三疣梭子蟹、凡纳滨对虾、菲律宾蛤仔和江蓠混养结构优化的实验研究

利用海水陆基实验围隔优化了三疣梭子蟹(Portunus trituberculatus)、凡纳滨对虾(Litopenaeus vannamei)、菲律宾蛤仔(Ruditapes philippinarum)和菊花江蓠(Gracilaria lichevoides)的混养结构。实验设6个处理组,分别为蟹-虾(CS)、蟹-虾-贝(CSB)、蟹-虾-贝-藻(CSBJ1、CSBJ2和CSBJ3),以梭子蟹单养为对照(C),每个处理组设置了4个重复。结果表明,各处理组梭子蟹个体规格差异不显著(P0.05),产量以CSBJ2处理组最高;CSBJ1和CSBJ2组凡纳滨对虾的平均体重、特定生长率以及成活率均显著高于其它处理组(P0.05);凡纳滨对虾产量以CSBJ2最高,显著高于其它处理组(P0.05);混养江蓠处理组菲律宾蛤仔成活率显著提高,但产量与其它处理差异不显著;养殖期间,CSBJ2处理组亚硝氮含量总体上显著低于其它处理(P0.05),混养江蓠各处理水体氨氮浓度养殖后期均显著降低(P0.05),而CSBJ1和CSBJ2组总氮和总磷浓度在中后期显著低于其它处理(P0.05)。本研究表明,虾蟹贝混养系统中搭配适宜密度的菊花江蓠,可以有效改善水质,减少水体中总氮、总磷浓度以及氨氮、亚硝氮等有害物质含量,提高梭子蟹和对虾的产量和成活率及菲律宾蛤仔的成活率。在本研究条件下,虾、蟹、贝和江蓠混养的最佳配比为三疣梭子蟹6ind/m2,凡纳滨对虾45ind/m2,菲律宾蛤仔30ind/m2和菊花江蓠0.36kg/m2。     

凡纳滨对虾幼虾低盐度粗养水体养殖容量的研究

试验设计6个凡纳滨对虾幼虾养殖密度:10、20、30、40、50、60 ind/m2,通过比较60 d养殖周期内的水环境因子、幼虾生长状况与消化酶活性,分析研究凡纳滨对虾幼虾低盐度粗养水体的养殖容量。结果表明:各试验组水质均符合凡纳滨对虾幼虾生长要求,单位水体载虾量主要影响pH、非离子氨氮(NH3-Nm)、溶解氧(DO)等水化指标;不同试验组水体pH、NH3-Nm含量、DO含量、总氮(TN)含量差异极显著(P0.01),浊度、总磷(TP)含量差异显著(P0.05);不同试验组对虾中肠腺蛋白酶活性差异显著(P0.05),表现出单位水体载虾量越高,对虾体内蛋白消化酶活性和氮利用率越低的规律,DO是低盐度粗养水体养殖容量的主要限制因子。综合各项评价指标,得到本试验条件下,凡纳滨对虾幼虾低盐度粗养水体的最佳养殖容量为46.0 g/m3或32.2 g/m2(322 kg/hm2)。     

海水养殖和低盐养殖凡纳滨对虾脂肪酸分析比较

对饲喂同一种配合饲料分别在海水及低盐水体中饲养的凡纳滨对虾Litopenaeus vannamei肌肉及肝胰脏的脂肪酸组成进行了分析比较.结果显示,(1)海水养殖凡纳滨对虾肌肉中的多不饱和脂肪酸(PUFA)和总含量低于低盐养殖凡纳滨对虾(P<0.05), 而单不饱和脂肪酸(MUPA)总含量高于低盐养殖凡纳滨对虾(P<0.05), 二者的饱和脂肪酸总量(SAFA)无显著差异(P>0.05).海水养殖凡纳滨对虾肌肉中ω3系列脂肪酸总量显著低于低盐养殖对虾(P<0.05),而ω6系列脂肪酸总量显著高于低盐养殖凡纳滨对虾(P<0.05),ω3/ω6比率海水养殖凡纳滨对虾显著低于低盐养殖对虾(P<0.05).(2)海水养殖凡纳滨对虾肝胰脏中多不饱和脂肪酸(PUFA)、单不饱和脂肪酸(MUFA)和饱和脂肪酸(SAFA)总含量均显著高于低盐养殖凡纳滨对虾(P< 0.05).海水养殖凡纳滨对虾肝胰脏中ω3系列脂肪酸总量显著高于低盐养殖凡纳滨对虾(P<0.05),而ω6系列脂肪酸显著低于低盐养殖对虾(P<0.05), ω3/ω6比率海水养殖凡纳滨对虾显著高于低盐养殖对虾(P<0.05).     

