Mid-shore Zone Research Articles

Impact of the 2011 Tohoku Earthquake on the species diversity of rocky intertidal sessile assemblages.

The impacts of large-scale disturbance events on the species diversity of rocky intertidal sessile assemblages across multiple spatial scales are not well understood. To evaluate the influence of the 2011 Tohoku Earthquake on alpha and beta diversities of rocky intertidal sessile assemblages, we surveyed sessile assemblages in the mid-shore zone from 2011 to 2019 and compared the data with those collected from 2003 to 2010 before the earthquake at the same region. The census was conducted across 22 study plots on five rocky shores along 30 km of the Sanriku Coast of Japan, which is located 150-160 km north-northwest of the earthquake epicenter. Alpha diversity was measured with three Hill numbers (H 0, H 1, and H 2), which represent the number of equally common species that would exist in a community with the same diversity as the sampled community, with higher values of the subscript indicating more weight placed on abundant species. Beta diversity was measured with two metrics (BD total at two spatial scales). Values were compared between the post-earthquake period (2011-2019) and the pre-earthquake period (2003-2010). The results show that the Tohoku Earthquake significantly altered the species diversity of intertidal sessile assemblages across multiple spatial scales. All diversity metrics obtained at multiple spatial scales (i.e., alpha diversities: H 0, H 1, and H 2; beta diversities: BD total at the shore and regional scales) decreased immediately after the earthquake and then increased in subsequent years. At 2 years after the earthquake, H 0 recovered to within the range of pre-earthquake values and H 1 and H 2 became significantly higher than pre-earthquake values. Most metrics of alpha and beta diversities recovered to pre-earthquake levels after several years, but regional BD total remained low for a longer period.

Effects of harvesting and an invasive mussel on intertidal rocky shore communities based on historical and spatial comparisons.

Intertidal rocky shores are the most accessible marine habitats and therefore heavily impacted by harvesting. In recent years, they have also been increasingly invaded by alien species, which compounds the effects of harvesting on rocky shore community composition and functioning. Recent survey data, combined with historical data from 1970, were used to assess temporal changes over the intervening period in rocky shore communities at two sites (Wireless Point and Wireless Island). Three kinds of changes emerged: (1) the appearance of alien species; (2) the effects of increased harvesting pressure; and (3) the direct and indirect effects of these changes on other species. A striking result was transformation of mid-shore zones on exposed shores by the appearance of the invasive Mediterranean mussel Mytilus galloprovincialis, and the indirect effects of this on the demography and vertical zonation patterns of the granular limpet Scutellastra granularis. Adult limpets have become excluded by the mussel, whereas juveniles find a secondary home on the shells of the mussel and their abundance has increased. To further disentangle the effects of harvesting from those of alien invasions, a spatial comparison was made between two currently unharvested no-take sites (Scarborough South and Scarborough North) and two regularly harvested sites (Kommetjie and Wireless Point). Harvesting has substantially depleted the granite limpet Cymbula granatina and Argenville's limpet Scutellastra argenvillei. This has led to the proliferation of opportunistic seaweeds, such as Ulva spp. The dual effects of alien invasive species and over-harvesting have major ecosystem effects but do not necessarily diminish biodiversity because the alternative habitats that have developed provide opportunities for colonisation by additional species.

Recovery of rocky intertidal zonation: two years after the 2011 Great East Japan Earthquake

To assess the course and status of recovery of rocky intertidal zonation after massive subsidence caused by the 2011 Great East Japan Earthquake, from 2011 to 2013 we censused the vertical distribution of 10 dominant macrobenthic species (six sessile and four mobile species) in the mid-shore zone of 23 sites along the Sanriku coastline, 150–160 km north-northwest of the earthquake epicentre, and compared the vertical distributions of each species with their vertical distributions in the pre-earthquake period. The dynamics of rocky intertidal zonation varied substantially among species. Among sessile species, one barnacle dramatically increased in abundance and expanded its vertical range in 2011, but then decreased and completely disappeared from all plots by 2013. Zonations of other sessile species shifted downward following the subsidence in 2011. With some species, there was no clear change in abundance immediately after the earthquake, but they then began to increase and move upward after a few years; with other species, abundance continuously decreased. There was no clear change in the vertical distribution of any of the mobile species immediately after the earthquake. Abundance of two mobile species was unchanged, but abundance of the others decreased from 2012 and had not recovered as of 2013.

