal con enCrUStingalgae ditions(13.0-17.6•Ž). EDeadleafDiverseand According to the development of the epiphytic wellgrowncover algae on Posidonia leaves(Mazzella&Russo1989; ofmacroscopic enCrUStingalgae Mazzella et al.1989;Mazzella&Spinoccia1992; 72 Activity patterns of epiphytic gastropods Mazzella et al.1995),five types of leaf fragments The natural light from the window of the laboratory (6cm long)were used in the experiments(Table1).De was not altered.During the night observations,a flash velopment of the algal cover on the leaf fragments was light was used.In preliminary observations,it was categorized,in increasing order,as‘A'<‘B'<‘C'<‘D' found that a dimmed flashlightcould be used for sev =‘E' .Macroscopic algae on the leaf fragments of types eral seconds to observe the behavior of the snails with ' ‘C ,‘D',and‘E'were mainly composed of encrusting out disturbing them.All the leaf fragments were re coralline algae.Width of the leaves(9.9mm±0.5SD, placed daily with new leaves in the morning(9:30- =96)was homogeneous among n the five types . 11:00h),without touching any snails.Ifa snailwas at We recorded four phases of behavior of the two tached to a leaf fragment,we waited until it moved off snail species:‘rest'involved resting in a place without the fragment before we replaced the fragment. any motion;‘tentacle shake'involved shaking the epipedalic tentacles slowly without any other motion; Second experiment move'involved moving straight(1.6-5.5cm/min) ‘ from one place to another without twisting the shell; In the second experiment,one group of leaf fragments twist'involved twisting the shell and shifting the‘ ce was placed vertically as in the first experiment and an phalic part around while moving slowly(0.1-0.9cm other group was placed horizontally on the bottom of /min).Thus,two‘inactive'phases(‘rest'and‘tentacle the aquarium.The natural curve of the leaf fragments shake')and two‘active'phases(‘move'and‘twist') allowed snails to use both sides of the fragments.In were recognized.These behaviors were common to the this experiment,three types of leaves(‘C',‘D',and‘E') two species and were clearly distinguishable.Snails fed were used for each group;thus,six leaves were offered rarely during the‘move'phase,and most often during in each aquarium.Leaf fragments were replaced with the‘twist'phase. new ones every day,as in the first experiment.Observa Two experiments were carried out.Interspecific tions on the position and the behavior of the snails were differences in the activity pattern and preference carried out as described above daily at12:00h,18:00h, among the leaf fragments(epiphytic algae)were ob and0:00h.In total,23snails(9specimens of G. served in both of the experiments.In addition,the day ,mean shell width=8.38mm,range=6.30 umbilicaris /night activity of snails and their preferences among -13 .00mm;14specimens of J.exasperatus,mean shell living leaf fragments was observed in the first experi height=9.21mm,range=8.20-9.85mm)were

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