Larval Shell Length Research Articles

Combined effects of short term exposure to seawater acidification and microplastics on the early development of the oyster Crassostrea rivularis

Ocean acidification and microplastics pollution are two consequences of anthropogenic activities. In regions such as estuarine areas, ocean acidification (OA) and microplastics (MPs) pollution are occurring simultaneously. The present study tested the combined effects of OA and MPs on the embryonic development and physiological response of the larval oyster Crassostrea rivularis in a short exposure duration. The fertilization process was exposed to six combinations of three MPs levels (0, 10 and 1000 items L−1) × two pH levels (7.3, 8.1) for 9 d after hatching. As a result, the hatching rate was not affected by either pH reduction or MPs exposure, while the deformation rate increased under low pH, MPs exposure and their combination. Larval shell length was reduced under low pH, MPs exposure and the combined exposure of the two factors. Furthermore, swimming speed decreased under low pH throughout the experiment but the MPs' effect was limited. Predictably, MPs in the body increased with the increase of MPs concentration. Compared to low pH, the oyster ingested more MPs under normal pH. But combined OA and MPs didn't affect the MPs intake. Moreover, no significant effects of OA and MPs on total antioxidant capacity and malondialdehyde were observed. However, alkaline phosphatase was significantly affected by low pH and MPs exposure independently. PCA showed that development of C. rivularis larvae changed over time and MPs concentration increase augmented intake of MPs by oysters. Consequently, we speculate that OA and MPs exposure may harm oyster C. rivularis embryonic and larval development, but not induce their antioxidant response in a short duration. Long term study about combined effects of OA and MPs on C. rivularis development is needed in the future.

Effect of larval density on growth and survival of the Pacific oyster Crassostrea gigas in a recirculation aquaculture system

The present study determined the effect of larval stocking density on larval yield (number of live larvae at the end of the experiment), competent larvae (number of live larvae at the end of the experiment retained on a sieve with mesh size of 236 μm (1st experiment) and 239 μm (2nd experiment)), and larval shell length of Crassostrea gigas in a recirculation aquaculture system (RAS). Two experiments were performed at different larval stocking densities: 50, 100 and 150 larvae mL−1 (1st experiment) and 50, 75 and 100 larvae mL−1 (2nd experiment). We used 250 L tanks and a randomized block design with three replicates per treatment (larval density). The physical and chemical parameters of the water were stable in all treatments during the experimental period. Our results demonstrated the feasibility of producing Crassostrea gigas larvae in a RAS at larval densities of 50 and 75 larvae mL−1; however, rearing larvae at densities of 50 larvae mL−1 resulted in the best growth and highest proportion of competent larvae. This RAS approach could be used to optimize larval rearing and improve both the resource efficiency and profitability of commercial hatcheries.

Acidification stress effect on umbonate veliger larval development in Panopea globosa

Ocean acidification generates a decrease in calcium carbonate availability essential for biomineralization in organisms such as mollusks. This effect was evaluated on Panopea globosa exposing for 7 days umbonate veliger larvae to two pH treatments: experimental (pH 7.5) and control (pH 8.0). Exposure to pH 7.5 affected growth, reducing larval shell length from 5.15–13.34% compared to the control group. This size reduction was confirmed with electron microscopy, also showing shell damage. The physiological response showed an increase in oxygen consumption in larvae exposed to low pH with a maximum difference of 1.57 nmol O2 h−1 larvae−1 at day 7. The gene expression analyses reported high expression values for nicotinamide adenine dinucleotide (NADH) dehydrogenase and Perlucin in larvae at pH 7.5, suggesting a higher energetic cost in this larval group to maintain homeostasis. In conclusion, this study showed that acidification affected development of P. globosa umbonate veliger larvae.

