Scopimera Research Articles

The complete mitochondrial genome of the sand bubbler crab Scopimera longidactyla Shen 1932 (Decapoda: Dotillidae) and its phylogenetic analysis

The sand bubbler crab, Scopimera longidactyla Shen, 1932 (Arthropoda: Malacostraca: Decapoda: Thoracotremata: Dotillidae), is commonly found along tropical and subtropical sandy shores of China, Korea, and Taiwan. Ecologically, it plays an important role in the productivity of sandy shores through their feeding and burrowing activities. In this study, the first complete mitochondrial genome (mitogenome) of S. longidactyla was analyzed using next-generation sequencer. Its mitogenome, circular in structure, spans 15,965 bp with a GC content of 29.97%, consisting of 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNA genes, and one putative control region. Its mitogenome arrangement and composition are identical to its two congeners, S. globosa and S. intermedia. Phylogenetic analysis fully supports for the monophyly of the genus Scopimera and the sister relationship between S. longidactyla and S. globosa. The complete mitogenome of S. longidactyla and its phylogenetic implications will provide valuable insights for further studies in phylogenetic and evolutionary biology.

Foraging in heterogeneous landscapes: variation in movement patterns of a tropical sand-bubbler crab

Abstract Acquiring food in heterogeneous landscapes presents a challenge to many foragers, as searching for food in an optimal manner is difficult in spatially and temporally variable environments. Investigating individual foraging patterns can elucidate how environmental variations at different scales constrain or select for energy-optimizing movements, which can inform conservation and management strategies by identifying spatio-temporal variations in species’ habitat use. To test how such movements vary with environmental conditions, we investigated foraging patterns of the deposit-feeding sand-bubbler crab, Scopimera intermedia Balss, 1934 at multiple spatial and temporal scales on soft sediment shores in Hong Kong. On a broad, annual, scale the crabs produced foraging tracks of different length and foraged over different areas around their burrows between hot and cool seasons. Although foraging movements of the crabs were slower and more restricted during the cool season, probably due to low environmental temperatures, foraging areas during the hot season were larger but limited by increasing conspecific interactions. Over a smaller scale at which the crabs make movement decisions, parameters such as turning angle and speed were variable, even within individual foraging excursions. Such variations appeared to be responses to small-scale variations in sediment food patches, which resulted in the crabs employing multiple movement modes. This context-dependent foraging strategy enables the crabs to feed for a longer time in food-rich patches compared with a fixed strategy and is, therefore, critical for species living in environments such as intertidal sediments, where food distribution is heterogeneous and foraging time is constrained by the tide.

Sand-bubbler crabs distinguish fiddler crab signals to predict intruders

There is growing evidence that animals gain information from heterospecifics, but utilising other species’ signals has rarely been reported in invertebrates. Herein, I conducted field experiments in a mixed colony of two crab species, Scopimera globosa and Austruca lactea, to test whether S. globosa discriminate several different displays of neighbouring male A. lactea. Three types of ‘intruders’ (male or female A. lactea, or a predatory crab, Helicana japonica) were exhibited to the ‘performer’ (male A. lactea) to elicit four different actions (courtship, aggressive, or defensive displays, or evasive behaviours). The actions of the performer, but not the presence of the intruder, were exhibited to the ‘audience’ (male A. lactea or S. globosa); the behaviours of the audience were recorded to analyse whether the audience could take advantage of the actions of neighbours. The evasive behaviour of performers elicited similar rates of evasive behaviours from conspecific and heterospecific audiences. Furthermore, courtship and aggressive displays of performers elicited significantly different rates of evasive behaviours from heterospecifics. The courtship displays of male A. lactea are more intensive than its aggressive displays; therefore, the reactions of S. globosa did not simply increase with the intensity of the performers’ displays. Presumably, S. globosa recognises that courtship displays by A. lactea do not indicate undesirable circumstances. Altogether, S. globosa were able to distinguish two types of displays as well as evasive behaviours of male A. lactea, an ability that may contribute to their estimation of the risk level of approaching intruders. By using field experiments, I demonstrated that sand-bubbler crabs distinguished the courtship and aggressive displays of fiddler crabs and showed evasive behaviours more frequently when they see the aggressive displays. The courtship displays of the fiddler crabs are more intensive than its aggressive displays; therefore, the reactions of the sand-bubbler crabs did not simply increase with the intensity of the performers’ displays. The displays of fiddler crabs were performed to other crabs (male or female fiddler crab, or a predatory crab) but not to the sand-bubbler crab; therefore, the sand bubbler crabs may have eavesdropped on heterospecific signals and utilised the heterospecific signals to predict the risk level of approaching intruders. To the best of my knowledge, distinguishing several signals of other species has been found only in some species of mammals, birds, or reptiles.

