Sanchez Jessica L, Trexler Joel C
Department of Biological Sciences Florida International University Miami FL USA.
Ecol Evol. 2018 May 7;8(12):5977-5991. doi: 10.1002/ece3.4133. eCollection 2018 Jun.
Herbivory is thought to be an inefficient diet, but it independently evolved from carnivorous ancestors in many metazoan groups, suggesting that plant-eating is adaptive in some circumstances. In this study, we tested two hypotheses to explain the adaptive evolution of herbivory: (i) the Heterotroph Facilitation hypothesis (herbivory is adaptive because herbivores supplement their diets with heterotrophic microbes); and (ii) the Lipid Allocation hypothesis (herbivory is adaptive because algae, which have high lipid concentrations, are nutritionally similar to carnivory). We tested these hypotheses using enclosure cages placed in the Everglades and stocked with Sailfin Mollies (), a native herbivore. Using shading and phosphorus addition (P), we manipulated the heterotrophic microbe and lipid composition of colonizing epiphyton and examined the effects of varying food quality on Sailfin Molly life history. Epiphyton grown in "shade only" conditions had a 55% increase in bacterial fatty acids and 34% lower ratios of saturated + monounsaturated to polyunsaturated fatty acids relative to the other treatments. Ratio of autotroph to heterotroph biovolume varied throughout the experiment, with a 697% increase at 3 weeks and 98% decrease at 6 weeks compared to the other treatments. Gut contents revealed that fish fed selectively on epiphyton to compensate for apparent deficiencies in the available food. Fish raised in "shade only" cages experienced the highest survival, which was best explained by autotrophic biovolume and algal- and bacterial-derived fatty acids at 3 weeks (2-6× more likely than alternative models with ∆AICc > 2.00), and by percentage of bacterial fatty acids in the diet at 6 weeks (3-8× more likely than alternative models with ∆AICc > 2.00). There were no differences in fish growth among treatments. Autotrophic lipids play a role in early fish life history, but we did not find these to be the best predictors of life history later in the juvenile period. Instead, heterotrophic lipids facilitated the herbivorous diet and enhanced survival of juvenile fish in our experiment. Bacterial fatty acid content of the diet promoted herbivore survival, consistent with the Heterotroph Facilitation hypothesis. This is the first study to explicitly contrast Heterotrophic Facilitation and Lipid Allocation hypotheses for the adaptive evolution of herbivory in an aquatic system.
食草被认为是一种低效的饮食方式，但在许多后生动物群体中，它是从肉食性祖先独立进化而来的，这表明在某些情况下，以植物为食具有适应性。在本研究中，我们测试了两个假说来解释食草的适应性进化：（i）异养促进假说（食草具有适应性，因为食草动物通过异养微生物补充其饮食）；以及（ii）脂质分配假说（食草具有适应性，因为脂质浓度高的藻类在营养上与肉食相似）。我们使用放置在大沼泽地的围隔笼并投放本地食草动物帆鳍茉莉（）来测试这些假说。通过遮荫和添加磷（P），我们操纵了定殖附生植物的异养微生物和脂质组成，并研究了不同食物质量对帆鳍茉莉生活史的影响。相对于其他处理，在“仅遮荫”条件下生长的附生植物的细菌脂肪酸增加了55%，饱和+单不饱和脂肪酸与多不饱和脂肪酸的比例降低了34%。在整个实验过程中，自养生物量与异养生物量的比例有所变化，与其他处理相比，在3周时增加了697%，在6周时减少了98%。肠道内容物显示，鱼类会有选择地以附生植物为食，以弥补可获得食物中明显的不足。在“仅遮荫”笼子中饲养的鱼类存活率最高，这在3周时最好由自养生物量以及藻类和细菌衍生的脂肪酸来解释（比∆AICc>2.00的替代模型可能性高2 - 6倍），在6周时则由饮食中细菌脂肪酸的百分比来解释（比∆AICc>2.00的替代模型可能性高3 - 8倍）。各处理之间鱼类生长没有差异。自养脂质在鱼类早期生活史中起作用，但我们发现它们并非幼年期后期生活史的最佳预测指标。相反，在我们的实验中，异养脂质促进了食草性饮食并提高了幼鱼的存活率。饮食中的细菌脂肪酸含量促进了食草动物存活，这与异养促进假说一致。这是第一项明确对比异养促进和脂质分配假说对水生系统中食草适应性进化影响的研究。