Effects of ocean acidification on toxicity of two trace metals in two marine molluscs in their early life stages

Ocean acidification (OA) is usually thought to change the speciation of trace metals and increase the concentration of free metal ions, hence elevating metal bioavailability. In this study, embryos of the oysterCrassostrea angulataand abaloneHaliotis discus hannaiwere cultured under 4 pCO2conditions (400, 800, 1500 and 2000 µatm) with Cu and Zn added. Fertilization rate was measured 2 h post-fertilization (hpf), while larval deformation and larval shell length were measured 24 hpf. Our results show that OA can alleviate Cu and Zn inhibition ofC. angulatafertilization by 86.1 and 26.4% respectively, and Zn inhibition ofH. discus hannaifertilization by 43.7%. However, OA enhanced the inhibitory effect of Cu on fertilization ofH. discus hannaiby 34.7%. OA enhanced the toxic effect of Cu on larval normality ofC. angulataby 22.0% and the effect of Cu and Zn on larval normality ofH. discus hannaiby 71.4 and 37.2%, respectively. OA also enhanced the inhibitory effects of Cu and Zn on larval calcification inH. discus hannaiby 8.8 and 8.6%, respectively. However, OA did not change the effect of Cu on the calcification ofC. angulatalarvae. OA decreased Zn inhibition of oyster larval calcification from 3.1 to 1.5%. Based on our results, the toxic effects of metal on early development of molluscs are not always increased by rising pCO2and differ across developmental stages, egg structure and species. This complexity suggests that caution should be taken when carrying out multiple environmental stressor tests on molluscan embryos.

Adaption potential of Crassostrea gigas to ocean acidification and disease caused by Vibrio harveyi

Abstract The survival and development of bivalve larvae is adversely impacted by ocean acidification and Vibrio infection, indicating that bivalves need to simultaneously adapt to both stressors associated with anthropogenic climate change. In this study, we use a half-dial breeding design to estimate heritability (h2) for survival to Vibrio harveyi infection and larval shell length to aragonite undersaturated and normal conditions in laboratory-reared Crassostrea gigas. Phenotypic differences were observed between families for these traits with heritability estimated to be moderate for survival to V. harveyi challenge (h2 = 0.25) and low for shell length in corrosive (Ωaragonite = 0.9, h2 = 0.15) and normal conditions (Ωaragonite = 1.6, h2 = 0.15). Predicted breeding values for larval shell length are correlated between aragonite-undersaturated and normal conditions (Spearman r = 0.63, p < 0.05), indicating that larger larvae tend to do better in corrosive seawater. Aquaculture hatcheries routinely cull slow-growing larvae to reduce and synchronize time taken for larvae to metamorphose to spat, thus inadvertently applying size-related selection for larger larvae. This indirect selection in the hatchery populations provides a plausible explanation why domesticated oyster populations are less sensitive to ocean acidification.

Transgenerational effects of short-term exposure to acidification and hypoxia on early developmental traits of the mussel Mytilus edulis

Transgenerational effects of multiple stressors on marine organisms are emerging environmental themes. We thus experimentally tested for transgenerational effects of seawater acidification and hypoxia on the early development traits of the mussel Mytilus edulis. Fertilization rate, embryo deformity rate, and larval shell length were negatively impacted by acidification, while hypoxia had little effect except for increasing deformity rates under control pH conditions. Offspring from low pH/O2 parents were less negatively affected by low pH/O2 conditions than offspring from control parents; however, low pH/O2 conditions still negatively affected developmental traits in offspring from acclimated parents compared to control seawater conditions. Our results demonstrate that experimental seawater acidification and hypoxia can adversely affect early developmental traits of M. edulis and that parental exposure can only partially alleviate these impacts. If experimental observations hold true in nature, it is unlikely that parental exposure will confer larval tolerance to ocean acidification for M. edulis.