The Variation on Sediment Reworking with Influencing Factors by a Sand Bubbler Crab, Scopimera globosa, in Intertidal Sediments of the Anmyeon Island, Korea

Though biological and ecological characteristics of Scopimera globosa have been intensively investigated, little has been understood on bioturbation, especially sediment reworking. This study was designed to evaluate variation on sediment reworking of S. globosa based on feeding pellet production (FP) and burrowing pellet production (BP) with influencing factors and estimating the chlorophyll content reduction within the surface sediment by its feeding. The FP and BP largely fluctuated according to chlorophyll a concentration and crab density, but both were not influenced by temperature. The FP was enhanced by chlorophyll a concentration, whereas both FP and BP were restricted by crab density. The daily individual production was highest in spring, followed by fall and summer, with values of 25.61, 20.70 and 3.90 g ind.−1 d−1, respectively, while the total daily production was highest in fall, followed by summer and spring 2150, 1660 and 660 g m−2 d−1, respectively. The daily sediment reworking based on the FP and BP of Scopimera was highest in fall, followed by summer and spring, with values of 1.91, 1.70 and 0.77 mm d-1 and the annual sediment reworking rate of this species was calculated 40 cm year−1 based on its density in this study area. The chlorophyll a reduction ratio was estimated from 11 to 24% in one day by its feeding. These results imply that the sediment reworking of S. globosa is regulated by food abundance and its density, and Scopimera is an important bioturbator, greatly influencing biogeochemical changes in the intertidal sediments.

Growth and population biology of the sand-bubbler crab Scopimera crabricauda Alcock 1900 (Brachyura: Dotillidae) from the Persian Gulf, Iran

BackgroundDotillid crabs are introduced as one common dwellers of sandy shores. We studied the ecology and growth of the sand bubbler crab Scopimera crabricauda Alcock, 1900, in the Persian Gulf, Iran. Crabs were sampled monthly by excavating nine quadrats at three intertidal levels during spring low tides from January 2016 to January 2017.ResultsPopulation data show unimodal size-frequency distributions in both sexes. The Von Bertalanffy function was calculated at CWt = 8.76 [1 − exp (− 0.56 (t + 0.39))], CWt = 7.90 [1 − exp (− 0.59 (t + 0.40))] and CWt = 9.35 [1 − exp (− 0.57 (t + 0.41))] for males, females, and both sexes, respectively. The life span appeared to be 5.35, 5.07, and 5.26 years for males, females, and both sexes, respectively. The cohorts were identified as two age continuous groups, with the mean model carapace width 5.39 and 7.11 mm for both sexes. The natural mortality (M) coefficients stood at 1.72 for males, 1.83 for females, and 1.76 years−1 for both sexes, respectively. The overall sex ratio (1:0.4) was significantly different from the expected 1:1 proportion with male-biased. Recruitment occurred with the highest number of annual pulse once a year during the summer.ConclusionsThe results, which show slow growth, emphasize the necessity of proper management for the survival of the stock of S. crabricauda on the Iranian coast of the Persian Gulf.