凡纳滨对虾与点带石斑鱼混养的试验研究

在2.5m×4.0m×2.0m的水泥池中,按3.0×106尾/hm2的密度放养体长2~3cm、4~5cm和8~9cm的凡纳滨对虾。每种规格的对虾3口池塘,其中两口池塘在对虾养殖10d后,分别放入5尾体长为12.3~13.6cm和17.6~18.8cm的点带石斑鱼,另一池单独饲养凡纳滨对虾作为对照组。60d的养殖结果显示,单养池中2~3cm、4~5cm的凡纳滨对虾的质量增加显著低于混养(P0.05),但存活率显著高于混养(P0.05);混养时对虾规格越大增长越快,存活率越低,其中凡纳滨对虾2~3cm,点带石斑鱼17.6~18.8cm及凡纳滨对虾4~5cm,点带石班鱼17.6~18.8cm混养组合的质量增加最大(分别为7.88g与7.67g)。单养和混养时,8~9cm的凡纳滨对虾存活率差异不显著(P0.05)。方差分析显示,对虾规格与养殖模式间的交互作用对对虾质量增加和存活率影响极显著(P0.01);混养时对虾规格对鱼的质量增加影响不显著(P0.05),17.6~18.8cm的点带石斑鱼平均质量增加显著大于12.3~13.6cm(P0.05),对虾规格越大,混养鱼的捕食量越小,凡纳滨对虾8~9cm,点带石班鱼17.6~18.8cm的混养组合对虾存活率高,鱼虾的交互作用对鱼的质量增加和捕食量均有极显著影响(P0.01)。凡纳滨对虾混养的利润显著大于单养(P0.05),其中凡纳滨对虾4~5cm、点带石斑鱼17.6~18.8cm,凡纳滨对虾4~5cm、点带石斑鱼12.3~13.6cm,凡纳滨对虾2~3cm、点带石斑鱼17.6~18.8cm混养组合的利润显著高于其他混养组合(P0.05),凡纳滨对虾8~9cm的养殖利益显著低于其他规格对虾。建议凡纳滨对虾体长达到4~5cm开始混养点带石斑鱼。     

海水和淡水养殖凡纳滨对虾肌肉营养成分的比较

测定和分析了海水和淡水养殖的凡纳滨对虾肌肉常规营养成分、氨基酸含量和脂肪酸组成。试验结果表明,海水养殖凡纳滨对虾肌肉中粗蛋白和粗灰分含量显著高于淡水养殖凡纳滨对虾(P0.05),而水分的含量显著低于淡水养殖凡纳滨对虾(P0.05);海水养殖凡纳滨对虾氨基酸总量、必需氨基酸量、鲜味氨基酸总量均显著高于淡水养殖凡纳滨对虾(P0.05);海水和淡水养殖凡纳滨对虾肌肉中均含有丰富的不饱和脂肪酸,总量分别达68.48%和69.80%,但海水虾肌肉中饱和脂肪酸和单不饱和脂肪酸总量显著高于淡水虾(P0.05),而多不饱和脂肪酸总量显著低于淡水虾(P0.01);海水虾肌肉中n-3多不饱和脂肪酸总量亦显著低于淡水虾(P0.05),表明在海水环境中凡纳滨对虾合成n-3多不饱和脂肪酸的能力较差。综合分析表明,海水养殖凡纳滨对虾肌肉的营养价值和风味稍优于淡水养殖凡纳滨对虾。     

