Influence of the gametogenic cycle on the biochemical composition of the ovary of the great scallop

The oogenic cycle and biochemical composition of the ovary of raft-cultured great scallop (Pecten maximus) were studied during the period April 1990–July 1991. The ovary condition index (FGI) and stereological studies showed the existence of two principal spawning periods, winter and late spring-early summer. No sexual resting period was found. Oocyte lysis was high throughout the year. Ovarian lipid levels displayed a clear seasonal pattern linked to the gametogenic cycle. Total lipid (TL, 16–21% dry weight), acylglycerol (AG, 20–65% TL) and free sterol (FS, 2.8–6.4% TL) levels were, generally, higher in the ripe ovary and a decrease coincided with spawning. Protein (59–63% dry weight), glycogen (<3% dry weight) and phospholipid (PL, 26–35% TL) levels showed no clear seasonal trend. The TL and AG were a good index of ovarian sexual maturity. The TL % of dry weight) correlated well with the female gonad condition index (r _s = 0.779, p < 0.001), and AG (% f TL) correlated well with the mean oocyte diameter (r _s = 0.630, p < 0.01) and the female gonad condition index (r _s = 0.443, p < 0.05).     

Bacterial nutrition of great scallop larvae

Bacterial nutrition of great scallop, Pecten moximus, larvae was investigated using the radioactive tracer technique. The bacterial labelling was studied initially to obtain a high and stable specific radioactivity (¹⁴C) of bacterial cells. A higher bacterial specific ¹⁴C activity was obtained when the tracer (amino acid) was introduced in the culture medium at the beginning of the exponential growth phase. After a 12 h labelling period in a rich nutritive medium, the bacteria were depurated in seawater for 5 h (chase) to prevent further ¹⁴C excretion and then added to larval rearing vessels. The larval labelling was followed for 12 h and then larvae were placed in new vessels without radioactive bacteria. The depuration of larvae was followed for 3 days. Data obtained on ingestion and assimilation efficiency show that bivalve larvae are able to ingest and digest bacteria.     

A seeding experiment of juvenile great scallops (Pecten maximus (L.)) off the Island of Jersey.

In 1995, landings of Great scallops, Pecten maximus(L.) increased dramatically in Jersey from around one tonne in the previousyear, to 66 tonnes and this continues to rise. This was caused by theintroduction of scallop diving permits and diversification of the fishingfleet.Due to this increase in effort it was decided that the feasibility of ranchingone-year-old juvenile scallops should be investigated. 100,000 scallops werepurchased from Ireland and seeded in specifically selected coastal sites. Thescallops grew from 22.8 mm shell length and 1.17 grams to 57.6mm and 23.2 grams in the first six months after seeding, and to93.3 mm and 88.9 grams during the subsequent 12 months. Growthrateslowed considerably during winter months. Given these growth rates the scallopswill reach market size in three years from settlement, less than the 4, 5 and 6years taken in Guernsey, the Eastern Channel and the offshore Irish Searespectively. Although growth rates are not unique and are comparable withotherinshore sites in the UK, they are significant for scallop farming in Jerseywaters. Mortality following re-seeding and predation rates by crab and starfishappears to be lower than reported by other workers. However this has not yetbeenquantified.     

The effect of seawater flow and temperature on metamorphosis and postlarval development in great scallop

Variations in growth and survival of hatchery-reared post-metamorphicjuveniles of great scallop Pecten maximus prompted anexamination of settlement and postlarval development. The effects ofseawater flow and temperature on great scallop metamorphosis andpostlarvae were studied over a 4–5 week period. In allexperiments, and regardless of environmental conditions, great scallopmetamorphosed after a 2–3 week period with values of 35 to70%. Subsequently, spat numbers increased slightly. Spatmortality generally occurred from the third week onward and reachedlevels as high as 30% by the fifth week under standardconditions. At 20 °C, however, 60% mortality levels wererecorded. Differences in spat growth rate, ranging from 37 to 45 mday^–1, were noticed at different seawater flow ratesbut no clear tendency could be discerned. Temperature affected spatgrowth with an increase in size from 24 m day^–1 at15 °C to 35 m day^–1 at 18 °C. Conversely,growth was suppressed at 20 °C (14 m day^–1).For optimal metamorphosis and postlarval development in great scallop, aseawater flow of 4.3 L h^–1 per sieve and a temperatureof 15 °C are recommended.     

