Illustration of crabs adapting to changing salinity in an estuary.

Crabs Under Stress: How Salinity Changes Impact Estuarine Life

Beau Callahan in Science & Nature December 2025 • 5 min read.

"Uncover the surprising ways estuarine crabs adapt to drastic salinity shifts – and what it means for their survival."

Estuaries are dynamic environments where freshwater meets saltwater, leading to dramatic and frequent salinity fluctuations. These changes pose significant challenges for the organisms that call these habitats home, requiring them to adapt at biochemical, physiological, and behavioral levels to maintain their internal balance.

Crustaceans, such as crabs, are particularly vulnerable to salinity changes. While some can regulate their internal salt concentration regardless of the external environment (hyper-regulators), others conform to the salinity of their surroundings. Understanding how these creatures adapt is crucial for predicting their survival in the face of increasing environmental stressors.

This article delves into a study examining the histochemical fiber type composition in the claw closer muscles of two estuarine crab species, Cyrtograpsus angulatus and Neohelice granulata, under reduced salinity conditions. By analyzing muscle fiber types, researchers gained insights into the crabs' adaptation mechanisms and their ability to cope with the energetic demands of osmoregulation.

Decoding Crab Muscle: Fiber Types and Salinity Stress

Illustration of crabs adapting to changing salinity in an estuary.

The study focused on the claw closer muscles, essential for feeding, defense, and social interactions. Researchers acclimated the crabs to a low salinity environment (10 psu) and then analyzed the muscle tissue using various histochemical techniques, including myosin-adenosine triphosphatase (m-ATPase) to identify different fiber types, succinic dehydrogenase (SDH) to assess oxidative capacity, and Periodic Acid Schiff (PAS) and Sudan Black B to evaluate glycogen and lipid content, respectively.

Four distinct fiber types were identified in Neohelice granulata: Type I (slow, weak reaction to histochemical tests), Type IV (fast, strong reaction to histochemical tests), and Types II and III (intermediate). Cyrtograpsus angulatus, however, only exhibited Types I, II, and III. This difference in fiber type composition suggests varying adaptation strategies between the two species.

Fiber Type I: Large fibers with weak reactions, indicating lower glycolytic and oxidative capacity.
Fiber Type IV: Small fibers with strong reactions, suggesting rapid contraction and high metabolic activity.
Fiber Types II & III: Intermediate characteristics, offering a balance between speed and endurance.

Interestingly, the study revealed that the proportion of fiber types differed significantly between the two species under reduced salinity. Cyrtograpsus angulatus showed a predominance of intermediate fiber types (II and III), while Neohelice granulata exhibited a higher proportion of Type II fibers. This suggests that Cyrtograpsus angulatus relies more on a balance of speed and endurance, whereas Neohelice granulata favors a faster, more glycolytic metabolism. Moreover, the diameters of fibers from both species was smaller than those at osmoconforming conditions (35 psu).

Adapting to Change: Implications for Crab Survival

The differential responses observed in the claw closer muscles of Neohelice granulata and Cyrtograpsus angulatus highlight the complexity of adaptation to salinity stress. These findings suggest that each species employs unique mechanisms to cope with the energetic demands of osmoregulation and maintain essential functions in a fluctuating environment.

While the study did not find significant changes in lipid concentration in the claw closer muscles, it did observe variations in glycogen reserves, suggesting different energy utilization strategies between the two species. Further research is needed to fully elucidate the biochemical pathways involved in salinity adaptation and to assess the long-term consequences of these changes on crab populations.

Understanding how estuarine crabs respond to salinity fluctuations is crucial for predicting their resilience in the face of ongoing environmental changes, including climate change and habitat degradation. By studying their physiological adaptations, we can gain valuable insights into the broader ecological impacts of environmental stressors on estuarine ecosystems.

About this Article -

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This article is based on research published under:

DOI-LINK: 10.4067/s0717-95022017000200025, Alternate LINK

Title: Influence Of Reduced Salinity On The Histochemical Fiber Type Composition Of The Claw Closer Muscle Of Two Estuarine Crab Species, Cyrtograpsus Angulatus And Neohelice Granulata (Decapoda: Varunidae)

Subject: Anatomy

Journal: International Journal of Morphology

Publisher: SciELO Agencia Nacional de Investigacion y Desarrollo (ANID)

Authors: María Victoria Longo, Alcira Ofelia Díaz

Published: 2017-06-01

Everything You Need To Know

Why are estuaries challenging environments for crabs and what are the two main strategies they use to deal with salinity changes?

Estuaries experience dynamic salinity changes where freshwater meets saltwater, posing a challenge for inhabitants like crabs. Some crabs, known as hyper-regulators, maintain a stable internal salt concentration, while others adjust to the salinity of their surroundings. Understanding these adaptations is essential for predicting survival amid increasing environmental stress.

What specific muscles did the study focus on and what techniques were used to analyze the muscle tissue of the crabs, Cyrtograpsus angulatus and Neohelice granulata?

The study analyzed claw closer muscles, crucial for feeding, defense, and social interactions, in Cyrtograpsus angulatus and Neohelice granulata. Researchers acclimated these crabs to low salinity (10 psu) and examined muscle tissue using histochemical techniques such as myosin-adenosine triphosphatase (m-ATPase), succinic dehydrogenase (SDH), Periodic Acid Schiff (PAS), and Sudan Black B to identify fiber types and assess oxidative capacity, glycogen, and lipid content.

What are the different muscle fiber types found in Neohelice granulata and Cyrtograpsus angulatus, and what do these fiber types indicate about their metabolic capabilities?

Four fiber types were identified in Neohelice granulata: Type I (slow, weak reaction), Type IV (fast, strong reaction), and Types II and III (intermediate). Cyrtograpsus angulatus exhibited only Types I, II, and III. Fiber Type I has lower glycolytic and oxidative capacity. Fiber Type IV suggests rapid contraction and high metabolic activity. Fiber Types II & III offer a balance between speed and endurance. The absence of Type IV fibers in Cyrtograpsus angulatus suggests different adaptation strategies compared to Neohelice granulata.

How did the proportion of different muscle fiber types change in Cyrtograpsus angulatus and Neohelice granulata under reduced salinity conditions, and what does this suggest about their adaptation strategies?

Under reduced salinity, Cyrtograpsus angulatus showed a predominance of intermediate fiber types (II and III), indicating reliance on balanced speed and endurance. Neohelice granulata exhibited a higher proportion of Type II fibers, favoring faster, more glycolytic metabolism. Also, the fiber diameters from both species were smaller than those at osmoconforming conditions (35 psu).

What are the broader implications of the different adaptation strategies seen in Neohelice granulata and Cyrtograpsus angulatus, and what further research could build upon these findings?

The differing muscle fiber compositions and responses to salinity stress in Neohelice granulata and Cyrtograpsus angulatus suggest unique adaptation mechanisms for coping with the energetic demands of osmoregulation. These findings highlight the intricate ways estuarine species can evolve to survive in fluctuating environments, but future research could explore the genetic basis of these adaptations or how other stressors, like temperature changes, might interact with salinity to affect crab physiology and survival. Also, how these adaptations impact the crabs' interactions within their community and the broader ecosystem.