鲜活蝇蛆对凡纳滨对虾生长和免疫的影响

用等质量的蝇蛆(按湿质量计)逐级替代对虾人工配合饲料,研究对凡纳滨对虾生长性能、肌肉营养成分及免疫参数的影响.试验分为5组(A～E),以凡纳滨对虾商品饲料A作为对照组,B、C、D、E分别替代A饲料总量的25%、50%、75%、100%.每组设2个重复,每个重复放养平均质量3.8 g的凡纳滨对虾50尾,养殖42 d.试验结果表明,B、C、D组凡纳滨对虾的体质量、体长、特定生长率与对照组差异不显著(P>0.05),E组与对照组差异显著(P<0.05).肌肉蛋白质、脂肪、灰分各组间差异不显著(P>0.05),E组水分显著高于其他组(P<0.05).各处理间红细胞密度无显著差异(P>0.05).酸性磷酸酶(ACP)和碱性磷酸酶(AKP)活性随着鲜活蝇蛆比重增加呈逐渐下降态势,E组ACP与对照组差异显著(P<0.05),其他处理组与对照组间差异不显著(P>0.05);除B组AKP与对照组无显著差异外(P>0.05),其他处理组与对照组间差异显著(P<0.05).在本试验条件下,用少量鲜活蝇蛆替代对虾配合饲料是可行的.     

密度和饲料种类对凡纳滨对虾Litopenaeus vannamei争胜行为和生长的影响

在水温(23.0±1.0)℃和盐度(26±1.0)‰下,将体长(46.1±3.2)mm的凡纳滨对虾Litopenaeus vannamei饲养在0.54m×0.36m×0.28m的塑料水槽中,密度为40尾/m~2(LLD)、80尾/m~2(MLD)、120尾/m~2(MHD),和160尾/m~2(HHD),投喂人工配合饲料;同时在密度50尾/m2下分别投喂人工配合饲料(ACF)、菲律宾蛤仔肉(RPF)和冰鲜杂鱼(FFF),测定投喂前1h、投喂时、投喂后1h对虾间的争斗数量、争斗时间、争斗发起方、胜利次数等指标,以探讨密度和饲料种类对凡纳滨对虾争胜行为和生长性能的影响。61d的饲养结果表明:凡纳滨对虾个体间的争斗次数和胜利次数随密度的升高而增加;不同密度间平均优势指数差异不显著(P0.05);饱食情况下凡纳滨对虾对三种饲料的选择无倾向性。     

硒源对凡纳滨对虾生长、体组成和抗氧化能力的影响

水温(29.0±1.3)℃,将初始体质量(0.41±0.01)g的凡纳滨对虾随机分为4组,每组3个重复,饲养在室内循环水养殖系统中,投喂基础饲料(对照组)及在此饲料中分别添加硒含量相等(0.30mg/kg)的亚硒酸钠、酵母硒和蛋氨酸硒的4种饲料,养殖56d,比较了不同硒源对凡纳滨对虾生长、体组成和抗氧化能力的影响。试验结果表明,蛋氨酸硒组对虾的生长性能显著高于对照组和亚硒酸钠组(P0.05)。外源硒可显著影响对虾体蛋白和脂肪含量(P0.05),蛋氨酸硒组对虾机体营养成分显著优于其他组(P0.05)。添加外源硒组对虾的血清总抗氧化能力和谷胱甘肽过氧化物酶活性显著高于对照组(P0.05)。蛋氨酸硒组对虾血清丙二醛含量显著低于其他组(P0.05)。由此得出,饲料添加0.3mg/kg的硒能在一定程度上提高凡纳滨对虾的生长和抗氧化能力,蛋氨酸硒优于酵母硒和亚硒酸钠。     