Seasonal differences in effect of broodstock diet on spawning success in the great scallop

Great scallop (Pecten maximus L.) broodstock collected at two different seasons, spring (May) and winter (December), were given four different diets during conditioning. The spring group was spawned in late June, and the winter group in February.The highest number of veligers in the summer spawning (30 millions per 10broodstock), was produced using a diet consisting of 80% Tahitian Isochrysis galbana. In the winter spawning, broodstock given a dietconsisting of 80% diatoms (Skeletonema costatum/Chaetocerosgracilis) produced the highest number of veligers (29 millions per 10broodstock). Fecundity and the number of egg-releasing individuals were higher in the early summer spawning (June) than in the winter spawning (February), however, the total number of three days old veligers produced was nearly the same in the two seasons.     

Abnormal melanization and microstructural distortions of the shell of great scallop living in shallow water

Modifications of great scallop (Pecten moximus) shells have been noticed in many sites of scallop fisheries in Brittany, especially in shallow waters. These calcification abnormalities are linked to the appearance of a brown colouration of the internal calcified shell layer, due to the presence of a eumelanin associated with the insoluble organic matrix of the biocrystals. The appearance of this pigment generates many disturbances of the calcified foliated microstructure of the scallop internal shell layer. The mantle structure is not modified in brown shells as compared with white ones. No pathogenic signs such as hyperplasia or haemocytic infiltration have been observed. According to this observation, we hypothesize that the brown colour phenomenon is more a result of environmental disturbance rather than a symptom of a pathogenic disease.The colour abnormalities of the internal shell layer can be detected by a spectral analysis of its reflectance before it can be detected with the naked eye. This method, correlated to microstructural observations, gives a rapid and precise analysis of the appearance of the pigmentation on adult or juvenile scallops. It may be a useful method for the evaluation of the influence of environmental parameters, for example, on calcification and its abnormalities.     

Fatty acids and egg quality in great scallop

Spawners of the great scallop, Pecten moximus, were conditioned on three microalgal diets: T-Isochrysis, a mixture of four species (PTSC) and Chaetoceros calcitrans.The polyunsaturated fatty acid (PUFA) composition of neutral and polar lipids of eggs was related to the fatty acid composition of the diet. However, the 20 and 22 carbon PUFAs were maintained at significant levels independent of those of the diet when these fatty acids were present in the diet at low levels. This effect was more pronounced in the polar than in the neutral lipids. A preferential incorporation of 22:6(-3) and 20:4(-6) was demonstrated in the polar lipids. This emphasizes their role in ovogenesis and embryogenesis.The 22:6(-3) requirement of P. maximus was better satisfied by T-Isochrysis, which favoured the highest incorporation of this essential fatty acid in the eggs. The good success in reproduction obtained with this monospecies diet led us to modify multispecies diets by increasing the level of 22:6(-3).     

Mortality control of scallop larvae in the hatchery

Larval mortalities occurring in molluscan hatcheries have often been associated with bacterial contamination, and more specifically with vibrios. Although batches of oyster and clam larvae have been routinely reared in the hatchery of Argenton (North Brittany, France) without antibiotics, high larval mortalities have been recorded with the great scallop, Pecten maximus, under similar conditions. For this species, an addition of antibiotics was found necessary and chloramphenicol was used at a concentration of 8 mg l^–1. However, this chemical has now been banned in Europe, making either substitution products or an improvement in the rearing procedures essential. Studies carried out have shown that neither a decrease in larval density (to 1 larva ml^–1) nor an increase in seawater change frequency (to one per day) had any positive effects. Furthermore, elective substances such as sugars were not suitable and the use of another antibiotic, erythromycin, led to inconsistent results. The only positive effects were obtained with lower levels of chloramphenicol, which does not resolve the problem. Because no alternative solutions have as yet been found, further research needs to be undertaken.     

Vitamin requirements in great scallop larvae

Vitamins were analysed in food (microalgae) and larvae of great scallop, Pecten maximus, during larval development. Microalgae used to feed larvae in hatcheries show great variability in their vitamin composition depending on both the species and culture condition (phase of growth). The microalgae used to feed Pecten maximus larvae were rich in vitamins; their content compared with diets used in fish culture appeared sufficient, with the possible exceptions of pyridoxine, biotin and pantothenic acid. Vitamins in bacteria, isolated from the larval rearing tank were also analysed, as they can also contribute to the diet. Vitamin B₁₂, -tocopherol and -carotene were detected in very low concentration in bacteria; however, some bacterial strains were rich in pantothenic acid, and the pattern of other vitamins was similar to that from microalgae. The presence of bacteria can complement the diet in panthothenic acid, as it has been demonstrated that bacteria are ingested by larvae. The vitamin content of Pecten maximus larvae was analysed from the second day after hatching to just before metamorphosis. The content of some vitamins, ascorbic acid (C), -tocopherol and -carotene, increased during larval development, suggesting that their requirement was satisfied. However, thiamin and riboflavin decreased during larval development and further studies, possibly using microencapsulated vitamins supplements, are needed to determine whether these vitamins are limiting during larval development.     

