Adult fiddler crabs Uca pugnax (Smith) enhance sediment-associated cues for molting of conspecific megalopae

Adult-associated chemical cues can stimulate settlement and metamorphosis of invertebrate larvae into habitats with an enhanced likelihood of juvenile and adult survival. For example, sediments from adult fiddler crab habitat stimulate fiddler crab megalopae to metamorphose (molt) sooner than sediments without adult cues. A similar stimulation of molting occurs after exposure to waterborne chemical cues from adult habitats and to exudates and extracts of adult crabs. We tested whether sediments from habitats without adult Uca pugnax (Smith), which do not stimulate molting of their megalopae, could become stimulatory through brief exposure to adult crabs. Sediments were collected from tidal flats at several distances (∼ 1 m, ∼ 50 m, and ∼ 5.4 km) from adult habitats, and incubated for 24 h with or without adult crabs. Molting rates of laboratory-reared megalopae exposed for 48 h to adult-conditioned sediments were compared to those for untreated controls. Sediments collected in or within 1 m of adult habitat elicited the highest molting rates, and natural sediments from 50 m and 5.4 km had little or no effect on molting. However, incubating sediments collected away from adult habitat with adult crabs produced a higher molting response, and the magnitude of the enhancement increased with distance from adult habitat. Results suggest that the chemical cues that adult crabs release are retained by sediments and consequently stimulate molting of megalopae, regardless of the nature of the sediments themselves. Lack of chemical cues may retard colonization of newly created or heavily disturbed habitats that are otherwise suitable settlement and adult habitat.     

Assessing changes in numbers and distribution of large whale entanglements in Newfoundland and Labrador,Canada1

Entanglements of large whales in commercial fisheries in Newfoundland and Labrador, Canada, have been consistently recorded since 1979, as part of a program aimed at releasing captured animals and reducing costs to fishermen. This data set represented an opportunity to identify fisheries posing particular entanglement risks to local whale populations. Data were assessed over the periods 1979–1992 and 1993–2008, corresponding to distinct phases in fisheries distribution and intensity. Between 1979 and 2008, 1,209 large whale entanglements were recorded in Newfoundland and Labrador. These were mostly humpback whales (Megaptera novaeangliae; 80%) and minke whales (Balaenoptera acutorostrata; 15%). Dramatic declines in reported inshore whale entanglement rates were observed following the 1992 moratorium on Atlantic cod (Gadus morhua) fisheries. Recently, more entanglements have been reported further offshore, largely due to expansion of fisheries targeting snow crab (Chionoecetes opilio). For all whale species, entanglement rates and associated mortality rates varied considerably in different fishing gear. Fractions of humpback and minke whales found dead in different fishing gear differed substantially, with minke whales far more likely to be found dead than humpback whales.     

Stimulus concordance and risk-assessment in hermit crabs (Coenobita clypeatus): implications for attention

Recent research has demonstrated that the topography of defensive reactions depends on factors that are extraneous to the stimulus that elicits the defensive response. For example, hermit crabs will withdraw more slowly to the approach of a simulated visual predator (i.e., the eliciting stimulus) when in the presence of a coincident acoustic stimulus. Multiple properties related to the magnitude (e.g., duration, amplitude) of the acoustic stimulus have been found to modulate the crabs' withdrawal response (Chan et al., 2010b). We demonstrate that the proximity in spatial location between a threatening visual stimulus and a potentially distracting extraneous auditory stimulus is an important determinant of anti-predator behavior in hermit crabs. We suggest that a distal relationship between the eliciting stimulus and an unrelated signal may produce greater distraction. This marks the first reported experimental evidence of this relationship in an invertebrate species.     

