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During laboratory and field experiments on Nacella concinna on the west coast of Admiralty Bay, King George Island (Antarctica) clear morphological and behavioural differences between two limpet forms (N. concinna polaris and N. concinna concinna) were found. They suggested presence of genetic divergence. AFLP (amplified fragment length polymorphism) profiling of N. concinna individuals representing the two forms revealed nearly 32% of polymorphic bands; only 2% of them differed between the forms. Our results suggest that the observed phenotypic variation seems to be a result of adaptation to environ− mental conditions and not of any genetic divergence.
Low genetic differentiation between two morphotypes
of the gastropod Nacella concinna from Admiralty Bay,
Katarzyna J. CHWEDORZEWSKA1, Małgorzata KORCZAK1, Piotr T. BEDNAREK2
1Zakład Biologii Antarktyki PAN, Ustrzycka 10/12, 02−141 Warszawa, Poland
2Instytut Hodowli i Aklimatyzacji Roślin, 05−870 Warszawa – Radzików, Poland
Abstract: During laboratory and field experiments on Nacella concinna on the west coast
of Admiralty Bay, King George Island (Antarctica) clear morphological and behavioural
differences between two limpet forms (N. concinna polaris and N. concinna concinna) were
found. They suggested presence of genetic divergence. AFLP (amplified fragment length
polymorphism) profiling of N. concinna individuals representing the two forms revealed
nearly 32% of polymorphic bands; only 2% of them differed between the forms. Our results
suggest that the observed phenotypic variation seems to be a result of adaptation to environ−
mental conditions and not of any genetic divergence.
Key wo rd s : Antarctica, Nacella concinna, genetic diversity, AFLP.
Herbivorous gastropod Nacella concinna is a characteristic representative of
the Maritime Antarctic benthos. It occurs in two forms: N. concinna polaris and
N. concinna concinna (Berry and Rugge 1973; Branch and Branch 1981). Mor
phological and behavioral differences between these two forms were observed by
many authors (Nolan 1991; Markowska and Kidawa 2007; de Aranzamendi et al.
2008). N. concinna polaris occurs on stony bed rock in the littoral zone, and
N. concinna concinna is present in the sublittoral and in the intertidal zones below
4 m down to 110 m depth (Walker 1972; Berry and Rugge 1973; Castilla and
Rozbaczylo 1985; Nolan 1991) along the Antarctic Peninsula and adjacent islands
(Picken 1980). The intertidal morphotype has a taller and heavier shell as com
pared to the rock pool morphotype having a lighter and flatter shell (Powell 1951;
Nolan 1991). These two morphotypes also differ in important physiological traits
(Waller et al. 2006; Weihe and Abele 2008).
Pol. Polar Res. 31 (2): 195–200, 2010
vol. 31, no. 2, pp. 195–200, 2010 doi: 10.4202/ppres.2010.11
In previous studies limpets Nacella concinna were gathered from rock pools
during the low tide and by divers from the subtidal area at the depths of 5–8 m.
Limpets collected from rock pools were significantly smaller than the subtidal
individuals (ttest, P< 0.005), ranging from 19.8 to 43.3 mm shell length (mean
= 21.2, SE = 0.32 mm, n= 222), in comparison to 23.7–47.0 mm (mean = 37.4,
SE = 0.49, n= 191) for the subtidal ones. Visible difference in reaction to preda
tors was also noted between the two described limpet populations in laboratory
and field experiments (Markowska and Kidawa 2007; Markowska 2008).
The aim of this study was to explore the genetic relationships between the two
forms described above: the rock pool N. concinna polaris and the subtidal N.
concinna concinna form from the Admiralty Bay.
Material and methods
DNA extraction. — Samples were collected during the previous experiment
run by Markowska (2008). Forty eight individuals from both forms were taken for
the AFLP analysis. DNA was extracted from about 20 mg of tissue, stored in 70%
ethanol. Samples were dried on Watman paper, then followed overnight proteinase
K digestion. Next steps of extraction followed the manufacturer’s protocol recom−
mendation (NucleoSpin®Tissue, Macherzy−Nagel, AQUA LAB). Purity and quan−
tity of the samples were determined spectrophotometrically. DNA integrity and lack
of RNA impurities were tested by agarose gels electrophoresis (1 × TBE buffer with
0.5 μg/ml ethidium bromide at 20 V/cm).