植物乳酸杆菌对凡纳滨对虾生长、消化酶活性和肠道组织形态的影响

将体质量(8.10±0.13)g的凡纳滨对虾放养在室内500L玻璃纤维水桶中,以不同剂量(0、0.5%、1.0%和2.0%)的植物乳酸杆菌菌液(密度109 cfu/mL)拌料投喂,观察凡纳滨对虾的生长、消化酶活性和肠道组织结构的变化。结果显示,投喂15d后,0.5%和1%菌液组对虾的平均体质量、质量增加率和特定生长率均显著高于对照组(P0.05),饲料系数显著降低(P0.05);0.5%菌液组对虾肝胰腺消化酶和肠道消化酶活性显著增强(P0.05),对虾肠上皮细胞高度显著增加(P0.05)。研究结果表明,投喂0.5%植物乳酸杆菌可显著提高凡纳滨对虾消化酶活性和肠上皮细胞高度,促进对虾生长。     

饲料中添加浒苔对凡纳滨对虾生长性能和肌肉品质的影响

试验旨在探究饲料中添加浒苔(Enteromorpha prolifera)对凡纳滨对虾(Litopenaeus vannamei)生长性能和肌肉品质的影响。在鱼粉含量分别为18% (A)和15% (B)的基础饲料中添加0%、3%、6%的浒苔(记为A0、A3、A6、B0、B3、B6),共6组等氮等能饲料,饲喂初始体重为(3.0 ± 0.1) g的凡纳滨对虾8周。结果显示：A0组具有最大增重率(654.2%)和最低饲料系数(1.61),在18% 、15%鱼粉饲料中添加3%浒苔,对虾体生长性能没有显著影响(P> 0.05),添加6%浒苔, 显著降低了增重率,升高了饲料系数 (P < 0.05),降低了虾体粗脂肪含量(P < 0.05);在18%、15%鱼粉饲料中添加3%、6%浒苔,显著降低了凡纳滨对虾的蛋白质效率、蛋白质沉积率、脂肪沉积率和蛋白质消化率(P < 0.05)。在肌肉品质方面,A3、A6、B6组肌肉硬度和咀嚼性均较A0、B0组显著提高(P < 0.05);饲料中添加6%浒苔显著提高了肌肉胶原蛋白含量,降低了肌肉煮失水率和冷冻失水率(P < 0.05);以上结果表明,在凡纳滨对虾饲料中添加3%浒苔,对虾体生长性能没有显著影响,添加6%浒苔,降低了虾体增重率和饲料利用,但可改善肌肉品质。     

Association between the interannual variation in the oceanic environment and catch rates of bigeye tuna (Thunnus obesus) in the Atlantic Ocean

The environmental processes associated with variability in the catch rates of bigeye tuna in the Atlantic Ocean are largely unexplored. This study used generalized additive models (GAMs) fitted to Taiwanese longline fishery data from 1990 to 2009 and investigated the association between environmental variables and catch rates to identify the processes influencing bigeye tuna distribution in the Atlantic Ocean. The present findings reveal that the year (temporal factor), latitude and longitude (spatial factors), and major regular longline target species of albacore catches are significant for the standardization of bigeye tuna catch rates in the Atlantic Ocean. The standardized catch rates and distribution of bigeye tuna were found to be related to environmental and climatic variation. The model selection processes showed that the selected GAMs explained 70% of the cumulative deviance in the entire Atlantic Ocean. Regarding environmental factors, the depth of the 20 degree isotherm (D20) substantially contributed to the explained deviance; other important factors were sea surface temperature (SST) and sea surface height deviation (SSHD). The potential fishing grounds were observed with SSTs of 22–28°C, a D20 shallower than 150 m and negative SSHDs in the Atlantic Ocean. The higher predicted catch rates were increased in the positive northern tropical Atlantic and negative North Atlantic Oscillation events with a higher SST and shallow D20, suggesting that climatic oscillations affect the population abundance and distribution of bigeye tuna.     

