Coenobitidae

Abstract: Background During the evolutionary radiation of Crustacea, several lineages in this taxon convergently succeeded in meeting the physiological challenges connected to establishing a fully terrestrial life style. These physiological adaptations include the need for sensory organs of terrestrial species to function in air rather than in water. Previous behavioral and neuroethological studies have provided solid evidence that the land hermit crabs (Coenobitidae, Anomura) are a group of crustaceans that have evolved a good sense of aerial olfaction during the conquest of land. We wanted to study the central olfactory processing areas in the brains of these organisms and to that end analyzed the brain of Coenobita clypeatus (Herbst, 1791; Anomura, Coenobitidae), a fully terrestrial tropical hermit crab, by immunohistochemistry against synaptic proteins, serotonin, FMRFamide-related peptides, and glutamine synthetase.

Abstract: 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.

Abstract: The motile macrofauna of coastal and estuarine sand beaches is represented nearly exclusively by arthropods, particularly semiterrestiral marine crustaceans. The permanent residents include amphipods (Talitridae; Hurley, 1968), isopods (Tylidae; Edney, 1968), and decapod crustaceans (Ocypodidae, Grapsiade, Mictyridae, Coenobitidae; Bliss, 1968). More occasional residents are wolf spiders (Lycosidae; Papi and Tongiorgi, 1963), beetles (Carabidae, Staphylinidae, Tenebrionidae; Papi, 1955a; Pardi, 1956), and mole crickets (Gryllotalpidae; Pardi, 1956). The groups successfully populating this environment do so in spite of the deleterious effects of variable and extreme physical conditions (Pearse et al., 1928; Moore et al., 1968). These include physical stresses posed by abrasion and removal of habitable substrate, or deposition of sediment, during storms. Physiological stresses, including insolation, osmotic imbalance, drying, anoxia, drowning, and poisoning by high concentrations of hydrogen sulfide, occur more regularly as a result of heavy rain, periodic tidal inundation, and aerial exposure. Biological pressure is also considerable, since the beach inhabitants are subject to predation from the land by mammals (rodents and raccoons) and toads (Bufo), from the air by shorebirds, and from the water by fishes (sciaenids) and portunid crabs, especially Callinectes spp. (Herrnkind, 1968a,b, and unpublished observations). The environmental conditions are such that Pearse et al. (1928) commented, “A marine sandy beach seems like an inhospitable place for ... animals to become established.”