This study demonstrated a clear seasonal pattern of varying site fidelity in C. guanhumi, that is most likely related to its reproduction cycle. Site fidelity is a key strategy for land crabs, where using the same site can reduce the risk of predation due to the optimized use and taking permanently possession of burrows and other shelters, and enhance the efficient long-term use of food resources in their territories. In contrast, migrating animals can completely extinguish local resources, such as in insects and herding mammals. Many fish and invertebrates define a constant home range except for seasonal reproductive migrations . The existence of seasonal migrations was observed in this study, for the first time, for this species, based on mark-recapture data.
Population size and density
This study estimated that there is a population of C. guanhumi in the study area that consists of only 1312 (± 417) individuals. Such a small, isolated, slow-growing population could be fully exterminated by fishermen within a few days if there was any failure or deactivation in the CMA surveillance system. As this mangrove is a restricted area, artisanal catch as a means of subsistence is currently close to zero, but natural mortality due to predation may be considerable.
Natural predation was also referred to by Gifford , citing raccoons as important predators of C. guanhumi.
On the other hand, in the middle of the mangroves and shrubs, there are refuges for this species which could reduce their vulnerability. Diele  observed that the low vulnerability of the mangrove crab (Ucides cordatus) to overfishing in the mangroves of Pará is due to these refuges in the middle of the extensive tangle of roots of Rhizophora mangle.
In Brazil, the National Management Plan for sustainable use of crabs pointed to a dramatic decline in the nationwide catch of C. guanhumi. In 1994, yearly catch of this species was 685 tons, while in 2007, total nationwide catch was 89.5 tons only .
One of the major threats besides overfishing is the devastation of refuges at the upper edge of mangroves by deforestation and landfills, pollution and eutrophication of the ecosystem. All these factors led this species to be recently included in the Brazilian “Official National List of Threatened Species of Endangered Species” as a “Critically Endangered” species , which was subsequently revoked after widespread protests, mainly from artisanal fishermen.
Mean densities of 2.23 ind. m−2 and 7.5 active openings m−2 found in this study are among the highest ever reported for this species or similar sized crabs, probably due to the protection from artisanal fishermen in this small, closed area. For instance, in the extensive, open-access mangroves of the Imburana peninsula, Northern Brazil, mean density of C. guanhumi was only 0.39 ind. m−2 .
Experiments I and II, seasonality and response of C. guanhumi to translocation
With experiments I and II, it was possible to conduct detailed investigations on the site fidelity of this species. In experiment I, 83% of the individuals were recaptured in the same sector of origin, similarly to experiment II, with 75% of the individuals recaptured in their home sector. It may seem trivial that in experiment II, most crabs returned to their home ranges, but moving through the territories of other aggressive crabs is far from simple, since they may actively hinder such movements. There are many other challenges for a crab that is trying to return to its home range, such as those related to the orientation through the complex labyrinth of prop roots and bushes and to the avoidance of common predators, such as humans, opossums, raccoons, monkeys, stray dogs and cats.
There was a significant difference between males and females regarding site fidelity. The number of recaptured males was 28% higher than that of females, but females presented on average 96% of fidelity to the home sector in the two experiments. These data indicate that males are more active and more prone to capture away from their home sector. Males are probably actively exploring new areas and competing for females, as observed in other land crabs. Sex-specific differences in behavior, such as habitat choice and food preference, have been described for Armases cinereum, another semi-terrestrial crab species, in coastal forests and saltmarshes on Sapelo Island, U.S.A. .
The seasonal pattern observed, with less site fidelity in October, is probably related to the beginning of the reproductive period of C. guanhumi, with the increase of temperature and the decrease of the winter rains, where the individuals begin their migration looking for mating partners. Previous studies [48, 49], which used tags in fish, found different movement patterns between species and between individuals of the same species, where some individuals of exploited species showed fidelity to the site, while others traveled long distances.
Gifford  observed greater activity in C. guanhumi in the reproductive period, in the months of October and November. In the state of Ceará (Brazil), Shinozaki-Mendes  did not find any reproductive activity in March for C. guanhumi similarly to the result found in this study, if we consider the decrease of migration events for this month.
Many decapod crustaceans use different habitat types throughout their life cycle [51, 52]. Different processes can influence the choice of a site. Few studies have tracked decapods for long periods in order to observe changes in land use.
