Resource allocation and population genetics of the bay scallop,Argopecten irradians irradians: effects of age and allozyme heterozygosity on reproductive output

The relationships among multiple-locus heterozygosity, age, reproduction and growth were examined over the ca. 2-yr lifespan of a cohort of northern bay scallops, Argopecten irradians irradians (Lamarck), from the Niantic River estuary, Connecticut, USA. Electrophoretic analyses revealed a relatively low proportion of polymorphic loci (=0.35) and low level of heterozygote deficiency ( D. across 6 loci=-0.05) in this population. Allele frequency distributions showed a high degree of temporal stability within and among cohorts. No significant correlation was found between multiple-locus heterozygosity and either somatic or reproductive growth. Our results lend support to the postulated linkage between low heterozygote deficit and lack of a heterozygosity/growth association in marine bivalves, and in the pectinid group in particular. Partial reproductive senility occurs in these scallops in their second year of life. Somatic tissue weight increased exponentially with increasing shell height, whereas pre-spawning gonad mass attained asymptotic values in 2-yr-old scallops. Thus, weight-specific reproductive effort was significantly lower in second- than first-year scallops (24 and 33% respectively). This pattern could be generated if high individual reproductive effort in the first year were correlated with accelerated (early) post-reproductive senescence. This kind of resource allocation, in which somatic growth is given precedence over reproductive growth in older individuals, has been previously reported in only one other bivalve, a pectinid (Chlamys islandica), in which the decline in reproductive effort occurs only after the scallops reach an age of ca. 20 yr.     

Genetics of growth in blue mussels: family and enzyme-heterozygosity effects

Shell length and electrophoretic heterozygosity at six enzyme loci were scored in juveniles of 30 hatchery-grown families and in adults of 10 field-grown families of the blue mussel Mytilus edulis. A total of 4 809 offspring were scored. We found no consistent pattern in the correlations between shell length and enzyme heterozygosity among sibs within families. However, a significant family effect on shell length was observed, suggesting an influence of background genotype on this character. A family effect on viability was also observed. When the environment and family effects were accounted for, a small positive correlation between heterozygosity and shell length at the juvenile stage remained. This correlation was significant in one of the three experiments. No such correlation was evident at the adult stage. Our interpretation of these results is that electrophoretic alleles appear to have no direct of independent effect of their own on growth. We suggest that the negative correlation of homozygosity with these characters, seen in natural populations, results from homozygosity for hidden recessive deleterious genes with which the electromorphs are in a steady state of quasilinkage disequilibrium.     

Population structure and growth of Donax trunculus (Bivalvia: Donacidae) in the western Mediterranean

A comparison of shell growth in Donax trunculus (collected between 1988 and 1990 of Cullera, Spain) has been carried out using an analysis of cohort progression in monthly length frequency distributions, hyaline surface shell growth rings and internal microgrowth bands. In the Mediterranean there are two periods of recruitment of D. trunculus, one in the summer (July to September) and the other in winter (December to February). Clams recruited to the population in winter display a clear cessation in shell growth during the following summer which may possibly be correlated with spawning, whereas individuals of the summer recruited cohort show no growth cessation the following summer and continue to deposit shell during this period. The normally opaque shell of D.trunculus reveals the presence of translucent hyaline growth rings when the shells are backlit by a strong light source, and these have been shown to be laid down in the shell during summer months. Formation of a hyaline ring is accompanied by a narrowing of the microgrowth patterns present in shell sections. Both the hyaline rings and the length frequency distributions have been used to determine the age and growth rate of D. trunculus.     

Growth,mortality and recruitment of the yellow clam Mesodesma mactroides on Uruguayan beaches

We studied some aspects of the population dynamics of the yellow clam Mesodesma mactroides (Deshayes, 1854) which inhabits the eastern sandy beaches of Uruguay. Observations were made from March 1983 through March 1985 (harvesting season), and additional data from January 1988 to December 1989 (closed season) were also included in the analyses. Parameters of growth and mortality were estimated by methods that make use of age or length information. An age/length relationship was constructed by counting growth rings. Results were consistent with those obtained with modal class progression analysis and other length/frequency methods. The growth curve obtained from the age/length relationship did not account for seasonality in growth rate, and a seasonal growth model explained growth oscillations in a better way. The instantaneous coefficient of natural mortality (M) estimated for the closed season was 1.64, whereas an increase in fishing mortality (F) was observed during 1984. Age composition did not change during the 2 yr of study, and the first 6 mm class accounted for over 50% of the population. The main contribution of recruits was observed between late summer and early fall (February to April), and the recruitment pattern suggested one main settlement period per year. Growth and recruitment success appeared to be regulated by density-dependence processes. Some implications for management are considered in this context. Methodological aspects concerning the reliability of empirical equations and programs for estimating growth and mortality are discussed.     

