Vibrant freshwater ecosystem in Maranhão, Brazil, teeming with biodiversity and freshwater crabs.

Unveiling the Hidden World of Freshwater Crabs: A Journey Through Maranhão's Ecosystems

Beau Callahan in Science & Nature June 2025 • 4 min read.

"Discover how new research is expanding our understanding of freshwater crab distribution in Northeastern Brazil and why it matters for conservation."

Freshwater crabs, often overlooked in the grand tapestry of aquatic life, play a crucial role in maintaining the health and balance of river ecosystems. These small crustaceans contribute to nutrient cycling, control populations of other invertebrates, and serve as a food source for larger animals. However, despite their importance, our knowledge of their distribution and diversity remains incomplete, especially in regions like Northeastern Brazil.

A recent study published in 'Nauplius, The Journal of the Brazilian Crustacean Society,' sheds light on the freshwater crab populations of Maranhão, a state in Northeastern Brazil. This research not only expands the known range of several crab species but also highlights the need for increased conservation efforts in this biodiverse region. Understanding the distribution of these creatures is the first step toward protecting them and their habitats.

Maranhão, a transitional zone between the Amazon, Caatinga, and Savanna biomes, boasts a complex network of rivers and streams. This unique geography supports a rich variety of aquatic life, including freshwater crabs belonging to the family Trichodactylidae. The study focuses on this family, which includes approximately 50 species ranging from southern Mexico to Argentina.

Expanding the Known Range

Vibrant freshwater ecosystem in Maranhão, Brazil, teeming with biodiversity and freshwater crabs.

The research team, led by Kele Sousa Pires Andrade, Marina de Sá Leitão Câmara de Araújo, and Jorge Luiz Silva Nunes, conducted surveys across eight streams in Maranhão's Savanna region. Using a combination of dip nets, sieves, gill nets, and drag nets, they collected and identified crab species, documenting their locations and physical characteristics. This meticulous approach revealed the presence of four distinct species:

The study confirmed the presence of Dilocarcinus septemdentatus, Goyazana castelnaui, Sylviocarcinus pictus, and Valdivia serrata. These findings are significant because they extend the previously known distribution ranges of these species. For instance, Valdivia serrata was recorded for the first time in the Preguiças river basin, while Dilocarcinus septemdentatus and Sylviocarcinus pictus were newly registered in the Munim river basin. This expansion of known habitats underscores the importance of continued research and monitoring.

Dilocarcinus septemdentatus: Previously known distributions are expanded in new river basins.
Goyazana castelnaui: New locations identified contribute to a better understanding of its habitat preferences.
Sylviocarcinus pictus: Findings add valuable data to the species' distribution map.
Valdivia serrata: First-time recording in specific river basins marks a significant discovery.

One interesting observation was the asymmetry in the number of teeth on the carapace (the upper shell) of some Goyazana castelnaui specimens. According to the research, some crabs had six teeth on one side and five on the other. Such variations highlight the genetic diversity within these populations and the adaptability of the species to different environmental conditions. This level of detail is crucial for understanding the overall health and resilience of these crab populations.

Why This Matters

The study emphasizes the urgent need for a comprehensive catalog of freshwater crab species in all hydrographic basins of Maranhão. Understanding the diversity and distribution of these crabs is essential for effective conservation strategies, especially in the face of environmental modifications such as pollution and habitat degradation. By gaining a deeper understanding of these ecosystems, we can work towards preserving the delicate balance of life within them.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1590/2358-2936e2018034, Alternate LINK

Title: Extension Range Of Four Species Of Freshwater Crabs (Decapoda: Trichodactylidae) In The State Of Maranhão, Northeastern Brazil

Subject: General Medicine

Journal: Nauplius

Publisher: FapUNIFESP (SciELO)

Authors: Kele Sousa Pires Andrade, Marina De Sá Leitão Câmara De Araújo, Jorge Luiz Silva Nunes

Published: 2018-12-06

Everything You Need To Know

Why are freshwater crabs important to river ecosystems, and what challenges exist in understanding their role?

Freshwater crabs are vital for maintaining the equilibrium of river ecosystems. They aid in nutrient cycling, regulate invertebrate populations, and serve as sustenance for larger animals. However, knowledge regarding their distribution, particularly in areas like Northeastern Brazil, is limited. Research is crucial to understanding and protecting these species and their habitats.

What specific freshwater crab species were identified in the Maranhão study, and how did the findings contribute to understanding their distribution?

The study identified four distinct species: *Dilocarcinus septemdentatus*, *Goyazana castelnaui*, *Sylviocarcinus pictus*, and *Valdivia serrata*. These findings expanded the known distribution ranges of these species. For example, *Valdivia serrata* was recorded for the first time in the Preguiças river basin, while *Dilocarcinus septemdentatus* and *Sylviocarcinus pictus* were newly registered in the Munim river basin. Further monitoring will help to inform conservation efforts.

Can you elaborate on the methodologies employed by Kele Sousa Pires Andrade, Marina de Sá Leitão Câmara de Araújo, and Jorge Luiz Silva Nunes to survey freshwater crabs in Maranhão?

The research team, consisting of Kele Sousa Pires Andrade, Marina de Sá Leitão Câmara de Araújo, and Jorge Luiz Silva Nunes, conducted surveys across eight streams in Maranhão's Savanna region. They employed dip nets, sieves, gill nets, and drag nets to collect and identify crab species. This thorough methodology facilitated the accurate documentation of the crabs' locations and physical attributes. The use of varied collection methods maximized the chances of capturing all species present.

What does the asymmetry in the number of teeth on the carapace of Goyazana castelnaui suggest about the species, and what are the implications of these variations?

The asymmetry observed in the number of teeth on the carapace of some *Goyazana castelnaui* specimens suggests genetic diversity within these crab populations. The study noted crabs with six teeth on one side and five on the other. Such variations indicate the species' adaptability to different environmental conditions, which is essential for the overall health and resilience of these crab populations. Further genetic studies could reveal the extent of this diversity and its implications.

Why is it important to create a comprehensive catalog of freshwater crab species in Maranhão, and how can this knowledge aid in conservation efforts?

A comprehensive catalog of freshwater crab species in all hydrographic basins of Maranhão is essential for effective conservation strategies, especially when addressing environmental modifications like pollution and habitat degradation. Understanding the diversity and distribution of crabs like *Dilocarcinus septemdentatus*, *Goyazana castelnaui*, *Sylviocarcinus pictus*, and *Valdivia serrata* enables targeted efforts to preserve the delicate balance of these ecosystems, and prevent species loss.