Photosynthetic activity and productivity of intertidal macroalgae: In situ measurements, from thallus to community scale

The photosynthetic activity and productivity of four dominant canopy intertidal macroalgae were measured under emersion and immersion, at saturating light levels (PAR>300μmolphotonsm−2s−1), and compared at two sites (eastern and western English Channel) in spring and summer. The photosynthetic activity of thalli was measured by the electron transport rate (ETR) using pulse-amplitude modulated (PAM) fluorescence and the productivity of individuals and of communities was measured by carbon fluxes in closed chambers. Under emersion, when thalli were still hydrated, the uppermost species Pelvetia canaliculata had higher photosynthetic activity (mean ETR between 327 and 460μmole−m−2s−1) and individual gross productivity (between 60 and 212μmolCgDW−1h−1) than the lowermost species Laminaria digitata (mean ETR between 24 and 53μmole−m−2s−1 and gross productivity between 2 and 38μmolCgDW−1h−1), whatever the site and season. P. canaliculata had higher ETR in air than underwater (averaged 146μmole−m−2s−1) and L. digitata had lower ETR in air than underwater (averaged 112μmole−m−2s−1), while they exhibited, respectively, 3 and 5 times higher gross productivity underwater. At the community scale, the low mid-shore zone of Fucus serratus had the highest mean gross productivity under emersion (47mmolCm−2h−1) while rates were higher for the uppermost than lowermost zone at the eastern site (average 20 and 6mmolCm−2h−1, respectively) and of the same order of magnitude for both zones at the western site (about 30mmolCm−2h−1). Finally, the variability of under emersion primary productivity among sites and seasons was reduced when the measurements were performed on entire communities compared to isolated individuals of the dominant species.

Abundance and Spatio-Temporal Distributional Pattern of Dotilla Myctiroides (Milne-Edwards) on Exposed Sandy Beach of Aksa, Mumbai

The abundance and distributional pattern of Ocypodid crab, Dotilla myctiroides (Milne-Edwards) was studied on a sandy beach of Mumbai from January 2005 to December 2006. The area experiences semidiurnal tides. The beach is intermediate dissipative type and consists of medium to fine grade sand. The data were collected after fixing a transect line from high to low shore zone based on tidal heights. A mean abundance of 40 ± 4.55 ind.m−2 was found during 2005, while it was 37.82 ± 7.31 ind.m−2 during 2006. Although no significant difference (p > 0.05) in abundance was observed on seasonal scale, there was a marked variation (p < 0.05) in the vertical distribution with an obvious increasing trend in abundance from high to low shore zone. The zonal variation in the abundance was distinct in different seasons and it increased from pre- to post-monsoon with decreasing tidal heights during both the years. A significant difference (p < 0.05) in the abundance was recorded between high and mid shore zone during all the seasons while mid shore zone showed significantly higher (p < 0.05) abundance during post-monsoon season. Data on distributional pattern of different maturity stages (juvenile, immature, young adult and adult) based on carapace width were also collected. The juveniles were mostly observed in mid and low shore zone, while adults dominated mostly the high shore areas. The factors like morphodynamic state of beach and behavioural ecology of this species seemed to play important role in its high abundance and vertical distribution on the sandy beach of Aksa, Mumbai.

