Environmental stress,competition and dominance of Crassostrea virginica near Beaufort,North Carolina,USA

Observations and experiments were made at 2 intertidal areas near Beaufort, North Carolina, USA from May 1977 to July 1978 to determine why the oyster Crassostrea virginica dominated the community in areas protected from wave action but not in areas directly exposed to waves. Barnacles, oysters, the green alga Enteromorpha sp. and the mussel Brachiodontes exustus were the main occupiers of primary space at the mid and low intertidal levels of exposed areas. The intertidal community at the protected site consisted of a mid intertidal occupied by the barnacles Balanus amphitrite and Chthamalus fragilis, and the oyster C. virginica, and a low intertidal dominated by C. virginica. The exposed area was highly variable with high colonization and mortality for all species producing large seasonal changes in structure. The protected site remained constant throughout the year; there was no evidence of further colonization of either barnacles or oysters and mortality was very low. Recolonization experiments, selective removal of species, and growth and survival data demonstrated that C. virginica does not become dominant at exposed locations because (1) the constant wave shock at the ocean site reduces growth and increases mortality of young and adult oysters and (2) oysters are outcompeted by the mussel B. exustus. The monopolization of space by C. virginica at protected sites contrasts with studies north of Cape Hatteras where the abundance of predators produces a more diverse and heterogeneous community. Predation was unimportant in Beaufort because predators were absent at the exposed areas and the oyster drill Urosalpinx cinerea was restricted to the subtidal zone at the protected site. This absence of predators indicates a higher level of environmental stress near Beaufort compared to areas farther north.     

Compressibility and shell failure in the European Atlantic <Emphasis Type="Italic">Patella</Emphasis> limpets

The specimens of Patella intermedia, Patella rustica, Patella ulyssiponensis and Patella vulgata were analysed for morphological and morphometric characters, and for the resistance to compression and crushing to a force applied at the apex. Shell shape in these species ranged from the high symmetrical cone, with a rounded base of P. rustica, to the flat, asymmetrical, narrow-pear-shaped base of P. ulyssiponensis. P. intermedia and P. vulgata showed intermediate morphologies. Shell thickness increased linearly with age, but differed in the four limpets. P. rustica had the thickest shells, and P. ulyssiponensis and P. vulgata had the thinnest shells. P. intermedia displayed intermediate shell thickness. Considering deformability and toughness, P. intermedia shells usually needed the highest force to compress in height, the highest pressure to collapse, and were appreciably deformed at collapse. On the opposite side, P. ulyssiponensis shells usually needed the lowest force to compress in height, the lowest pressure to collapse, and were much less deformed at collapse. P. intermedia shells were therefore the most deformable and tough, and P. ulyssiponensis, the most stiff and fragile. P. rustica and P. vulgata shells displayed intermediate behaviour. However, numerical simulations based on the finite element method using the experimentally determined shells’ geometry and thickness, but considering similar shells’ material and structure in the four species, predicted that shell toughness should be decreased in the order P. rustica >> P. intermedia > P. vulgata >> P. ulyssiponensis. P. rustica shells’ geometry (a high and centred cone) and thickness (very thick) were therefore, theoretically, the most fitted for shells to resist crushing by compression. Yet, in the experimental tests, they were not the most resistant shells. It was concluded that resistance to crushing was not a direct function of shell morphology and morphometry, but appeared to be mainly determined by shell deformability. This is most probably related to differences in the internal composition and architecture of the shell in the four species. By comparison with data reported in the literature on the pressures normally exerted by ocean waves, it was concluded that these limpets have resistances to crushing in far excess to pressures normally endured in nature, being therefore unlikely that these species are crushed by the action of even very strong waves on shores. Hard objects, like logs and boulders, can be hurled onto the shore by waves, and constitute a much greater threat to limpets than the waves themselves. The high toughness of the limpet shell can be related to resistance to the impact of incidental hard objects. There was no direct relationship between the habit preferences of these limpets and the resistance to crushing. Other factors are involved in the distribution of these species in the shores.     