Growth, Survival, and Feeding Rates for the GeoduckPanopea globosaDuring Larval Development

Developmental larval stages of the geoduck Panopea globosa (Dall, 1898) are described for the first time. Growth rate, survival, and the length—size relationship were also assessed at 22°C in five independently reared cohorts. Clearance rates (CR), ingestion rates (IR), dry weight, and organic weight of select larval stages were quantified in two additional runs. Clearance rates and IR were evaluated as a function of cell concentration (50, 100, 200 and 300 cells/µL) of the chrysophyte Isochrysis sp. (clone T-ISO) using static systems. Conspicuous larval stages appeared after 20 h (trochophore), 24 h (straight hinge or “D”), 5–6 day (early umboned), 9–10 days (intermediate umboned), 11–12 days (late umboned), and 12–14 days (pediveliger). The larval period for P. globosa lasted between 12 days and 14 days, and the shell length recorded at settlement ranged from 332–356 µm (mean, 343 ± 4.2 µm). The overall average gross growth rate of the five larval cohorts was ∼20 µm/day. The relationship between larval shell length (in micrometers) and organic weight (in micrograms) followed a power function with an exponent value of 2.43, whereas the relationship between dry weight and organic weight was linear. Survival rate was approximately 50% in three larval cohorts and 20% in two cohorts. Clearance rates were inversely related to microalgal concentration regardless of larval size, and ranged from 0.3–1.5 µL/h (D larvae) to 15–59 µL/h (pediveliger, 330µm). On the other hand, IR increased with increasing algal concentration, and reached maximum values at 200 cells/µL, except in D-hinge larvae, which exhibited a maximum IR at all concentrations tested. At the highest concentration (300 cells/µL), premetamorphic larvae inhibited their IR. Overall, IR increased from 57–85 cells/h (D-hinge larvae) to 40- or 50-fold when larvae reached the premetamorphic stage. The current study revealed that P. globosa has a short larval period and high survival rate, and withstands a wide range of food concentrations without inhibiting its IR. Therefore, this species appears to be an excellent candidate for hatchery production.

Effects of ocean acidification caused by rising CO2 on the early development of three mollusks

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 23:147-157 (2015) - DOI: https://doi.org/10.3354/ab00615 Effects of ocean acidification caused by rising CO2 on the early development of three mollusks Guo X.1,2, Huang M.1,2, Pu F.1,2, You W.2, Ke C.1,2,* 1State Key Laboratory of Marine Environmental Sciences, Xiamen University, Xiamen 361005, PR China 2College of Ocean and Earth Sciences, Xiamen University, Xiamen 361005, PR China *Corresponding author: chke@xmu.edu.cn ABSTRACT: Increasing atmospheric CO2 can decrease seawater pH and carbonate ions, which may adversely affect the larval survival of calcareous animals. In this study, we simulated future atmospheric CO2 concentrations (800, 1500, 2000 and 3000 ppm) and examined the effects of ocean acidification on the early development of 3 mollusks (the abalones Haliotis diversicolor and H. discus hannai and the oyster Crassostrea angulata). We showed that fertilization rate, hatching rate, larval shell length, trochophore development, veliger survival and metamorphosis all decreased significantly at different pCO2 levels (except oyster hatching). H. discus hannai were more tolerant of high CO2 compared to H. diversicolor. At 2000 ppm CO2, 79.2% of H. discus hannai veliger larvae developed normally, but only 13.3% of H. diversicolor veliger larvae. Tolerance of C. angulata to ocean acidification was greater than the 2 abalone species; 50.5% of its D‑larvae developed normally at 3000 ppm CO2. This apparent resistance of C. angulata to ocean acidification may be attributed to their adaptability to estuarine environments. Mechanisms underlying the resistance to ocean acidification of both abalones requires further investigation. Our results suggest that ocean acidification may decrease the yield of these 3 economically important shellfish if increasing CO2 is a future trend. KEY WORDS: Ocean acidification · Embryo · Larva · Haliotis diversicolor · Haliotis discus hannai · Crassostrea angulata Full text in pdf format Erratum Supplementary material Supplementary material PreviousNextCite this article as: Guo X, Huang M, Pu F, You W, Ke C (2015) Effects of ocean acidification caused by rising CO2 on the early development of three mollusks. Aquat Biol 23:147-157. https://doi.org/10.3354/ab00615 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 23, No. 2. Online publication date: February 10, 2015 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2015 Inter-Research.