Influence of acute and chronic disturbance on macrophyte landscape zonation

ABSTRACT: Although the roles of physical disturbance and successional recovery from such distur-bances in structuring natural communities are well known, recent studies have begun to uncover thepotential for alternate outcomes or climax states in a number of systems. Here, we examine the dis-tribution of tropical macrophytes at a site with heavy wave exposure and explore equilibrium (micro-habitat) and non-equilibrium (patch-dynamic) hypotheses to explain the observed pattern. The exis-tence of a large and distinct zone of prolific macroalgae, undescribed 30 yr earlier, situated between2 zones with a relatively high abundance of seagrass, challenges the classic successional regimewithin Caribbean macrophyte beds. Significantly smaller mean size and lower frequency of acutedisturbances within the macroalgal-dominated zone, as compared to the outer, mixed macrophytezone, appears to contradict the highly disturbed environment of classic colonizers. The dominance ofmacroalgae in the mid-shore zone may be enhanced by the presence of large sediment sizes that areaccumulated through the effect of chronic wave stress. Potential causes of overall changes in thehydrodynamic forces at the study site include rise in sea level, increased wave exposure and associ-ated erosion of reefs protecting the coast. We propose a model of macrophyte bed development anddistribution that includes the important chronic ‘stress’ category of wave energy and potential domi-nance by macroalgae. Increased average wave energy can impose an elevated level of stress leadingto modified patterns of macrophyte distributions and changes in the deterministic endpoint of thesuccessional sequence. KEY WORDS: Succession · Colonization · Competition · Waves · Stress · Gaps · Seagrass ·Macroalgae

Rivets or bolts? When single species count in the function of temperate rocky reef communities

There is considerable controversy about the role of individual species in ecosystem functioning. Most models stress the role of species richness and diversity in ecosystem function, but it is also recognised that individual species or functionally similar species can play prominent roles in assessments of function. There have been relatively few tests of functional replacement by similar species in the marine environment. On intertidal reef platforms in southern New Zealand, six species of fucoid algae co-occur. Two of these, Cystophora torulosa and Hormosira banksii, have extensive cover in the mid-tidal zone. These species were removed from their areas of dominance in the mid-tidal zone at two sites and followed for several years to determine how resilient they are to disturbance, what processes determine functional recovery, the role of the dominant grazer, the gastropod Turbo smaragdus, in the recovery process, and whether there is functional replacement of species. In the C. torulosa removal experiment, H. banksii recruited rapidly into removal treatments within the first year at one site, but this took a year longer at the other site. A press removal of C. torulosa developed almost 100% cover of H. banksii after 3 years, but only at one site. In the pulse treatment, there was a mixed stand of the two species after 3–4 years. In the H. banksii removal experiment, there was no functional replacement by other species. The rate of recovery for H. banksii was variable. Bare space increased following canopy removal, particularly on the higher shore treatment at one site, because of the burn-off of newly exposed turfing coralline algae, which had not fully recovered by the end of the experiment. After 10 months, there were 20–40% fewer species in the removal plots in both experiments than in controls. T. smaragdus grazing had great effects on ephemeral algae, but not on the fucoids. The conclusion is that these fucoid species have no functional equivalents in their areas of dominance. They are the autogenic engineers or foundation species on which most other species in their communities rely. There is little buffering overall in these mid-shore zones should these species be lost or severely reduced in abundance. They are, therefore, key species in the function of the intertidal system and must be understood if the functioning of these areas is to be managed effectively. These results are discussed in the spirit of this festschrift for AJ Underwood.

