Institute of Zoology, Department of Cytology and Evolution, University of Greifswald, Johann-Sebastian-Bach-Straße 11/12, D-17487 Greifswald, Germany.
Justus-Liebig-Universität Gießen, Fachbereich 06 Psychologie und Sportwissenschaft, Abteilung für Entwicklungspsychologie, Otto-Behaghel-Strasse 10F, D-35394 Giessen, Germany.
Front Zool. 2010 Sep 10;7:25. doi: 10.1186/1742-9994-7-25.
Several lineages within the Crustacea conquered land independently during evolution, thereby requiring physiological adaptations for a semi-terrestrial or even a fully terrestrial lifestyle. Birgus latro Linnaeus, 1767, the giant robber crab or coconut crab (Anomura, Coenobitidae), is the largest land-living arthropod and inhabits Indo-Pacific islands such as Christmas Island. B. latro has served as a model in numerous studies of physiological aspects related to the conquest of land by crustaceans. From an olfactory point of view, a transition from sea to land means that molecules need to be detected in gas phase instead of in water solution. Previous studies have provided physiological evidence that terrestrial hermit crabs (Coenobitidae) such as B. latro have a sensitive and well differentiated sense of smell. Here we analyze the brain, in particular the olfactory processing areas of B. latro, by morphological analysis followed by 3 D reconstruction and immunocytochemical studies of synaptic proteins and a neuropeptide.
The primary and secondary olfactory centers dominate the brain of B. latro and together account for ca. 40% of the neuropil volume in its brain. The paired olfactory neuropils are tripartite and composed of more than 1,000 columnar olfactory glomeruli, which are radially arranged around the periphery of the olfactory neuropils. The glomeruli are innervated ca. 90,000 local interneurons and ca. 160,000 projection neurons per side. The secondary olfactory centers, the paired hemiellipsoid neuropils, are targeted by the axons of these olfactory projection neurons. The projection neuron axonal branches make contact to ca. 250.000 interneurons (per side) associated with the hemiellipsoid neuropils. The hemiellipsoid body neuropil is organized into parallel neuropil lamellae, a design that is quite unusual for decapod crustaceans. The architecture of the optic neuropils and areas associated with antenna two suggest that B. latro has visual and mechanosensory skills that are comparable to those of marine Crustacea.
In parallel to previous behavioral findings that B. latro has aerial olfaction, our results indicate that their central olfactory pathway is indeed most prominent. Similar findings from the closely related terrestrial hermit crab Coenobita clypeatus suggest that in Coenobitidae, olfaction is a major sensory modality processed by the brain, and that for these animals, exploring the olfactory landscape is vital for survival in their terrestrial habitat. Future studies on terrestrial members of other crustacean taxa such as Isopoda, Amphipoda, Astacida, and Brachyura will shed light on how frequently the establishment of an aerial sense of olfaction evolved in Crustacea during the transition from sea to land. Amounting to ca. 1,000,000, the numbers of interneurons that analyse the olfactory input in B. latro brains surpasses that in other terrestrial arthropods, as e.g. the honeybee Apis mellifera or the moth Manduca sexta, by two orders of magnitude suggesting that B. latro in fact is a land-living arthropod that has devoted a substantial amount of nervous tissue to the sense of smell.
甲壳动物的几个谱系在进化过程中独立地征服了陆地，因此需要适应半陆地甚至完全陆地生活方式的生理适应。椰子蟹（Birgus latro Linnaeus，1767），又名巨型陆寄居蟹或强盗蟹（十足目，寄居蟹科），是最大的陆生节肢动物，栖息在印度洋-太平洋岛屿，如圣诞岛。椰子蟹一直是甲壳动物征服陆地相关生理方面研究的模型。从嗅觉的角度来看，从海洋到陆地的转变意味着需要在气相中检测分子，而不是在水溶液中检测。先前的研究提供了生理证据，表明椰子蟹等陆生寄居蟹（寄居蟹科）具有敏感而分化良好的嗅觉。在这里，我们通过形态分析，随后进行 3D 重建和突触蛋白以及神经肽的免疫细胞化学研究，分析椰子蟹（Birgus latro）的大脑，特别是嗅觉处理区。
初级和次级嗅觉中枢主宰着椰子蟹（Birgus latro）的大脑，总共占其大脑神经间质体积的约 40%。成对的嗅觉神经节是三分叉的，由 1000 多个柱状嗅觉小球组成，这些小球围绕嗅觉神经节的外围呈放射状排列。每个小球都被大约 90000 个局部中间神经元和约 160000 个投射神经元支配。这些嗅觉投射神经元的轴突靶向二级嗅觉中枢，即成对的半椭球神经节。投射神经元轴突分支与半椭球神经节（每侧）相关联的约 250000 个中间神经元接触。半椭球神经节的神经节板组织成平行的神经节板层，这种设计对于十足目甲壳动物来说是非常不寻常的。视神经节和与天线二相关的区域的结构表明，椰子蟹（Birgus latro）具有与海洋甲壳动物相当的视觉和机械感觉能力。
与之前关于椰子蟹（Birgus latro）具有空中嗅觉的行为发现一致，我们的结果表明，它们的中央嗅觉通路确实最为突出。与亲缘关系密切的陆生寄居蟹 Coenobita clypeatus 的类似发现表明，在 Coenobitidae 中，嗅觉是大脑处理的主要感觉模态，对于这些动物来说，探索嗅觉景观对于它们在陆地栖息地的生存至关重要。对其他甲壳动物类群（如等足目、端足目、十足目和短尾目）的陆生成员的未来研究将揭示甲壳动物从海洋向陆地过渡过程中空中嗅觉的建立在多大程度上经常发生。分析椰子蟹（Birgus latro）大脑中的嗅觉输入的中间神经元数量达到约 100 万个，超过了其他陆生节肢动物，如蜜蜂（Apis mellifera）或飞蛾（Manduca sexta）的数量两个数量级，这表明椰子蟹（Birgus latro）实际上是一种陆生节肢动物，它投入了大量的神经组织来感知嗅觉。