Reproductive function of Selenoprotein M in Chinese mitten crabs (Eriocheir sinesis)

Selenoproteins are present in all major forms of life, including eukaryotes, bacteria and archaea. In eukaryotic animals, selenoproteins often function as antioxidants, but rare or absent in other phyla, such as plants and fungi (except for the green alga Chlamydomonas). Selenoprotein M (SelM) is a selenocysteine containing protein with redox activity, which is involved in the antioxidant response. However, information remains limited about SelM physiology and function in marine invertebrates, particularly in crustaceans. Hence, we investigated the reproductive functionality of SelM in the Chinese mitten crab (Eriocheir sinensis), which is a commercially important yet disease vulnerable aquaculture species. The full-length SelM cDNA (928bp) strand was cloned by using PCR, based on an initial expressed sequence tag (EST) that was isolated from a hepatopancreatic cDNA library. The SelM cDNA contained a 390bp open reading frame (ORF) that encoded a putative 129 amino acid (aa) protein. SelM mRNA expression in E. sinensis was (a) tissue-specific, with the highest expression observed in the hepatopancreas, testis, ovaries and intestines. Based on this information, we then detected the different stages of tissue expression for SelM in the testis, ovary, and male crab hepatopancreas and hemolymph, and the enzyme activity of SelM in the testis. Overall, SelM was isolated successfully from the Chinese mitten crab, and its involvement in the regulation of reproduction during the period of rapid development in E. sinensis was confirmed.     

Melatonin: neuritogenesis and neuroprotective effects in crustacean x-organ cells

Melatonin has both neuritogenic and neuroprotective effects in mammalian cell lines such as neuroblastoma cells. The mechanisms of action include receptor-coupled processes, direct binding and modulation of calmodulin and protein kinase C, and direct scavenging of free radicals. While melatonin is produced in invertebrates and has influences on their physiology and behavior, little is known about its mechanisms of action. We studied the influence of melatonin on neuritogenesis in well-differentiated, extensively-arborized crustacean x-organ neurosecretory neurons. Melatonin significantly increased neurite area in the first 24 h of culture. The more physiological concentrations, 1 nM and 1 pM, increased area at 48 h also, whereas the pharmacological 1 μM concentration appeared to have desensitizing effects by this time. Luzindole, a vertebrate melatonin receptor antagonist, had surprising and significant agonist-like effects in these invertebrate cells. Melatonin receptors have not yet been studied in invertebrates. However, the presence of membrane-bound receptors in this population of crustacean neurons is indicated by this study. Melatonin also has significant neuroprotective effects, reversing the inhibition of neuritogenesis by 200 and 500 μM hydrogen peroxide. Because this is at least in part a direct action not requiring a receptor, melatonin's protection from oxidative stress is not surprisingly phylogenetically-conserved.     

Alternative camouflage strategies mediate predation risk among closely related co-occurring kelp crabs

Although camouflage is a common predator defense strategy across a wide variety of organisms, direct tests of the adaptive and ecological consequences of camouflage are rare. In this study, we demonstrated that closely related crabs in the family Epialtidae coexist in the same algal environment but use alternative forms of camouflage––decoration and color change––to protect themselves from predation. Decoration and color change are both plastic camouflage strategies in that they can be changed to match different habitats: decoration occurs on a short timescale (hours to days), while color change accompanies molting and occurs on longer timescales (months). We found that the species that decorated the most had the lowest magnitude of color change (Pugettia richii); the species that decorated the least showed the highest magnitude of color change (Pugettia producta), and a third species (Mimulus foliatus) was intermediate in both decoration and color change, suggesting a negative correlation in utilization of these strategies. This negative correlation between color change and decoration camouflage utilization mirrored the effectiveness of these camouflage strategies in reducing predation in different species. Color camouflage primarily reduced predation on P. producta, while decoration camouflage (but not color camouflage) reduced predation on P. richii. These results indicate there might be among-species trade-offs in utilization and/or effectiveness of these two forms of plastic camouflage, with important consequences for distribution of these species among habitats and the evolution of different camouflage strategies in this group.     