Effects of the red tide dinoflagellate, Karenia brevis, on early development of the eastern oyster Crassostrea virginica and northern quahog Mercenaria mercenaria

The brevetoxin-producing dinoflagellate, Karenia brevis, adversely affects many shellfish species including the commercially and ecologically important bivalve molluscs, the northern quahog (=hard clam) Mercenaria mercenaria and eastern oyster Crassostrea virginica, in the Gulf of Mexico, USA. This study assessed the effects of exposure of these bivalves to K. brevis during their early development. In separate experiments, embryos of 2–4 cell stage of M. mercenaria and C. virginica were exposed to both whole and lysed K. brevis cells isolated from Manasota Key, Florida. Low bloom concentrations of 500 to 3000 cellsmL−1 were simulated for 96h. Shell length, percent abnormality (and normality), and percent mortality of resulting larvae were measured. Percentages were recorded after 6, 24, and 96h of exposure; larval shell length was measured at 24 and 96h. For both quahogs and oysters, the effects of exposing embryos to K. brevis on all larval responses were generally dose- and time-dependent. Percent mortalities and abnormalities of both clam and oyster embryos increased significantly after only 6h of exposure to whole cells of K. brevis. For clams, these parameters were significantly higher in whole and lysed treatments (at 3000 cellsmL−1) than in controls. Percent mortalities of oysters were significantly higher in the whole-cell treatment (3000 cellsmL−1) than under control conditions. After 24h of exposure, mean larval shell length of both bivalve species was significantly reduced relative to controls. This was evident for clam larvae in both the lysed treatment at 1500 cellsmL−1 and in whole and lysed treatments at 3000 cellsmL−1, and for oyster larvae in the lysed treatment at 3000 cellsmL−1. After 96h, both species exposed to the lysed cell treatment at 3000 cellsmL−1 had significantly smaller larvae compared to those in the control. Overall, lysed cells of K. brevis had a more pronounced effect on shell length, percent abnormality, and mortality in both clams and oysters than did whole cells. Given the fact that blooms of K. brevis overlap with the spawning periods of these two bivalves, and that cells of this naked dinoflagellate are readily lysed by wave action, these results suggest that exposure to K. brevis during the early life history stages of clams and oysters could adversely affect their population recruitment. Further, the presence of whole or lysed cells of K. brevis in hatcheries could have a major negative impact on production.

Effects of ocean acidification on the early developmental stages of the horned turban, Turbo cornutus

To estimate the impact of CO2-driven ocean acidification on the early life stages of gastropods, the effects of increased partial pressure of seawater carbon dioxide (pCO2) (800–2,000 μatm) on the early developmental stages and larval shell length of the commercially important gastropod, the horned turban snail, Turbo cornutus were investigated. Increase in experimental seawater pCO2 had an increasingly negative impact on the early developmental rate; the proportion of embryos or larvae displaying retarded development increased at higher pCO2. The proportion of embryos that developed to the 4-cell stage at 2 h after fertilization decreased linearly with increasing pCO2. At ~1,000 μatm pCO2, retarded development was observed in ~50 % of larvae. No embryos developed to the 4-cell stage at 2,000 μatm pCO2 within 2 h of fertilization. A similar trend continued until 24–26 h after fertilization; the proportion of larvae attaining veliger stage by 24–26 h also decreased with increasing pCO2. The shell length of T. cornutus veligers decreased gradually as seawater pCO2 increased, but markedly decreased in seawater under nearly unsaturated and unsaturated conditions (≤1.04) of the aragonite saturation state (Ωaragonite). The results indicate that increased pCO2 seawater has a progressive and acute effect on embryonic and larval T. cornutus, and imply that the extended early developmental period and/or the downsized larval shell produced by ocean acidification will have a negative impact on survival, settlement and recruitment well into the future.

