Moran Amy L, Woods H Arthur
Department of Biological Sciences, Clemson University, Clemson, SC 29634 USA.
J Exp Biol. 2007 Feb;210(Pt 4):722-31. doi: 10.1242/jeb.02702.
Embryos of many marine invertebrates are encased in gelatinous masses for part or all of development. Because gel and intervening embryos retard oxygen flux, such a life-history mode profoundly affects partial pressures of metabolic gases surrounding embryos. However, little is known about relationships between egg-mass structure and the opportunities and constraints imposed on structure by metabolic gas transport. We examined the effects of four factors (temperature, embryo age, embryo density and egg-mass size) on the metabolism of egg masses using both natural egg masses of a nudibranch and artificial egg masses made from sand dollar embryos and low-melting point agarose. Both temperature and embryo age strongly affected metabolic rates of nudibranch embryos. For embryos of a given age (stage), rates of oxygen consumption roughly doubled between 12 and 21 degrees C; from early cleavage to the veliger stage, consumption rose two- to fourfold, depending on temperature. Oxygen profiles in egg masses showed that advanced embryonic age, and to a lesser extent high temperature, both led to steeper oxygen gradients into egg masses. Egg masses containing advanced embryos at 21 degrees C had very low central oxygen levels. Small-diameter artificial masses (2 mm diameter) had virtually no internal oxygen gradients regardless of embryo density or temperature, while medium (4 mm) and large diameter (10 mm) artificial masses had oxygen profiles that depended strongly and interactively on embryo density and temperature. Together, our data on natural and artificial egg masses suggest that (i) multiple factors have strong effects on metabolic rate; (ii) rates of oxygen transport are relatively invariant with temperature in simple, artificial systems but may vary more strongly with temperature in natural egg masses; and (iii) the four factors--temperature, embryo age, embryo density and egg-mass size--interact in important ways bearing on egg mass design. A simple mathematical model is developed to provide a quantitative means of estimating primary and interactive effects of the different factors. We also show that in T. diomedea the gel itself is the main barrier to oxygen transport into egg masses, and that the metabolic activity of embryos increases substantially when embryos are artificially released from the capsules that contain them within the gel mass.
许多海洋无脊椎动物的胚胎在发育的部分或全部阶段被包裹在凝胶状物质中。由于凝胶和其间的胚胎会阻碍氧气通量，这种生活史模式会深刻影响胚胎周围代谢气体的分压。然而，关于卵团结构与代谢气体运输对结构所施加的机会和限制之间的关系，我们知之甚少。我们使用海蛞蝓的天然卵团以及由海胆胚胎和低熔点琼脂糖制成的人工卵团，研究了四个因素（温度、胚胎年龄、胚胎密度和卵团大小）对卵团代谢的影响。温度和胚胎年龄都强烈影响海蛞蝓胚胎的代谢率。对于给定年龄（阶段）的胚胎，耗氧率在12至21摄氏度之间大约翻倍；从早期卵裂到担轮幼虫阶段，耗氧量增加两到四倍，具体取决于温度。卵团中的氧气分布表明，胚胎发育晚期以及在较小程度上的高温，都会导致进入卵团的氧气梯度更陡。在21摄氏度下含有发育晚期胚胎的卵团，其中心氧气水平非常低。小直径人工卵团（直径2毫米）无论胚胎密度或温度如何，几乎没有内部氧气梯度，而中等（4毫米）和大直径（10毫米）人工卵团的氧气分布则强烈且相互依赖地取决于胚胎密度和温度。总之，我们关于天然和人工卵团的数据表明：（i）多种因素对代谢率有强烈影响；（ii）在简单的人工系统中，氧气运输速率相对不受温度影响，但在天然卵团中可能随温度变化更大；（iii）温度、胚胎年龄、胚胎密度和卵团大小这四个因素以重要方式相互作用，影响卵团设计。我们开发了一个简单的数学模型，以提供一种定量方法来估计不同因素的主要和交互作用。我们还表明，在双角塔螺中，凝胶本身是氧气进入卵团的主要障碍，并且当胚胎从凝胶团中包含它们的胶囊中人工释放出来时，胚胎的代谢活性会大幅增加。
