Variation in shell morphology of Calliostoma zizyphinum (Gastropoda: Trochidae)

Differences in shell morphology in the intertidal prosobranchmollusc Calliostoma zizyphinum were studied from a number ofsites within four geographical regions of the British Isleswith varying exposures to wave action and crab predation. Meanvalues of damage scarring were highest in shells sampled fromStrangford Lough, Northern Ireland, and lowest in individualsfrom the Atlantic Coast. Shells collected from the Isle of Manwere smaller than those from either the Atlantic coast of Irelandor Strangford Lough. Shells from Strangford Lough had tallershells (higher aspect ratio) than shells from the County Downand Atlantic Coast and shells from the County Down coast weremore squat (lower aspect ratio) than those from all other areas.Shells from Strangford Lough were significantly thinner thanshells from all other geographical areas. The relationshipsbetween shell damage scarring and shell size and tallness werenot consistent among areas. Shells from Strangford Lough arenotable in showing a steep, positive relationship between shellsize and scarring and a steep, negative relationship betweenscarring and tallness. The unusual shell phenotypes observedin shells from Strangford Lough may be explained by rapid shellgrowth, which would not only allow Calliostoma to attain a sizerefuge from crab predators but also to recover successfullyfrom repeated crab attacks on the shell lip. Such a strategywould result in larger, thinner shells with a high number ofdamage scars. (Received 6 June 2006; accepted 20 December 2006)     

SHELL COLOUR IN CALLIOSTOMA ZIZYPHINUM (L.) FROM STRANGFORD LOUGH, N. IRELAND

Shells of Calliostoma zizyphinum taken from Strangford Lough,N. Ireland are divisible into three distinct colour forms: white(=var lyonsi) intermediate (a pale variegated form) and purple(a dark variegated form). The predominance of white and paleshelled individuals within and the absence of white Calliostomaoutside the lough was confirmed. The proportions of animalswith white and variegated shells at selected sites were almostidentical with those previously documented, suggesting a temporalstability of colour form ratios. No differences in shell thicknessand pedal adhesion were demonstrated between these forms. Snails with white shells reflect radiant heat better, have lightercoloured feet, move more rapidly, show a greater incidence ofshell repair and are more frequently exposed on weed at lowtide, than those with either intermediate or purple shells.Increased proportions of white individuals may be associatedwith high population densities. Under such circumstances, itis suggested that increased mobility may, by increasing dispersion,reduce intraspecific competition. In the event of exposure atlow tide a white shell would help minimize thermal stress. (Received 9 February 1987;     

AN ANALYSIS OF SHELL SHAPE VARIATION IN THE PAINTED TOPSHELL CALLIOSTOMA ZIZYPHINUM (L.) (PROSOBRANCHIA: TROCHIDAE

Littoral and sublittoral Calliostoma zizyphinum (L.) from fourteensites around Ireland are compared by a new method of shell shapeanalysis. The apparatus used is described and results show clearvariation between populations. It is also used to show similaritybetween the variegated and white morphs found in StrangfordLough. Patterns of growth are shown to be largely in responseto the habitat conditions encountered at each site. The meanapical angle of each population is shown to be of great importancein distinguishing populations of topshells, as it varies withthe relative exposure of each site. (Received 30 July 1986;     

PREDATION ON YOUNG PECTEN MAXIMUS (L.) (BIVALVIA), BY THE ANEMONE ANTHOPLEURA BALLII (COCKS)

On the S.W. coast of Ireland at Lough Hyne (Ine) to the southof the rapids, an area of strong water flow, the anemone Anthopleuraballii (Cocks) was found preying on young escallops, Pectenmaximus (L.), in situ and under controlled conditions. Completedead shells were commonly found beside or on the oral surfaceof anemones. Anemones fed under controlled conditions digestedyoung escallops (8 - 17 mm shell height) within 7 - 15 h andthe ejected shell was distributed the same way. An undisturbedanemone accepted and devoured three spat presented to it insuccession. This species of anemone is locally common and isoften found in areas where escallops are encountered, and maythere cause significant mortalities. (Received 16 October 1982;     