Protein and amino acid composition from the mantle of juvenile O ctopus vulgaris exposed to prolonged starvation

Protein and amino acid composition of the mantle of juvenile O ctopus vulgaris (Cuvier, 1797) during fasting for 27 days were determined. Average protein content of octopus mantle was of 711.19 ± 46.80 g kg^?1 DW, and it decreased with increasing fasting days. The non‐essential amino acids content was higher (486.18 ± 11.08 g kg^?1 protein) than essential amino acids (425.82 ± 9.15 g kg^?1 protein) at the start of the experiment (unstarved animals). The results suggest that the amino acid profile of the mantle where the most abundant amino acids are Arg, His, Lys, Gly, Leu and Pro could indicate a prolonged fasting condition (>20 days) or poor nutrition of O . vulgaris. This study supports the idea of using mantle for metabolic needs of starved O . vulgaris suggesting that the degradation pathway of amino acids to pyruvate and tricarboxylic acid cycle intermediates was favoured contrary to the degradation pathway of ketogenic amino acids. Special considerations should be taken concerning Thr, Ile, Ser, Ala, Asx (Asp, Asn), Glx (Glu, Gln) (because of their fast intake) and Lys and His (due to their stable contents) during a prolonged period of fasting.     

Growth performance,digestive enzyme activity and immune response of Japanese sea bass,Lateolabrax japonicus fed with fructooligosaccharide

In this experiment, a feeding trial was performed to determine the effects of fructooligosaccharide (FOS) on growth performance, digestive enzyme activity and immune response of Japanese sea bass, Lateolabrax japonicus juveniles (initial weight 38.3 ± 0.5 g), and the fish were examined following feeding with six levels of FOS (0, 0.5, 1, 2, 4 and 6 g/kg) for 28 days. Significant enhancement of weight gain (WG) and specific growth rate (SGR) was found in fish fed 1 g/kg FOS incorporated diets (p < .05), while the feed conversion ratio (FCR) in the 1, 2 g/kg FOS groups reduced significantly compared with the control (p < .05). Besides, the crude lipid in the 4, 6 g/kg FOS groups increased significantly compared with the control (p < .05). On the other hand, the erepsin and lipase activities significantly elevated in intestine of fish fed 2 g/kg FOS (p < .05) and the lysozyme activity in serum of fish fed 2 g/kg FOS were significantly higher than that in the control (p < .05). Moreover, the alkaline phosphatase activities in serum of fish fed 0.5, 1, 2 g/kg FOS were significantly higher than in control (p < .05). Regression analysis showed that the relationships between dietary FOS levels and either SGR, FCR, erepsin or lysozyme activities were best expressed by regression equations, and the optimal inclusion levels are 1.37, 1.80, 3.06, 3.11, 1.93 and 1.80 g/kg for SGR, FCR, erepsin, lipase, lysozyme and total superoxide dismutase activities, respectively. Overall, this study revealed that FOS incorporated diets could beneficial for L. japonicus culture in terms of increasing the growth, digestion and immune activities. Under the present experimental condition, the optimal supplementary level of FOS in the diet of L. japonicus is 1–3 g/kg.     

美人鱼发光杆菌对凡纳滨对虾非特异性 免疫功能及抗病力的影响

在基础饲料中分别添加0.1%和1%美人鱼发光杆菌灭活菌、0.1%美人鱼发光杆菌活菌配制成3种免疫实验饲料,以基础饲料为空白对照组饲料,每组设3个平行样。对个体质量为(4.83±0.36)g的凡纳滨对虾进行为期20 d的饲养实验,分别在0、5、10、15和20d进行取样,以血清中的酚氧化酶(PO)、酸性磷酸酶(ACP)、碱性磷酸酶(AKP)、超氧化物歧化酶(SOD)和溶菌酶(UL)活性为免疫指标,探讨了美人鱼发光杆菌作为免疫制剂对凡纳滨对虾非特异性免疫效应的影响;在投喂免疫饲料后的第22天,按0.004 2 kg/kg体重的剂量,直接投喂对虾白斑综合征病毒(WSSV)病料,并记录累积死亡率。结果表明,美人鱼发光杆菌免疫实验组对凡纳滨对虾血清中PO、ACP、AKP、UL和SOD活性影响明显高于对照组,并且在饲料中添加美人鱼发光杆菌后,明显提高了对虾抵御WSSV感染的能力。其中0.1%美人鱼发光杆菌活菌实验组的抗病毒感染能力最强,WSSV感染14d内累计死亡率为63.3%±5.8%;而对照组为96.7%±3.3%。研究表明,美人鱼发光杆菌添加在对虾饲料中能提高凡纳滨对虾非特异性免疫水平,增强抵抗疾病的能力,将其作为对虾免疫增强剂具有良好的应用前景。     