General and specific combining abilities of larval and juvenile growth and viability estimated from natural oyster populations

Diallel crosses of oysters from three geographically isolated natural populations were produced to evaluate the relative importance of genetic, maternal, and environmental effects on larval and juvenile growth and viability. Significant additive genetic effects were observed only in larval viability at Day 12 and larval shell length at Day 2. The presence of significant male and female mean square for larval viability (suggesting non-additive genetic variance) is consistent with fitness related characters. Important maternal effects were observed for the larval and juvenile shell length and viability characters. These female mean squares are probably affected by both real and spurious maternal effects and potential contributing influences are discussed. The performance of the crosses can be largely explained by two factors: parental performance and the heterotic gene effects. This is based on an apparent positive correlation of mean values between the parental populations and their crosses. The crosses' mean viability at the end of the larval phase was 14.0% lower than the pure matings and support a previous observation of lower heterozygote viability in the larval phase (Mallet and Haley, 1983b).     

Gamete production,somatic growth and multiple-locus enzyme heterozygosity inMytilus edulis

Mussels,Mytilus edulis L., were collected from the lower shore in Stony Brook Harbor, New York, USA, in April 1984. The mussels were in spawning condition at the time of collection. Mantle weight was used as an index of fecundity and somatic tissue weight and growth rate were measured so that the relative production of gametes and somatic tissue with age could be calculated. For each individual mussel the genotypes at loci coding for five enzymes were determined. Somatic tissue production exceeds gamete production for the first two and a half years of life, but thereafter it declines and is exceeded by gamete production. There is a positive correlation between multiple-locus enzyme heterozygosity and fecundity in mussels which have grown beyond the size at which gamete production starts to exceed somatic production. There is no correlation between heterozygosity and fecundity in younger individuals. We conclude that the higher scope for growth of more heterozygous bivalves, described elsewhere, is translated into allocation of energy to somatic growth during early life and into gamete production in later life. Our observations perhaps explain why, in other studies, the correlation between growth rate and heterozygosity in bivalves is manifested most clearly when young individuals are examined.     

Genetic identity determines risk of post-settlement mortality of a marine fish

Longitudinal sampling of four cohorts of Neopomacentrus filamentosus, a common tropical damselfish from Dampier Archipelago, Western Australia, revealed the evolution of size structure after settlement. Light traps collected premetamorphic individuals from the water column ("settlers") to establish a baseline for each cohort. Subsequently, divers collected benthic juveniles ("recruits") at 1-3-month intervals to determine the relative impacts of post-settlement mortality during the first three months. Growth trajectories for individual fish were back-calculated from otolith records and compared with nonlinear mixed-effects models. Size-selective mortality was detected in all cohorts with the loss of smaller, slower growing individuals. Three months after settlement, recruits showed significantly faster growth as juveniles, faster growth as larvae, and larger sizes as hatchlings. The timing and intensity of post-settlement selection differed among cohorts and was correlated with density at settlement. The cohort with the greatest initial abundance experienced the strongest selective mortality, with most of this mortality occurring between one and two months after settlement when juveniles began foraging at higher positions in the water column. Significant genetic structure was found between settlers and three-month-old recruits in this cohort as a result of natural selection that changed the frequency of mtDNA haplotypes measured at the control region. The extent of this genetic difference was enlarged or reduced by artificially manipulating the intensity of size-based selection, thus establishing a link between phenotype and haplotype. Sequence variation in the control region of the mitochondrial genome has been linked to mitochondrial efficiency and weight gain in other studies, which provides a plausible explanation for the patterns observed here.     

Comparative effects of two antifouling paints on the oyster Crassostrea gigas

The effects of two antifouling paints, one containing organotin compounds, the other copper oxide, were studied from September 1983 to October 1984 on Crassostrea gigas (Thunberg) grown under natural field conditions in the Bay of Arcachon, France. The organotin paint reduced growth rate expressed as weight, length and width, but did not affect shell height; it drastically decreased the dry condition factor and shell density, but did not affect the viability of embryos and larvae from exposed oysters. However, some decrease in larval growth rate was observed. The copper paint had no effect on oyster growth, but lowered the condition factor compared to controls. Neither viability of embryos or larvae nor larval growth were affected by this paint.     