GENETIC AND CONCHOLOGICAL COMPARISON OF SNAILS (HELIX ASPERSA) DIFFERING IN SHELL DEPOSITION OF LEAD

Populations of snails inhabiting areas with different historiesof Pb contamination differed in their deposition of Pb in shellrelative to soft tissues. Genetic variation, measured usingisozymes, was not related to Pb history nor geographic distancebetween populations. Shell characteristics were significantlydifferent among sites; shell dry weight was strongly relatedto soil calcium levels. Shells of snails from areas with longhistories of Pb contamination were significantly more robust(greater shell width/shell height ratio) than snails from otherlocations. H. asprsa adaptation to Pb contamination may involvesignificant changes in shell characteristics but these do notcorrelate with genetic traits assessed with allozymes (Received 29 December 1994; accepted 15 October 1995)     

RELATIONSHIPS BETWEEN SHELL SHAPE, WATER RESERVES, SURVIVAL AND GROWTH OF HIGHSHORE LITTORINIDS UNDER EXPERIMENTAL CONDITIONS IN NEW SOUTH WALES, AUSTRALIA

The shell morphologies of the highshore littorinids, Littorinaunifasciata Gray and Nodilittorina pyramidalis (Quoy & Gaimard)have previously been shown to vary at a variety of spatial scales,including among replicate sites at the same height, from heightto height and from shore to shore. In this study, the relationshipsbetween morphology of the shell, the reserves of water heldwithin the shell, the size of the foot and survival on differentshores and rates of growth in different habitats were examinedfor L. unifasciata and, to a lesser extent, N. pyramidalis.Reserves of water were not consistently related to size or shapeof the shell, but did increase as relative weight of shell increased.This may be due to the relatively smaller body providing moreinternal volume for extra-corporeal water. Water reserves andthe amount of free water held in the shell were also not relatedto loss of water or survival during extended periods of emersion.Although the shape of the shell on sheltered and exposed shoreswas correlated with size of the foot, with the snails on anexposed shore having larger apertures and feet than those ona sheltered shore, transplant experiments did not show differentialmortality between morphs from the different shores. All translocatedand transplanted snails disappeared from the exposed shore ata greater rate than from the sheltered shore, but this was probablydue to the snails dispersing out of the experimental areas ratherthan due to mortality. Therefore, many of the large-scale modelsthat have previously been used to describe patterns of shellshape in intertidal gastropods do not appear to be importantin these highshore littorinids. Finally, field experiments ongrowth of juvenile L. unifasciata indicated that rate of growth,largely governed by opportunity to feed rather than type andquantity of food, is the most likely explanation for the small-and large-scale patterns of shell shape that have been previouslydescribed in this species. (Received 22 October 1996; accepted 19 February 1997)     

SHELL SCARRING IN CALLIOSTOMA ZIZYPHINUM (PROSOBRANCHIA: TROCHIDAE) FROM STRANGFORD LOUGH, NORTHERN IRELAND

Pink and white shells of Calliostoma zizyphinum show both undamagedand damaged scars. White shelled individuals predominate inStrangford Lough, Northern Ireland where Calliostoma densitiesand scarring levels are high. No white shells were found intertidally,outside the entrance channel to the Lough. Significant differencesoccur in the size of Calliostoma shells between sampling sitesbut not between different colour morphs at each site. Howeverin pooled samples, pink individuals are significantly largerthan white individuals. When sites and morphs are consideredseparately, pink Calliostoma density is negatively correlatedwith water movement. At sites where pink Calliostoma occur,the percentage of pink shelled individuals is negatively correlatedwith total Calliostoma density. Damaged and undamaged scarring values per unit area of shell,show highly significant differences between sites and damagedscars are significantly higher in white individuals. Undamagedscars are not correlated with any of the environmental parametersrecorded, but are positively correlated with damaged scars suggestinga common causative factor. The level of damaged scarring ispositively correlated with crab/total Calliostoma ratio at allsites and where each colour morph was considered separately.Multiple regression analyses reveal that crab/Calliostoma ratiosaccount for 42% of the between site variation in damaged scars.Significantly higher levels of damaged scars are found at siteswith high crab densities and significantly larger individualsare found at sites where crab densities are intermediate invalue. The largest and most highly scarred individuals occur at siteswith most coarse substrata where Calliostoma are present intheir lowest densities. The higher scarring levels and smallersize of white individuals reflect either higher mortality orreduced growth in white shelled Calliostoma. (Received 25 November 1991; accepted 5 November 1992)     