However, some studies that used a marking method, such as Rodríguez-Fourquet and Sabat  that evaluated the impact of capture on the abundance, survival and demographics of C. guanhumi and Forsee and Albrecht , who estimated the population of C. guanhumi, both in Puerto Rico, reported at least one recapture event at the same first catch location. In this work, fidelity to the sites was evident, 77% of the individuals were recaptured in their area of origin, and the remaining individuals were recaptured in adjacent areas.
Diel cycle and behavior
The level of low activity of C. guanhumi during the night period observed in this study agrees well with previous studies [18, 53, 54] that indicated the prevalence of C. guanhumi individuals collected mainly during the hours of the early morning and near sunset. The dietary habits of this species observed in this study agree with the observations made by Herreid . In Florida, he noted that C. guanhumi fed on 35 different plant species. They collected the vegetation near the burrows and carried them inside, as observed in the present study. He also observed that in shaded places and away from human presence, feeding activity spread throughout the day, but in places exposed to direct sunlight, individuals became active at dawn and dusk only.
In the present study, blue land crabs were seen feeding only on leaves and fruits. No carnivorous or insectivorous feeding behaviour was observed, in contrast to Herreid , where such non-herbivorous feeding was described. High rainfall coincided with the period of smaller catches in the cooler months of the year (June and July 2015). Our data indicate that rainfall can inhibit the migration of C. guanhumi, since activity was reduced during the rainy months.
Capture and visualization of juveniles was very rare, probably since juveniles of this population shelter themselves in less accessible areas, in the low-lying core of the mangrove, where they are protected by mangrove roots.
Juveniles may be less active outside their burrows than adults and less prone to be captured. In Neohelice granulata the displacement of small individuals of was less intense than those of adult individuals due to the vulnerability to cannibalization by larger individuals .
The largest and smallest size classes were represented by males. This was also observed by Bozada and Páez , Silva and Oshiro , Shinozaki-Mendes et al. , Silva et al.  and Gil . Males of this species tend to be larger than females [34, 50, 56, 59, 60].
In Brazil, current legislation regulates the appropriate size for the capture of male C. guanhumi individuals in each state. For instance, the Normative Instruction no. 90 02/02/2006  allows only catches of individuals larger than 60.0 mm CW in the state of Pernambuco, and the capture of males only. The months from December to March are closed to capture, considered to be the reproductive period of this species. Since 2002, capture of C. guanhumi females is forbidden in the states of Ceará, Rio Grande do Norte, Paraíba, Pernambuco, Alagoas, Sergipe and Bahia.
Carapace width of the largest individual captured in the CMA mangrove was 70.0 mm, much smaller than that found by Shinozaki-Mendes et al.  in the estuary of the Jaguaribe river, Ceará State (92.2 mm) and by Silva et al.  in the Potengi estuary (94.0 mm). Overfishing has probably affected stocks of C. guanhumi in Itamaracá Island, decreasing the size of crabs on this nearshore costal island. The small average CW of 43.4 mm indicates a population unfit for capture and consumption.
During the year of study, an occurrence of ovigerous females at some moment could have been expected, but none were collected nor observed. Silva  made similar observations, with absence or rarity of ovigerous females of this species, possibly due to behavioral changes of females after the extrusion of the egg mass. Shinozaki-Mendes  obtained very few specimens of ovigerous females, attributing this rarity to a form of predator protection and energy saving. Increased timidity and reduction of activity in ovigerous females is a common feature in land crabs. For example, in N. granulata, ovigerous females were almost inactive, suggesting that activity patterns are strongly correlated with the reproductive period .
Probably, these females, once they extrude the egg mass, become more timid and less active, and thus less propitious to be captured, or possibly, there is a migration to spawning areas that are closer to the river mouths.
In brachyurans, sex ratio generally tends to be 1:1. Diele and Koch  report that for well-preserved populations, the relative abundance of males tends to be higher than of females. Hernández-Maldonado and Campos  found a sex ratio of 3:1 (males:females) of C. guanhumi at San Andrés Island. According to these authors, this island is a refuge for this species due to the lack of natural predators and there is little catch for consumption. The same was observed by Sato et al. , which found a ratio of 3:1 for males in B. latro.