Behavioural plasticity in the monsoonal tropics: implications for thermoregulatory traits in sandy shore crabs

Behavioural time budgets of organisms reflect their reaction norms to environmental conditions and, as such, indicate their behavioural plasticity. In environments which limit the activity time of organisms, efficient organization of behaviours is critical to enhancing performance to achieve necessary functions and minimize time and/or energetic costs. We investigated variations in behavioural time budgets of a common deposit-feeding crab, Scopimera intermedia, to the spatio-temporal scales of environmental changes experienced on sandy shores in the monsoonal tropics, where activity times are restricted by the tides. Activity levels of the crabs varied strongly with air temperature, with crabs emerging from the sediments only when air temperature exceeded ~ 20 °C. Behavioural water uptake increased in frequency with both feeding rate and environmental temperature, and was often followed by waving displays. Water uptake and waving, therefore, appear to be multifunctional behaviours, as not only do the crabs replenish water for feeding and wave their chelipeds to court females, but these behaviours can also contribute to thermoregulation, enhancing cooling when taking up water from the sediment surface. Such plastic and multifunctional behavioural traits are likely a widespread solution for survival and reproductive success in harsh and dynamic environments, where specialized traits are energetically costly and time-inefficient to optimize performance. Surviving in extreme and variable environments is challenging due to the potentially substantial energetic cost animals need to undertake in order to maximize performances under contrasting conditions. This study reveals a multitasking strategy adopted by the sand-bubbler crab to survive hot and variable temperatures within a limited activity time on tropical sandy shores. Behaviours of the crabs are strongly temperature dependent; and when active whilst the tide is out the crabs flexibly adjust their activity times to feed, stay cool and attract mates using a limited set of multifunctional behaviours. Specialized traits are therefore not necessary to achieve these critical functions and, as such, this study highlights the importance of this plastic, multitasking strategy as a common solution for animals to endure highly variable environments with minimal cost.

Osmotic response of Dotilla fenestrata (sand bubbler crab) exposed to combined water acidity and varying metal (Cd and Pb)

This study assessed the interactive effects of near-future coastal acidification in combination with varying sub lethal metal concentrations on the haemolymph osmolality of Dotilla fenestrata. Crabs were exposed to acute combination of near-future pH scenarios of estuarine systems (7.2, 7.4 and 7.6) by bubbling CO2 into holding tanks and metal concentrations (Cd = 0.50, 0.75, and 1.00 mg/l), (Pb = 6.50, 8.50 and 10.50 mg/l) and (Cd & Pb = 4.50, 5.75 and 7.00 mg/l) at 32 psu salinity and 18 °C for 96 h and compared with the control group that were acclimated in water medium (salinity 32 psu, temperature 18 °C and pH 8.1). Mean haemolymph osmolality of crabs exposed to a combination of varying pH and metal concentrations were not significantly different (ANOVA HSD: df 9; p > 0.05) from the crabs acclimated close to background water parameters. The study showed that near-future coastal pH has no significant effect on the haemolymph osmolality of the crab Dotilla exposed to sublethal concentrations of Cd and Pb at salinity level of 32 ppt.

The complete mitochondrial genome of the sand bubbler crab Scopimera globosa and its phylogenetic position

The mitochondrial genome has become commonly used for the molecular phylogenetic analysis of animals. Most phylogenetic studies on brachyurans using mitogenome sequences have indicated the paraphyly of superfamilies Grapsoidea and Ocypodoidea but taxon sampling remains limited. The phylogenetic position of Scopimera has been tested in several previous studies using nuclear and/or mitochondrial DNA sequences, but the phylogenetic relationship within the family remains to be resolved. We newly sequenced the complete mitochondrial genome of the sand bubbler crab Scopimera globosa (Ocypodoidea: Dotillidae). Scopimera globosa was clustered with Ilyoplax despite the morphological similarity between Scopimera and Dotilla. The mitochondrial gene order of S. globosa was unique, whereas that of other genera in the family was the same. These results suggest that phylogenetic analysis based on mitogenome sequences and gene order comparison would provide a more robust phylogeny of Dotillidae.