COMPARISONS OF THE BIOLOGY OF THE INTERTIDAL SUBANTARCTIC LIMPETS NACELLA CONCINNA AND KERGUELENELLA LATERALIS

Two limpet species occur intertidally on subantarctic SouthGeorgia, the patellid Nacella concinna and the siphonarlid Kerguelenellalateralis. N. concinna is confined to the lower shore closeto LWS; K. lateralis occurs in middle shore pools, so theirdistributions do not overlap. N. concinna has a much narrowerthermal niche (–12.9°C to +15.6°C) than K. lateralis(–17.8°C to +31.8°C). Environmental data are presentedto show that the upper lethal temperature of N. concinna islow enough to prevent the limpet living higher on the shore.Both limpet species are slow-moving, but K. lateralis showsincreasing speed with rising temperature, peaking at 15–20°C.In contrast, N. concinna moves actively down to –1.9°C(when sea water freezes), but there is a steady decrease inspeed of locomotion above +2°C. Locomotion ceases at 14°Cin N. concinna (c.f. 30°C in K. lateralis). Both speciesexhibit very low tenacities, but in N. concinna tenacity decreaseswith increasing shell length. In K. lateralis there is no effectof temperature on tenacity. Both species show a positive allometricrelationship between foot area and shell length. N. concinnafeeds upon microbial films and microepiflora, but K. lateraliseats colonial diatoms and Enteromorpha bulbosa. Observationson shell middens of the kelp gull Larus dominicanus showed thatthe gulls did not eat K. lateralis, though they ate great quantitiesof the less accessible N. concinna. Gulls ate N. concinna assmall as 11 mm shell length (within the size range of K. lateralis).Experiments on gulls demonstrated an unwillingness to eat K.lateralis, probably because the siphonariid extrudes a viscidwhite mucus when the foot is touched. (Received 9 May 1996; accepted 8 July 1996)     

LIFE CYCLE AND HABITAT SHIFT OF THE TROCHID SNAIL DILOMA SUAVIS (PHILIPPI) WITHIN INTERTIDAL MUSSEL ZONES

The life cycle of the trochid snail Diloma suavis (Philippi, 1849),was studied on an intertidal rocky shore at Shirahama, Wakayama Prefecture,where two mytilid bivalves, Septifer virgatus (Wiegmann) andHormomya mutabilis (Gould), formed vertically contiguous musselbeds in the upper-middle and lower zones, respectively. At lowtide in April, the snail density increased with decreasing shoreheight and was greatest at the middle level of the H. mutabilisbed. Then, the density decreased towards the lower littoralfringe. Newly settled juveniles smaller than 2 mm in shell heightappeared abundantly in late summer and autumn within algal turfon the lower shore. As snails grew larger than 2 mm, they appearedwithin the gaps of the H. mutabilis bed and the S. virgatusbed. They increased in size monotonically towards the next summer,but rate of growth in shell height tended to be great in autumnand small in winter. Seasonal change in the density of snailsfound within the gaps of the mussel beds was remarkable during athree year period, increasing from autumn to winter and then decreasingtowards next summer. Reproduction occurred in summer, and adultsnails disappeared by September. It is thus suggested that this specieshas a one year lifespan and shows a habitat shift from algal turfto the gaps of the mussel beds with growth. (Received 12 October 1998; accepted 2 March 1999)     

TIDALLY PRODUCED GROWTH BANDS IN THE SUBTIDAL BIVALVE SPISULA SUBTRUNCATA (DA COSTA)

Shells of the subtidal bivalve Spisula subtruncata (da Costa)were marked by treatment with cold shock and allowed to growunder different environmental conditions. In animals kept inthe intertida] zone at low water of spring tides (LWST) theshell banding showed a pattern related to the spring and neaptides. During spring tides when the animals were emersed distinctsemi-diurnal bands were laid down in the shell. However duringneap tides the bands were weaker and ill-defined with an approximatecirca-tidal periodicity. Similarly, animals grown in the laboratoryin a simulated semi-diurnal tidal règime of 11 hourshigh tide, 1 hour low tide and under different light règimes,deposited a number of growth bands which coincided with thenumber of tidal emersions. Individuals continuously immersedin the laboratory and in subtidal boxes in the natural environmentshowed ill-defined growth bands which approximate to a semi-diurnalfrequency. In the laboratory the number of bands deposited wasindependent of any change in the light règimes. Thereis a possibility that an endogenous rhythm of band productionexists which persists when the animals are kept continuouslyimmersed in the laboratory (Received 21 May 1987;     

