Crustacean Connections: The Roles of Crabs in Coastal Forest Ecosystems

Written by Sabrina Dookie

Crabs play a crucial role in numerous fisheries and have received considerable focus on conservation efforts through sustainable fishing practices and management strategies. Crabs serve essential functions in coastal forests such as mangrove wetlands, acting as ecosystem engineers which facilitate nutrient cycling and enhance soil health through behaviours such as burrowing and organic matter processing. Bioturbation by these organisms aerates soil, enhances nutrient availability, and facilitates sediment flushing. Their scavenging of leaf litter and organic matter recycles nutrients, influencing plant growth and community composition through seed consumption and the creation of fertile microhabitats [1].

Crabs such as fiddlers are known as ‘coastal engineers’ or ‘forest floor architects’ due to their ability to create elaborate burrows under the soil, which facilitates aeration and water flow within and around the forests (Photo: Pxhere via CC0 attribution licence)

Cardisoma guanhumi (Blue Land Crab) (Photo: Connor Long via iNaturalist – distributed under https://creativecommons.org/licenses/by-sa/4.0/) 

Research on environmental gradients in coastal systems often fails to acknowledge the significant role of crabs in wetlands and coastal forests. Crabs influence the quantity and occasionally the quality (species) of recruitment in tropical coastal forests through the excavation of burrows, collection and accumulation of leaf litter within these burrows, and predation on seeds and seedlings. They significantly impact mangrove community dynamics by converting organic nitrogen to ammonia, enhancing the decomposition of organic matter, grazing on leaf material, aerating anoxic soils through burrowing, and modifying soil microtopography by creating mounds. Furtherplay, the predation of propagules and seeds by crabs play a significant role in regulating recruitment [2]. The distribution of crabs along environmental gradients can distinctively influence mangrove seed germination or propagule growth initiation through mechanisms such as direct consumption, damage, or burial. The primary families included in the crab fauna of mangroves are Gecarcinidae, Grapsidae, and Ocypodidae. Grapsid crabs serve as the principal consumers of propagules in the Indo-West-Pacific region, while in the Atlantic, Eastern Pacific, and Caribbean, gecarcinids (e.g., Cardisoma spp.) and Ocypodids (e.g., Ucides spp.) are more significant than grapsids [3]. Additional noteworthy families include Grapsidae, Portunidae, and Varunidae.

Crab burrow architecture exhibits species-specific characteristics; however, crabs possess the ability to modify this architecture to accommodate various environmental conditions across different habitats. These crabs exhibit notable interspecific differences in burrow morphology influenced by various environmental and biological factors within their habitats. Their burrows exhibit variability in shape and size, which are influenced by biological factors, including crab species, sex, size, and reproductive stages [4]. The architecture of burrows is influenced by abiotic factors, including sediment composition, shore elevation, tidal variation, sub-surface root systems, vegetation, and human disturbances. These burrows function as a refuge from environmental challenges and provide shelter for food storage and reproduction. This represents a significant bioengineering impact which modifies the physicochemical processes within mangrove forests. Burrow functionality differs across architectures and is constructed variably by different crab species [5]. 

Top: Atlantic Mangrove Ghost Crab (Ucides cordatus) – a commonly exploited South America and the Caribbean (Photo: Luciano Bernardes via iNaturalist – distributed under https://creativecommons.org/licenses/by-nc/4.0/). Bottom: Scylla serrata (Giant mud crab) – commonly exploited in the Indo-Pacific Region (Photo: David White via iNaturalist – distributed under https://creativecommons.org/licenses/by-nc/4.0/)  

The overexploitation of mangrove crabs, widely considered a profitable trade, is fuelled by significant market demand and serves as the primary income source for thousands of small-scale fishermen globally. Consequently, unsustainable fishing practices are resulting in declining populations, reduced crab sizes, and habitat degradation. Crab exploitation significantly impacts ecological and economic systems, leading to shifts in ecosystem balance and increased population fragility. Overharvesting results in diminished catch sizes and heightened fishing pressure, whereas selective harvesting of specific individuals, such as large males, can modify population structure and impact reproductive success. Environmental factors at middle and small scales significantly influence crab density, size, and sex-ratio [6].

The overharvesting of crabs can affect their population dynamics over time, leading to ecological and economic shifts within coastal wetlands (Photo: Vengolis via Animalia – distributed under CC BY-SA).