Beta diversity of tropical marine benthic assemblages in the Spermonde Archipelago, Indonesia

AbstractIn order to preserve diversity it is essential to understand how assemblages change across space. Despite this fact, we still know very little about how marine diversity is spatially distributed, especially among lesser‐studied invertebrate taxa. In the present study beta‐diversity patterns of sea urchins, sponges, mushroom corals and larger foraminifera were assessed in the Spermonde Archipelago (Indonesia). Using ordinations we showed that the inshore zone (&lt;5 km offshore), midshore zone (5 &lt; x &lt; 30 km offshore) and distance offshore zone (&gt;30 km offshore) all contained distinct assemblages of sponges and corals, while only foraminifera assemblages from the inshore (&lt;5 km offshore) zone were distinct. There was a significant spatial pattern of community similarity for all taxa surveyed, but this pattern proved to be wholly related to environmental variables for sponges and foraminifera, and primarily for mushroom corals and sea urchins. The lack of a pure spatial component suggests that these taxa may not be dispersal limited within the spatial scales of this study (c. 1600 km2). The analyses of the corals and foraminifera were additionally tested at two spatial scales of sampling. Both taxa were primarily associated with local‐scale environmental variables at the local scale and larger‐scale variables at the larger scale. Mean inter‐plot similarity was also higher and variation lower at the larger scale. The results suggest that substantial variation in similarity can be predicted using simple locally assessed environmental variables combined with remotely sensed parameters.

Structural gradients in an intertidal hard-bottom community: examining vertical, horizontal, and taxonomic clines in zoobenthic biodiversity

Measures of biodiversity along environmental gradients have long been the focus of marine ecological research. However, a general lack of comparability between studies and under-appreciation of co-occurring, less obvious clines has often undermined any general conclusions. Latitudinal, vertical, horizontal, and taxonomic gradients in intertidal biodiversity were assessed directly and indirectly using a large data set from one locality in southwest Ireland. A total of 153 epifaunal species, from 106 families and 13 phyla, was recorded from over 200,000 individuals in a sample area of 63 m2. Multivariate analysis revealed that the vertical gradient of immersion was the primary structuring factor in the community but that a flow-driven horizontal gradient was also important in structuring low- and mid-shore zones. Crustaceans, annelids, and molluscs were most numerous over the entire site, but bryozoans and sponges may have played the most significant role in driving the horizontal (flow-related) trend in species richness at lower levels on the shore. Comparison of species richness with sites from other locations around the world proved inconclusive in assessing a global trend in richness from the poles to the tropics. However, interpolation of this and similar studies may prove useful in future latitudinal investigations of hard-substratum intertidal biota. Higher-taxon surrogacy for species trends proved significant and useful at the scale investigated here, whereby analysis of data from family or even order level may prove an efficient method of future monitoring. More data are required to assess if the ‘top-down’ taxonomic approach will sufficiently mirror species trends at larger spatial scales. Overall, the generality of effects of environmental gradients on intertidal organisms may be more conclusive where entire communities are studied and consideration is given to other, less dominant clines.

Nonallopatric and parallel origin of local reproductive barriers between two snail ecotypes.

Theory suggests that speciation is possible without physical isolation of populations (hereafter, nonallopatric speciation), but recent nonallopatric models need the support of irrefutable empirical examples. We collected snails (Littorina saxatilis) from three areas on the NW coast of Spain to investigate the population genetic structure of two ecotypes. Earlier studies suggest that these ecotypes may represent incipient species: a large, thick-shelled 'RB' ecotype living among the barnacles in the upper intertidal zone and a small, thin-shelled 'SU' ecotype living among the mussels in the lower intertidal zone only 10-30 m away. The two ecotypes overlap and hybridize in a midshore zone only 1-3 m wide. Three different types of molecular markers [allozymes, mitochondrial DNA (mtDNA) and microsatellites] consistently indicated partial reproductive isolation between the RB and the SU ecotypes at a particular site. However, each ecotype was related more closely to the other ecotype from the same site than to the same ecotype from another site further along the Galician coast (25-77 km away). These findings supported earlier results based solely on allozyme variation and we could now reject the possibility that selection produced these patterns. The patterns of genetic variation supported a nonallopatric model in which the ecotypes are formed independently at each site by parallel evolution and where the reproductive barriers are a byproduct of divergent selection for body size. We argue that neither our laboratory hybridization experiments nor our molecular data are compatible with a model based on allopatric ecotype formation, secondary overlap and introgression.

