Fu Amy L, Hammerschlag Neil, Lauder George V, Wilga Cheryl D, Kuo Chi-Yun, Irschick Duncan J
Department of Biology, 221 Morrill Science Center, University of Massachusetts at Amherst, Amherst, Massachusetts, 01003.
Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida.
J Morphol. 2016 May;277(5):556-64. doi: 10.1002/jmor.20515. Epub 2016 Feb 11.
How morphology changes with size can have profound effects on the life history and ecology of an animal. For apex predators that can impact higher level ecosystem processes, such changes may have consequences for other species. Tiger sharks (Galeocerdo cuvier) are an apex predator in tropical seas, and, as adults, are highly migratory. However, little is known about ontogenetic changes in their body form, especially in relation to two aspects of shape that influence locomotion (caudal fin) and feeding (head shape). We captured digital images of the heads and caudal fins of live tiger sharks from Southern Florida and the Bahamas ranging in body size (hence age), and quantified shape of each using elliptical Fourier analysis. This revealed changes in the shape of the head and caudal fin of tiger sharks across ontogeny. Smaller juvenile tiger sharks show an asymmetrical tail with the dorsal (upper) lobe being substantially larger than the ventral (lower) lobe, and transition to more symmetrical tail in larger adults, although the upper lobe remains relatively larger in adults. The heads of juvenile tiger sharks are more conical, which transition to relatively broader heads over ontogeny. We interpret these changes as a result of two ecological transitions. First, adult tiger sharks can undertake extensive migrations and a more symmetrical tail could be more efficient for swimming longer distances, although we did not test this possibility. Second, adult tiger sharks expand their diet to consume larger and more diverse prey with age (turtles, mammals, and elasmobranchs), which requires substantially greater bite area and force to process. In contrast, juvenile tiger sharks consume smaller prey, such as fishes, crustaceans, and invertebrates. Our data reveal significant morphological shifts in an apex predator, which could have effects for other species that tiger sharks consume and interact with.
形态如何随体型变化会对动物的生活史和生态产生深远影响。对于能够影响更高层次生态系统过程的顶级捕食者而言，此类变化可能会对其他物种产生影响。虎鲨（居氏鼬鲨）是热带海域的顶级捕食者，成年后具有高度的洄游性。然而，人们对其体型发育过程中的形态变化知之甚少，尤其是与影响运动（尾鳍）和进食（头部形状）的两个形态方面相关的变化。我们拍摄了来自佛罗里达州南部和巴哈马群岛、体型各异（因此年龄不同）的活体虎鲨头部和尾鳍的数字图像，并使用椭圆傅里叶分析对每个部位的形状进行了量化。这揭示了虎鲨在个体发育过程中头部和尾鳍形状的变化。体型较小的幼年虎鲨尾巴不对称，背叶（上叶）明显大于腹叶（下叶），而在体型较大的成年虎鲨中则转变为更对称的尾巴，不过成年虎鲨的上叶仍然相对较大。幼年虎鲨的头部更呈圆锥形，在个体发育过程中逐渐转变为相对更宽的头部。我们将这些变化解释为两种生态转变的结果。首先，成年虎鲨能够进行广泛的洄游，更对称的尾巴可能在长距离游泳时更有效率，尽管我们没有测试这种可能性。其次，成年虎鲨随着年龄增长会扩大食谱，食用更大、更多样化的猎物（海龟、哺乳动物和板鳃亚纲鱼类），这需要更大的咬食面积和力量来处理。相比之下，幼年虎鲨食用较小的猎物，如鱼类、甲壳类动物和无脊椎动物。我们的数据揭示了顶级捕食者显著的形态转变，这可能会对虎鲨所捕食和互动的其他物种产生影响。