Effects of elevated pCO2 on the early development of the commercially important gastropod, Ezo abalone Haliotis discus hannai

We used an up-to-date, a high accuracy CO2 manipulation system to investigate the sensitivity of organisms to CO2 acidification, rearing marine calcifiers under elevated CO2 in running water. We evaluated the effects of elevated partial pressures of carbon dioxide (pCO2) in seawater on larvae of the commercially important marine gastropod Ezo abalone Haliotis discus hannai. In larval Ezo abalone, no effect of exposure to <1100 μatm pCO2 seawater was observed in fertilization, malformation, or mortality rates until 15 h after fertilization. However, compared to control larvae in seawater (450 or 500 μatm pCO2), the fertilization rate and the hatching rate (15 h after fertilization) decreased with increased pCO2 exposure (1650 and 2150 μatm pCO2) and the malformation rate increased significantly, with the larval shell length being smaller 75 h after hatching. These results suggest that ocean acidification will potentially impact the marine population of Ezo abalone as a human food source in the future.

Realized Heritability and Genetic Gain Estimates of Larval Shell Length in the Chinese Pearl Oyster Pinctada martensii at Three Different Salinities

AbstractThe second‐generation selected (SS) and control (SC) lines of the Chinese pearl oyster Pinctada martensii were established by selecting the 10% of breeders with the largest and mean sizes, respectively, from the first‐generation selected group. Larval offspring of the SS and SC lines were reared at 30, 24, and 18‰. Heritability and genetic gains of larval shell length were estimated on the basis of data measured at days 8, 14, and 21. At days 8 and 14, there were no significant differences in mean shell length between the SS and SC lines at 30, 24, and 18‰ (P &gt; 0.05). On day 21, however, the SS line displayed significantly larger mean shell length than the SC line at 30‰ and 24‰ (P &lt; 0.05). Heritability estimates and genetic gains for larval shell length ranged from 0.22 to 0.64 and from 3.68% to 14.93%, respectively. The results from this study indicate that considerable genetic variability exists in the base population and that mass selection for the second generation is still effective.

Can bivalve veligers escape feeding currents of adult bivalves?

While the stock of introduced Pacific oysters ( Crassostrea gigas) increased in the Oosterschelde estuary (SW Netherlands), so did the filtration pressure of all bivalve species together. In the same period, stocks of native bivalves declined slightly. The expansion of Pacific oysters in Dutch estuaries might be partially due to better abilities of their larvae to avoid or escape filtration, compared to larvae of native bivalves. In this context, escape and swimming abilities of Pacific oyster larvae and the larvae of the native blue mussel ( Mytilus edulis) were compared. Swimming behaviour of C. gigas larvae and larvae of M. edulis was recorded in still water and in a suction current mimicking a bivalve feeding current, in a horizontal and in a vertical plane. Larval swimming behaviour in a suction flow field was reconstructed by subtracting local water movement vectors from the total movement of larvae, yielding movement paths due to larval swimming alone. Swimming speeds and the rate of displacement in vertical direction of C. gigas and M. edulis larvae were related to larval shell length, and to the pitch of up- or downward swimming. Larvae of both species did not show escape reactions in a suction flow field. With increasing shell length, larval swimming speeds of both species increased significantly. Swimming speeds of C. gigas larvae were significantly higher than swimming speeds of M. edulis larvae, resulting in a faster vertical displacement. The ability to migrate to more favourable water layers faster may offer C. gigas an advantage over native bivalves with slower swimming larvae.

Laboratory hybridizations between Mytilus species and performance of pure species and hybrid veliger larvae at lowered salinity

Laboratory hybridizations between Mytilus species and performance of pure species and hybrid veliger larvae at lowered salinity