Comparative analysis of trace metal levels in the crab Dotilla fenestrata, sediments and water in Durban Bay harbour, Richards Bay harbour and Mlalazi estuary, Kwazulu-Natal, South Africa

Durban Bay and Richards Bay Harbours are the largest and most economically active shipping harbours in South Africa supporting a diversity of ecosystems and biota of ecological importance. This study assessed and compared levels of metals in selected tissues of the sand bubbler crab (Dotilla fenestrata), water and sediments from anthropogenically impacted Durban and Richards Bay Harbours with those of Mlalazi estuary, a considered pristine site due to its sheltered catchments. Metal concentrations (Cd, Cu, Pb and Zn) were investigated using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Metals concentrations in crab tissues were as follows; exoskeleton > gill > digestive gland, metal concentrations in tissues followed the order Cu > Zn > Pb > Cd. Metal levels in crabs from Durban Harbour (Cd 0.42, Cu 83.8, Pb 2.43and Zn 6.4 μg/g) were significantly higher than Richards Bay (Cd 0.22, Cu 27.7, Pb 1.23 and Zn 9.54 μg/g) and Mlalazi estuary (Cd 0.17, Cu 18.7, Pb 3.53 and Zn 6.91 μg/g). Metal levels in sediments followed the order Zn > Cu > Pb > Cd. Mlalazi had significantly elevated metal levels in sediment (Cd 6.83, Cu 35.63, Pb 33.43 and Zn 56.27 μg/g) compared to Durban Harbour (Cd 2.73, Cu 16.07, Pb 12.20 and Zn 38.70 μg/g) and Richards Bay (Cd 3.10, Cu 16.00, Pb 11.43 and Zn 26.07 μg/g). Metal concentrations in water were Cu > Zn > Pb > Cd save Mlalazi estuary with significantly higher Zn levels. Mlalazi estuary, the considered pristine site had significantly elevated metal levels in the sediments and water compared with the impacted sites; however, had significantly lower metal levels in the crabs’ tissues due to lower metal bioavailability. Factors such as high natural metal concentrations, metal speciation, sediment grain and organic matter content could possibly account for high metal concentrations without corresponding bioaccumulation and magnification in crabs from Mlalazi estuary.

On the two 'forms' of Dotilla intermedia De Man, 1888 (Crustacea: Brachyura: Dotillidae) from the Bay of Bengal, India, with description of a new species.

Two distinct morphotypes of sand bubbler crab (Dotillidae) from eastern India were assigned to Dotilla intermedia De Man, 1888, by British carcinologist, Stanley Kemp in 1919, due to similarities in the areolation of the carapace and the occurrence of these two forms in the same locality. Kemp, however, also illustrated the differences in the morphology of the male thoracic sternums, chelae, and first gonopods and, accordingly, divided D. intermedia into two subcategories which he called the "high form" and the "low form". The recent designation of a lectotype for D. intermedia, which corresponds with Kemp's "high form", makes it possible to recognize the "low form" as a distinct and new species, D. fraternalis n. sp., which is described here. Furthermore, Kemp's synonymization of Dotilla clepsydrodactylus Alcock, 1900, under Dotilla intermedia is affirmed here after examination of the surviving syntypes. A lectotype is selected to stabilize the taxonomy of this species.

Small-scale population genetic structure of the sand bubbler crab Scopimera ryukyuensis in the Ryukyu Islands, Japan.

Generally, the gene flow of marine organisms is well maintained, but some local populations of coastal species are genetically differentiated even on a small scale (genetic patchiness). Small-scale isolation can be crucial for understanding genetic diversity within a species. The present study examined the population genetic structure of the sand bubbler crab Scopimera ryukyuensis, which is endemic to the Ryukyu Islands in the northwestern Pacific. A total of 52 haplotypes of mitochondrial cytochrome c oxidase subunit I were recovered from 197 specimens collected from four islands. The haplotype and nucleotide diversities were relatively high in thecentral Ryukyus (Amami-Oshima and Okinawa Islands) with some exceptions but were low at the southern edge of the geographical distribution of the species, i.e., the southern Ryukyus (Ishigaki and Iriomote Islands). Pairwise FST analysis suggested that the gene flow of S. ryukyuensis was largely restricted. The local populations of the species are differentiated among islands, except for stations on Ishigaki Island and a station on Iriomote Island. Moreover, a clear intra-island population genetic structure was observed within Amami-Oshima and Iriomote Islands, e.g., only 20 km between stations. Small-scale isolation among local populations may be a common tendency for coastal species in the Ryukyu Islands, considering the results of previous studies on corals.