AFLP analysis. — The AFLP technique was performed according to the previ−
ously described procedures with minor modifications (Chwedorzewska et al. 2006).
Briefly, 250 ng of genomic DNA was digested simultaneously with two restrictive
enzymes EcoRI and MseI. This was followed by ligation of the appropriate adaptors,
preselective and, finally, selective amplification steps (core sequence Table 1). For
196 Katarzyna J. Chwedorzewska et al.
Table 1
Adapter and primer sequences
Adapter/Primer code Sequence (5’−>3’)
EcoRI preselective primer GACTGCGTACCAATTCA
MseI preselective primer GATGAGTCCTGAGTAAC
E−Azz − selective primer GACTGCGTACCAATTCAzz
M−Cyy − selective primer GATGAGTCCTGAGTAACyy
E and M are for any selective primer complementary to EcoRI and MseI adaptors respectively; −zz,
−yy − any combination of the nucleotides at the primers 3’ends.
the selective amplification seven primer pair combinations (EcoRI – AGG/MseI–
CAC, EcoRI – ACG/MseI – CAC, EcoRI – ACT/MseI – CGC, EcoRI – ACC/MseI
–CAC,EcoRI – AAA/MseI – CAG, EcoRI – AGT/MseI – CAA, EcoRI –
AAA/MseI – CGG) were used (Table 1). The EcoRI compatible primers were la
belled at their 5’−ends with gamma – 32P ATP. PCR products were separated on 5%
PAGE and X−ray films were exposed to the gels at −70°C overnight.
Data analysis and statistics. — Reproducible (experiment was run twice), in
dividual AFLP fragments were scored as either present (1) or absent (0). Their fre
quencies were calculated for all the markers. Xlstat v.7.5.2 excel add−in software
(, Addinsoft) was used to perform clustral analysis (Aggregation
criterion: unweighted pair−group average UPGMA, Jaccard coefficient of dissimi
larity) and construct the dendrogram. Principal Component Analysis (PCA) and
Analysis of Molecular Variance (AMOVA) was carried out using GeneAlex5.1
excel add−in software (Peakall and Smouse 2001).
Results and discussion
DNA profiling of the samples representing animals collected from rock pools
and from subtidal zone revealed 374 scoreable fragments amplified by seven
primer pair combinations. The number of the DNA fragments generated by an in−
dividual primer pair varied from 31 to 72 with an average of 53. In total all primer
pairs generated 121 (32% of all signals) polymorphic fragments with an average of
17 per primer combination (Table 1).
Clustral and PCA analyses using all the polymorphic bands failed to differentiate
among all samples collected from rock pools and from subtidal. Nevertheless, the first
main axis of the PCA explained 88.00% and the second 4.34% of the identified vari
ability (Fig. 1). Based on the mentioned methods samples were randomly distributed
and formed no separate groups. The similarity matrix was analysed using UPGMA.
Dendrogram was based on all DNA fragments obtained with seven selective primer
pairs for all individuals. Cluster analysis did not reveal any groups (the results of both,
cluster and PCA analysis were similar, thus only PCA diagram is presented).
Although molecular profiling of N. concinna individuals revealed nearly 32%
of the polymorphic bands, few (2%) were discriminated between the analyzed
populations as demonstrated by AMOVA (Table 2).
Genetic diversity of Nacella concinna 197
Table 2
Results of AMOVA of N. concinna samples from all analysed populations. Significance
tested by a permutation analysis against alternative random partitioning of individuals
(999) across populations.
Source of variation df SS Estimate variance Total variance (%) P−value
Among populations 1.0 5.312 0.066 1.7 <0.001
Within populations 46.0 172.001 3.739 98.3 <0.001
Genetic differentiation of the two forms was studied by de Aranzamendi et al.
