Morphological defences of plankton can include armour, spines and coloration. Spines defend from gape‐limited fish predators, while pigmentation increases visibility to fishes but defends from ultraviolet radiation (UVR). Planktonic crab larvae (zoeae) exhibit inter‐ and intraspecific variability in the lengths of defensive spines, extent of pigmentation and body size. The determinants of this variability and the relationships among these traits are largely unknown. Larvae may employ generalized defences against the dual threats of UVR and predation or specialized defences against their primary threat, with an unknown role of allometric or phylogenetic constraints. Generalization would result in longer spines compensating for the increased predation risk imposed by darker pigments, while specialization would lead to more investment in either defence from predation (long spines) or UVR (dark pigments), at the expense of the other trait. We examined (a) the relationship between spine lengths and pigmentation, (b) the scaling of spine lengths with body size, and (c) phylogenetic constraint in spine lengths, pigmentation, and body size, among and within 21 species of laboratory‐hatched and 23 species of field‐collected crab larvae from Panama and California. We found a negative relationship between spine length and pigmentation among species from laboratory and field. Within species, we found a marginally significant negative relationship among field‐collected larvae. Spine lengths showed positive allometric scaling with carapace length, while spine and carapace lengths, but not pigmentation, had significant phylogenetic signals. The negative relationship we observed between pigmentation and spine length supports our defence specialization hypothesis. Positive allometric scaling of spine lengths means larger larvae are better defended from predators, which may indicate that larvae face greater predation risk as they grow larger. Phylogenetic constraint may have arisen because related species encounter similar predation threats. Conversely, phylogenetic constraint in the evolution of spine lengths may induce convergent behaviours resulting in related species facing similar predation threats. Our results improve understanding of the evolution of the larval morphology of crabs, morphological defences in the plankton and evolutionary responses of morphology to multiple spatially segregated selective forces. A free Plain Language Summary can be found within the Supporting Information of this article.

中文翻译：

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浮游幼虫形态防御的适应性特化和约束

浮游生物的形态防御可以包括盔甲、刺和着色。棘刺可以抵御受限制的鱼类捕食者，而色素沉着增加了鱼类的可见度，但可以抵御紫外线辐射 (UVR)。浮游蟹幼虫（zoeae）在防御棘的长度、色素沉着程度和体型方面表现出种间和种内变异性。这种变异性的决定因素以及这些特征之间的关系在很大程度上是未知的。幼虫可能会针对 UVR 和捕食的双重威胁采用广义防御或针对其主要威胁的专门防御，具有异速生长或系统发育约束的未知作用。泛化将导致更长的刺，以补偿深色颜料带来的增加的捕食风险，而专业化会导致在防御捕食（长刺）或 UVR（深色色素）方面进行更多投资，而以牺牲其他特征为代价。我们检查了（a）脊柱长度和色素沉着之间的关系，（b）脊柱长度与体型的比例，以及（c）脊柱长度、色素沉着和体型的系统发育约束，在实验室孵化的 21 个物种之间和内部。和 23 种从巴拿马和加利福尼亚现场采集的蟹幼虫。我们发现实验室和野外物种的脊柱长度和色素沉着之间存在负相关。在物种内，我们发现野外采集的幼虫之间存在略微显着的负相关。脊椎长度与甲壳长度呈正异速生长，而脊椎和甲壳长度，但没有色素沉着，具有显着的系统发育信号。我们观察到的色素沉着和脊柱长度之间的负相关支持我们的防御专业化假设。脊柱长度的正异速缩放意味着较大的幼虫可以更好地抵御捕食者，这可能表明幼虫随着它们长大而面临更大的捕食风险。由于相关物种遇到类似的捕食威胁，系统发育约束可能已经出现。相反，脊柱长度进化中的系统发育约束可能会导致趋同行为，导致相关物种面临类似的捕食威胁。我们的研究结果提高了对螃蟹幼虫形态进化、浮游生物形态防御以及形态对多种空间分离选择力的进化反应的理解。