Factors shaping the macrobenthic animal assemblages in a tidal flat newly-formed by the Great East Japan Earthquake

In 2011, the Great East Japan Earthquake and tsunami struck the Pacific coast of northeastern Japan and created new intertidal habitats, which are ideal places to examine the roles of spatial and environmental variables shaping community structure since historical effects caused by resident species are precluded. In this study, therefore, we examined the relative importance of environmental and spatial variables in determining the community structure of the benthic macrofauna in the Higashi-Yachi tidal flat. During the study period, from July 2015 to June 2016, a total of 36 macrobenthic invertebrate taxa were found in the tidal flat. Among these, 14 taxa were abundant and in general significantly different in the abundance among the stations, but not among seasons. Redundancy analysis showed that most polychaetae such as Heteromastus spp. and Notomastus spp. were abundant in stations near the destroyed points of embankments, while the sand bubbler crab Scopimera globosa was abundant at stations with higher elevations. In addition, the analysis showed that 39% of the spatial and temporal variations in the community structures was explained by the examined environmental and spatial variables. The effects of spatial variables such as the elevation of ground level and distance from the destroyed parts of the embankment explained 18.7% of the variations, while the effects of temporally changing variables such as temperature and nutrient contents in the sediment and overlapping effects of these variables explained 7.4 and 12.7% of the variation, respectively. These results suggest that the macrobenthic animal community in this tidal flat was assembled by not only the environmental conditions but also the spatial configuration of the seawater flow and/or bottom elevation in the tidal flats.

Tropical sand-bubblers heading north? First discovery of Scopimera curtelsona Shen, 1936 (Crustacea: Decapoda: Dotillidae) populations in Hong Kong: possible range expansion from Hainan, China.

A sand-bubbler crab, Scopimera curtelsona Shen, 1936, previously known only from Hainan, China, is reported from Hong Kong. By searching past taxonomic records and examining local material, an increasing trend of S. curtelsona abundance was observed in comparison with another common sympatric dotillid, S. intermedia Balss, 1934, in Hong Kong since the early 2000s. This scenario is hypothesized to represent a northward range expansion of this tropical species from Hainan, China, by 500 km, coinciding with an increase in sea surface temperatures in Hong Kong, which are now at comparable levels with previous type locality temperatures. At a smaller scale, S. curtelsona was sympatric with two other dotillids: S. intermedia and Dotilla wichmanni De Man, 1892, on local sandflats, where the three species occupied different zones along the tidal gradient possibly due to the interaction between the variation in sediment characteristics and the crabs' maxillipedal setation.

To hide or to feed: an evaluation of personality traits in the sand bubbler crab, Dotilla wichmanni, when responding to environmental interference

Behaviour plays a crucial role in a species’ ability to cope with environmental challenges. However, this ability may be affected by repeatable individual differences in behaviour, a pattern described as animal personality. The consideration of animal personality is therefore essential when understanding how a species copes with its environmental stressors. For sand bubbler crabs, feeding is often disrupted by environmental interference, in the forms of predatory events and human recreational activities. How these crabs deal with such disruption is, however, not well documented. Here, we characterised the foraging and risk-taking behaviours of Dotilla wichmanni when responding to induced disruption. Whether these are personality traits and if they form part of a behavioural syndrome were also examined. We quantify both behaviours by taking four measures (two per behaviour). All behavioural measures were consistently different among individuals, suggesting that D. wichmanni exhibits personality. Results further suggest that they could cope with some environmental interference, although this is limited. Crabs did not vary the time spent hiding in burrows with each repeated disruption nor did behavioural plasticity differ between individuals. Notwithstanding these, the absence of support for a foraging–risk propensity behavioural syndrome points to possible complexity in the crabs' coping ability.

Suction-induced habitat selection in sand bubbler crabs.

We show that a decapod crustacean, the sand bubbler crab (SBC) Scopimera globosa, uses suction, which is the tension of moisture in the sediment, to select habitats at normal times and at the time of disaster events, through a range of controlled laboratory experiments and field observations at various sandflats in Japan. When SBCs are released on fields with no spatial suction gradient, their direction of movement is random. However, the situation clearly changes with increasing suction gradients, in which case the SBCs move to suitable zones for burrowing. Predictions based on suction–burrowing relationships coupled with the knowledge of geophysical state changes induced by suction dynamics are consistent with the observed formation of habitats throughout the seasons. Such suction-induced habitat selection in SBCs manifests itself in a robust way even following sudden events such as typhoons, where erosion and deposition processes distinctly alter the geomorphological profiles, as well as the states of suction, yet consistently yielding habitats at the newly formed, suitable suction environments. Repeated battles were observed in a suitable suction environment over burrows, with the competition rate more than seven times as high as that in a critical suction environment for burrowing.