In this study, sex ratio seemed to be in balance over the months, since the number of males of C. guanhumi was only slightly higher than that of females (males:females = 1.13:1). Several studies also obtained sex ratios close to unity for C. guanhumi, e.g., in Pernambuco, , Ceará , Mexico , Rio de Janeiro , Rio Grande do Norte  and São Paulo .
Patterns of burrow distribution and burrow density
Considering the density of individuals (2.43 ± 0.71 ind. m−2) and the density of burrow openings obtained in this study (7.49 ± 1.8 openings/m2), the ratio of openings/individuals estimated was 3.4:1. This would indicate that on average there are 3.4 actively maintained and inhabited burrow openings for each individual of C. guanhumi. Several authors already suggested that this species may dig burrows with various openings [14, 15, 17, 66].
In local artisanal crab harvesting, two openings that are very close to each other are sought, one is sealed with surrounding sediment and the other one is used to place the trap. This harvesting strategy also hints at the existence of multiple openings for one burrow. It was also possible to observe some abandoned burrows (absence of feces and mud) and many closed burrows, probably in the period in which these individuals perform their molt. These results clearly indicate that the ratio openings/individuals is considerably above 1:1. However, one must consider such ratios with caution, especially when obtained with different methods and temporal and spatial and scales.
The diameters of the burrows showed a distribution pattern quite different from the size distribution of C. guanhumi. There were 4 extremely small burrows, less than 30 mm in diameter, which formed only 4% of the burrows found. The occurrence of these small burrows may indicate the presence (in very low numbers) of small, cryptic C. guanhumi individuals (which would be absent in the samples due to selectivity of the traps), or possible identification errors, leading to counting burrows of other cryptic small crab species, such as Sesarma sp., Armases sp., or Uca spp. Gifford  also found several other crab species in the habitat of C. guanhumi, such as Uca spp. and Ocypode albicans (= Ocypode quadrata).
The presence of some very large burrows (even in very small numbers, N = 3 very large burrows) could be due to the existence of few, large, cryptic individuals. This is a real possibility if we consider the possibility that the diameter of the largest measure (max. burrow diameter = 152 mm), converted to the size of the individual (BD/CW = 1.46), would result in an estimated carapace width of BDmax/1.46 = 104 mm. This size would be well above the maximum crab size (CWmax = 70 mm) found, but still within the size range reported in the literature for this species.
On the other hand, several other factors, such as burrows more widely excavated than usual, or enlarged by predators (mammals) in search of food, or eroded by rainfall, can lead to erroneous measurements with exaggeratedly large burrows openings with much larger diameters than CW * 1.46. Therefore, caution is advised in the interpretation of these data, especially for unusually large burrows.
Due to the problems mentioned above and the difficulty of associating the burrows with absolute certainty to a particular species, the usefulness of this method for the study of population dynamics should be carefully evaluated, and ideally restricted to areas with a single species of brachyurans, such as the sand banks inhabited by Uca leptodactyla .
In contrast, burrow diameter data can be obtained very easily and with low impact, and may lead to new approaches and hypotheses, especially if compared with data from captured and properly identified individuals.
Other studies have already used the approach of analyzing burrow openings of C. guanhumi . In an area of environmental preservation on the coast of São Paulo, Gil  found burrow diameters between 36.4 and 155.0 mm and Oliveira-Neto et al.  found burrow diameters ranging from 12.0 to 165.0 mm and a population density of 0.43 ind. m−2, in coastal forest areas adjacent to an estuary in southern Brazil. In these two studies, C. guanhumi burrows with larger diameters were found in protected mangrove environments, suggesting that in this kind of habitat, this species can reach larger sizes.
The burrows found in the Itamaracá mangrove were usually inhabited only by a single individual of C. guanhumi, in contrast to the results Schmidt and Diele  for the mangrove crab Ucides cordatus, where burrows with up to four cohabitants were reported.
Usefulness of PIT tags
Recently, tagging with PTs was used for several species of crustaceans, such as M. rosenbergii and C. magister , P. camtschaticus [30, 31], U. cordatus , B. latro  and C. guanhumi [18, 34], but studies using this method for land crabs are still scarce. The loss of PIT tags observed in some individuals in this study did not influence our population size calculations, since all animals with heat scars were considered, and loss of heat scars was considered to be zero for this study period.
The mark-recapture approach applied in this study proved to be effective and appropriate for C. guanhumi, considering the difficulty of marking land crabs using other methods.