The occurrence ofabnormalembryos intwo ovoviviparous barnacle-dwellinglittorinids LisaJ. Johnson*, PeterJ. Mill O , Susan L. Hull* andJean-Paul Ducrotoy* *UniversityCollegeScarborough,FileyRoad,Scarborough,NorthYorkshire,YO113SN. O SchoolofBiology,UniversityofLeeds, Leeds,LS29JT The occurrence of developmental abnormalities intwo ovoviviparous littorinids  Littorina neglecta andabarnacle-dwelling formof L. saxatilis)inhabitingthebarnaclezoneatPeakSteel,Ravenscaronthenorth-eastcoastofEnglandwasinvestigated. Ingeneraldevelopmentalabnormalitiesappearedtobearareoccurrenceinbothtaxa.However, L. neglecta wasfoundtohave asigni¢cantlygreaterproportionofdeformedembryosintheirbroodpouchesthan L. saxatilis BFigure1).Adistinctseasonal variation in the number of developmental abnormalities was observed in L. neglecta, and it is suggested this is a result of broodsdevelopingoutsideofthemainreproductiveseasonbeingmorepronetodeformity. Littorina saxatilis Olivi)and L. neglecta Beanarebothovovivi- parous intertidal gastropod molluscs, retaining embryos within the brood pouch and releasing miniature snails at the crawling stage Reid, 1996). The presence of deformed embryos in L. neglecta hasnotappearedintheliterature.However,deformed embryos have been shown to be a common occurrence in the brood pouches of some populations of L. saxatilis Clyne & Du¡us, 1979; Dixon & Pollard, 1985; Janson, 1985; Sokolova, 1995). A number of potential causes of such abnormalities have been suggested, for example, environmental water quality Dixon & Pollard, 1985; Sokolova, 1995) which can in turn activate latent diseases present in L. saxatilis), temperature Sokolova, 1995) and restricted gene £ow Janson, 1985). The aim of this study is to determine the level of abnormal developmentofembryosintwoovoviviparousbarnacle-dwelling littorinids found at Peak Steel, Ravenscar on the north-east coast of England 54825'N 00827'W). Until recently it was assumed that any small mature littorinid found inthe barnacle zonewas L. neglecta Warmoesetal.,1988).Thetruetaxonomic statusofthistaxonishighlycontested,severalworkersclaiming ittobeabarnacle-dwellingecotypeof L. saxatilis ratherthana species in its own right Johannesson & Johannesson, 1990; Reid,1996). However, at Peak Steel, a barnacle-dwelling form of L. saxatilis referredtointhispaperas L. saxatilis B)hasbeen discovered which is quite distinct from L. neglecta Grahame et al.,1995).Therehasbeenlittleresearchintohowthisbarnacle- dwellingformof L. saxatilis di¡ersfrom L. neglecta. Using a 10cm 2 quadrat three replicate samples were taken from four sites on Peak Steel, Ravenscar, at monthly intervals fromJune 1995 to August 1996, and then at intervals of three months until May 1997. The specimens were relaxed in 10% magnesium chloride, then boiled. The snails were identi¢ed using a combination of shell characteristics and the structure of their reproductive systems in agreement with the diagnostic featuresgiveninCaley1994)andReid1996)).Thecontentsof eachbroodpouchwerethencarefullyremovedandthejuveniles counted under a dissecting microscope. The number of deformed embryos was noted. Eggs with granulated cytoplasm, multiple embryos and embryos with rudimentary, funnel shaped,uncoiledorplanorbidshellswereclassi¢edasabnormal. Any occurrence of trematode parasites in the brood pouch was alsonoted.Replicateswerecollectedbecausetheywererequired by a concurrent study Johnson,1998).The replicate data were pooledinthisstudyandtheresultspresentedherere£ectthis. Littorina neglecta females were found to have a signi¢cantly greater number of deformed embryos in their brood pouches than L. saxatilis B, in which only a small proportion were abnormal Figure 1) Mann^Whitney U-test, df  17, U245, P50.01).Thepercentageofabnormalembryosshowedadistinct seasonal variation in L. neglecta, with greater numbers of defor- mities occurring during the autumn and winter months. Few broodpoucheswerefoundtocontaintrematodeparasitesduring this study  L. neglecta N6, L. saxatilis BN4), and none of thesecontainedabnormalembryos. Throughout this study only low numbers of deformed eggs and embryos were found in L. neglecta and L. saxatilis B.Thisis in sharp contrast to the ¢ndings of Sokolova 1995), who found developmental abnormalities to be common in theWhite Sea, with 20^90% of L. saxatilis females containing broods entirely composed of abnormal eggs. Low levels of salinity were suggested as a critical factor for such high levels of abnormal- ities. Sokolova 1995) also proposed that low temperatures may cause an increase in the number of abnormal eggs. Other workershavealsoobservedgreaterfrequenciesofdevelopmental abnormalities than this current study Clyne & Du¡us,1979; Dixon & Pollard,1985; Janson,1985). A number of factors have been suggested which may result in such increased levels of abnormalities: pollution Dixon & Pollard, 1985); environ- mental stress Clyne & Du¡us, 1979); and disease Dixon & Pollard, 1985; Sokolova, 1995). The small area of Peak Steel provides a uniform habitat, in the form of the barnacle zone, apparently without such exceptional stressful environmental conditionse.g.lowlevelofsalinity,relativelyhighlevelofpollu- tion), and this may account for the low level of abnormalities observed.This is supported by the ¢ndings of Clyne & Du¡us 1979), who observed a complete absence of abnormal embryos at Ham on the north coast of Scotland), a site they considered toberelativelyfreefromenvironmentalstress. The samples of L. neglecta and L. saxatilis Busedinthisstudy were all taken fromthe same habitat, however, the number of abnormalities found in the brood pouches of female L. neglecta was greater than in L. saxatilis B. The di¡erences observed Journal of the Marine Biological Association of the United Kingdom 2000) J. Mar. Biol. Ass. U.K. 2000), 80,547^548 PrintedintheUnitedKingdom