Variability of shell growth and morphology of the wall-plate junctions of the intertidal barnacle Tesseropora rosea (Cirripedia: Tetraclitidae)

Variability in shell growth and morphology of the wallplate junctions of Tesseropora rosea (Krauss) were studied in 1983 in New South Wales, Australia, at two sites with differing wave exposure. The wall-plate junctions, a combination of dado (geometrically interlocking) and lap bints, increase in complexity with age, but do not differ between sites. The number of parietal tubes is fixed at settlement and does not differ between sites, but differs among wall plates in a manner consistent with their evolutionary history (rostrum > laterals > carina). The number of secondary septa increases with age, but does not differ between sites. In epilithic T. rosea, growth of the alae contributes to orifice enlargement. Epizoic T. rosea (on limpets) have conspicuous radii and display diametric growth. At the more exposed site, the shell wall was sleeper, and the aperture length, height and weight of the shell increased at a faster rate than at the less exposed site, but the maximum height of the shell was less. These differences appear to be due to a combination of greater feeding rates and greater erosion rates at the exposed site. The greater maximum height attained at the more sheltered site may be due to greater individual longevity. Juveniles at the sheltered site had thinner septa in the laterals than those at the exposed site. Septa in adults were rhinner at the exposed site, but this is due to age differences between sites in the adult populations.     

An energy budget for individual barnacles (Balanus glandula)

An annual energy budget was constructed for individual adult barnacles (Balanus glandula Darwin) for the first year after settlement. The production of body tissue, egg, shell, aquatic and aerial respiration, molting and faecal production was determined and consumption was derived from the summation of these budget items. To provide an estimation of the accuracy of the budget equation, energy budgets were constructed for three small groups of barnacles (n=40) kept under laboratory conditions, in which the budget items, including consumption, were determined independently. The results of the laboratory energy budgets indicated that consumption values derived from the summation methods for the three groups of barnacles were 7.4% higher and 16.2 and 15.6% lower than those determined by actual feeding experiments. The average consumption, assimilation and production of individual barnacles were estimated to be 699.5, 647.3 and 159.6 cal year^–1, respectively. B. glandula has an exceptionally high assimilation efficiency (92.5% from the annual budget and 99.3% from the laboratory budgets) but a low gross production efficiency (22.8%) and net production efficiency (24.7%). A very large proportion of energy (67.4%) was lost in respiration. The second most important budget item was egg production (12.3%); followed in decreasing order by: shell production (6.6%)> production of body tissue (3.9%)>molting (2.3%).This paper was presented at the 58th Annual Meeting of the American Society of Limnology and Oceanography, San Francisco, 1977, and received the Award for Excellence for the best student paper.     

<Emphasis Type="Italic">Semibalanus</Emphasis><Emphasis Type="Italic">balanoides</Emphasis> in winter and spring: larval concentration,settlement, and substrate occupancy

This study measured the progression from pelagic larvae to juvenile barnacles, and examined whether recruitment of barnacles, Semibalanus balanoides Linnaeus, at two intertidal sites in contrasting hydrodynamic regimes was determined by pre-settlement or post-settlement processes. The two sites were 1.5 km apart in the vicinity of Woods Hole, Mass., USA. Quantitative plankton samples were taken twice weekly from December 1997 to May 1998 at a nearby site as an estimate of nearshore larval abundance. The presence of S. balanoides nauplii was noted, and cyprids were enumerated and measured. Larval settlement at the two sites [Gansett Point, Buzzards Bay (GP) and Little Harbor, Vineyard Sound (LH)] was estimated from examination of replicate settlement plates exposed for 2 or 3 days throughout the settlement season, and from replicate plots on marked rock quadrats at each site. On both plates and rocks settled cyprids and metamorphs were enumerated. Space occupancy on unmanipulated rock quadrats by all stages from cyprids to adult barnacles was also examined. Settlement occurred from 2 January to 20 May, and major settlement peaks coincided with peaks in pelagic cyprid concentration at LH, but not at GP. Space occupied by juvenile barnacles was close to zero up until late February despite substantial settlement prior to that. At LH, juvenile barnacle cover was zero at the end of the observations; all settlement failed. Almost 100% of settled cyprids failed to metamorphose within 2 days from late January to late March. Then the proportion metamorphosing increased sharply coinciding with a sudden increase of 3°C in water temperature. Observed site differences in space occupancy by juvenile barnacles suggest that while cyprid supply is a necessary condition for barnacle settlement, other factors affecting metamorphosis of settled cyprids and early juvenile mortality determine recruitment.     