Overharvesting of crabs frequently employs traditional methods such as traps and manual harvesting; however, the rising harvesting pressure encourages unsustainable fishing practices. To what extent do current management practices influence the sustainability of crustacean populations in coastal ecosystems? Numerous monitoring and management methods are available; however, many may be ineffective due to insufficient regulations, coordination, and enforcement over time. When traditional management practices and fisheries policies do not ensure sustainable natural resource exploitation over the long term, collaborative efforts among communities, researchers, and decision-makers may serve as an alternative to achieve the desired outcomes in the near future [7]. Establishing long-term and regular monitoring is essential for evaluating the biological aspects of the targeted crab species, including population size, dynamics, and productivity in harvests. Researchers can provide scientific information and educational practices to local gatherers and decision-makers while also integrating community members into the research process, facilitating knowledge exchange through relatable language [8].  As we move forward, it is important to continuously emphasise the importance, resilience, and adaptability of crabs in coastal wetlands and be aware that simple actions can have a ripple effect on the entire ecosystem. By supporting sustainable practices and conservation initiatives, we can help to ensure the long-term survival of these incredible crustaceans and preserve the delicate balance of our coastal ecosystems. 

References 

[1]  Wing, M., Vorsatz, L. D., Stefano Cannicci, & Not, C. (2023). The role of mangrove crabs, the key macrofaunal bioengineers, in microplastic production in tropical coastal forests. Regional Studies in Marine Science, 63, 103012–103012. https://doi.org/10.1016/j.rsma.2023.103012

[2] Egawa, R., Sharma, S., Nadaoka, K., & MacKenzie, R. A. (2021). Burrow dynamics of crabs in subtropical estuarine mangrove forest. Estuarine Coastal and Shelf Science, 252, 107244–107244. https://doi.org/10.1016/j.ecss.2021.107244

[3] Lindquist, E. S., Krauss, K. W., Green, P. T., O’Dowd, D. J., Sherman, P. M., & Smith, T. J. (2009). Land crabs as key drivers in tropical coastal forest recruitment. Biological Reviews/Biological Reviews of the Cambridge Philosophical Society, 84(2), 203–223. https://doi.org/10.1111/j.1469-185x.2008.00070.x

[4]  Sen, S., & Homechaudhuri, S. (2016). Comparative Burrow Architectures of Resident Fiddler Crabs (Ocypodidae) in Indian Sundarban Mangroves to Assess Their Suitability as Bioturbating Agents. Proceedings of the Zoological Society, 71(1), 17–24. https://doi.org/10.1007/s12595-016-0178-7

[5]  Agusto, L. E., Fratini, S., Jimenez, P. J., Quadros, A., & Cannicci, S. (2020). Structural characteristics of crab burrows in Hong Kong mangrove forests and their role in ecosystem engineering. Estuarine Coastal and Shelf Science, 248, 106973–106973. https://doi.org/10.1016/j.ecss.2020.106973

[6]  Dumas, P., M. Léopold, L. Frotté, & C. Peignon. (2011). Mud crab ecology encourages site-specific approaches to fishery management. Journal of Sea Research, 67(1), 1–9. https://doi.org/10.1016/j.seares.2011.08.003

[7]  Evans, L., Cherrett, N., & Pemsl, D. (2011). Assessing the impact of fisheries co-management interventions in developing countries: A meta-analysis. Journal of Environmental Management, 92(8), 1938–1949. https://doi.org/10.1016/j.jenvman.2011.03.010

[8]  Côrtes, L. H. de O., Zappes, C. A., & Di Beneditto, A. P. M. (2018). The crab harvest in a mangrove forest in south-eastern Brazil: Insights about its maintenance in the long-term. Perspectives in Ecology and Conservation, 16(2), 113–118. https://doi.org/10.1016/j.pecon.2018.02.002

‌Author biography

Dr. Sabrina Dookie is a lecturer and mangrove ecologist attached to the University of Guyana, Turkeyen Campus (Department of Biology). Her research interests include mangrove restoration, microbial functionality in mangroves, ecophysiological adaptions of mangroves in stressed environments, coastal and marine life management, hydroperiod and sedimentation dynamics, sustainable forestry management, and traditional ecological knowledge and its applications.