The Colonization of Land by Crustaceans

虽然有几群动物在陆地上定居，但使这成为可能的进化变化只是表面上的理解。这在很大程度上是由于难以解决特定类群内陆生分类群之间的进化关系，以及它们与水生同类的关系，这些问题使水生祖先和过渡性或“原型”陆生物种的地位都无法解决。在最成功的三种陆地分类动物——四足脊椎动物、昆虫和蛛形纲动物——的例子中，很明显，这些祖先群体的现存后代已不复存在。这极大地限制了我们对使陆地生命成为可能的适应性转变的理解。陆生等足类，通常被称为木虱或虫，可能为研究伴随水陆过渡的生理系统的进化提供了最好的“模型系统”。与其他主要陆生动物相比，它们的生活群体占据着从海洋水生到“过渡性”潮间带和完全陆生的各种栖息地。现存物种的进化关系最近通过比较解剖学和分子系统发育技术得到了相当详细的阐明，这些表明水生和潮间带物种更基础，因此与其他海洋等足类动物的关系更密切，而不是更“衍生”的陆生物种。此外，它们表明至少有三个独立的陆生习性起源。综上所述，这些因素为比较生理学研究提供了良好的基础。通过重建从海洋到完全陆地的进化谱系，并比较来自该谱系的物种的特定生理特征，有可能理解所涉及的生理转变。以这种方式建立生理变化的图景，不仅使我们能够研究伴随地球的具体变化，而且还可以比较独立的地球辐射之间的适应解决方案。研究将集中在五个主要的生理系统上。只有一种陆生等足类动物具有水蒸气吸收（WVA）能力，它允许动物在环境湿度超过约87%的情况下恢复水分。水蒸气的冷凝涉及到鳃分泌浓缩的盐溶液以产生降低的蒸汽压。解释WVA进化的一个关键问题是解开潮间带或陆生物种盐分泌的祖先功能。与WVA一样，氮排泄和离子调节都与水分平衡密切相关。陆生等足类动物以氨气的形式排出含氮废物，这是一种有趣的节水解决方案，它有可能消除同时发生的水损失。然而，氨挥发确实会引起鳃表面的酸化，而积累的酸的缓冲是该过程工作所必需的。因此，了解缓冲机制及其可能的预适应功能对于解释氨挥发如何演变至关重要。大多数陆生动物都具有有效的调节血液溶质浓度的机制，保护组织免受渗透水合作用或脱水以及由此导致的细胞体积变化。水生等足类动物能够在高（高渗透）和低（低渗透）外部盐度下调节血盐浓度。最近的研究表明，潮间带物种通过通过鳃排泄盐分来抵消因脱水而增加的血盐，就像水生物种一样。然而，完全陆生的物种不能排泄盐，因为饮食中的盐是贫乏的。一组已被证明在脱水过程中通过后肠的隔离来下调血盐。尽管在其他群体中已经证实了这一规律，但其机制仍有待进一步研究。至少有一个群体似乎已经失去了祖先的渗透调节能力。除了脱水造成的渗透浓度外，等足类动物还可能在阵雨期间面临渗透稀释，这种危险因其可渗透的角质层而加剧。尽管最近的研究表明上颌腺在产生稀尿中的作用，但消除多余水分的机制基本上尚未得到研究。其余的研究领域将集中于生殖生理学，特别是母体对胚胎环境的调节和卵子对胚胎周围液的离子调节。卵和新孵化的幼崽（蝠鲼）在一个充满液体的有袋类动物中孵化。海洋等足类动物用海水冲洗，但陆地物种要么从外部水源或血液中填充。干燥和热胁迫对土地的需求可能是母性环境调节或有袋动物早期发育阶段的高生理耐受性。最近的研究表明，陆生等足类动物的胚胎对渗透压、pH值、温度和氨的极端耐受能力极强，pH值和离子调节似乎利用了一个大大扩大的胚胎“背器官”。研究将探讨离子在该器官中的转运机制。谱系之间的比较将揭示背部器官的扩大是否是一种独特的陆地创新，并将检验其祖先功能是角质层矿化钙摄取的假设。

Although several groups of animals have colonized land, the evolutionary changes that have made this possible are only superficially understood. This is due in large part to difficulties in resolving the evolutionary relationships among terrestrial taxa within a particular group, and their relationships to aquatic counterparts, problems that leave the status of both the aquatic ancestors and transitional or 'prototypal' terrestrial species unresolved. In the case of the three most successful terrestrial taxa, the tetrapod vertebrates, the insects, and the arachnids, it is clear that living descendents of these ancestral groups no longer survive. This greatly limits our understanding of the adaptive transitions that have made terrestrial life possible. Terrestrial isopods, commonly known as woodlice or sowbugs, provide possibly the best 'model system' for studying the evolution of physiological systems accompanying the aquatic-terrestrial transition. In contrast to the other major terrestrial animals, they are represented by living groups that occupy diverse habitats from marine aquatic to 'transitional' intertidal and fully terrestrial. The evolutionary relationships of living species have recently been elucidated in considerable detail from comparative anatomy and molecular phylogenetic techniques, and these indicate that the aquatic and intertidal species are more basal, and thereby more closely related to other marine isopods, than the more 'derived' terrestrial species. Furthermore, they indicate at least three independent origins of a terrestrial habit. Taken together, these factors provide an excellent basis for comparative physiological studies. By reconstructing an evolutionary lineage from marine to fully terrestrial, and comparing specific physiological traits across species taken from that lineage, it is possible to understand the physiological transitions involved. Building a picture of the physiological changes in this way allows us not only to study specific changes that