Mortality of scallop spat in cultivation, infested with tube dwellingbristle worms, Polydora sp.

Between 1995 and 1997, a traditional Norwegian oyster poll with an attachednursery was used for the production of scallop (Pecten maximus) spat.During summer 1997, four batches of 2 mm scallop spat were placed in thenursery. In June, hydroids (Obelia) caused problems with fouling andaccumulation of sediment in the nursery. Thereafter, the scallop spat wereheavily infested with tube dwelling polychaets (Polydora sp.). Thepolychaets were both introduced with poll water and reproduced inside thenursery. Infested spat suffered high mortalities. Although the spat could have been affected by the hydroids, as well as shortage of food, Polydorainfestations were considered the main cause of the mortalities. Dead andinfested spat were continuously sorted out and discarded, but the infestationproblems persisted. As a result, the scallop spat did not represent anycommercial value for the company, and in September, the nursery wasemptied. In total, approximately one million spat – representing onethird of Norway's intensive scallop spat production in 1997, was lost.     

Effect of air exposure on scallop behaviour, and the implications for subsequent survival in culture

Scallop spat produced for continued culture normally requiretransportation between sites, and the associated stresses may causemortalities. In the present experiment, scallops of 40–55 mm shell heightwere emersed in air for time intervals up to 24 h and their behaviourexamined once re-immersed. Scallops were placed upside-down and thenumber of movements and the cumulative numbers righting in 5 min timeblocks were recorded. The greatest frequency for all behaviouralresponses was found at 15–17 °C in August. Responses were reducedin November and June (9–11 °C) and least at 5 °C in January.All effects of treatment, temperatures and season, and increases in airexposure were significantly different. Following the treatments, mortalitiesafter 10 days in culture was about 10–30% for scallops emersed for 18 and24 h in August and June. Scallops did not show significantly differentbehaviour whether they were emersed upright or inverted. However,scallops emersed at 15 °C had fewer responses than scallops held at <10 °C, so chilling during transport may prolong scallop vitality. Theresults suggest that air exposure >12 h should be avoided. At temperatures>9 °C, behavioural responses may be a simple and effective method toassess vitality which can assist in the management of scallop culture.     

Particle clearance and selection in three species of juvenile scallops

Juvenile scallops (<2 mm shell height) of three species (Placopecten magellanicus, Patinopecten yessoensis, Argopecten irradians) were fed mixed, unialgal cultures. Scallops were fed a total of six algal clones simultaneously and clearance rates were monitored using flow cytometric techniques. In another experiment, scallops were presented with natural assemblages of particulate matter as a food source. Data are presented on differences in clearance rates for the individual algal species as well as size-related differences of algal clones, and uptake of chlorophyll vs. non-chlorophyll cells, both within and between scallop species. Significant differences in clearance rates of individual algal species have been found within and between scallop species. Particle selection does not appear to be based upon size alone and is apparently based on other characteristics of the algae as well. The results demonstrate pre-ingestive sorting.     

Great scallop, Pecten maximus, research and culture strategies in Norway: a review

The great scallop, Pecten maximus is a potentialaquaculture species in Norway, but production to date has been low. Greatscallop landings in 2000 were 571 tonnes, exclusively from harvesting wildstocks, landed by divers. Approximately two million juveniles of 15mm shell height have been produced annually in a hatchery since1998, and distributed to farmers. Research projects on larvae and juvenilesinclude studies of various antibacterial treatments, improved culture systemdesigns and application of probiotics. Results from hanging cultures underdifferent environmental conditions indicate a strong correlation between lowtemperature and poor survival, and point out the importance of careful selectionof cultivation sites. Good growth potential of scallops in Norwegian waters hasbeen shown, and it is possible to reach market size of 10 mm shellheight in three to four years. Experiments with fences and other strategiesprotecting cultured scallops on the seabed from predation by crabs are inprogress.     

Effect of photoperiod on conditioning of the great scallop

A 45 days experiment was conducted by exposing adult scallops with empty gonads to three different photoperiod regimes under constant food and temperature conditions: Simulated natural/decreasing photoperiod (N), constant photoperiod (C) and increasing photoperiod (I). At the end of the experiment the I and C groups had significantly higher gonad indexes and larger oocyte diameters compared to the N group. No significant changes were found in the N group during the experiment. The adductor muscles and digestive glands did not change significantly during the experiment and no differences were found between the groups. These results suggest that photoperiod affects early rebuilding of gonads in scallops from Western Norway.     

