John J. Manzi

Hard clam, Mercenaria mercenaria, broodstocks: Genetic drift and loss of rare alleles without reduction in heterozygosity☆

Abstract Two nursery stocks of the hard clam, Mercenaria mercenaria, selected for fast growth were compared to corresponding wild populations in regard to allele frequencies at seven polymorphic enzyme loci. Although as few as 30–60 parents were spawned at each of four generations to produce these two broodstocks, neither line exhibited any reduction in heterozygosity. Both lines, however, showed evidence of genetic drift and loss of rare alleles, suggesting that a cross between them could result in a third genetically distinct line.

Growth of seed clams, Mercenaria mercenaria, at various densities in a commercial scale nursery system☆

Abstract Hatchery-raised Mercenaria mercenaria (mean size = 3.9 mm) were placed in commercial nursery raceways at densities approximating 740, 2220, 6660 and 19 980 seed/m 2 . Each density was replicated eight times in the nursery and the highest and lowest densities were replicated four times in adjacent subtidal field controls. All replicates were monitored monthly from February to August 1981 to determine growth and survival. Temperature and salinity were measured daily and inflow and outflow water were sampled monthly to determine chlorophyll α concentrations. Results indicated that growth was significantly affected by planting density in both raceways and field controls. Although total mean growth for the raceway and the field was similar, a number of observations indicated that different factors influence growth in the two locations. Growth in the raceways was inversely proportional to both distance from inflows and planting density. Greatest growth was observed in the lowest density nearest the inflow and alowest growth was observed in the highest density nearest the outflow. Growth rates were analyzed in relationship to effective water flow rate (volume water/volume clama/min), effective density (number clams/unit water) and chlorophyll α stripping rates.

Realized heritability of growth rate in the hard clam Mercenaria mercenaria

Abstract Native South Carolina wildstock clams ( Mercenaria mercenaria ) were mass-spawned to produce a large initial population of parents for a directed breeding program. At 2 years of age, the largest 10% of this population, and an equal number of mean size clams, were segregated to become selected and control-line parents. Three separate experiments were performed, usually involving 20–40 selected and control parents. Offspring were reared under standard hatchery, nursery and field grow-out conditions. Realized heritability was determined at 2 years of age. In one experiment, no response to selection was observed at 2 years, possibly due to reduced effective breeding number. Conservative estimates of realized heritability of growth rate for the other two experiments were consistent and high: 0.42±0.10 and 0.43±0.06. Mass selection appears to be a promising technique for improvement of hard clam broodstocks.

Effect of algal ration and substitution of algae by manipulated yeast diets on the growth of juvenile Mercenaria mercenaria

Abstract The development of a cost-effective algal substitute would greatly reduce the operating costs of bivalve hatcheries. Previous work has resulted in the development of a yeast diet with improved digestibility and nutritional composition. The use of this yeast product as a partial algal substitute for the culture of juvenile hard clams Mercenaria mercenaria was investigated in a series of growth experiments. Juveniles of the hard clam (1 mg live weight) were batch cultured in an 18 l recirculating system for 2 to 3 weeks. The optimal weight-specific daily ration for a mixture ( 50 50 on dry weight basis) of Isochrysis galbana (clone T-Iso) and Chaetoceros gracilis was found to be 1.5 to 2% dry weight per wet weight of seed. Replacing 50% of the algal ration by yeast did not result in a significant decrease in growth rate relative to the algal-fed controls. The substitution of 80% of the algal diet resulted in growth rates reaching 90% compared with those obtained for the algal-fed controls. The incorporation in the yeast diet of fat-soluble vitamins, rice starch, or an extract from macro-algae did not improve its nutritional value. However, the addition of kaolin occasionally resulted in a significantly higher growth rate. The present results are compared with previous reports on the use of yeasts for nursery rearing of bivalves.

Genetics and shell morphology in a hybrid zone between the hard clams Mercenaria mercenaria and M. campechiensis

Populations of Mercenaria mercenaria (L.) from South Carolina, USA, and M. campechiensis (Gmelin) from the Gulf of Mexico, Florida, USA, were sampled in 1987. The two species differed at all of seven enzyme loci tested, as well as in the thickness of shell ridges and nacre color. The difference in lunule shape was not great, although differences in relative shell width, shell weight, and lunule size make morphometric discrimination between the species possible. Shell ridges, nacre color, and multivariate morphometrics in a sample of clams collected from the Indian River Lagoon on the Atlantic coast of Florida in 1985 do not assort independently. Individuals with thick ridges, white nacre, and/or campechiensis-like morphometrics have significantly different allele frequencies at most enzyme loci from individuals with thin ridges, purple nacre, and/or mercenaria-like morphometrics. The deviations are in the direction predicted from the analysis of the allopatric populations of M. mercenaria and M. campechiensis. M. mercenaria outnumber M. campechiensis in the Indian River sample, but the majority of the clams seem to be hybrids.