Role of post-settlement mortality and 'supply-side' ecology in setting patterns of intertidal distribution in the chthamalid barnacles Chthamalus montagui and C. stellatus

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 249:207-214 (2003) - doi:10.3354/meps249207 Role of post-settlement mortality and Œsupply-side¹ ecology in setting patterns of intertidal distribution in the chthamalid barnacles Chthamalus montagui and C. stellatus J. Delany1,*, A. A. Myers2, D. McGrath3, R. M. O¹Riordan2,4, A. M. Power2 1Dove Marine Laboratory, Department of Marine Sciences and Coastal Management, University of Newcastle upon Tyne NE30 4PZ, United Kingdom 2Department of Zoology and Animal Ecology, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland 3Department of Life Sciences, Galway-Mayo Institute of Technology, Galway, Ireland 4Department of Biological Sciences, National University of Singapore, 117543 Singapore *Email: j.e.delany@ncl.ac.uk ABSTRACT: The chthamalid barnacles Chthamalus montagui and C. stellatus occupy distinctive distributional patterns on European rocky shores. To explore whether these patterns are determined primarily by larval settlement or by post-settlement mortality, we examined the abundance of each species from cyprid attachment through successive stages in its intertidal life history and thus attempted to determine the critical stage for the establishment of adult patterns of dominance. Sampling was carried out on 4 shores in Cork, SW Ireland. On 2 of the shores, sampling sites were in zones in the upper shore dominated by C. montagui, and on the other 2 shores, sites were in zones in the midshore dominated by C. stellatus. The patterns of cyprid settlement and recruitment of early metamorphs in these zones did not reflect that of adult abundances. Patterns of mortality of the 2 species were determined from photographs. Mortality of recruits between 6 and 12 mo after settlement differed between the 2 zones. Mortality of C. montagui was higher than that of C. stellatus in the midshore zone, while the reverse trend was observed in the upper C. montagui-dominated zone, with C. stellatus recruits suffering higher rates of mortality than those of C. montagui. Post-settlement mortality, rather than larval supply, was therefore deemed to be the more significant factor in determining the characteristic patterns of these chthamalid species on SW Ireland shores. KEY WORDS: Larval supply · Post-settlement mortality · Chthamalus montagui · C. stellatus Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 249. Online publication date: March 10, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

HYBRID FITNESS SEEMS NOT TO BE AN EXPLANATION FOR THE PARTIAL REPRODUCTIVE ISOLATION BETWEEN ECOTYPES OF GALICIAN LITTORINA SAXATILIS

Partial reproductive isolation between closely related groups of organisms is suggested to be of central importance during speciation. Galician populations of Littorina saxatilis are phenotypically differentiated into an upper-shore and a lower-shore morph. These mate assortatively in the mid-shore zone of overlap, and genetic assessment shows an impeded gene flow between the parental morphs. The traditional explanation as to why assortative mating occurs is that reproductive isolation is reinforced due to hybrid unfitness. Earlier studies have, however, not found hybrids to be less viable. Likewise, growth and migratory behaviours are merely intermediate between those of the parental morphs. In the present study we compared male and female fertility components of the parental morphs and the hybrids to test hypotheses of decreased hybrid fertility. The results showed that hybrid males were as fertile as other males, and hybrid females did not produce fewer embryos, nor aborted embryos at a higher rate, than the parental morphs.

THE MAINTENANCE OF A CLINE IN THE MARINE SNAIL LITTORINA SAXATILIS: THE ROLE OF HOME SITE ADVANTAGE AND HYBRID FITNESS.