Effects of air emersion on survival and growth of hatchery rearedgreat scallop spat

Scallop spat production normally requires transfer between growthsystems. Simulated transport experiments were carried out in April, June,December and February to evaluate effects of transport time on greatscallop (Pecten maximus) spat growth and survival. The spat (1.7–1.8 mm in shell-height and 21–25 µg ash free dry weight [AFDW]) wereheld in moist coffee filters at a temperature of 10 °C for up to 24 h,before being replaced into sieves in rearing tanks at 15 °C. The studyshowed that by increasing air emersion time, survival and growthdecreased. No significant difference in the results between 0 and 4 h of airemersion was found, while the effects after 12 and 24 h differed betweenspat groups. Survival and growth rates showed seasonal differences. Meansurvival was 35–71% in April and 77–99% from June to February. In Junemean growth rates attained were 115–128 µm shell-height and 15–18µg AFDW per day compared with 49–69 µm and 3.8–7.0 µgper day for the other spat groups. Great scallop spat may survive atransfer time of 24 h, but transportation for longer than 12 h is notrecommended if subsequent high survival and growth rates are to beensured.     

Yields of great scallop, Pecten maximus, larvae in a commercial flow-through rearing system in Norway

Survival, growth and yield of competent great scallop (Pecten maximus) larvae were investigated during a full production season in a commercial hatchery in western Norway. Broodstock were collected from natural scallop beds and 12 groups were induced to spawn during the period December 2002 to July 2003. Larvae were reared on a large scale in 36 flow-through tanks (3500 l) at 17±1 °C and continuously fed a mixture of five algal species produced in an indoor continuous-flow system. Large variations in larval performance between spawning groups and tanks were observed, but the results were as good as earlier results using the batch system and prophylactic addition of chloramphenicol. Growth from days 3–24 averaged 4.8 μm day⁻¹±0.8 (sd) and survival 22.4%±21.8 (sd). Mean yield of day 3 larvae was 7.1%±10.0 (sd) and 26.6%±25.9 (sd) for those surviving to day 24. Yield was significantly correlated to larval survival. Larval success was related to initial larval density, algal concentration and season. It was found that the best production regime had an initial larval density lower than 6 ml⁻¹ and algal concentration of less than 12 μl⁻¹ regardless of season. Seventeen tanks met these criteria and produced a mean yield of 0.5 larvae ml⁻¹ to settlement. Flow-through systems are currently regarded as the only feasible method for viable hatchery production of P. maximus larvae in Norway.     

Post-transport recovery of cultured scallop (<Emphasis Type="Italic">Pecten maximus</Emphasis>) spat,juveniles and adults

High mortality associated with transport operations in scallop culture has been a major problem faced by European farmers. Simulated transport with Pecten maximus L. spat <2 mm, spat 15–30 mm, juveniles 30–50 mm and adults >100 mm were carried out in Spain, Ireland and Norway. Different time and temperature combinations were studied in order to maximise post-transport survival and establish best practices. Out-of-water transport could result in 100% survival if conditions were right, but the response to emersion stress depended on size, season and location. Post-transport recovery decreased with emersion time and was strongly influenced by temperature. Air exposure was tolerated for a longer time by adult scallops than spat and juveniles, but the results differed among trials in the different countries. The maximum emersion time that gave post-transport survival ≥80% was 12 h for the smallest spat, 18 h for larger spat and 24 h for juvenile and adult scallops. Adults were less affected by transport temperatures that deviated from ambient seawater temperature than spat and juveniles. In general post-transport recovery was high when sea temperature was <10°C, but during warm-water seasons special care should be taken to avoid stressful and lethal transport conditions. A transport temperature <12°C was recommended, though not more than 10°C below ambient culture temperature. A maximum transport time of 9 h was suggested for spat and juveniles to attain post-transport survival close to 100%, but 12–24 h was feasible during the cold-water season or at favourable transport temperatures.     

Age, size distribution and growth of native and cultured Japanese scallops in Possjet Bay, Sea of Japan, Russia

Stock size, distribution, size and age composition, and growth of Japanese scallops,Mizuhopecten yessoensis, were studied at eight sites in Possjet Bay, Sea of Japan, Russia. At seven sites, most of the scallops were cultured animals (seeded as one-year-olds in 1986–1989). At the eighth site, only native (i.e. naturally settled) scallops were present. Cultured scallops had an irregular strip-like distribution at all locations. Maximum growth of scallops occurred in the northwestern part of Reid Pallada Bight. Only at Temp Bight did both native and the majority of cultured scallops attain the harvest size of 100 mm shell height at age 3 years. At all other sites in Possjet Bay, cultured scallops reached harvest size at an age of 4 years.