Bilateral linkage of monomorphic and dimorphic limb sizes in fiddler crabs

In this study, the subject of whether investment in one bilateral structure was linked to investment in the homologous bilateral opposite structure was investigated. Male fiddler crabs (genus Uca, family Ocypodidae) displayed strong bilateral claw differentiation of function and size, which are used for feeding (minor claw) or display/combat (major claw). Females had similar‐sized feeding claws. Linkage between claw size was investigated by estimating the deviations from an overall fitted regression of claw length to body size. The positive correlations of the deviations of claw size for major and minor claws of males and between right and left claws of females, relative to body size, suggested a linkage in investment between one claw and the corresponding claw on the other side of the body, for both monomorphic females and dimorphic males. A signal to send resources may be effectively gated to the claw complex, suggesting that positively correlated resources are allocated to both claws. Positive correlations were also found at the interspecific level. The fiddler crab model, described here, gives access to study the linkage in symmetric and asymmetric bilateral structures in the same species with a connection to the macroevolutionary level.     

The presence of neighbors affects waving display frequency by Scopimera globosa (Decapoda, Ocypodidae)

Scopimera globosa, a small ocypodid crab, rhythmically raises and lowers its body and both chelae in a waving display when it is not interacting directly with another individual. To determine whether waving is a social signal and to deduce its possible function, we manipulated the sex and density of crabs in a field enclosure and recorded the waving frequency of males. Males with abundant female neighbors waved significantly more often than when the same males were caged with abundant male neighbors. Males caged with fewer neighbors of either sex seldom waved. Thus, males waved most in the presence of females, especially at high density, and least in the presence of other males, suggesting that waving may function in female acquisition. Received: August 18, 1998 / Accepted: October 2, 1999     

Long-term response of fishes and other fauna to restoration of former salt hay farms: multiple measures of restoration success

This synthesis brings together published and unpublished data in an evaluation of restoration of former salt hay farms to functioning salt marshes. We compared nine years of field measurements between three restored marshes (Dennis, Commercial, and Maurice River Townships) and a reference marsh (Moores Beach) in the mesohaline portion of Delaware Bay. In the process, we compared channel morphology, geomorphology, vegetation, sediment organic matter, fish assemblages, blue crabs, horseshoe crabs, benthic infauna, and diamondback terrapins. For fishes we compared structural (distribution, abundance) and functional (feeding, growth, survival, reproduction, production) aspects to evaluate the restored marshes in an Essential Fish Habitat context. Marsh vegetation and drainage density responded gradually and positively with restored marshes approximating the state of the reference marsh within the nine-year study period. The fauna responded more quickly and dramatically with most measures equal or greater in the restored marshes within the first one or two years after restoration. Differences in response time between the vegetation and the fauna imply that the faunal response was more dependent on access to the shallow intertidal marsh surface and intertidal and subtidal creeks than on characteristics of the vegetated marsh. The fishes in created subtidal creeks in restored marshes responded immediately and maintained fish assemblages similar to the reference marsh over the study period. The intertidal creek fish assemblages tended to become more like the reference marsh in the last years of the comparison. Overall, these results document the success of the restoration and how marshes function for both resident and transient fauna, especially fishes.     

An “Ethnic” Arboretum

Part I of this three-part series deals generally with sound and its pollution. Part II will move on to measurement, effects, and indoor learning activities. Outdoor activities and references will make up Part III.     

A functional analysis of the shift from gill- to lung-breathing during the evolution of land crabs (Crustacea, Decapoda)

The evolution of air-breathing in land crabs is associated with a progressive shift in the primary site of respiratory gas exchange from the diffusion-limited gills used for water-breathing, via a simple 'cutaneous' lung surface to the perfusion-limited, invaginated lung described in the mountain crab, Pseudothelphusa garmani. The reduced diffusion limitation over the lungs facilitates oxygen transfer from air to the tissues at lower ventilation rates but is associated with accumulation of carbon dioxide. A potential respiratory acidosis is buffered by the respiratory pigment haemocyanin and by elevation of haemolymph bicarbonate levels. These changes parallel those described in vertebrates but air-breathing crustaceans maintain relatively low carbon dioxide levels in the haemolymph, either by retaining an aquatic route for its elimination over the reduced gills or by blowing it off across the lung. Maintenance of low carbon dioxide levels may be associated with a limited capacity to buffer against an acidosis due to low levels of circulating haemocyanin (i.e. crustaceans lack red blood cells). This may ultimately limit their survival in air as an acidosis will reduce oxygen transport due to a marked Bohr effect on haemocyanin. The primary role of an invaginated lung may be to reduce rates of water loss in air.     

