Short Communication |
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Corresponding author: Hilton Entringer Jr ( hiltonentringer@hotmail.com ) Academic editor: Cássio Cardoso Pereira
© 2023 Hilton Entringer Jr, Ana Carolina Srbek-Araujo.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Entringer Jr H, Srbek-Araujo AC (2023) Habitat use, non-breeding groupings and chromatic pattern in Johngarthia lagostoma (H. Milne Edwards, 1837) (Decapoda, Gecarcinidae) in Trindade Island, South Atlantic Ocean. Neotropical Biology and Conservation 18(1): 83-95. https://doi.org/10.3897/neotropical.18.e101409
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The land crab Johngarthia lagostoma is endemic to Trindade Island, Atol das Rocas, Fernando de Noronha and Ascension Islands. The natural history of the species in non-breeding periods is little known. Therefore, here we reported the formation of non-breeding groups and evaluated the chromatic populational pattern of J. lagostoma in Trindade Island. Records were obtained between April and June 2015. The groups were characterized according to their location, terrain elevation, environmental characteristics and specimens’ behavior. The chromatic pattern was defined by the classification of individuals between yellow and purple, and the proportion of each color was compared between populational units (previously defined based on genetic differences). Non-breeding groups were recorded in four locations in Trindade Island, at altitudes < 40 m, and all of them were in locations with food resources and sediment suitable for the construction of shelters. Isolated individuals or the absence of the species were observed in the most inhospitable places, indicating that the maintenance of the species depends on portions of suitable habitat amid the currently arid matrix. Yellow individuals (96.4%) were predominant on Trindade Island and the chromatic pattern differed from the other populations. Color patterns seem to follow genetic differences between populations, and the founder effect may account for current patterns. From the data obtained, we emphasize that the maintenance of the species may depend on food, sediment suitable for shelters construction, humidity and shade. Due to the significant population decline in other regions, the need to define guidelines for the conservation of the species on Trindade Island is highlighted. In this context, the regeneration of insular vegetation and prohibiting the known anthropic consumption of individuals may represent important strategies for the maintenance of the species.
Chromatic variation, founder effect, insular environment, land crab, natural history, oceanic islands
The land crab, Johngarthia lagostoma (H. Milne Edwards, 1837), occurs in the South Atlantic and is endemic to the Brazilian archipelagos of Trindade, Fernando de Noronha and Atol das Rocas, and to the British island of Ascension (
The species has a generalist diet and, due to its complex life cycle, participates at different levels of the trophic chain, acting as prey during the marine larval stage and as a top predator in terrestrial environments when adults (
In Brazil, J. lagostoma occurs in less than 5,000 km2, with an evident reduction in its original occupied area, for example, in Fernando de Noronha due to urban growth (
Here we aim to report the formation of non-breeding groups, provide information on habitat use and evaluated the chromatic populational pattern of J. lagostoma in Trindade Island, providing subsidies for its conservation.
Trindade Island is located 1,200 km from Vitória (Espírito Santo state), in southeastern Brazil (Fig.
Records of J. lagostoma were obtained by non-systematized sampling (visualization of the specimens from non-predefined transects) between April 3 and June 20, 2015. During this period, most of the altitudinal ranges (including some peaks of the higher regions), and different environments were sampled, including most of the perimeter of the island (rocky and sandy beaches), water courses, herbaceous fields, cloud forest and buildings (“Posto Oceanográfico da Ilha da Trindade” – POIT, Trindade Island Oceanographic Outpost). The sampled places were accessed by trails, with the most remote beaches (western portion) accessed by sea with the aid of a boat (Fig.
Purple (left) and yellow (right) individuals of Johngarthia lagostoma (H. Milne Edwards, 1837) recorded in Trindade Island (A). Grouping J. lagostoma recorded in a watercourse (B), with detail of individuals in foraging activity (C). Soil exposed by intense grazing of Cyperus atlanticus Hemsl. by the species (D – lower portion), with detail of the rhizomes of consumed plants, accumulation of feces and individuals near burrows (E). Consumption of a newly hatched individual of Chelonia mydas (Linnaeus, 1758) (F). Photographs by Hilton Entringer Jr.