(2008) with use of ISSR−PCR markers and indicated that the two analyzed forms
can be considered as genetically distinct populations maintaining low levels of
gene flow. Our results, obtained by a different genetic method focused on total
genomic DNA, showed much smaller differences between the analyzed forms (ac
cording to AMOVA less than 2%) and did not confirm results of de Aranzamendi
et al. (2008) study. These differences may be linked to too small sample size taken
for analysis by de Aranzamendi (only eleven individuals per population).
Thus, our results may point to specificity of this species reproduction physiol
ogy. Nacella concinna produces and disperses long−lived planktonic larvae, which
can survive for two months in the water column (Bowden et al. 2006). The pelagic
198 Katarzyna J. Chwedorzewska et al.
Fig. 1. Principal Component Analysis (PCA) of two population of Nacella concinna collected from
rockpools (A) and from subtidal zones (B), based on AFLP data.
long−lived larvae are able to colonise new sites rapidly, thus recruitment occurs
within a common gene pool and there is little opportunity for local genetic diver
gence. In N. concinna the reproduction takes place about 3 weeks after the water
temperature exceeds −1.4°C (Picken 1980, Picken and Allan 1983). Thus spawn
ing appears to be synchronized with the increasing temperature and probably the
availability of food (Brathes et al. 1994). However, it does not seem to be a suffi
cient barrier to avoid gene exchange in form of partial reproductive isolation.
Moreover, the intertidal morphotype (equivalent of rock pool in presented study)
is also migratory, moving seasonally between the intertidal and subtidal zones
(Walker 1972). Probably these phenotypic and behavioural variations (Mar
kowska 2008), specific to these populations, might be the result of phenotypic
plasticity expressed in particular environmental conditions, rather than it reflects
genetic differences. The heterogeneous environment that characterises intertidal
region (different wave exposures, temperature, ultraviolet irradiation and variable
chemical−physic parameters over a short vertical distance, different predators)
could be responsible for such a significant morphological variability.
Acknowledgments. — We would like to thank two anonymous reviewers for constructive
advice that has improved our paper.
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Received 21 October 2009
Accepted 28 April 2010
200 Katarzyna J. Chwedorzewska et al.
... Population genetic studies using different molecular markers were conducted to analyse the contemporary gene flow between the two ecotypes in this species, but with disparate results (Beaumont and Wei, 1991;de Aranzamendi et al., 2008;Hoffman et al., 2010;Chwedorzewska et al., 2010). Utilizing inter simple sequence repeat (ISSR) markers, de Aranzamendi et al. (2008) detected low but significant genetic structure between ecotypes. ...
... By using AFLPs, Hoffman et al. (2010) did not detect genetic structure between ecotypes, attributing the morphological separation to phenotypic plasticity. However, Chwedorzewska et al. (2010) also with AFLPs found low but significant genetic differences but attributed the morphological divergence to phenotypic plasticity expressed in particular environmental conditions, rather than to genetic differentiation. Taking the two ecotypes as a single panmictic population, genetic differentiation was found mostly among geographically distant populations of N. concinna with different demographic history and levels of genetic diversity (Hoffman et al., 2011;González-Wevar et al., 2013). ...
... Interestingly, replicate ecotype-pairs in given environments might experience different gene flow levels than populations in other environments, resulting in the magnitude of divergence being differentially constrained (Hendry and Taylor, 2004;Butlin et al., 2014). Thus, although previous genetic studies did not concord (Beaumont and Wei, 1991;de Aranzamendi et al., 2008;Chwedorzewska et al., 2010;Hoffman et al., 2010), the migratory behaviour of the L individuals and the reproduction of ecotypes occurring spatially separated (Walker, 1972;Brêthes et al., 1994;Clark et al., 2018) still generate multiple questions, and an incipient genetic divergence cannot be ruled out hitherto. Interestingly, at Ryder Bay Littoral individuals have also been found to remain in the littoral during winter, suggesting that not all individuals migrate to the sublittoral (Waller et al., 2006;Obermüller et al., 2011), strengthening the differentiation between littoral and sublittoral individuals due to a more likely reproductive isolation. ...