THE ECOLOGY OF THE HONG KONG LIMPETS CELLANA GRATA (GOULD, 1859) AND PATELLOIDA PYGMAEA(DUNKER, 1860): DISTRIBUTION AND POPULATION DYNAMICS

Cellana grata is a high-zoned, exposed-shore limpet with a lifespanof 3 years. Patclloida pygmaeais a low-zoned, sheltered-shorelimpet with a lifespan of I year. C. grata migrates up-shorein winter, but occurs lower on the shore in summer. This migrationis synchronized with variations in algal availability high onthe shore; such variations are, however, themselves relatedto seasonal changes in wave height. P. pygmaea is restrictedto the low shore, year round. The movements upwards in winterand downwards in summer are still evident. Algal species richnessincreased from September to reach a peak between December andMarch, and to abruptly diminish between May and June. The encrustingalgae Brachytrichia maculans, Hildenbrandia prototypus, Lithothamnionsp. and Ralfsia vcrrucosa were present year round. The timingof juvenile recruitment of C. grata and P. pygmaea was fromOctober to July and February to October, respectively. Temperaturemay be important in enhancing juvenile survival by affectingthe seasonal pattern of algal occurrence on the shore. Seasonalfluctuations in the rate of growth were recorded for both specieswith the time of fastest growth of C. grata and P. pygmaea occurringbetween October and March and September and February, respectively.Mortality of both species occurred mainly in summer. Summerheat, predation and intra-specific competition for food appearto be the major causes of death. (Received 1 December 1992; accepted 5 May 1993)     

SPATIAL PATTERNS OF SHELL SHAPE OF THREE SPECIES OF CO-EXISTING LITTORINID SNAILS IN NEW SOUTH WALES, AUSTRALIA

The shape and relative weight of the shell have been shown tovary intraspecifically and interspecifically in a number ofspecies of gastropods, including many different littorinids.These differences give rise to different shell forms in differenthabitats. In those species which have non-planktotrophic development,differences in shell form among shores have been usually explainedin terms of natural selection because exposure to waves supposedlyfavours light shells with large apertures, while predation bycrabs on sheltered shores favours elongated, thick shells withsmaller apertures. Differences in shell shape among speciesfound at different heights on the shore have been explainedin terms of resistance to desiccation and temperature. Suchvariables would tend to act on a relatively broad-scale, i.e.causing differences among heights on a shore or among shores.Rates of growth, which might vary at much smaller scales withina shore, have also been shown to affect the shapes of many shells. In this study, the shape and relative weight of shells of threespecies of co-existing littorinids (Littorina unifasciata, Bembiciumnanum and Nodilittorina pyramidalis) were measured. These speciesall haveplanktotrophic development and they are found on manyshores where there is no evidence that they are preyed uponby crabs. Before explanations of shell shape are proposed, itis necessary that patterns of variation, within different partsof ashore and among different shores are clearly documented.These patterns were measured at a number of different spatialscales within and among replicate shores with different amountsof wave exposure. Large and small specimens were included toallow intraspecific comparisons among snails of different sizesfound at different heights on the shore. The results showedsignificant differences among shores in shape and relative weightof shells, but these differences could not be explained by exposureto waves. In addition, snails of different sizes and differentspecies did not show the same patterns although they were collectedfrom the same sites. Importantly, the shell shape of Liuorinaunifasciata varied significantly among sites at approximatelythe same height within a shore. These differences could notbe clearly correlated with density, mean size nor exposure towaves. The only consistent pattern was a decrease in relativeaperture size in specimens living higher on the shore. Modelsthat have commonly been proposed to explain shape and relativeweight of shells in other species of gastropods are not adequateto explain the small- and large-scale variation of the measurementsdescribed here. It is proposed that any selective advantageof shell morphology and the effects of any variables on thedevelopment of shell morphology in these species can only beidentified after appropriately designed and replicated fieldexperiments. (Received 4 March 1994; accepted 13 September 1994)     

THE RATIO OF CALCAREOUS AND ORGANIC SHELL COMPONENTS OF FRESHWATER SPHAERIID CLAMS IN RELATION TO WATER HARDNESS AND TROPHIC CONDITIONS

