‘Photography’ as a useful method for estimating the density, sex-ratio and body size of the surface-active Austruca perplexa (H. Milne Edwards, 1852) (Brachyura, Ocypodidae)

Estimations of crab density, sex-ratio, and body sizes are difficult. Though the ‘burrow excavation’ method is widely used to estimate these parameters in surface-active crabs, it is destructive to crab populations. Therefore, an alternative, non-destructive method is desirable. This study compared the non-destructive ‘photography’ method with the ‘burrow excavation’ method in a fiddler crab (Austruca perplexa (H. Milne Edwards, 1852)) population. Twenty 0.25 m2 quadrats were set out and 4 photos were taken of the surface-active crabs in each quadrat. All crab burrows were then excavated, and the crabs were collected to estimate their numbers, sexes, and body and claw sizes. Afterward, the photographs were analysed to estimate the same parameters by using the GIMP program. These parameters were then compared between the two methods. The results showed that these parameters were not different between the methods compared. This study thus reveals that the ‘photography’ method could safely be used instead of the ‘burrow excavation’ method, and yield the same results.

PENGARUH KARAKTERISTIK HABITAT MANGROVE TERHADAP KEPADATAN KEPITING ( SCYLLA SERRATA ) DI PANTAI UTARA KABUPATEN DEMAK, JAWA TENGAH

Rehabilitation of mangrove forests is very important to maintain ecosystem stability and increase the productivity of mangrove forests. The success of mangrove forest rehabilitation can be seen by the density of mangrove vegetation and the increase in mangrove crabs. Mangrove crab is the main commodity for farming communities because in addition to playing an important role in the mangrove ecosystem, it also has high economic value so it needs to be developed to increase people's income. This research was conducted with the aim of knowing the density of mangrove vegetation and crab density in different planting years and the effect of mangrove habitat characteristics on crab density. The research was conducted in three different planting years, namely 2003, 2005, and 2006. Each planting year consisted of a front zone, a middle zone and a back zone. In each zone a measuring plot with a size of 5 m x 5 m was made to collect data on vegetation density, temperature, mud thickness, salinity, acidity, and dissolved oxygen (DO). Then in the 5 m x 5 m plot, a 1 m x 1 m plot was made to collect data on the number of crabs. Data analysis used regression equation and ANOVA difference test. The results showed that the highest vegetation density was in the back zone of the 2006 planting year with a density of 2160 individuals/ha, while the highest density of crabs was in the 2005 planting year with a density of 12111.11 individuals/ha. Regression analysis showed the effect of temperature, mud thickness and salinity variables on crab density with the equation Y = 3.447 - 0.044X1 - 0.009X2 - 0.230X3 with R = 0.809. In the different ANOVA test, only the thickness of the mud had a significant difference at three years of planting (0.00 < 0.05).

Patch use and departure rules by gull-billed tern Gelochelidon nilotica

The gull-billed tern Gelochelidon nilotica feeds on the fiddler crab Leptuca uruguayensis, which settles forming patches in south-eastern temperate mudflats of Argentina. Through field observations, we evaluated whether gull-billed terns used patches following the marginal value theorem (MVT). Gull-billed tern residence time in a patch was not related to crab density or travel times. The number of captures was also unrelated to crab density. Most of the times (44.6%) terns captured only one crab from each patch, and 35% of the times they left without a capture. However, crab density was lower when terns left the patches than when they arrived. This suggests that following several capture attempts by terns, crabs hide, producing a temporary decrease in their availability, forcing tern departure from the patches, which are no longer profitable. Thus, when prey availability is affected more by predator activity than by consumption, the MVT may not necessarily apply.

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.

The density of the Atlantic marsh fiddler crab (Minuca pugnax, Smith, 1870) (Decapoda: Brachyura: Ocypodidae) in its expanded range in the Gulf of Maine, USA

The Atlantic marsh fiddler crab, Minuca pugnax (Smith, 1870), is a climate migrant that recently expanded its range northward into the Gulf of Maine. We tracked the M. pugnax population within the Great Marsh, in northeastern Massachusetts, USA, since it was first detected in 2014 using burrow counts. Because burrow counts can overestimate fiddler-crab density, we used camera traps to determine the relationship between burrow densities and fiddler-crab densities in 2019. The burrow count surveys show a six-fold increase in the density of M. pugnax in the Great Marsh from 2014 to 2019. Results indicates that the fiddler-crab population in the expanded range is established and growing. Based on burrow counts, however, the density of M. pugnax in the expanded range (6 burrows m–2) remains much lower than those found in the historical range (up to 300 burrows m–2). Based on the camera traps, we determined that burrow counts overestimated fiddler-crab densities by 47% in 2019. There was, on average, one crab detected for every two burrows observed. This result suggests that estimates of densities of M. pugnax based on burrow counts should be reduced by half. Minuca pugnax is an ecosystem engineer that can influence saltmarsh functioning and the magnitude of that influence is related to its density. Our results imply that the populations of M. pugnax in the expanded range are currently having minor impacts on marshes relative to larger populations in the historical range, but their impact will increase as the populations grow.

Predicting the distribution of a portunid crab in Patagonian coastal waters

Ovalipes trimaculatus is a commercially important, cosmopolitan portunid crab. However, environmental conditions that drive its distribution have never been studied. Thus, we aimed to assess the habitat preferences of this species in northern Patagonia by developing a species distribution model. We obtained spatial quantitative data of the crab and its prey species from a benthic survey performed prior to commercial fishing in the area. We used measurements of environmental condition, biomass, richness and evenness of its prey as predictors, and modeled the density of O. trimaculatus through a delta-gamma model, and produced a map of its predicted biomass. Prey biomass, depth and substrate type were the most important variables for predicting the distribution of O. trimaculatus. The model predicts a restricted depth range, with maximum crab density at 10 m (the shallowest depth sampled) in bottom substrates of relatively fine granulometry, such as silty sand and silt. Prey biomass does not determine crab presence, but, where the crab is present, higher prey biomass correlates with higher crab abundance. Our results accurately predicted 1 of the 2 actual fishing grounds; hence, the model may be a useful tool for local fishers and managers.

Reproductive biology of the land crab Epigrapsus politus Heller, 1862 (Brachyura: Gecarcinidae) on Iriomote-jima, Japan

The breeding cycle of the land crab Epigrapsus politusHeller, 1862 (Gecarcinidae) was studied through a mark-recapture experiment on subtropical Iriomote-jima, Japan between September and November 2017. The abundance of E. politus at the cobble shore increased during the breeding season, and then decreased. Crab density was estimated to be 423.5 over 20 m2. The population showed a bimodal and a monomodal body-length frequency for males and females, respectively. Lifespan was estimated to be between one and two or three years. Ovigerous (egg bearing) females were detected from early September to late November, with the highest abundance occurring between late September and early November. The proportion of embryonic development (non-eyed and eyed eggs) of ovigerous females and the presence or absence of embryos and embryonic development of recaptured females showed that larvae were released on the days after the full moon. In comparison, egg extrusion occurred during the period of the previous full moon, i.e., eggs were incubated for four weeks. Females produced one brood within a single breeding season. The breeding ecology of E. politus was characterized as being short and occurring in a cooler season than that of other gecarcinids inhabiting the subtropical islands of the northwest Pacific.