Steep clinal transitions in one or several inherited characters between genetically distinct populations are usually referred to as hybrid zones. Essentially two different mechanisms may maintain steep genetic clines. Either selection acts against hybrids that are unfit over the entire zone due to their mixed genetic origin (endogenous selection), or hybrids and parental types attain different fitness values in different parts of the cline (exogenous selection). Survival rate estimates of hybrids and parental forms in different regions of the cline may be used to distinguish between these models to assess how the cline is maintained. We used reciprocal transplants to test the relative survival rates of two parental ecotypes and their hybrids over microscale hybrid zones in the direct-developing marine snail Littorina saxatilis (Olivi) on the rocky shores of Galicia, Spain. One of the parental forms occupies upper and the other lower shores, and the hybrids are found at various proportions (1-38%) along with both parental forms in a midshore zone a few meters wide. The survival rate over one month was 39-52% of the native ecotype on upper shores, but only 2-8% for the lower-shore ecotype. In contrast, only 4-8% of the upper-shore ecotype but 53% of large (> 6 mm) and 8% of small (3-6 mm) native lower-shore ecotype survived in the lower shores. In the midshores, both the two parental ecotypes and the hybrids survived about equally well. Thus there is a considerable advantage for the native ecotypes in the upper and lower shores, while in the hybrid zone none of the morphs, hybrids included, are favored. This indicates that the dimorphism of L. saxatilis is maintained by steep cross-shore selection gradients, thus supporting the selection-gradient model of hybrid zones. We performed field and laboratory experiments that suggest physical factors and predation as important selective agents. Earlier studies indicate assortative mating between the two ecotypes in the midshore. This is unexpected in a hybrid zone maintained by selection gradients, and it seems as if the reproductive barrier compresses the hybrid zone considerably.

Spatial Variation of Intertidal Macrofauna on a Sandy Ocean Beach in Australia

Spatial variation of macrofauna on a sandy beach was examined simultaneously over two scales across-shore (among and within zones) and three nested scales along-shore over the entire length of the beach. Prior to the main study, pilot studies were performed to determine: (1) the relative efficiency, accuracy and precision of combinations of core size, depth of sampling, and sieve mesh size; and (2) the likely distribution of macrofauna across-shore so that stratification of sampling in the main study would be meaningful. From this, three zones were defined across-shore, namely: (1) the high-shore zone which extended 10 m downshore of the drift line and was dominated by two species of isopod; (2) the mid-shore zone which extended across the beach from the bottom of the high-shore zone to the top of the swash zone and was dominated by the glycerid polychaete Hemipodussp.; and (3) the swash zone which contained more species than the other two zones and was dominated by amphipods, Hemipodussp., the bivalve Donax deltoidesand a species of cumacean. In the main study, multivariate analyses confirmed that assemblages of macrofauna varied significantly among zones despite smaller scale variation within zones and along-shore variation. Significant along-shore variation was detected in assemblages of macrofauna from each zone and occurred at different scales for different zones. Only assemblages in the swash zone showed a pattern of along-shore variation that was consistent with a gradient in wave exposure along the beach. Univariate analyses showed that significant variation in populations of individual taxa occurred at both large and small scales. Significant variation was detected across-shore within zones for nearly all variates and this demonstrated the importance of formally assessing variation withinzones when making comparisons amongzones. Significant variation was also detected along-shore in analyses of particular taxa, and interactions of across- and along-shore variation also occurred. These results illustrate the necessity of considering both across- and along-shore variation for describing spatial patterns in assemblages or individual species of macrofauna. Unfortunately, sampling a single transect across a beach, which is common in many published descriptions of spatial patterns, will not provide an adequate nor representative description of the macrofauna of that beach because this approach fails to consider all important sources of variation and confounds large- and small-scale variation. The authors conclude that a better understanding of small-scale variation, both along- and across-shore within beaches, is required in order to provide better descriptions of patterns, provide a basis for larger scale studies, allow unconfounded comparisons among beaches and, ultimately, to improve our understanding of the ecology of sandy beaches.