Parallel phenotypic divergence is the independent differentiation between phenotypes of the same lineage or species occupying ecologically similar environments in different populations. We tested in the Antarctic limpet Nacella concinna the extent of parallel morphological divergence in littoral and sublittoral ecotypes throughout its distribution range. These ecotypes differ in morphological, behavioural and physiological characteristics. We studied the lateral and dorsal outlines of shells and the genetic variation of the mitochondrial gene Cytochrome Oxidase subunit I from both ecotypes in 17 sample sites along more than 2,000 km. The genetic data indicate that both ecotypes belong to a single evolutionary lineage. The magnitude and direction of phenotypic variation differ between ecotypes across sample sites; completely parallel ecotype-pairs (i.e., they diverge in the same magnitude and in the same direction) were detected in 84.85% of lateral and 65.15% in dorsal view comparisons. Besides, specific traits (relative shell height, position of shell apex, and elliptical/pear-shape outline variation) showed high parallelism. We observed weak morphological covariation between the two shape shell views, indicating that distinct evolutionary forces and environmental pressures could be acting on this limpet shell shape. Our results demonstrate there is a strong parallel morphological divergence pattern in N. concinna along its distribution, making this Antarctic species a suitable model for the study of different evolutionary forces shaping the shell evolution of this limpet.
... Intertidal and subtidal N. concinna collected on the islands around the Antarctic Peninsula could not be distinguished genetically by allozyme loci (Beaumont and Wei 1991), AFLPs (Chwedorzewska et al. 2010;Hoffman et al. 2010) or the mitochondrial cytochrome oxidase subunit I gene (González-Wevar et al. 2011). In contrast, the two morphotypes could be distinguished by intersimple sequence repeat (ISSR) markers (Aranzamendi et al. 2008). ...
... In contrast, the two morphotypes could be distinguished by intersimple sequence repeat (ISSR) markers (Aranzamendi et al. 2008). However, due to small samples size, their results were contested by Chwedorzewska et al. (2010) and Hoffman et al. (2010). Samples for these genetic studies were collected at the South Orkney Islands (Beaumont and Wei (1991), King George Island, South Shetlands (Aranzamendi et al. 2008;Chwedorzewska et al. 2010;González-Wevar et al. 2011, 2012 and Adelaide Island (Hoffman et al. 2010). ...
... However, due to small samples size, their results were contested by Chwedorzewska et al. (2010) and Hoffman et al. (2010). Samples for these genetic studies were collected at the South Orkney Islands (Beaumont and Wei (1991), King George Island, South Shetlands (Aranzamendi et al. 2008;Chwedorzewska et al. 2010;González-Wevar et al. 2011, 2012 and Adelaide Island (Hoffman et al. 2010). Nolan (1991) concluded that differences in shell morphology between littoral and sublittoral individuals were induced by a combination of several factors including environmental (wave action, ice-film formation, air exposure), biological (peak marks of gull attack, endolithic algae infestation on shells) and behavioral factors (intraspecific shell grazing). ...
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Physiological studies suggest that Antarctic marine organisms are adversely affected by rising global temperatures and ocean acidification and have poor prospects for survival. However, according to ecological studies, their vulnerability might be less severe than initially thought. Thus, a realistic forecast of species survival and Antarctic biodiversity should be based on studies from a variety of species under consideration of ecological factors. The limpet Nacella concinna is often found in the rocky intertidal and sublittoral zones of the Antarctic Peninsula and adjacent subantarctic islands. This review summarizes most of the available information on the biology of this limpet, one of the most conspicuous invertebrates of the intertidal zone. There is some evidence that adult N. concinna are physiologically flexible and can acclimate to 3 °C. However, the requirements of the larval stage are poorly known, thus precluding realistic predictions of how elevated temperatures will affect N. concinna populations. Data on physiological performance (righting ability, tenacity and radula rasping rate) under different temperatures could provide a useful baseline for further field investigations on the effects of warming. The species could be used as model organism for investigating the biological effects of ongoing global warming on slow-growing Antarctic ectotherms. Nacella concinna might also be a better biomonitor for polycyclic aromatic hydrocarbons than other Antarctic mollusks.