Diet and feeding regimens for larval doughboy scallops, Mimachlamys asperrima

Nine microalgal species commonly cultured as food in mariculture were assessed as food for larvae of Mimachlamys asperrima. Larvae were cultured for seven to nine days in a shaker incubator cabinet and fed a range of monospecific, binary and ternary algal diets. When fed a single algal species, the greatest increases in shell length occurred with Pavlova lutheri. When fed the remaining eight algal species in combination with P. lutheri, Chaetoceros calcitrans and Tahitian Isochrysis aff. galbana produced the greatest shell length increases. In both these experiments, increases in larval shell length were correlated with larval survival. The greatest larval growth recorded in these trials was with a diet combining the three previously listed algae. To optimise the use of this ternary diet, the maximum proportion of C. calcitrans in the diet before significant reductions in growth occurred was estimated at 44%. However, synergistic improvements in growth of larvae fed ternary diets were achieved with the inclusion of as little as 10% C. calcitrans. Larval growth decreased when the frequency with which larvae were fed each of the three algal species in the ternary diet was reduced from daily, to diets in which six days passed before larvae received each species. The concentration and storage (2–7 days) of the ternary diet before feeding did not affect larval growth or survival, nor did the change from discontinuous (twice daily) to continuous feeding regimens.

Heritability estimates of larval shell length in the Chilean blue musselMytilus chilensis, under different food densities

Heritability of larval growth rate in diferent food environments and genotype-food environment interaction (during larval growth) are estimated in Mytilus chilensis, based on a nested design, involving 24 full-sib families (8 males each mated to 3 separated females) grown under three different algal concentrations. The heritability estimates for larval shell length ranged between 0.2 ± 0.1 and 0.9 ± 0.3 and showed a decreased towards the low food treatment; but the values suggest that selection would be effective. However, since there is a significant genotype-food interaction, selection should be done in each environment in which the animals are expected to perform. The significant interaction term (p<0.001) associated with a significant genetic variability for larval growth suggest the presence of genes in the population whose expression depends upon local food conditions.

Quantitative genetic analysis of growth in larval scallops (Placopecten magellanicus)

We estimated the broad-sense heritability of larval size in 20 full-sib families of the giant scallop, Placopecten magellanicus (Gmelin, 1791) grown in laboratory culture in August and September 1991. The goal was to compare scallops with other bivalves which have been shown to have significant heritabilities for larval growth. Secondly, we estimated the lipid content of occytes from female parents, since this is hypothesized to affect larval growth and survival. Finally, we estimated the among-family variation in mortality from 4 to 21 d as a test of possible genetic variation for viability among larval scallops. Significant genetic variation (h2=1.10 to 1.24) was estimated for larval shell length at 4, 14, and 21 d. There was a significant correlation (r=0.66) between larval size at 4 d and lipid content of oocytes, but only when two females with high levels of lipid oocyte−1 were excluded as outliers. There was no correlation between larval size at 14 and 21 d and lipid content of oocytes. Mortality among families from 4 to 21 d was high (69 to 97%), and was significantly different among families. These results indicate that there is significant heritability for larval growth which is largely independent of the lipid content of the oocytes. A high heritability for larval growth may indicate that this trait is only weakly correlated with fitness.

Hybrid vigor in Pacific oysters: an experimental approach using crosses among inbred lines