Lignocellulose and lignin in the salt marsh grass Spartina alterniflora: initial concentrations and short-term,post-depositional changes in detrital matter

Individuals of Mytilus edulis of the same age (ca 2 months) were collected as spat from natural populations. Relative growth rates were determined among individuals differing in heterozygosity at five enzyme loci. Growth rate was positively correlated with individual heterozygosity and each of the five loci contributed about equally to the relatinship. More heterozygous individuals also achieved more uniform average growth rates. Although there was a deficiency of heterozygotes at each locus, relative to Hardy-Weinberg expectations, the magnitude of the deficiency, measured as F_IS, was less among faster growing mussels. Our results conform closely with those of Zouros et al. (1980) on the American oyster. We conclude that the relationship between multiple locus heterozygosity and growth rate is one that is general to a diversity of outbreeding plant and animal populations. Other studies indicate that this relationship is due to a greater average metabolic efficiency of more heterozygous individuals. This relationship does not emerge from experimental designs in which there has been limited genetic sampling of the natural genetic variation.     

Cephalopod prey of the wandering albatross Diomedea exulans

Cephalopod beaks from the stomach contents of wandering albatross (Diomedea exulans L.) chicks from Bird Island, South Georgia, were sampled between May and September in 1983 and 1984. Lower beaks were identified and measured, and allometric data were used to calculated mantle length and biomass of the species consumed. A total of 3421 lower beaks were examined, representing 35 species in the 1983 sample and 45 species in the 1984 sample. Eight of the twenty families contributed over 95% of the biomass. In 1984 there were less Onychoteuthidae and more Ommastrephidae than in 1983 and a decrease in the number of species known to occur south of the Antarctic Polar Front. There was a difference in the size-frequency distribution of the cephalopod diet in the two years; in 1984 there was a higher frequency of intermediate-sized specimens, reflecting the greater importance of ommastrephids, especially Illex sp. The energy content of cephalopods in 1984 may have been greater than in 1983. Serial sampling of cephalopod beaks during the austral winter did not reveal evidence of growth. By the age of 200 d, wandering albatross chicks have consumed a total of approximately 100 kg wet weight of cephalopods each.     

Ontogenetic change in the radula of Conus magus (Gastropoda)

Specimens of the Indo-Pacific piscivorous gastropod Conus magus Linnaeus, 1758 were either collected by SCUBA in the field in Palau (1983–1984) or raised in the laboratory from egg capsules. C. magus captures adult fish with a specially modified radula tooth. Radula morphology was analyzed in a size range of individuals from 4.1 to 43.7 mm shell length, which encompassed early postmetamorphic juveniles to adults. Post-metamorphic C. magus juveniles are too small to consume fish, and all individuals below 9 mm possess a juvenile radula tooth totally different from that of the adult and resembling that of some vermivorous Conus species. The only food remains found in the digestive tracts of juveniles were the setae of syllid polychaetes. All individuals above 10.5 mm possessed adult teeth and had only fish remains in the gut. Two specimens, 10.1 and 9.2 mm in shell length, had intermediate-type radula teeth.     

Demography and reproductive biology ofMelinna palmata (Ampharetidae: Polychaeta) in Inner Galway Bay on the west coast of Ireland

A high-density population of the polychaeteMelinna palmata Grube was the focus of a two-year study at a shallow-water location in Inner Galway Bay on the west coast of Ireland. Mean densities were highest in the second year of the study following successful recruitment. Size-frequency analysis of population structure (April 1983 to March 1985) revealed the presence of four cohorts, with the 1983 settlement becoming the dominant modal class. Size comparison of this cohort with the 1982 settlement showed that growth rates were lower and mortality higher in the 1983 settlement, which may be related to density dependent intra-specific competition.M. palmata at this location live for 2 to 2.5 yr with a small number surviving to 3 yr of age.M. palmata in Galway Bay is dioecious and has an even sex ratio. Reproductively mature worms were approximately 2 yr old when they first spawned. Spawning took place between May and July, when mature oocytes had a modal size of ca. 290µm. Proliferation of previtellogenic oocytes (ca. 10µm) from the gonads occurred following spawning. Growth over the winter was slow and was followed by a period of rapid growth between March and May, coincident with rising water temperatures. The presence of mature unspawned oocytes, which were resorbed after commencement of new proliferation, was taken as evidence of the species propensity for polytely. However, post-spawning mortality appeared to be high, so that only a small number of individuals survived to spawn the following year. The highest abundance of mature males was encountered between May and August, but lower levels of mature stages were present throughout the year.     