... However, contrary to expectations, several lines of evidence point towards populations of this species being spatially structured. For example, de Arazamendi et al. [12] reported statistically significant genetic differences between intertidal and subtidal morphs of this species within a single locality using 35 binary inter-simple sequence repeat (ISSR) markers, although two subsequent studies using larger panels of Amplified Fragment Length Polymorphism (AFLP) loci were unable to replicate this finding at other locations [13,14]. Similarly, Beaumont and Wei [15] detected genetic differences between limpets from the South Orkney Islands and South Georgia using five allozymes, while more recently Hoffman et al. [16] found surprisingly strong population structure using AFLPs along a latitudinal gradient spanning the Antarctic Peninsula and the outlying islands of Signy and South Georgia. ...
... those found in this study, despite sample sizes being far smaller (total n = 108 individuals distributed over 7 populations). However, two subsequent studies conducted independently at other localities found no genetic differences between intertidal and subtidal N. concinna individuals using larger panels of AFLPs [13,14]. Furthermore the two morphs have also recently been shown to be part of a continuous cline in both morphology and physiology with depth [13,35]. ...
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Several recent empirical studies have challenged the prevailing dogma that broadcast-spawning species exhibit little or no population genetic structure by documenting genetic discontinuities associated with large-scale oceanographic features. However, relatively few studies have explored patterns of genetic differentiation over fine spatial scales. Consequently, we used a hierarchical sampling design to investigate the basis of a weak but significant genetic difference previously reported between Antarctic limpets (Nacella concinna) sampled from Adelaide and Galindez Islands near the base of the Antarctic Peninsula. Three sites within Ryder Bay, Adelaide Island (Rothera Point, Leonie and Anchorage Islands) were each sub-sampled three times, yielding a total of 405 samples that were genotyped at 155 informative Amplified Fragment Length Polymorphisms (AFLPs). Contrary to our initial expectations, limpets from Anchorage Island were found to be subtly, but significantly distinct from those sampled from the other sites. This suggests that local processes may play an important role in generating fine-scale population structure even in species with excellent dispersal capabilities, and highlights the importance of sampling at multiple spatial scales in population genetic surveys.
... The extension of this feature would strength differenciation between littoral and sublittoral individuals due to a more likely reproductive isolation. Interestingly, population genetic studies between ecotypes obtained contradictory results ranging from genetically similar to low but significant genetic differences; the former study was performed in Ryder Bay where Littoral individuals seems to stay in the intertidal also during winter (Chwedorzewska et al., 2010;de Aranzamendi et al., 2008;Hoffman et al., 2010). ...
Ocean acidification (OA) could become a serious threat for the Antarctic marine ecosystem over coming years, as the solubility of atmospheric CO2 and CaCO3 minerals increases at lower temperatures. We evaluated the effect of OA on the stress response of the limpet Nacella concinna by measuring gene expression levels. The experiment was performed with the two ecotypes (Littoral and Sublittoral) of the species during 54 days (IPCC, 2019 scenario RCP8.5; control, ~375 ppm; low-pH treatment, ~923 ppm). Exposure to low-pH treatment during 15 days triggered the down-regulation of two heat-shock protein genes (HSP70A, HSP70B) only in sublittoral individuals. Little variation in the relative expression values of all genes in both ecotypes was observed probably, due to a historical exposure to the substantial daily natural pH fluctuations recorded in the study area during the experiment. This study provides relevant baseline data for future OA experiments on coastal species in Antarctica.
... From that point, the potential genetic differentiation between the two morphotypes has been investigated, some of the studies concluding an absence of genetic distinction (Wei 1988, Beaumont and Wei 1991, Nolan 1991) while contrarily, de Aranzamendi et al. (2008) reported significant differences based on inter-simple sequence repeat (ISSR) markers. More recently, this last method was questioned (Hoffman et al. 2010) and several studies using different markers and populations (Chwedorzewska et al. 2010, Hoffman et al. 2010, González-Wevar et al. 2011 have concluded an absence of genetic differentiation between the two morphotypes. ...