Two competing genetic hypotheses for heterosis, dominance and overdominance, have been championed to explain positive correlations between allozyme heterozygosity and fitness-related traits for bivalve molluscs. To begin to test these hypotheses, we made controlled crosses among inbred lines of the Pacific oyster Crassostrea gigas. In such mating experiments, heterosis ( h p ) can be defined and quantified through ANOVA as Q L > 1.0 or < − 1.0 , where L is the trait-difference between the two parental inbred lines and Q is twice the deviation of the hybrid from the mid-parent value (Griffing, 1990). Inbred lines of the Pacific oyster were made by selfing hermaphrodites; brood stock pedigrees and a grand mean fixation index of F = 0.5 were confirmed by allozymes. In two separate experiments pairs of inbred lines were crossed in 2 X 2 fashion to produce two inbred and two hybrid progeny genotypes. Each genotype was replicated by pairwise matings and replicates were grown in multiple containers (8 1 plastic bags), from which data on larval mortality (proportional decrement in population number per day) and size (shell-height) were obtained. In the first experiment, the two inbred genotypes differed significantly from each other in daily larval mortality (−0.115 vs. −0.881) and both hybrids had lower mortalities than inbred genotypes (−0.034 and −0.078); mean h p , 1.15, was significantly greater than 1.0. By Day 14, larvae of one hybrid group were significantly smaller than those of the inbred genotypes (241.0 μm vs. 253.5 and 259.2 μm, respectively); only a very small part of this difference could be attributed to negative correlation between larval density and growth. Negative heterosis for larval size was highly significant ( h p = −5.36, P < 0.001) and persisted to the juvenile stage (mean h p = −1.34, ns, for shell-height at Day 154 and h p = −7.77, P < 0.001, for live-weight at Day 163). In the second experiment, inbred genotypes again differed significantly in daily larval mortality (−0.341 vs. −0.654). One hybrid group had significantly less daily mortality than either parent (−0.110), but the other was similar to the best parent (−0.347), so that for one hybrid, h p = 2.47, P < 0.01, and for the other, h p = 0.96, ns). Variance among genotypes for larval shell-length was highly significant on Days 2, 7, and 14, with heterosis evident on Days 2 and 7 ( h p = 1.04 and h p = 3.84, respectively). At 11 months of age, hybrid shell-height averaged 150% of the shell-height for the best inbred parent and h p ≈ 7.7. These measurements of heterosis, both positive and negative, implicate a third explanation of heterosis, epistasis. Intercrosses of F 1 hybrids can be used to discriminate among dominance, overdominance and epistasis hypotheses for quantitative trait loci (QTL) causing hybrid vigor.

Genetic variation and covariation during larval and juvenile growth in Mercenaria mercenaria

Quantitative genetic variances and covariances were estimated for shell length of the hard clam Mercenaria mercenaria (L.) at three larval stages (prodissoconch I, 2 d and 10 d post-fertilization) in 1987 and in 1988 after ca. 9 mo of growth. At each sample interval additive genetic variance was a highly significant component of the total size variation. Narrow sense heritability estimates for shell length ranged between 0.58 (±0.10) for prodissoconch I and 1.08 (±0.29) for 2-d-old larvae. There was significant and positive genetic covariance in prodissoconch I and 2-d larval shell length which resulted in a highly significant genetic correlation (r g=0.74) between these two traits. This covariance is not surprising since the prodissoconch I comprises the majority of the larval shell of a 2-d-old larvae. The genetic covariances between 2-d-old and 10-d-old larvae and between 10-d-old larvae and 9-mo-old juveniles were low and not significantly different from zero. These results indicate that there is substantial genetic variation for shell growth in M. mercenaria but this variation is not stable during development; the genetic variation in shell growth at one stage of development is not strongly related to the genetic variation in growth during other ontogenetic periods. In this study there were no evident constraints to natural selection for increased shell growth rate during development, which coupled with the high levels of genetic variation may suggest that in nature high rates of larval growth may not be normally subject to significant selective pressure.

Shell morphology of larval and post-larval mytilids from the north-western Atlantic

Comparison of early shell morphological features of six mytilids from the north-western Atlantic revealed differences useful for species identification and classification. Brachidontes exustus, lschadium recurvum, Geukensia demissa, Amygdalum papyrium, Mytilus edulis, and Modiolus modiolus larvae and post-larvae were cultured in the laboratory. Scanning electron micrographs of the shell and hinge during early ontogenetic stages showed that all species had a long provinculum with taxodont dentition. In addition, provinculum length and number of teeth increased during the larval period in the six species. The small, numerous provincular teeth of Mytilus edulis and the bold, comparatively few provincular teeth of Amygdalum papyrium clearly differentiated these two species. Most species had a low umbo, round posterior margin, and more pointed anterior margin, although Amygdalum papyrium was distinguished by a high, prominent umbo. Distinction of Geukensia demissa and lschadium recurvum larval shells was difficult due to similarity in their shapes and hinge dentition. However, discriminant analysis using larval shell length, shell height, provinculum length, and number of teeth aided in classification of these and other sympatric species.