Effects of Spirulina platensis as a feed additive on growth and coloration of blue dolphin cichlids (Cyrtocara moorii Boulunger, 1902)

This study was carried out to investigate the effects of replacing fish meal with dietary Spirulina as a feed supplement on the growth performance and coloration of blue dolphin cichlids (Cyrtocara moorii). Five isonitrogenous (47% crude protein) and isocaloric (17.36 kJ/g digestible energy) diets were for formulated to replace FM with 0, 5, 10, 15 and 20% Spirulina (designated as Control, SP5, SP10, SP15 and SP20 respectively) and fed to the fish (initial body weight, 3.15 ± 0.01 g). Fish were randomly distributed into fifteen 120 L aquariums (26.5 ± 1.00°C), 15 fish per aquarium. The diets were tested in triplicate for 12 weeks. Experimental groups were fed twice daily (09:00 and 17:00) by hand to satiation. At the end of the feeding trial, significantly (p < 0.05) higher weight gain (WG), specific growth rate (SGR), protein efficiency ratio (PER) and lower feed conversion ratio (FCR) were observed in fish fed the SP10 diet when compared to the SP20 diet. There was no significant difference in these parameters between the other groups. The skin coloration of blue dolphin cichlid fed a diet containing Spirulina meal was enhanced. The best coloration was observed in the SP15 group. These impressions were objectively validated by chemical determinations of carotenoids extracted from fish skins and passed statistical tests of significance. The study findings show that Spirulina meal does not diminish growth rates except at very high levels.     

Dietary thiamin and pyridoxine requirements of fingerling Indian major carp,Cirrhinus mrigala (Hamilton)

Two experiments were conducted to quantify the dietary thiamin (experiment I) and pyridoxine (experiment II) requirements of fingerling Cirrhinus mrigala for 16 weeks. In experiment I, dietary thiamin requirement was determined by feeding seven casein–gelatin‐based diets (400 g kg^?1 CP; 18.69 kJ g^?1 GE) with graded levels of thiamin (0, 0.5, 1, 2, 4, 8 and 16 mg kg^?1 diet) to triplicate groups of fish (6.15 ± 0.37 cm; 1.89 ± 0.12 g). Fish fed diet with 2 mg kg^?1 thiamin had highest specific growth rate (SGR), protein retention (PR), RNA/DNA ratio, haemoglobin (Hb), haematocrit (Hct), RBCs and best feed conversion ratio (FCR). However, highest liver thiamin concentration was recorded in fish fed 4 mg thiamin kg^?1 diet. Broken‐line analysis of SGR, PR and liver thiamin concentrations exhibited the thiamin requirement in the range of 1.79–3.34 mg kg^?1 diet (0.096–0.179 μg thiamin kJ^?1 gross energy). In experiment II, six casein–gelatin‐based diets (400 g kg^?1 CP; 18.69 kJ g^?1 GE) containing graded levels of pyridoxine (0, 2, 4, 6, 8 and 10 mg kg^?1 diet) were fed to triplicate groups of fish (6.35 ± 0.37 cm; 1.97 ± 0.12 g). Fish fed diet containing 6 mg kg^?1 pyridoxine showed best SGR, FCR, PR, RNA/DNA ratio, Hb, Hct and RBCs, whereas maximum liver pyridoxine concentration was recorded in fish fed 8 mg kg^?1 dietary pyridoxine. Broken‐line analysis of SGR, PR and liver pyridoxine concentrations reflected the pyridoxine requirement from 5.63 to 8.61 mg kg^?1 diet. Data generated during this study would be useful in formulating thiamin‐ and pyridoxine‐balanced feeds for the intensive culture of this fish.     