Shells from 14 populations of sphaeriid clams including Sphaeriumstriatinum, S. simile, Pisidium walkeri, Musculim partumeiumand M. iransversum were analyzed for organic carbon (µgCmg^–1 shell), nitrogen (µg,N mg^–1 shell) andCaCO_s (%CaCO₃ of total clam dry weight). Habitat waters wereassessed for total hardness (expressed as ppm CaCo₃), ppm Ca,ppm Mg, conductivity (µmho) and suspended organic Carbon(µgCl^–1). For all populations, shell organic C andN are positively correlated and there is an inverse relationshipbetween the amounts of shell CaCO₃ and shell organic carbon.Trophic considerations give the best correlation with shelltype at the generic level of consideration since species ofMusculium are found at the opposite end of the trophic scale(eutrophic) from all other populations studied. For S. striatinum,the most extensively studied species, the amount of shell CaCO₃is inversely related to water hardness. The selection of shellsin the Sphaeriidae is discussed in relation to structural-functionalneeds and habitat conditions ¹ Present Address: Department of Biology, Syracuse University,Syracuse, New York 13210, U.S.A. ² Present Address: Department of Zoology, Miami University,Oxford, Ohio 45056, U.S.A. (Received 5 December 1978;     

THE INFESTATION OF MYTILUS EDULIS LINNAEUS BY POLYDORA CILIATA (JOHNSTON) IN THE CONWY ESTUARY, NORTH WALES

The incidence of the shell-boring parasite Polydora ciliatain Mytilus edulis from the Conwy estuary, North Wales is described.Amongst mussels >40 mm in shell length both the occurrenceand abundance of P. ciliata increased steeply with host size.Infestation was highest amongst mussels near the mouth of theestuary where around 60–65% of the population was parasitized;no systematic relationship between the incidence of P. ciliataand tidal elevation was observed. Mussel condition was negativelycorrelated with the degree of infestation. Moderately and heavilyinfested mussels, however, never accounted for more than ca.10% of this population and the overall detrimental effects ofP. ciliata on growth and production is thus probably quite small.Whilst heavily infested mussels were evidently more vulnerableto predation, the shore crab, Carcinus maenas, selected smaller,non-infested mussels whenever these were available. No obviousrelationships were found between the presence of P. ciliataand the incidence of either pearls or pea crabs. (Received 14 January 1991; accepted 2 April 1991)     

SIZE, SHAPE AND SHELL MORPHOLOGY IN THE ANTARCTIC LIMPET NACELLA CONCINNA AT SIGNY ISLAND, SOUTH ORKNEY ISLANDS

Comparison of the size, shape and shell morphology in littoraland sub-littoral morphs of the Antarctic limpet Nacella concinnareveal differences in shell morphology which are enhanced bystructural anomalies within the shells of the two types. Infestationof sub-littoral shells by the conchocelis phase of an endolithicalga significantly affects shell density and total chlorophylllevels in the two shell morphs. The surface sculpture of sub-littoralshells is characterised by a series of grooves, the configurationof which closely resembles that of the radular teeth in N. concinna.Limpets utilise the available food supply within the shell matrixof other limpets by grazing the shell material. Epibiotic growthof calcareous algae prevent erosion and preserve underlyingshell layers. In severe cases, where protection is lacking,intraspecific shell grazing may remove parts of the shell exposingthe internal tissues. The Dominican Gull, Larus dominicanus, is a major shore predatorof both shell morphs. Gull middens contain both shell typesbut are dominated by the more accessible littoral shells. Comparisonof living populations and midden assemblages indicates thatsize and shape selection of prey occurs, with pear-shaped limpetsbetween 21 mm and 29 mm in length being taken preferentially. Apparent differences in shell form are induced by physical,biological and behavioural influences. Littoral animals arerobust in nature, resist avian pre-dation and are not extensivelygrazed whereas those of the sub-littoral are not subject tothe same degree of predatory attention but suffer a gradualdepletion of their shallower shell form through a combinationof algal infection and intraspecific shell grazing. (Received 21 February 1990; accepted 5 July 1990)     

COMPETITIVE GRAZERS AND THE PREDATORY WHELK LEPSIELLA FLINDERSI (GASTROPODA: MURICIDAE) STRUCTURE A MUSSEL BED (XENOSTROBUS PULEX) ON A SOUTHWEST AUSTRALIAN SHORE