Patch dynamics of mussel beds near São Sebastião (São Paulo), Brazil

In permanent quadrats on exposed and protected shores near São Sebastião (São Paulo), Brazil, changes in percent cover of mussels [Brachidontes solisianus (d'Orbigny)] and barnacles [Chthamalus stellatus bisinuatus (Pilsbury)] were followed from November 1979 through December 1982. Mussels gradually disappeared from both shores, while barnacles came to dominate the exposed, but not the protected, shore. Decreases in percent cover of mussels were related to periods of higher temperatures. Mussels were only transient dominants of the mid-intertidal zone due to lack of recruitment and high summer mortality. This is probably because the study was carried out near the northern limit of their occurrence as a dominant of mid-intertidal communities.Deceased     

Growth,mortality and shell morphology of cultivated mussel (Mytilus edulis) stocks cross-planted between two Scottish sea lochs

One-year-old rope-grown blue mussels (Mytilus edulis L.) were held in experimental lantern nets and cross-transplanted between two sea lochs (Lochs Etive and Leven) on the west coast of Scotland. Growth and mortality were monitored from May 1991 to May 1992, as well as shell morphology in native and transplanted mussels. There were highly significant differences (P0.001) in growth rates between lochs; both native and transplanted mussels performed better in all shell and tissue growth-parameters in Loch Etive than in Loch Leven. Stock also had a significant (P0.05) influence on shell length and live weight, but its contribution to total variance was considerable only in the latter case. Wet and ash-free dry meat weights were governed mainly by site and to some extent by site x stock interaction. Mortality rates were quite low (6 to 7% yr^-1) and did not show any significant variation either between sites or stocks. There were significant morphological differences (ratios of shell length, height and width) between the Loch Etive and Loch Leven populations (P0.001) which persisted for one year after transfer. Stock appeared to be the main factor affecting shell morphology, as opposed to site. Linear regressions of shell height and width on length were significantly different between the native stocks, but became indistinguishable from those of the recipient populations one year after reciprocal transfer. This did not, however, conceal the effect of stock origin on dimensional ratios. It is concluded that site differences related to environmental factors, notably chlorophyll a levels and currents, and also possible stress caused by high levels of Zn and Cu, are major determinants of growth, which are of main interest to aquaculture, while morphological differences most probably result from genotypic variation.     

Environmental and structural control of trace elements in barnacle shells

The shell composition of 5 species of barnacles [Balanus balanoides (L.), B. crenatus Bruguière, B. hameri Ascanius, Elminius modestus Darwin and Chthamalus stellatus (Poli)] collected from four different sites on the coast of North Wales and one site in the Irish Sea were studied using chemical, X-ray diffraction and electron-microprobe analyses. All the shells consisted of calcite. The trace substances studied were Mg, Sr, Na, Mn, PO₄, and SO₄. Other trace elements detected included Si, Li, Ba, Zn, Cl, and Al. For each species, Mn was the only element in the shell which varied significantly from one locality to another. Variation of concentration of Mn in the shells may be related to the variations of the concentration of Mn in the water, since concentrations in the shells were highest in (estuarine) waters containing high concentrations of Mn. In Balanus balanoides L. and Elminius modestus Darwin, the Mn:Ca and Mg:Ca ratios decreased with decreasing shore level and Sr:Ca varied in the opposite direction. When B. crenatus Bruguière, a subtidal and lower littoral species, and B. hameri Ascanius, a Continental-Shelf species, were compared with the intertidal, species B. balanoides and E. modestus, the Mn:Ca and Mg:Ca ratios followed the same decreasing trend, and the Sr:Ca ratios the same increasing trend with increasing depth. These variations may be related either to increased growth rate of the shells of each species or maxium temperature to which the species is exposed, or both. The differences between species are more consistently reflected, in the concentration of Mn. However, since the concentration, of Mn is correlated with the amount of organic matter in the shell, the specific variations may reflect only the variations in the amount of organic material in the shell of each species.     

Growth in the bivalve Yoldia eightsi at Signy Island,Antarctica, determined from internal shell increments and calcium-45 incorporation