Sea bream culture in Egypt; status, constraints and potential

Marine fish farming in Egypt began in 1976 with the culture of gilthead sea bream, (Sparus aurata) as this fish was notably adaptable to brackish and marine pond conditions. Today, marine fish and shrimp farms amount to about 19,000 ha, out of which 42% is already in production while the rest, i.e., 58% is still under construction. In 1997, cultured gilthead sea bream production of 2,250 tons made up 3% of the 75,000 tons total aquaculture catch. In polyculture, usually with the grey mullet and sea bass, gilthead sea bream contributed 440 kg ha^–1 to the total yield of 1,700 kg ha^–1 (26%) over a period of 16 months. For the same period, the yield of monoculture ponds averaged 100 kg ha^–1, while in marine cages, yields ranged from 4–10 kg m³. In 1996–1997, fry of 0.25–1 g and fingerlings 1–10 g with a total of 3 million, were collected from the wild and 1 million fry were produced in the three marine hatcheries out of the four existing ones. The development of sea bream culture in Egypt is now severely inhibited by the shortage of seeds and adequate feeds. Exports of both sea bream and sea bass, during 1994–1996 averaged 1,300 tons per year.     

A Minchinia mercenariae‐like parasite infects cockles Cerastoderma edule in Galicia (NW Spain)

The cockle Cerastoderma edule fishery has traditionally been the most important shellfish species in terms of biomass in Galicia (NW Spain). In the course of a survey of the histopathological conditions affecting this species in the Ria of Arousa, a haplosporidan parasite that had not been observed in Galicia was detected in one of the most productive cockle beds of Galicia. Uni‐ and binucleate cells and multinucleate plasmodia were observed in the connective tissue mainly in the digestive area, gills and gonad. The parasite showed low prevalence, and it was not associated with abnormal cockle mortality. Molecular identification showed that this parasite was closely related to the haplosporidan Minchinia mercenariae that had been reported infecting hard clams Mercenaria mercenaria from the Atlantic coast of the United States. The molecular characterization of its SSU rDNA region allowed obtaining a fragment of 1,796 bp showing 98% homology with M. mercenariae parasite. Phylogenetic analysis supported this identification as this parasite was clustered in the same clade as M. mercenariae from the United States and other M. mercenariae‐like sequences from the UK, with bootstrap value of 99%. The occurrence of M. mercenariae‐like parasites infecting molluscs outside the United States is confirmed.     

Involvement of gonadal steroids in final oocyte maturation of white perch (Morone americana) and white bass (M. chrysops): in vivo and in vitro studies

Plasma estradiol-17 (E₂), testosterone (T), 17,20-dihydroxy-4-pregnen-3-one (DHP) and 17,20,21-tri-hydroxy-4-pregnen-3-one (20-S) levels were measured by radioimmunoassay (RIA) in white perch (Morone americana) and white bass (M. chrysops) that were induced to undergo final oocyte maturation (FOM) with human chorionic gonadotropin (hCG). Plasma DHP levels increased in females of both species in association with oocyte germinal vesicle migration (GVM) and germinal vesicle breakdown (GVBD) and decreased thereafter. Plasma 20-S levels also increased with oocyte GVM in white bass, but were several-fold lower than DHP levels. Circulating E₂ and T levels were greatest during GVM and GVBD in both species and decreased to low levels during oocyte hydration and ovulation. Follicles from white perch and white bass which received a priming injection of hCG in vivo, produced both DHP and 20-S in vitro after exposure to hCG and their oocytes underwent GVBD. Ovarian incubates from unprimed fish of either species produced only E₂ and T and their oocytes did not complete GVBD. Oocytes from unprimed bass, but not perch, matured when follicles were exposed to hCG in vitro. Both trilostane and cycloheximide blocked in vitro production of DHP and 20-S and oocyte GVBD by white perch follices. DHP and 20-S were equipotent inducers of FOM in the GVBD bioassay. None of several other structurally-related steroids tested were effective within a physiological range of concentrations. These results indicate a role for DHP and 20-S in the control of FOM in white perch and white bass.