Pelster Bernd, Wood Chris M, Speers-Roesch Ben, Driedzic William R, Almeida-Val Vera, Val Adalberto
Institut für Zoologie, Leopold-Franzens-Universität Innsbruck, Technikerstr. 25, 6020, Innsbruck, Austria,
J Comp Physiol B. 2015 Feb;185(2):225-41. doi: 10.1007/s00360-014-0879-z. Epub 2014 Dec 21.
Three closely related characids, Tambaqui (omnivore), black Piranha (carnivore), and Pacu (herbivore), all Serrasalmidae, inhabit the ion-poor, acidic Rio Negro. We compared O2-consumption and N excretion rates in vivo, and sodium, chloride, glucose, and ammonia transport characteristics of gut sac preparations in vitro. The Pacu had a significantly higher weight-specific oxygen consumption, and a lower N/Q ratio than the omnivorous Tambaqui, and a significantly lower urea-N excretion rate than the carnivorous black Piranha, suggesting N-limitation in the herbivorous Pacu. With a value of 2.62 ± 0.15, gut to fork length ratio in the Pacu was about 2.5 times higher than in the black Piranha, and 2.0 times higher than in the Tambaqui. Anterior intestinal activities of three enzymes involved in N-fixation for amino acid synthesis (glutamate dehydrogenase, glutamate-oxaloacetate transferase, and glutamate-pyruvate transferase) were generally greatest in the carnivore and lowest in the herbivore species. In all three species, sodium, chloride, glucose, and ammonia were taken up at high rates from the intestine, resulting in an isosmotic fluid flux. Comparing the area-specific fluid flux of the anterior, mid, and posterior gut sections, no difference was detected between the three sections of the Pacu, while in the Tambaqui, it was highest in the anterior section, and in the black Piranha highest in the middle section. Overall, the area-specific uptake rates for sodium, chloride, glucose, and ammonia of anterior, mid, and posterior sections were similar in all three species, indicating that there is no difference in the area-specific transport rates associated with trophic position. The net ammonia uptake flux from gut interior was not significantly different from the net ammonia efflux to the serosal fluid, so that the ammonia removed from the intestine by the mucosal epithelium was quantitatively transferred through the tissue to the serosal side in all three species. Thus, metabolic activity of gut tissue did not significantly influence the net ammonia transfer. Due to the much higher gut to fork length ratio, the overall transport capacity of the gut of the herbivorous Pacu by far exceeded the transport capacity of their carnivorous and omnivorous relatives, thus compensating for the lower digestibility and the low Na(+), Cl(-), and N-content of the plant diet. Accordingly, in order to cope with the more difficult digestible plant material and the very low nitrogen content of plants, herbivorous fish have not evolved more effective area-specific transport capacities, but rather have increased the length of the gut.
三种亲缘关系密切的脂鲤科鱼类，巨脂鲤（杂食性）、黑色食人鱼（肉食性）和帕库鱼（草食性），均为锯脂鲤科，栖息于离子含量低、呈酸性的内格罗河。我们比较了它们体内的氧气消耗和氮排泄率，以及体外肠囊制剂对钠、氯、葡萄糖和氨的转运特性。帕库鱼的体重特异性耗氧量显著高于杂食性的巨脂鲤，氮商比则较低，且尿素氮排泄率显著低于肉食性的黑色食人鱼，这表明草食性的帕库鱼存在氮限制。帕库鱼的肠长与叉长之比为2.62±0.15，约为黑色食人鱼的2.5倍，巨脂鲤的2.0倍。参与氨基酸合成固氮过程的三种酶（谷氨酸脱氢酶、谷草转氨酶和谷丙转氨酶）在前肠的活性通常在肉食性鱼类中最高，在草食性鱼类中最低。在这三种鱼类中，钠、氯、葡萄糖和氨均从前肠被高速吸收，导致等渗液体通量。比较前肠、中肠和后肠段的单位面积液体通量，帕库鱼的三段之间未检测到差异，而巨脂鲤在前肠段最高，黑色食人鱼在中肠段最高。总体而言，三种鱼类前肠、中肠和后肠段对钠、氯、葡萄糖和氨的单位面积摄取率相似，这表明与营养级相关的单位面积转运率没有差异。从肠腔内吸收的氨净通量与向浆膜液中排出的氨净通量无显著差异，因此在这三种鱼类中，黏膜上皮从肠道清除的氨在数量上通过组织转移到了浆膜侧。因此，肠道组织的代谢活性对氨的净转移没有显著影响。由于草食性帕库鱼的肠长与叉长之比高得多，其肠道的总体转运能力远远超过其肉食性和杂食性亲属，从而弥补了植物性食物消化率较低以及钠、氯和氮含量较低的不足。因此，为了应对更难消化的植物性物质和植物中极低的氮含量，草食性鱼类没有进化出更有效的单位面积转运能力，而是增加了肠道长度。