On a southwest Australian exposed rocky shore within King GeorgeSound, the predatory muricid Dicathais orbita was observed feedingnon-selectively on an array of sessile prey, including Austromegabalanusnigrescens, Galeolaria caespitosa, Patelloida nigrosulcata,Serpulorbis sipho, Siphonaria sp. and the mussel Xenostrobuspulex. On a more sheltered, contiguous, platform of the same shore,however, a mat of Xenostrobus pulex occurred which was beingpredated by another muricid, Lepsiella flindersi. X. pulex occupieda wide vertical range on this shore. Higher-zoned, supralittoral,individuals were small and occurred as clusters in crevicesand pits in the rock. A mat of mussels in the lower eulittoralzone was heavily predated by the drilling L. flindersi whichattacked selectively the largest individuals. Between, in thehigh eulittoral, the mussel bed was formed into patches whichwere isolated by the grazing activities of an array of herbivorousgastropods. Recruitment here by X. pulex was to the patch edgesand the grazer's habits of clustering against them, furtherkept them discrete. Little predation by L. flindersi occurredat this shore level. The Xenostrobus pulex community was, therefore, highly structureddown the shore sequentially by: (a), physiological stresses;(b), competitive grazing pressures and (c), predation. (Received 2 February 1998; accepted 5 January 1999)     

INTRA-POPULATION VARIATION OF THE SHELL OF LITTORINA RUDIS (MATON) (MOLLUSCA: PROSOBRANCHIA)

Shell variation within a single population of Litlorina rudis(Maton), collected near the Biological Station at Trondheim,Norway, was studied. The variation of the shell features studied, which includedthe shape of the basal part of the outer lip, and the relativeheight of the spire, width of the shell and of the aperture,were found to be at least partially related to shell height.Furthermore, the shell shape in the population investigated,besides varying with age, also varies due to shell damage. Nosignificant differences were found between the shapes of theshells of males and females. The shell characteristics of different populations of L. rudisalso vary greatly, one of the causes being differences in theenvironmental conditions. When studying this variation betweendifferent populations, however, it must be emphasized that onlyundamaged shells of individuals of about the same age shouldbe compared, in order to obviate the effects of the variabilitydue to age differences and shell damage within each individualpopulation. ^*Contribution from Trondhjem Biological Station no. 199 (Received 18 February 1980;     

GENETIC AND MORPHOLOGICAL VARIATION OF POPULATIONS BELONGING TO THE BULINUS TRUNCATUS/TROPICUS COMPLEX (GASTROPODA; PLANORBIDAE) IN SOUTH WESTERN ZIMBABWE

Aquatic snails from south western Zimbabwe belonging to theBulinus trunscatus/tropicus complex vary widely in shell formsuggesting that more than one taxon could be present. This possibilitywas investigated by making observations on snail samples from13 populations from the Plumtree area, in respect of allozymevariation (5 polymorphic loci), shell morphology (9 variables),copulatory organ and chromosome number. Comparative data wereobtained from snails from north western Zimbabwe identifieddefinitely as B. tropicus. Analysis of the genetic structurerevealed a high degree of polymorphism (P) ranging from 0.29–0.80among populations from Plumtree and expected heterozygosity(H_e) from 0.02–0.22. No enzymatic diagnostic loci werefound which could differentiate the different morphs or populationsand discriminant function analysis on the morphological datashowed an overlap of morphs among populations. Snails analyzedfor chromosome number were all diploid (2n = 36). Snails exposedto Schistosoma haematobium mira-cidia were all refractory. Thisinformation supports the view of a single species, B. tropicus,which is differentiated due to migration barriers and whereenvironmental variables might be implicated in the morphometricdivergence. (Received 31 July 1995; accepted 15 January 1998)     

RECRUITMENT AND GROWTH OF THE BLUE-RAYED LIMPET, HELCION PELLUCIDUM (L.), IN SOUTH EAST IRELAND

This account reports on recruitment, growth and population structureof the macroherbivorous gastropod Helcion pellucidum (L.) onan exposed rocky shore in S.E. Ireland, with particular referenceto the pellucidum form. Recently settled Helcion spat were foundinitially on living crustose lithothamnia in pools and on openrock, on the lower shore, during February-April and subsequentlyrecruited to macro-algal fronds. By May they were found in abundanceon Mastocarpus stellatus and Laminaria spp. Helcion was markedlyseasonal in its occurrence on Mastocarpus, being most abundantduring May-August. A complementary pattern was evident on growingHimanthalia receptacles, with limpets abundant from July onwards.It is suggested that Helcion may migrate from Mastocarpus toHimanthalia as they grow. Data on growth rates are presentedfor Helcion in the various habitats studied. Growth rates werehighest on the Laminaria spp. (Received 21 February 1992; accepted 8 June 1992)