Patterns of co-occurrence and interactions between age classes of the common triplefin,Forsterygion lapillum

Many marine organisms have pelagic larval stages that settle into benthic habitats occupied by older individuals; however, a mechanistic understanding of intercohort interactions remains elusive for most species. Patterns of spatial covariation in the densities of juvenile and adult age classes of a small temperate reef fish, the common triplefin (Forsterygion lapillum), were evaluated during the recruitment season (Feb–Mar, 2011) in Wellington, New Zealand (41°17′S, 174°46′E). The relationship between juvenile and adult density among sites was best approximated by a dome-shaped curve, with a negative correlation between densities of juveniles and adults at higher adult densities. The curve shape was temporally variable, but was unaffected by settlement habitat type (algal species). A laboratory experiment using a “multiple-predator effects” design tested the hypothesis that increased settler mortality in the presence of adults (via enhanced predation risk or cannibalism) contributed to the observed negative relationship between juveniles and adults. Settler mortality did not differ between controls and treatments that contained either one (p = 0.08) or two (p = 0.09) adults. However, post hoc analyses revealed a significant positive correlation between the mean length of juveniles used in experimental trials and survival of juveniles in these treatments, suggesting that smaller juveniles may be vulnerable to cannibalism. There was no evidence for risk enhancement or predator interference when adults were present alongside a heterospecific predator (F. varium). These results highlight the complex nature of intercohort relationships in shaping recruitment patterns and add to the growing body of literature recognizing the importance of age class interactions.     

Settlement and recruitment of sea urchins (Strongylocentrotus spp.) in a sea-urchin barren ground and a kelp bed: are populations regulated by settlement or post-settlement processes?

I sampled recruitment of very small sea urchins (Strongylocentrotus spp.) by using the anesthetic magnesium chloride to remove individuals from substrata collected in sea-urchin barren grounds (barrens) and kelp beds at Naples Reef near Santa Barbara, California, USA. Preliminary sampling found low numbers of newly settled individuals(<0.6 mm test diam) from April–July in 1984 and 1985, and in April, 1986. In early May, 1986, I found many newly settled seaurchins (0.3 to 0.6 mm, 5 to 17 d old), and I compared the densities of the cohort on several types of natural substrata in barrens and kelp-bed habitats. Newly settled individuals of both purple sea urchins (S. purpuratus) and red sea urchins (S. franciscanus) were present in similar, high densities (1 000 S. purpuratus m^-2) on foliose red algal turf, a dominant substratum ofthe kelp bed, and on crustose coralline algae, the dominant substratum of an adjacent barrens. Larvae of S. purpuratus reared and tested in the laboratory showed high rates of settlement on both red algal turf and on crustose coralline algae, but significantly lower rates on rock. Larvae also settled in response to a partiallypurified extract of coralline algae. The reduced settlement on natural rock surfaces relative to either algal treatment and the significant settlement in response to the extract of coralline algae indicate that larvae discriminate between natural substrata and probably respond to a settlement cue other than, or in addition to, a simple microbial (bacterial) film. The similar densities of young recruits of S. purpuratus on dominant substrata of barrens and kelp bed show that, at least in this case, differential settlement cannot explain the high densities of sea urchins in the barrens habitat. Movement between barrens and kelp bed is unlikely given the small sizes of the newly recruited sea urchins relative to the large distances often involved. Reduced post-settlement mortality of newly settled individuals in the barrens remains the most likely mechanism leading to the higher densities of sea urchins in barrens relative to kelp-bed habitats.     

Spatial variability in habitat associations of pre- and post-settlement stages of coral reef fishes at Ishigaki Island,Japan

Successful settlement of pelagic fish larvae into benthic juvenile habitats may be enhanced by a shortened settlement period, since it limits larval exposure to predation in the new habitat. Because the spatial distribution of marine fish larvae immediately prior to settlement versus during settlement was unknown, field experiments were conducted at Ishigaki Island (Japan) using light trap sampling and underwater visual belt transect surveys to investigate the spatial distribution patterns of selected pre- and post-settlement fishes (Acanthuridae, Pomacentridae, Chaetodonidae and Lethrinidae) among four habitats (seagrass bed, coral rubble, branching coral and tabular coral). The results highlighted two patterns: patterns 1, pre- and post-settlement individuals showing a ubiquitous distribution among the four habitats (Acanthuridae) and pattern 2, pre-settlement individuals distributed in all habitats, but post-settlement individuals restricted to coral (most species of Pomacentridae and Chaetodontidae) or seagrass habitats (Lethrinidae). The first pattern minimizes the transition time between the larval pelagic stage and acquisition of a benthic reef habitat, the latter leading immediately to a juvenile lifestyle. In contrast, the second pattern is characterized by high settlement habitat selectivity by larvae and/or differential mortality immediately after settlement.     