Studying the influence of changing environmental conditions on Antarctic marine benthic invertebrates is strongly constrained by limited access to the region, which poses difficulties to performing long-term experimental studies. Ecological modelling has been increasingly used as a potential alternative to assess the impact of such changes on species distribution or physiological performance. Among ecological models, the Dynamic Energy Budget (DEB) approach represents each individual through four energetic compartments (i.e. reserve, structure, maturation and reproduction) from which energy is allocated in contrasting proportions according to different life stages and to two forcing environmental factors (food resources and temperature). In this study, the example of an abundant coastal limpet, Nacella concinna (Strebel 1908), was studied. The species is known to have intertidal and subtidal morphotypes, genetically similar but physiologically and morphologically contrasting. The objectives of this paper are (1) to evaluate the potential of the DEB approach, and assess whether a DEB model can be separately built for the intertidal and subtidal morphotypes, based on a field experiment and data from literature and (2) to analyse whether models are contrasting enough to reflect the known physiological and morphological differences between the morphotypes. We found only minor differences in temperature-corrected parameter values between both populations, meaning that the observed differences can be only explained by differences in environmental conditions (i.e. DEB considered variables, food resources and temperature, but also other variables not considered by DEB). Despite the known morphological difference between the populations, the difference in shape coefficients was small. This study shows that even with the amount of data so far available in the literature, DEB models can already be applied to some Southern Ocean case studies, but, more data are required to accurately model the physiological and morphological differences between individuals.
... Two studies using allozymes and AFLPs did not find genetic differences between the two ecotypes, attributing the morphological separation to phenotypic plasticity (Beaumont & Wei, 1991;Hoffman et al., 2010). By using AFLPs, Chwedorzewaska et al. (2010) also found significant genetic differences, but not above 2%, and thus attributed the morphological separation to environmental reasons. By contrast, de Aranzamendi et al. (2008), using inter simple sequence repeat (ISSR) markers, reported statistically significant genetic differences, suggesting that the two morphs can be considered as genetically distinct populations maintaining low levels of gene flow. ...
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Antarctic limpets, Nacella concinna, from the Admiralty Bay (King George Island, South Shetlands) for at least part of the year (austral winter) co-exist with predatory sea stars Lysasterias sp. Our laboratory and field experiments established that the presence of Lysasterias sp. or its odour had considerable influence upon their behaviour. Limpets’ responses, consisting of shell mushrooming, shell rotation and flight, were distinctly different from their reaction to other stimuli, such as food and conspecific odours, or mechanical stimulation. Moreover, a significant impact of sea star presence on limpets’ activity was observed, with limpets fleeing to a distance of 60cm from the predator. Such reactions allow limpets to lower the incidence of sea star predation, but at the cost of presumptive disrupting of foraging and an additional energy expended for locomotion. A visible difference was noted between two limpet populations, with the rockpool limpets responding only after physical contact with being touched by a sea star, and the subtidal ones responding at a distance of up to 20cm.
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The present work addresses the effect of environmental factors (icing, water temperature, food availability) on the ecology of the patellid limpet Nacella (Patinigera) concinna, in a bay on the Antarctic Peninsula. Sampling was conducted at three depths (intertidal, 5m, 10m) from February 1987 to January 1988. Temperature was recorded and concentrations of Chlorophyll a were measured on the bottom, in the water and in the ice-water layer. The limpets were measured, weighed and a condition coefficient for somatic and gonadal mass was calculated. Their ages were estimated through size frequency distribution analysis and a seasonalized von Bertalanffy growth model was applied. The intertidal subpopulation migrated to deeper levels at the beginning of the icing season and recolonized the intertidal zone after ice retreat. The growth rate was very low (von Bertalanffy K 0.08). Growth rates showed important seasonal variations, with maxima during December and January. Nacella (P.) concinna spawns once a year and spawning coincided with raising water temperature (from -1.33C to-0.84C), and probably was also related to increasing spring food availability. Body mass increased during periods of high standing stock of microphytobenthos, revealing that ice-algae and phytoplankton were of minor importance as food sources for limpets in Esperanza Bay.