Differentiation in radular and embryonic characters, and further comments on gene flow, between two sympatric morphs ofLittorina saxatilis(Olivi)

Abstract On exposed rocky shores of Galicia, two morphs of the direct-developing snail Littorina saxatilis are found associated with different microhabitats. A ridged and banded (RB) morph is found in upper, less wave-exposed parts of the shore, while a smooth and unbanded (SU) morph is present in low, heavily wave-stressed parts. They overlap in a mid-shore zone where hybrids are found, usually at low frequencies (<20%). We compared shells of embryos from upper-shore RB females with those from low-shore SU females. We also compared embryonic shells from mid-shore RB and SU females, at two sites. The size of the nuclear whorl, the aperture width, the shell diameter and the number of whorls differed between embryos of the two morphs both in allopatry and in sympatry. Populations of the same morph from different microhabitats differed only in 1–2 of the charaeters. The number of cusps on the outer marginal tooth of the radula of adult females differed between both allopatric and sympatric populations of the...

Morphology, flowering and seed production of Zostera capricorni Aschers. in subtropical Australia

The vegetative morphology, flowering characteristics, seed production, and seed distribution in the sediment of the seagrass Zostera capricorni Aschers. were studied for 18 months by monthly sampling. Three seagrass zones were studied. In each of these zones there was a different morphological type of Z. capricorni: small, dense plants in the midshore zone; large, sparse plants in the offshore zone; medium size and medium density plants in the inshore zone. The distribution of seeds in the sediment was also studied in bare areas adjacent to these vegetated zones. Flowering lasted for 8 months, from September to April, with the highest percentage of shoots flowering in September and November. Flowering lasted longest in the midshore zone (7 months), compared with the inshore (3 months) and offshore (4 months) zones. The density of flowering shoots was also higher in the midshore zone (341 ± 14.0 flowering shoots m −2) than in the inshore (4 ± 1.0) or offshore (12 ± 4.0) zones. Both the mean number of spathes per shoot (3.1 ± 0.07), and the number of female flowers per spathe (4.8 ± 0.20) did not vary significantly between zones. However, there were more male flowers per spathe in the offshore (10.5 ± 1.55) than in the inshore (5.6 ± 1.60) or midshore (5.4 ± 0.88) zones. Although most seeds were found in the midshore zone during the flowering season (177m −2 ± 28.4), some seeds remained in the sediment for 4 months after seed production had ceased. Few seeds were found outside the seagrass beds, of these most were in bare areas long-shore of the seagrass.

MORPHOLOGICAL DIFFERENTIATION AND GENETIC COHESIVENESS OVER A MICROENVIRONMENTAL GRADIENT IN THE MARINE SNAIL LITTORINA SAXATILIS.

The marine gastropod Littorina saxatilis has different ecotypes in shores only a few meters apart. This has both taxonomic and evolutionary implications. Here we report on an extreme type of within-shore dimorphism in shell characters. In the wave-exposed rocky shores in northwestern Spain, we found one form of L. saxatilis in the upper-level barnacle zone. It had a white, ridged shell, with black bands in the grooves. Another form confined to the lower-shore mussel belt had a smooth shell that was either white and tessellated or darkly colored. These two forms cooccured in a narrow midshore zone together with individuals that had combined characters, but were present in low frequencies (11%-29%). We used principal-component analysis of metric shell characters to study variation in shell size and shape. We found that the upper-shore form was larger than the lower-shore form. We also found small but significant differences in shell shape. Experiments in a common laboratory environment suggested the differences in shell ornamentation and color are inherited, but the individuals did not develop the morph-specific characters until a shell height of about 3 mm. The occurrence of mainly two distinct forms may suggest the presence of two species that hybridize. An analysis of five polymorphic enzyme loci in populations of snails from three geographically separated sites indicated, however, that there was no positive correlation between morphological distances and genetic distances among populations on a geographic scale (tens of kilometers). Thus, we rejected the hypothesis of two species. However, on a microgeographic scale (meters), genetic differentiation between groups with the same form was less than differentiation between forms. This indicated a partial barrier to gene flow between the two forms, and preliminary mate choice data suggested this was caused by nonrandom mating in the midshore zone of overlap.