Allozyme and physiological variation in the scallop Placopecten magellanicus and a general model for the effects of heterozygosity on fitness in marine molluscs

Heterozygosity and growth rate have been correlated in many molluscs, although the phenomenon is not universal. Enhanced growth of heterozygous molluscs has been attributed to lower energetic requirements for basal metabolism. We have investigated heterozygote deficiency, and the correlation between single-locus and multiple-locus heterozygosity and phenotype in juveniles of the scallop Placopecten magellanicus (Gmelin). Six samples were collected between 22 November 1984 and 11 December 1985 at Passamaquoddy Bay, New Brunswick, Canada. On average, heterozygote deficiency was small, although somewhat higher at the octopine dehydrogenase (Odh) locus, and decreased with age. No correlation was observed between genotype and growth rate. This result and published records indicate that allozyme heterozygosity of pectinids does not influence growth to the same degree as in other bivalves. However, we have observed in P. magellanicus a correlation between allozyme heterozygosity and octopine accumulation, a trait that relates to functional anaerobiosis, and may be related to the scallop's scope for movement. We combine these results and results from the literature into a model that relates the hypothesis of associative overdominance (at the population genetics level) with the hypothesis of energetic efficiency (at the physiological level). The model assumes that energy savings attributed to heterozygosity are used to maximize fitness. In freely moving molluscs this results in enhanced activity (such as searching for prey or swimming away from a predator), and in sessile molluscs either in enhanced somatic growth in juveniles or gonadal growth in adults.     

Effects of the 1983-El Niño on growth and carbon and nitrogen metabolism ofPleurophycus gardneri (Phaeopyceae: Laminariales) in the northeastern Pacific

Growth and compounds of carbon and nitrogen of the northeast Pacific kelpPleurophycus gardneri (Laminariales, Laminariaceae) were assessed before, during and after the 1983-El Niño Souther Oscillation event. This 2-yr study provides evidence that reduced ambient levels of nitrate and phosphate from March until August/September, 1983, in combination with elevated seawater temperature, caused a significant impairment of growth. Blade surface area, blade length, total fresh weight, and dry weight on an areal basis were significantly reduced in 1983 compared to 1982 (P<0.01). They were also significantly smaller in 1984 than in 1983 (P<0.05). The C:N values obtained both in May and July 1983 were considerably higher than in 1982, while those in May 1984 were strikingly smaller than in both 1982 and 1983. Mannitol content of the mibrid in May 1983, exhibited the highest levels recorded during the 2-yr study period, with ca. 19% dry weight and significantly exceeded mannitol content measured in May 1982 (average: 13%) and in May 1984 (average: 8%). The concentration factor for internal vs external nitrate in the midrib was 700 x ambient in May 1982 and 1984 compared with 20 x in May 1983. Ninhydrin positive substances (NPS) averaged 117 mol g dry wt^-1 in May 1984 compared with ca. 50 mol in May 1982 and 1983. The pigment ratios chlorophyllc: chlorophylla and fucoxanthin: chlorophylla in May 1983 were similar to those obtained in 1982, but were significantly different in May 1984. It is suggested that the interactive effects of temperature stress and nutrient limitation in 1983 caused a disruptive influence on C- and N-metabolism ofPleurophycus gardneri, which still affected growth performance in 1984.     

Recruitment and ontogenetic habitat shifts of the yellow snapper (<Emphasis Type="Italic">Lutjanus argentiventris</Emphasis>) in the Gulf of California

We examined recruitment and ontogenetic habitat shifts of the yellow snapper Lutjanus argentiventris in the Gulf of California, by conducting surveys and collections in multiple mangrove sites and major marine coastal habitats from 1998 to 2007. Over 1,167 juvenile individuals were collected and 516 otoliths were aged to describe the temporal pattern of the settlement. L. argentiventris recruits in mangroves, where juveniles remain until they are approximately 100 mm in length or 300-days-old. Back-calculated settlement dates and underwater surveys indicated a major recruitment peak during September and October, around 8 days before and after the full moon. The majority of mangrove sites in the Gulf of California had a similar L. argentiventris average size at the beginning of the settlement season for the cohort of 2003; although there were significant differences in individual sizes at the end of the nursery stage. When sub-adults leave mangroves, they live in shallow rocky reefs and later become abundant in deeper rocky reefs. The density of migratory individuals (10–20 cm SL) decreased exponentially as the distance between a reef and a nearby mangrove site increased. This finding has important implications for local fishery regulations and coastal management plans.     