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The limpet Nacella concinna (Strebel 1908) was the focus of numerous studies dealing with Antarctic benthos. One of the main characteristics of the species is the presence of two distinguishable morphotypes, one inhabiting the intertidal (during summer) and the other inhabiting the subtidal. For a long time these forms were considered as an expression of phenotypic plasticity, since previous studies did not found genetic differences between them. In the present work, we performed both a morphometric and a genetic differentiation analysis (using ISSR-PCR markers) of these two forms in three stations sampled at Potter Cove, South Shetland Islands. The results confirmed the morphological differences between intertidal and subtidal forms reported in other Antarctic localities. The genetic differences detected indicate that the two forms can be considered as genetically distinct populations maintaining low levels of gene flow. The degree of reproductive isolation of the ecotypes is discussed, as well as the possible origin of the divergence. The genetic differentiation observed can also have behavioral and physiological correlates, pointing out the importance of taking into account the potential differences in the response of both populations to different conditions in future studies in this species.
L. unifasciata dominates the upper levels of rocky shores at Cape Banks, New South Wales. Peak densities of up to 9600 m-2 occur near the centre of the littorine's vertical zonation and are associated with small body sizes. Density declines, and mean size rises, further down and more particularly further up the shore. Largest animals occur at the top of the shore and in areas experiencing strong wave action, where densities are low. Field experiments showed that increase in density results in a decrease in body weight and an increase in mortality. However, even at a density four times that of the natural population, mortality remained remarkably low, and this is probably a key feature allowing L. unifasciata to penetrate high up the shore. L. unifasciata feeds mainly on lichens and food levels are low over most of the zone occupied by this littorine, rising above and below this zone and being particularly high in the supralittoral immediately above the range of L. unifasciata. Thus, food cannot be a factor limiting the height that L. unifasciata extends up the shore. Experimental caging shows that the standing stock of lichen is inversely related to the density of L. unifasciata. The zonation pattern and size gradient of L. unifasciata may be due to a combination of two factors: a decline of body size due to increasing intraspecific competition at higher densities, and the tendency of L. unifasciata to migrate (probably upwards) away from areas of low food availability. The latter was experimentally demonstrated. L. unifasciata suffers from intense intraspecific competition and is responsible for limiting the availability of its food. Its populations are seemingly not regulated by predators. It borders on a zone of high food availability in which there are no important competitors. These are all circumstances favouring range expansion of the species, to the limits of physiological tolerance, to allow the species to capitalize on the adjacent rich food source.
Twenty-one monthly collections of the Antarctic limpet Nacella (Patinigera) concinna (Strebel, 1908) were obtained by divers at Signy Island, South Orkney Islands. A mean monthly population density of 123.7 ± 21.2 · m−2, mean biomass of 13.7 ± 2.7 g dry tissue wt · m−2, and annual production of 2.9 g · m−2 were recorded in the depth range 2–12 m below mean low water.Shell growth was slow with a maximum growth rate, in the first 3–5 yr of life, of 3 4 mm per year. Maturity was attained at 7–8 yr (21 mm length), and maximum size (41 mm length) at about 21 yr. Unique spawning behaviour was observed in two Austral springs, and data relating spawning to the spring increase in sea temperature were obtained.
The Antarctic limpet Nacella (Patingera) concinna (Strebel, 1908) forms temporary “stacks” before spawning, a behaviour which ensures that gametes are released in close proximity. This promotes the fertilization of a large proportion of eggs, so reducing the energy outlay necessary to ensure reproductive success. The annual reproductive output of Nacella is low in comparison with other Patellidae. Rapid fertilization may also help to contain larvae in shallow coastal waters suitable for settlement. Evidence of this unique spawning behaviour has now been obtained in a series of photographs taken at Signy Island, South Orkney Islands (60°S : 43°W).