The growth rate of the infaunal nuculanid bivalve Yoldia eightsi at Factory Cove, Signy Island, South Orkney Islands (maritime Antarctica), was estimated from internal shell increments and ⁴⁵Ca incorporation of individuals collected monthly from December 1987 to April 1989. Acetate peels of etched shells revealed clear first-order increments, with less well defined, narrower, second-and third-order increments. The first-order increments were assumed to be annual, although there is no independent confirmation of this assumption. Unfortunately abrasion of the umbo region and the small thin shells of Y. eightsi meant that in no case could a complete sequence of increments be measured realiably on any individual shell. Measurements of 1043 first-order increments from 130 shells where a minimum of two consecutive increments could be detected were therefore pooled, and a population growth curve constructed from a Ford-Walford plot. This indicated a slow growth rate, with a maximum shell height of 22.3 mm (equivalent to a shell length of 35.6 mm) being reached at an age >60 yr. The size-frequency distribution of 1521 individuals pooled from winter (July to October) samples revealed a distinct lack of smaller (younger) individuals, possibly reflecting poor recruitment in areas of dense adult populations. The largest shell recovered in the samples was 33.5 mm in length, with an estimated age of 52 yr. Short-term ⁴⁵Ca-incorporation experiments indicated a mean daily rate of growth increment of 3.8 m for individuals of 12 mm shell height, which matches the proposed annual growth rate if growth is assumed to occur for about 150 d each year and the first-order increments are assumed to be annual.     

Diversity of gastropods at Pudhumadam coast region and unusual occurrence of Tetraclita rubescens on conus shell

The marine environment is continuously subjected to chemical pollution, which may exert a detrimental effect on aquatic organisms. Biodiversity of marine invertebrates has been proven to be difficult to assess accurately using traditional morphological methods. Major problems include dearth of characters in many taxa. Hence, it is necessary to assess the eco-health of this marine invertebrate using reliable measurements of environmental quality as required in the present study. With the backdrop of information, the Pudhumadam coastal area was surveyed for the diversity of ruined/washed out gastropods species. As a competitor for space with gastropods, these barnacles settle on the conus for its food and habitat. In our study, the occurrence of acorn barnacles was observed on gastropod shells. The pink adult and many white small barnacles were noted on the shells. If these barnacles increase their habitat this may lead to destruction of diversity of gastropods.     

Mating success of male hermit crabs in shell generalist and shell specialist species

Summary The reproductive behavior of two species of diogenid hermit crabs was studied in Hawaii. In the shell generalist, Clibanarius zebra, male reproductive success varied little with size, although the largest males were less successful in obtaining copulations than were medium-large males. Male and female size were positively correlated, in successful pairs, thus larger males had the potential to fertilize more eggs when they were successful in obtaining a copulation. Female fecundity in C. zebra was not affected by species of gastropod shell inhabited once female size was taken into account. Male copulatory success was very strongly influenced by the species of gastropod shell inhabited. Males in Trochus or Nerita shells had greatly reduced reproductive success compared to males in Turbo or Nassarius shells. This result was due both to (1) males in Trochus especially dropping and otherwise poorly handling females during precopulatory behavior and (2) females not responding to precopulatory behavior patterns executed by males in Trochus and Nerita. Transferring males from good to bad shapes of shells and vice versa showed that male success was a function of shell type inhabited and not some correlated feature of the crabs. In the shell specialist, Calcinus seurati, which is found primarily in Nerita shells as an adult, males in Nerita shells were quite successful in obtaining copulations.     

Effect of shell repair on shell strength in the gastropod Littorina irrorata

Repair of damage to the outer lip of gastropod shells results in a scar whose presence could weaken the shell against crushing forces. Shell strength in Littorina irrorata (Say), measured as force required to break the shell between a moving bar and a stationary steel plate, was found to be similar for scarred and unscarred individuals. This suggests that scars do not affect crushing resistance adversely. Cracks tended to propagate along sutures and along growth lines. These seams appear to be more important than scars as lines of weakness in the shell of L. irrorata.     

Examples of post-mortality alteration in Recent brachiopod shells and (paleo)ecological consequences

Post-mortality alteration of brachiopod shells under normal environmental conditions leads to high taphonomic loss, and to a poor contribution to the biodetrital soft sediment. The successive stages of alteration which shells undergo are: (bio)degradation of the organic matrix shell softening structural disaggregation or/and mechanical fragmentation; these processes depend on the shell structure (number of layers) and composition (organic and inorganic components), but very little on environmental conditions, except for the dissolution of inorganic shell constituents. Among the Brachiopoda, three types of alteration occur to different types of shells — Type I: two-layered chitino-phosphatic shell (species ofLingula andGlottidia) displays a rapid degradation of the organic matrix and mechanical abrasion, leading to total disappearance of the shell in 2 to 3 wk; Type II: two-layered carbonate shell, e.g.Terebratulina spp. [but notNotosaria nigricans (Sowerby) which may constitute a fourth shell type], exhibits degradation of the organic matrix of the secondary layer, shell softening, and structural disaggregation leading to shell disintegration in 6 to 7 mo with a concomitant contribution of calcitic microfibres to the sediment; Type III: three-layered carbonate shell, e.g.Gryphus vitreus (Born), undergoes organic degradation of the secondary layer, fragmentation of the anterior two-thirds of the shell, and slow degradation (because of the thick tertiary layer) of the posterior portion of the shell, with dissolution of the inorganic components (mainly in the tertiary layer) which make a relatively minor contribution to the sediment.     