Microsatellite analysis of genetic diversity and population structure of Chinese mitten crab （Eriocheir sinensis）

Chinese mitten crab (Eriocheir sinensis) has higher commercial value as food source than any other species of Eriocheir in China.To evaluate the germplasm resources and characterize the genetic diversity and population structure of the crabs in different water systems,two stocks and two farming populations were assessed with 25 polymorphic microsallite loci available in public GenBank.Basic statistics showed that the average observed heterozygosity (Ho) amongst populations ranged from 0.5789 to 0.6824.However,a remarkable presence of inbreeding and heterozygote deficiencies were observed.To analyze population structure,pairwise FST coefficients explained only ～10.3% variability from the subdivision of mitten crab populations,the remaining variability stems from the subdivision within subpopulations.Although the four populations had slight differentiation,different allelic frequencies resulted in distinct population structures.Two stocks and one farming population were clustered together to the phylogenetic branch of Yangtze crab,with an approximate membership of 95%.Whereas,another fanning population was clustered singly to the phylogenetic branch of the Liaohe crab,with a membership of 97.1%.The tests for individual admixture showed that Yangtze crab had probably been contaminated with individuals from other water systems.Genetic relationships between populations also supported the conclusion that Yangtze crab and Liaohe crab had different gene pools in spite of the origins of the same species.     

Faunal impact on vegetation structure and ecosystem function in mangrove forests: A review

The last 20 years witnessed a real paradigm shift concerning the impact of biotic factors on ecosystem functions as well as on vegetation structure of mangrove forests. Before this small scientific revolution took place, structural aspects of mangrove forests were viewed to be the result of abiotic processes acting from the bottom-up, while, at ecosystem level, the outwelling hypothesis stated that mangroves primary production was removed via tidal action and carried to adjacent nearshore ecosystems where it fuelled detrital based food-webs. The sesarmid crabs were the first macrofaunal taxon to be considered a main actor in mangrove structuring processes, thanks to a number of studies carried out in the Indo-Pacific forests in the late 1970s and early 1980s. Following these classical papers, a number of studies on Sesarmidae feeding and burrowing ecology were carried out, which leave no doubts about the great importance of these herbivorous crabs in structuring and functioning Old world ecosystems. Although Sesarmidae are still considered very important in shaping mangrove structure and functioning, recent literature emphasizes the significance of other invertebrates. The Ocypodidae have now been shown to have the same role as Sesarmidae in terms of retention of forest products and organic matter processing in New world mangroves. In both New and Old world mangroves, crabs process large amounts of algal primary production, contribute consistently to retention of mangrove production and as ecosystem engineers, change particle size distribution and enhance soil aeration. Our understanding of the strong impact of gastropods, by means of high intake rates of mangrove products and differential consumption of propagules, has changed only recently. The role of insects must also be stressed. It is now clear that older techniques used to assess herbivory rates by insects strongly underestimate their impact, both in case of leaf eating and wood boring species and that herbivorous insects can potentially play a strong role in many aspects of mangrove ecology. Moreover, researchers only recently realized that ant–plant interactions may form an important contribution to our understanding of insect–plant dynamics in these habitats. Ants seem to be able to relieve mangroves from important herbivores such as many insects and sesarmid crabs. It thus seems likely that ants have positive effects on mangrove performance.     

Occurrence of additional Zoea-VI larvae in the mud crab, Scylla paramamosain (Estampador), reared in the laboratory

Mud crabs, Scylla spp. , are commercially important in many Indo-Pacific countries. The larval development of mud crabs has been reported previously as five zoeal and one megalopal stages. This paper reports larval rearing experiments that revealed variability in larval developmental stages in the mud crab Scylla paramamosain, one of four mud crab species. In addition to normal five zoeal stages, an alternative pathway of developing through six zoeal stages was observed for the crab. There were evidences suggested that the appearance of the additional Zoea-VI larvae was associated with unfavourable dietary conditions, including poor quality of diet, inadequate quantity of dietary supply and a period of starvation for newly hatched larvae. Based on exuviae and larval specimens, the morphology of the additional Zoea-VI larvae was described.