Groupings were also analyzed considering the color of the individuals’ cephalothorax for chromatic characterization of the population. For this, each individual was classified qualitatively, based on photography, as yellow (also considering orange individuals), purple and intermediate (predominantly yellow with large purple patches on the carapace; according to the classification proposed by
J. lagostoma was observed along most of the trails. Isolated individuals were commonly recorded in rocky environments with little vegetation or non-vegetated sites dispersed throughout the island and distributed in almost all altitudinal ranges sampled (~ 570 m of maximum sampled altitude). Groups were observed in four locations, which differed from the physiognomic arid matrix. At “Praia dos Cabritos”, visited in mid-April and late May, a group was observed on rocky and wet substrate along a watercourse next to a turtle breeding area (alt. ~ 30 m; Figs
Two groups had the chromatic variation of the individuals evaluated. The first one was located at “Praia dos Cabritos”, and from the total number of individuals counted (n = 23), 22 (95.6%) were yellow and one (4.4%) was classified as purple. In the second grouping, located in “Ponta das Tartarugas”, 21 individuals were evaluated, 20 (95.2%) of them were categorized as yellow and only one (4.8%) was purple. In total, 44 individuals were evaluated, most of them yellow (95.4%). The chromatic pattern of individuals did not differ from that observed by
Chromatic variation among the three populational units of Johngarthia lagostoma (H. Milne Edwards, 1837) in the South Atlantic Ocean.
| Populations | Number of individuals | % of individuals | |||
|---|---|---|---|---|---|
| Total | Yellow | Purple | Yellow | Purple | |
| Trindade Islanda | 84 | 81 | 3 | 96.4 | 3.6 |
| Atol das Rocasb | 1550 | 682 | 868 | 44.0 | 56.0 |
| Ascension Islandc | 1798 | 1551 | 247 | 86.3 | 13.7 |
The habitat use by J. lagostoma on Trindade Island may have been influenced by availability of food resources and more adequate habitat conditions (sediment suitable for shelters construction, humidity and shade), and these factors may also have induced the formation of groupings during non-breeding periods. In its turn, the color patterns differed between populations and can be attributed to the founder effect, highlighting genetic populational differences.
Regarding the influence of resources on the spatial organization of the species, it is noteworthy that, when the distribution of resources is irregular, it is expected that the activity of the species is concentrated in places more favorable to obtaining resources (
Habitat use and group formation by J. lagostoma on Trindade Island may have been influenced by the spatial distribution of food resources, such as sea turtle eggs and hatchlings, plants and waste discarded by humans. In our study, three of the four groupings were recorded near the main nesting beaches of C. mydas (except POIT grouping). This suggests that the use of space by J. lagostoma could be influenced by the reproductive activity of migratory species. Eggs and hatchlings of C. mydas can represent an important source of food and induce a more intense use of certain areas, with consumption of the species observed in Trindade Island, Fernando de Noronha and Atol das Rocas (
In addition to the consumption of animals, the consumption of native plants, such as C. atlanticus, C. obtusifolia and Ipomea spp., and exotic plants, such as T. catappa, was also recorded. Plant items are also consumed and may be important for other species of Johngarthia, such as omnivores and opportunists J. malpilensis, which regularly consume microalgae obtained from rocky surfaces, in addition to bird droppings and feathers (
Beyond the groupings registered in places with less anthropogenic influence, there were also groups recorded in the POIT. Areas inhabited by humans can favor the presence of J. lagostoma when they offer adequate resources and conditions for the species. On Ascension Island, the Georgetown city gardens, where the soil is irrigated and vegetation is cultivated, is frequently used by the species (
All groupings recorded on Trindade Island were located at altitudes below 40 m, while isolated individuals were observed in all altitudinal ranges, including the highest areas of the island (> 500 m). The species also occupies different altitudinal ranges on Ascension Island (max. alt. 859 m;
Since the environments of Trindade Island have been historically and profoundly altered, we suggested that the current distribution of J. lagostoma may be different from the original. The same can be proposed for the other islands, since the species is restricted to a small area in Fernando de Noronha (
The chromatic pattern differed between populations, with Trindade Island and Ascension Island more similar to each other (predominance of yellow individuals) and more different in relation to Atol das Rocas (smaller difference between the color categories, but with more purple individuals). The variation in the chromatic pattern reflects the genetic structure observed for each populational unit, and it varies between the evaluated locations (
According to the data presented, the conservation of localities with characteristics favorable to the establishment of the species, individually or in groups, is essential for the maintenance of the population of J. lagostoma on Trindade Island, a key species in the insular trophic chain. Therefore, we suggest that the maintenance of natural food resources and native vegetation, as well as stimulating plant restoration, can contribute to the maintenance of the studied population. We also highlight the need to reduce threats that contribute to populational decline, such as the anthropogenic consumption of these crabs, which should be effectively prohibited in Trindade Island. These actions can benefit the species in general, as it is restricted to a few isolated islands, many of which have had their terrestrial environment significantly impacted after human colonization.
We thank the Projeto TAMAR and the Programa de Pesquisas Científicas na Ilha da Trindade (PRO-TRINDADE) for the logistical support and accommodation on Trindade Island. We also thank Andrea Santarosa Freire for the suggestions and revision of the manuscript.