Multi-objective optimization design for a sand crab-inspired compliant microgripper

A large working travel and a minimal stress are the most critical characteristics of a microgripper but they are conflicted each other. This paper develops a new efficient hybrid algorithm to solve the multi-objective optimization design for a sand bubbler crab-inspired compliant microgripper. The structure of sand bubbler crab-inspired compliant microgripper is inspired from the profile of sand bubbler crab. A surrogate-assisted multi-objective optimization is conducted by developing a hybrid approach of finite element analysis, response surface method, Kigring metamodel and multi-objective genetic algorithm. First, the data are collected by integrating the finite element analysis and response surface method. Subsequently, in the types of common surrogates, Kigring metamodel is adopted as an efficient tool to approximate the objective functions. And then, the Pareto-optimal fronts are found via the multi-objective genetic algorithm. The results indicated that the optimal results are at the displacement of 5999.9 µm and stress of 330.68 MPa. The results revealed that the optimized results are highly consistent with both the validation results. The accuracy of the surrogate models showed that the regression model is a good prediction. The proposed approach is useful tool to solve complex optimization designs.

Experience matters: context-dependent decisions explain spatial foraging patterns in the deposit-feeding crab Scopimera intermedia.

Behavioural decisions are often context-dependent, where information from immediate experience is incorporated into an individual's decision-making, particularly in complex environments. To test whether such mechanism is adopted by foragers in heterogeneous environments, we investigated the foraging behaviour of the deposit-feeding sand-bubbler crab, Scopimera intermedia An individual-based model was constructed, based on an optimal-patch selection criterion, which implicitly assumed that individuals adjust foraging decisions based on immediate past experience. The model's predictions were tested on the shore by manipulating the location of food patches, where the crab showed a strong context-dependent foraging pattern. When resources were randomly distributed, the crab responded by spending 56% of time in enriched patches compared with only 28% in the same area when patches were composed of natural sediments. Shore manipulations varying resource distribution supported the underlying principles of the model mechanism, and highlighted the benefits of such a strategy in heterogeneous environments such as intertidal sediments where food resources vary at different spatial and temporal scales. The proposed model therefore provides a mechanistic process, based on optimal foraging, to predict foraging decisions and movement patterns of animals feeding in heterogeneous landscapes.

Population structure and morphometric variation in the sand-bubbler crab Scopimera crabricauda (Brachyura: Dotillidae)

In the present study, population ecology and relationships between various morphometric characters of the sand-bubbler crab Scopimera crabricauda from the Persian Gulf (Iran) were studied. Crabs were collected monthly by excavating nine quadrats in high-density areas of open burrows at low, mid and high intertidal levels during spring low tides for one year. A total of 534 crabs was collected, of which 70% were males (and 30% females). Mean carapace width and total weight in both sexes showed significant differences. Crabs with a carapace width ranging from 5 to 7 mm were the dominant crabs in the population. The highest numbers of crabs were found in the higher intertidal area. The mean size of crabs decreased towards the sea. The aggregation of small crabs was found towards sea in female crabs. Juveniles were abundantly found from January to March whereas the sub-adults and adults were mostly found from April to January. The carapace length to carapace width relationship differed between males and females, as did the carapace width and carapace length to total weight relationships. Finally, the relationship between carapace width and weight for both sexes showed that the growth of this species is allometric.

