Rawlings T A
Biol Bull. 1990 Dec;179(3):312-325. doi: 10.2307/1542323.
Intraspecific variation in the morphology of egg capsules is ideal for assessing the costs and benefits of encapsulation, yet little is known about the extent of such variation among populations of a single species. In the present study, I compared capsule morphology among three populations of the intertidal gastropod, Nucella emarginata. Significant differences were found both in capsule wall thickness and capsule strength. Mean capsule wall thickness varied as much as 25% among populations, with the dry weight of capsular cases differing accordingly. Capsule strength, measured as resistance to puncturing and squeezing forces, also varied among populations, but did not directly reflect differences in capsule wall thickness. Despite extensive variation in capsule morphology within this species, the number and size of eggs contained within capsules of equal volume did not differ significantly among populations. I also compared the type of capsule-eating predators that were present at each site. Shore crabs, Hemigrapsus spp., were abundant at all three sites; however, the predatory isopods Idotea wosnesenskii were only present at sites containing relatively thick-walled capsules. Although Hemigrapsus and Idotea were able to chew through both thick- and thin-walled capsules, laboratory experiments revealed that Idotea preferentially opened thin-walled capsules. These results suggest that variation in capsule morphology among populations of N. emarginata may, at least in part, reflect selection for the protection of embryos against predation.
卵囊形态的种内变异对于评估包囊化的成本和收益而言是理想的，但对于单一物种不同种群间这种变异的程度却知之甚少。在本研究中，我比较了潮间带腹足动物边缘扁玉螺三个种群之间的卵囊形态。在卵囊壁厚度和卵囊强度两方面均发现了显著差异。不同种群间卵囊壁的平均厚度差异高达25%，卵囊壳的干重也相应不同。卵囊强度以对穿刺力和挤压力的抵抗力来衡量，不同种群间也存在差异，但并不直接反映卵囊壁厚度的差异。尽管该物种的卵囊形态存在广泛变异，但等体积卵囊内所含卵的数量和大小在不同种群间并无显著差异。我还比较了每个地点存在的取食卵囊的捕食者类型。滨蟹属的蟹类在所有三个地点都很常见；然而，捕食性等足类动物沃氏艾氏蜾蠃蜚仅出现在卵囊壁相对较厚的地点。尽管滨蟹和艾氏蜾蠃蜚都能咬穿厚壁和薄壁卵囊，但实验室实验表明，艾氏蜾蠃蜚更倾向于打开薄壁卵囊。这些结果表明，边缘扁玉螺不同种群间卵囊形态的变异可能至少部分反映了为保护胚胎免受捕食而进行的选择。