中国鲎胚胎血细胞生成的初步电镜观察

人工孵育的鲎胚,以光镜和电镜观察其血细胞发生过程。血细胞包括变形细胞和蓝细胞二系列,均可分为原始、幼稚和成熟三阶段。血细胞生成开始于胚胎的附肢原基形成阶段,变形细胞最早见于胚区,蓝细胞先出现于胚外部分。鲎胚中未发现具备组织结构的造血器官,造血系在结缔组织和血腔中进行。     

Homing in fiddler crabs (Uca lactea annulipes and Uca vomeris : Ocypodidae)

Fiddler crabs emerge from burrows on intertidal sand- and mudflats to feed during low tide. In the species studied here (Uca lactea annulipes, Uca vomeris) a crab normally wanders no more than about 1 m away from its burrow and, when frightened, dashes back along a straight line to take cover. Feeding crabs tend to move sideways, without changing orientation, along paths radiating from the burrow. When they move along circumferential paths they adjust their orientation so that one side continues to point towards the burrow. The crabs do not need to see the burrow in order to stay aligned with the home vector, and they are not misled by a dummy hole close to their own burrow unless they have come to within about 10 cm of it. The home runs of crabs end within a few centimeters of a burrow that is covered with a sheet of sandpaper and then give way to search runs, centred upon a position slightly short of the burrow location. Feeding crabs can be displaced on sandpapers and their subsequent home runs end at a position where the burrow would be, had there been no displacement. Landmarks close to the burrow do not influence the home runs of displaced crabs. Crabs that are rotated on a sheet of sandpaper, counter-turn to keep their original orientation constant. Fiddler crabs thus employ path integration with external compass information and close range visual guidance for homing. Accepted: 11 May 1998     

Freshwater crab origins—Laying Gondwana to rest

We review the available data on the phylogeny, palaeontology and divergence time estimation of primary freshwater crabs in relation to a hypothesized Gondwanan origin of these brachyurans, as postulated by some workers in recent decades. Known phylogenetic relationships within the Old World freshwater crabs do not correspond to the successive fragmentation of the Gondwana continent. This is strong evidence against an ancestral Gondwanan distribution of Afrotropical Potamonautidae and Asian-Australian Gecarcinucidae. The fossil record of freshwater crabs (no older than the Oligocene) and heterotreme brachyurans also postdate the initial break up of Gondwana. Molecular-clock based time estimates for the most common recent ancestor of freshwater crab families differ profoundly, depending on the method of calibration used, and whether freshwater or marine brachyuran fossils are used as calibration points. As such, molecular clock estimates calibrated on freshwater crab fossils favour a post-Gondwanan evolution of freshwater crabs whereas calibration based on marine brachyuran fossils date their last common ancestor before the fragmentation of Gondwana.     

Recruitment dynamics in a rainforest seedling community: context-independent impact of a keystone consumer

The influence of keystone consumers on community structure is frequently context-dependent; the same species plays a central organising role in some situations, but not others. On Christmas Island, in the Indian Ocean, a single species of omnivorous land crab, Gecarcoidea natalis, dominates the forest floor across intact rainforest. We hypothesised that this consumer plays a key role in regulating seedling recruitment and in controlling litter dynamics on the island, independent of the type of vegetation in which it occurred. To test this hypothesis, we conducted crab exclusion experiments in two forest types on the island and followed the dynamics of seedling recruitment and litter processing for six years. To determine if these effects were likely to be general across the island, we compared land crab densities and seedling abundance and diversity at ten sites across island rainforest. Surveys across island rainforest showed that seedlings of species susceptible to predation by land crabs are consistently rare. Abundance and diversity of these species were negatively correlated to red crab abundance. Although red land crabs may be important determinants of seedling recruitment to the overstorey, differences in overstorey and seedling composition at the sites suggests that recruitment of vulnerable trees still occurs at a temporal scale exceeding that of this study. These “windows” of recruitment may be related to infrequent events that reduce the effects of land crabs. Our results suggest that unlike the context dependence of most keystone consumers in continental systems, a single consumer, the red land crab, consistently controls the dynamics of seedling recruitment across this island rainforest.     

Complete Mitochondrial Genomes of Carcinoscorpius rotundicauda and Tachypleus tridentatus (Xiphosura,Arthropoda) and Implications for Chelicerate Phylogenetic Studies

Horseshoe crabs (order Xiphosura) are often referred to as an ancient order of marine chelicerates and have been considered as keystone taxa for the understanding of chelicerate evolution. However, the mitochondrial genome of this order is only available from a single species, Limulus polyphemus. In the present study, we analyzed the complete mitochondrial genomes from two Asian horseshoe crabs, Carcinoscorpius rotundicauda and Tachypleus tridentatus to offer novel data for the evolutionary relationship within Xiphosura and their position in the chelicerate phylogeny. The mitochondrial genomes of C. rotundicauda (15,033 bp) and T. tridentatus (15,006 bp) encode 13 protein-coding genes, two ribosomal RNA (rRNA) genes, and 22 transfer RNA (tRNA) genes. Overall sequences and genome structure of two Asian species were highly similar to that of Limulus polyphemus, though clear differences among three were found in the stem-loop structure of the putative control region. In the phylogenetic analysis with complete mitochondrial genomes of 43 chelicerate species, C. rotundicauda and T. tridentatus were recovered as a monophyly, while L. polyphemus solely formed an independent clade. Xiphosuran species were placed at the basal root of the tree, and major other chelicerate taxa were clustered in a single monophyly, clearly confirming that horseshoe crabs composed an ancestral taxon among chelicerates. By contrast, the phylogenetic tree without the information of Asian horseshoe crabs did not support monophyletic clustering of other chelicerates. In conclusion, our analyses may provide more robust and reliable perspective on the study of evolutionary history for chelicerates than earlier analyses with a single Atlantic species.