Symbols in sand

Ao Manao Bay, a place of serene beauty just south of Prachuap Khiri Khan on Thailand's gulf coast, is the stage for a wildlife spectacle that occurs twice every day. Go quietly along its shore as the tide recedes, and you will see veritable works of art begin to form on the wet strand, produced as if out of thin air. Little balls of sand will materialize all around you, forming ever larger and more mystifying patterns. Gradually, the entire canvas of the intertidal zone will be filled by them. And if you get on all fours, keep perfectly still, and stare long and hard at the ground, you will be rewarded with the knowledge of what lies behind the appearance of these fascinating motifs: tiny sand bubbler crabs (Scopimera sp). Enjoy them and their creations all you can before the tide returns, for it will inevitably wash their labors away. Sand bubblers are indeed little crabs. The largest among them may be just a centimeter across. Some could happily sit under a lentil. Although they are air breathers, they have no lung cavity around their gills; rather, they exchange gases at membranous windows on the first segments of their legs. The vast works of sand art that these crabs produce are partly made possible by their huge numbers, but also by their rapid workmanship. With lightning speed they scrape up the surface sand from around their burrows, sift it in an upward motion between their mouthparts to extract its organic material, and in so doing produce a “bubble” of spent grains (sometimes half as large as themselves) that is dropped to the ground. Then, with a dexterous back-heel, they kick it behind them. And this they repeat every few seconds for as long as the tide is down. Sometimes they work in straight lines, sometimes in sweeping curves and spirals, but always keeping a clear track home to their burrows. Growing ever more numerous, these sand spheres and unobstructed pathways result in a fascinating landscape of shapes and designs, as inspiring as any artist's purposeful composition – at least for the hours they exist. Some human sand artists have sought ways to guarantee the continuity of their creations. Benjamin Zobel (1762–1831) developed a method of fixing sand on glued boards, and produced intriguing works such as Tiger and Crocodile that can still be seen today (http://bit.ly/2bTqYpJ). Later in the 19th century, Andrew Clemens (1857–1894) created delicately crafted works of art with colored sand in apothecary bottles (http://bit.ly/2bTrJza), building his pictures with infinite care, grain by carefully positioned grain. But like bubbler crabs, most past and present sand artists have embraced the ephemeral, their works lasting only until naturally or purposely disturbed. Practitioners of the Japanese art of bonseki have for centuries used feathers, brushes, and wooden wedges to shape scenes out of sand on darkly lacquered trays. Intended to be temporary, most are swept away by a feather stroke soon after their completion – a type of ancient, oriental Etch-a-Sketch. Performance sand artists such as Charlene Lanzel (sand-artist.com) and Didi Rodan (didisand.com) produce scenes with their hands in the sand spread over a light box, their works changing as fast as the next idea enters their heads. But ephemeral sand art must surely reach its maximum expression in the mandalas created by the Buddhist monks of Tibet. After days of painstaking work laying millions of colored sand grains and powder particles to finish complex designs, commonly representing the universe, they are ritualistically destroyed to symbolize the impermanence of all things, a central tenet of Buddhism. The sand is then placed in a vessel and poured into a river, the water carrying it away to distribute the “healing energy” it is said to contain. The artwork of bubbler crabs, with its own delicate intricacies, is similarly destroyed and washed away by the returning sea, transmitting that same message of impermanence. As for its healing effect, watching it dissolve in silence into the ocean as the moon comes up might well do wonders for a stressed mind. The art of sand bubbler crabs. M Wolsza; license: CC BY-NC 2.0

Activity and feeding of Dotilla fenestrata (Brachyura: Ocypodidae) in a warm, temperate South African estuary

The activity of and consumption of organic material by the sand-bubbler crab Dotilla fenestrata was studied over neap and spring tides on a sheltered sand bank close to the mouth of the warm, temperate Kowie Estuary, South Africa. Crabs emerged from their burrows only after the tide receded, and it was light. Time to emergence therefore varied from about 30 minutes to three hours after exposure, depending on the time of low water in the early morning vs at midday. General activity of the crab population was longer on a spring tide (about five hours) than on a neap tide (about three hours). Maximum densities of active crabs were 140 m–2 and 41 m–2 on spring and neap tides, respectively. After emergence, crabs spent 60% to 80% of their time feeding. In the Kowie Estuary, D. fenestrata produced between 7 and 12 pseudofaecal pellets, average weight 0.0358 g per pellet, per minute. These pellets had a significantly lower organic and chlorophyll a content than the substratum, and it was estimated that crabs removed about 25% of the organic content from the sediment.