Comparing community structure on shells of the abalone Haliotis midae and adjacent rock: implications for biodiversity

This paper concerns the effects on biodiversity of depletion of the South African abalone Haliotis midae, which is a long-lived species with a large corrugated shell that provides a habitat for diverse benthic organisms. We compared community structure on H. midae shells with that on adjacent rock at three sites (Cape Point and Danger Point sites A and B) and at two different times of the year at one of these sites. Shells of H. midae consistently supported communities that were distinctly different from those on rock. In particular, three species of non-geniculate (encrusting) corallines, Titanoderma polycephalum, Mesophyllum engelhartii and Spongites discoideus, were all found either exclusively or predominantly on shells, whereas another non-geniculate coralline, Heydrichia woelkerlingii, occurred almost exclusively on adjacent rock. The primary rocky substratum, however, supported a higher number of species than abalone shells. Possible reasons for the differences between the two substrata include the relative age, microtopography and hardness of the substrata; the abundance of grazers on them; and the relative age of different zones of the abalone shell, which support communities at different stages of succession. Diversity on shells was lowest in zones that were either very young or very old, in keeping with the intermediate disturbance hypothesis. The distinctiveness of shell epibiota will increase β diversity despite having a lower α diversity than that of adjacent rock. Decimation of H. midae by overfishing therefore has implications for biodiversity conservation.     

Modification of shell formation in the giant clam Tridacna gigas at elevated nutrient levels in sea water

The effect on shell formation of Tridacna gigas by sea water supplemented for 3 mo with ammonium (5, 10 M, N) and phosphate (2, 5, 10 M, P), separately or in combination, was examined. Exposure to N and N+P significantly enhanced shell-extension rates, but significantly reduced shell weights at equivalent size. Scanning-electron microscopy further revealed structural alterations in the outer shell layer, such as misshapen aragonite crystals, irregular crossed-lamellar orientation, and relatively porous shell microstructure. These observations are consistent with results of X-ray diffractometry on the shells which show distinct shifts in the positions of reflections from the (012) and (200) crystal planes relative to the control, indicating changes in crystal lattice parameters following addition of nutrients.     

Shell parameters in relation to zonation in mediterranean limpets

The relation of shell form to habitat in the Mediterranean limpets Patella caerulea (L.) and P. lusitanica (Gmelin) was investigated. P. caerulea shells were significantly longer, broader, but lower in height than P. lusitanica shells. The regression of the ratio (length plus breadth)/2 on the height of the shell was found to characterize the two limpet populations, with a small degree of overlap reflecting the distribution of the two limpets on the shore.     

Shell shape variation in populations of Mytilus chilensis (Hupe 1854) from southern Chile: a geometric morphometric approach

The pattern of shell shape variation in populations of the mussel, Mytilus chilensis (Hupe 1854) from Southern Chile was analyzed as a function of sample origin (cultivated vs. wild) and latitude, using standard tools of geometric morphometrics for landmark data. Additionally, posterior adductor muscle index (PAMI), Freeman condition index and shell thickness were measured in each sample. Highly significant differences in shell shape components were found among mussel populations. These differences are related to the origin of samples (expansion of the posterior adductor muscle scar, elongation of the lateral ligament and of the ventral umbo position in non-cultivated samples) and to latitude (more elongated shells and more extended posterior adductor muscle scar in most southern samples when compared with the northernmost ones). PAMI and shell thickness were statistically higher in wild population, and Freeman condition index was higher in cultivated shells. It is suggested that in wild populations of M. chilensis, the mussels may face higher predator pressures and other environmental stress factors. Consequently, individuals may be using higher energy fraction to reinforce shells and to promote adductor muscle growth at the expense of somatic growth. In contrast, individuals found in calm aquaculture environments are relatively protected from predators and use most of their assimilated energy in somatic growth. In turn, this growth depends on changes that covariate with shell morphology.     