Seed clam, Mercenaria mercenaria, culture in an experimental-scale upflow nursery system

Abstract Upflow nursery culture of bivalve seed, while in widespread use in France and England, has experienced only limited application in commercial-scale nurseries in the United States. Reported studies on appropriate flow rates, stocking densities, and carrying capacities for upflow culture either have used water from fertilized ponds or have provided supplemental feeding with cultured algae. There have been no published results of upflow use in natural waters. This study was conducted to evaluate growth of seed clams, Mercenaria mercenaria , in an upflow nursery culture system in South Carolina relying on natural productivity as the only food source. Experimental-scale passive upflow cylinders were stocked with small seed clams (initial mean size ∼ 4.0 mm) at various densities (2.5, 5.0, 10.0, 20.0, 30.0, and 40.0 kg/m 2 ) in trials initiated at different times of the year (∼ quarterly). Cultures received a continuous uniform water flow rate of 2.5 l/min from an adjacent estuary, at ambient phytoplankton concentration, temperature and salinity. Most rapid growth was obtained with seed stocked in April and October, when water temperatures were between 18 and 22°C. Monthly biomass increases as high as 267 g/100 g were achieved. Growth was positively correlated with flow rate in all seasons except winter. A flow:biomass ratio of 15:1 resulted in a doubling of biomass in 30 days, while a ratio of 30:1 resulted in a tripling over the same period. Under favorable environmental conditions, a biomass doubling could be achieved at stocking densities as high as 20 kg/m 2 . Maximum production over a 3-month period was 495 g (309 g/100 g) of 7 mm seed, corresponding to 62 kg/m 2 . Although water requirements were similar to those previously reported for raceway culture in South Carolina, results indicated much greater biomass carrying capacities per unit area with upflow culture systems.

Hard clam, Mercenaria mercenaria, broodstocks: growth of selected hatchery stocks and their reciprocal crosses

Abstract Crosses between and within two hatchery stocks of hard clams, Mercenaria mercenaria , were created in three independent experiments and the offspring reared to 2 years of age. Environmental variation between experiments strongly influenced early growth, but mean sizes at 2 years were similar for the three experiments. Offspring from all crosses had above-average growth rates. Offspring were assessed electrophoretically at seven enzyme loci, and gene frequencies and overall heterozygosities were compared to parental stocks. There was no correlation between heterozygosity and size at 2 years, nor between heterozygosity and variance of mean size. Reciprocal crosses were not consistently faster growing nor more heterozygous than purebred lines, nor was any relationship apparent between cross and variance of mean size. However, offspring from the reciprocal crosses were genetically distinct from each other and from the purebred lines and possessed the desirable traits of the parental stocks.

Enzyme heterozygosity and growth rate in nursery populations of Mercenaria mercenaria (L.)

Abstract Crosses between and within two lines of the hard clam Mercenaria mercenaria (L.) were reared to age 1 yr under typical commercial upflow nursery conditions. Within these crosses, little relationship was detected between shell length and heterozygosity averaged over seven enzyme loci. However, significant differences between the largest and smallest clams were detected at individual loci in 10 of 42 tests. Results were consistent neither with the hypothesis that the alleles themselves were affecting growth, nor with the hypothesis that these enzyme loci are tightly linked to other loci affecting growth. Rather, it appears that alleles are marking the entire genomes of their parents, and that variation in the growth rates of the offspring from individual clams may be obscuring any relationship with overall heterozygosity.

A comparison of growth and survival of subtidal Crassostrea virginica (Gmelin) in South Carolina salt marsh impoundments

Four South Carolina salt marsh impoundments and their associated tidal creeks were assessed for the culture of subtidal Crassostrea virginica. The impoundments were chosen primarily for their diversity and ranged from old large impoundments with appreciable tidal exchange and surrounded by extensive low marsh, to new small impoundments with little tidal exchange and surrounded by maritime forest. Floating and bottom hardware cloth trays (1.22 × 0.61 × 0.14 m) each holding 200 seed oysters (initial length, y = 43.8 mm) were placed at each location and sampled monthly for growth and survival. Coincidental monthly estimates of primary production (14C), phytoplankton concentrations and total organic carbon were performed. Ancillary data collected biweekly at all locations included standard hydrographic information (temperature, salinity, pH and turbidity) and nutrient determinations (nitrates, nitrites, orthophosphates and silicates). Results indicated that over a 6-month period (October–April) growth at all locations was significantly (α = 0.001) greater in ponds than in adjacent creeks and greater in floating than bottom trays. Growth means ranged from as little as 1.00 mm/month in the Wando River, to median values of approximately 2.25 mm/month in tidal creeks, to a relatively high growth rate of 3.11 mm/month in Blue Heron Pond (Kiawah Island). Survival was high in all areas ranging from 85.0% at Blue Heron Pond to 94.5% at Kiawah Creek. There was no significant difference in survival in comparisons among locations. A direct correlation of growth in oysters, primary production and phytoplankton biomass was established. This relationship was reiterated by indications of an inverse correlation between nutrient concentrations and growth.