have accompanied terrestriality, but also to compare adaptive solutions among the independent terrestrial radiations. Studies will focus on five main physiological systems. Water vapor absorption (WVA) is known from only one group of terrestrial isopods and allows animals to recover water in ambient humidities exceeding approximately 87%. The condensation of water vapor involves the secretion of concentrated salt solution by the gills to generate reduced vapor pressure. A key question in explaining the evolution of WVA is to unravel the ancestral functions of salt secretion in intertidal or terrestrial species. Like WVA, both nitrogenous excretion and ion regulation are closely associated with water balance. Terrestrial isopods excrete nitrogenous waste as ammonia gas, and this presents an intriguing solution to water conservation by potentially eliminating simultaneous water losses. Ammonia volatilization does, however, cause acidification of the gill surface and buffering of accumulated acid is essential for the process to work. Understanding the buffering mechanism, and its possible preadaptive functions, is thus essential in explaining how ammonia volatilization evolved. Most terrestrial animals possess efficient mechanisms for regulating blood solute concentration, protecting the tissues against osmotic hydration or dehydration and resultant changes in cell volume. Aquatic isopods are capable of regulating blood salt concentration in both elevated (hyper-osmotic) and depressed (hypo-osmotic) external salinities. Recent studies have shown that intertidal species counteract increasing blood salinities imposed by dehydration by excreting salts across the gills, as in aquatic species. Fully terrestrial species cannot excrete salts, however, since dietary salts are impoverished. One group has been shown to down-regulate blood salts by sequestration in the hindgut during dehydration. Although regulation has been demonstrated in other groups, the mechanisms await investigation. At least one group appears to have lost the ancestral capacity for osmoregulation. As well as osmotic concentration imposed by dehydration, isopods may face osmotic dilution during rain showers, a danger exacerbated by their permeable cuticles. The mechanisms by which excess water is eliminated are essentially unstudied, though recent work indicates a role of the maxillary glands in producing dilute urine. The remaining areas of study will focus on reproductive physiology, specifically maternal regulation of the embryonic environment and ion regulation of periembryonic fluid by the eggs. The eggs and newly hatched juveniles (mancas) are brooded in a fluid-filled marsupium. Marine isopods flush this with seawater, but terrestrial species either fill it from external water sources or from the blood. Desiccation and thermal stresses on land demand either maternal regulation of the marsupial environment or high physiological tolerance in early developmental stages. Recent studies have shown extreme tolerance of osmolality, pH, temperature and ammonia extremes in embryos of terrestrial isopods, and pH and ion regulation appears to employ a greatly enlarged embryonic 'dorsal organ'. Studies will investigate ion transport mechanisms in this organ. Comparisons within lineages will reveal whether the enlargement of the dorsal organ is a unique terrestrial innovation and will test the hypothesis that its ancestral function was calcium uptake for mineralization of the cuticle.