Incidence and distribution of phototrophic shell-degrading endoliths of the brown mussel Perna perna

The incidence, distribution and infestation sequences of four endolithic cyanobacteria in the shells of the brown mussel Perna perna (L.) were studied along the south coast of South Africa. The incidence of endolith-infested shells varied significantly among the 21 study sites (∼23 to 95%), with highest infestation rates occurring on promontories and headlands as compared to sites within bays. At a smaller scale, the incidence of infested shells also varied with tidal height, being high at the upper tidal levels of mussel distribution and low or absent on the lowshore. The observation that small-scale variability in infestation rates was positively related to water movement suggested that physical damage to the outer protective periostracum of mussels may have facilitated colonisation by endoliths. This hypothesis was supported by the fact that shells with artificially damaged periostraca became infested at a greater rate than did control shells. Once colonisation by the filamentous cyanobacteria Plectonema terebrans, Hyella caespitosa and Mastigocoleus testarum had taken place, endoliths spread throughout the shell, causing progressive shell deformation and damage. Only older shells that were infested by the late successional, cavity-forming Pleurocapsa sp., however, exhibited severe shell deformations, became brittle and eventually fractured near the structurally important site of adductor muscle attachment. Heterotrophic endoliths typically associated with shell degradation in previous studies were extremely rare and if present did not contribute to shell disintegration. Although it is generally perceived that endolithic algae and cyanobacteria are restricted to the surface layers of shells by light limitation within the substratum, it is clear from this study that the interaction of a combination of factors (i.e. erosion of the periostracum, successional sequence of colonists and mechanical properties of the shell) may result in phototrophic endoliths causing severe shell degradation and eventually mussel mortality. Received: 18 December 1998 / Accepted: 18 June 1999     

Effects of seawater temperature and pH on the boring rates of the sponge Cliona celata in scallop shells

Warmer, more acidic water resulting from greenhouse gas emissions could influence ecosystem processes like bioerosion of calcifying organisms. Based on summer-maxima values (temperature = 26 °C; pH = 8.1) at a collection site in New York (40°56″ N, 72°30″ W), explants of the boring sponge Cliona celata Grant, 1826 were grown for 133 days on scallop shells in seawater ranging from current values to one scenario predicted for the year 2100 (T = 31 °C; pH = 7.8). High water temperature had little effect on sponge growth, survival, or boring rates. Lower pH slightly reduced sponge survival, while greatly influencing shell boring. At pH = 7.8, sponges bored twice the number of papillar holes and removed two times more shell weight than at pH = 8.1. Greater erosion resulted in weaker scallop shells. This study suggests that lower seawater pH may increase boring rates of C. celata in shellfish, with potentially severe implications for wild and farmed shellfish populations.     

Large-scale patterns of shell variation in Littorina striata, a planktonic developing periwinkle from Macaronesia (Mollusca: Prosobranchia)

Littorina striata King and Broderip, 1832 is a strictly Macaronesian, intertidal periwinkle with planktonic development. The species displays a high degree of shell variation involving size and sculpture (nodulose vs smooth shells). The present work provides a preliminary account of some aspects of this shell variation on wave-exposed shores over the entire geographical range of the species. Based on morphological patterns observed among other prosobranchs it was predicted that southern specimens of L. littorina should on the average be larger, heavier, more nodulose, and should show more shell repair marks, than northern specimens. These expectations were confirmed for shell size and weight. In contrast, there was no consistent pattern in nodulosity between archipelagos, even though there were differences at much smaller scales. Shell repair marks were more prevalent in northern populations, but this trend was only due to a significant N–S difference among nodulose shells. This is surprising as nodulose shells displayed significantly fewer shell repair marks than smooth shells. These observations were tentatively interpreted as a function of presumed differential N–S patterns of wave action and ambient temperatures. In this context, wave action in Macaronesia seems to increase in the south (contrary to what current theories predict). This atypical situation may confound the interpretation of morphological patterns in L. striata so that firm conclusions cannot be drawn without further experimental work at different spatial scales. Nevertheless, it seems that generalisations about macrogeographic shell morphology patterns, based on interspecific comparisons, are not directly applicable to intraspecific patterns, and may strongly depend on local conditions which make adequate sampling and data treatment very difficult. Received: 10 September 1997 / Accepted: 15 January 1998