Population dynamics and growth of <Emphasis Type="Italic">Nassarius reticulatus </Emphasis>(Gastropoda: Nassariidae) in Rhosneigr (Anglesey,UK)

Seasonal changes in catch rate, growth and mortality of Nassarius reticulatus from an intertidal lagoon and a wave-exposed beach at Rhosneigr (Anglesey, North Wales, UK) are described. The number of N. reticulatus caught in baited traps from the lagoon was significantly higher (>125 individuals trap⁻¹) during the summer (>18°C), than at <12°C (<65 individuals trap⁻¹), and the numbers caught in the lagoon were an order of magnitude greater than on the beach, >13 individuals trap⁻¹ in July (>16°C), and <5 individuals trap⁻¹ between December and April (<9.5°C). Predictions of shell growth attained by N. reticulatus annually in the lagoon using graphical modal progression analysis (MPA) of length frequency data, were similar to the growth of marked and recaptured lagoon N. reticulatus. Predictions of shell growth using computerised length frequency distribution analysis (LFDA), however, did not reflect the growth as accurately as MPA. Modal progression analysis demonstrated that N. reticulatus from the lagoon achieved a higher asymptotic maximum shell length (L _∞) and a lower growth constant (K) than animals from the beach. Shell growth was seasonal with growth of the lagoon individuals slowing down towards the end of September and resuming in early April, about a month later than the beach individuals. Mortality of N. reticulatus was greater during the summer, and survival was lower in the lagoon than on the beach. Recruitment patterns were similar in the lagoon and on the beach, and MPA and LFDA predicted that larval N. reticulatus settled between late summer and early autumn, with juveniles (7–8.9 mm) appearing in the population the following year, between February and April. Growth of male and female N. reticulatus in the laboratory was similar and was temperature and size dependent. The different growth patterns between N. reticulatus from the two habitats, predicted using MPA, were maintained when individuals were reared under laboratory conditions for ∼6 months; N. reticulatus <21 mm from the beach grew faster than individuals from the lagoon, although N. reticulatus >21 mm from the lagoon grew faster and attained a larger length (26 mm) than individuals from the beach (24 mm). Low food availability did not affect N. reticulatus survival in the laboratory but significantly suppressed shell growth.     

Age and growth of the naticid gastropod <Emphasis Type="Italic">Polinices pulchellus</Emphasis> (Gastropoda: Naticidae) based on length frequency analysis and statolith growth rings

In Red Wharf Bay, UK the naticid gastropod, Polinices pulchellus, was more abundant and more highly aggregated during the summer months (June–August 2001) than during the winter (December 2000). Whilst small numbers of juvenile P. pulchellus (4–6 mm shell length) were present throughout the year the population consisted mainly of individuals of 12–14 mm shell length. Juvenile snails grew rapidly in size during the winter and early spring; growth then virtually ceased between May and June, following which there was a further period of rapid growth between August and February. Densities ranged between 57 and 4,073 ha⁻¹ and the largest individual collected during this investigation measured 16.2 mm in shell length. Statoliths from adult P. pulchellus revealed the presence of a settlement ring and two prominent growth rings (rings 1 and 2). A curvilinear relationship exists between statolith diameter and shell length in snails up to 16 mm in length. Settlement rings ranged in diameter from 19.7 to 45.2 μm (mean 29.8 μm; SE=0.41) giving an estimated shell length of the settled juvenile of 1.1 mm. The diameter of ring 1 and ring 2 were significantly correlated indicating that rapid growth during the first year is maintained during year 2. Shell lengths estimated from the diameters of the prominent statolith rings and those obtained from length frequency data analysis (LFDA), were broadly congruent strongly suggesting an annual periodicity to the statolith rings. The largest snails (>15 mm) present within this population were estimated to be between 2 and 3 years old. Von Bertallanfy seasonal growth curves obtained from the LFDA predicted values of L∞, K and t ₀ of 14.32 mm, 1.54 and −0.14 years, respectively, suggesting that P. pulchellus rapidly attains its maximum asymptotic size.