Stress and mineralization responses to environmental stress in Mantis Shrimp
Calcified marine organisms face a double edged sword as global warming increases not only the temperature of ocean waters, but also the level of acidification. Many such organisms that live within the intertidal zone are already adapted to cope with the broad fluctuations in temperature and pH that occur with each tidal cycle – but precisely how the species cope with the stressors of increased temperature and decreased pH varies widely and have costs that can include increased energy demands, growth restriction, physiological stress, and changes in mineralization. As the planet warms, climate scientists believe that these broad tidal fluctuations will be even greater, and the stress on intertidal organisms that much greater. As we look to understand the changes facing our planet it is important to study the responses of the indicator species living in these highly changeable zones as they can reveal a great deal about what types of adaptations might be most likely to indicate success and survival, and what types of adaptations might be less successful and even eventually lead to extinction.
Mantis shrimp are predatory crustaceans found in intertidal tropical and subtropical marine ecosystems throughout the world including coral reefs, mangroves, and sand flats. They possess a unique and specialized raptorial appendage that produces one of the fastest and most powerful strikes known in the animal kingdom. The power behind this strike comes from precise and specialized structure, mineral composition, mechanical properties in different layers of exoskeleton. Because of this high degree of specialization, mantis shrimp are an interesting indicator species to look at the potential impact of changes due to global warming.
deVries et al. (Scientific Reports, December 2016) examined the long term effects of moderate increases in pCO2 and temperature (based in predicted future climate conditions) on mantis shrimp stress physiology as well as the exoskeleton morphology and the mechanical properties of the raptorial appendage as compared to that of the carapace, a less specialized segment of exoskeleton.
To address the question of stress they examined protein damage due to oxidative stress as well as assessing the activity of the stress response enzymes Superoxide Dismutase (SOD, Arbor Assays Cat.# K028-H1) and Catalase (Arbor Assays Cat.# K033-H1). Across their range of treatment conditions with 3 and 6 month exposure they found no change in either SOD or Catalase activity, and no increase in protein damage. Taken together these present significant evidence that the animals were not physiologically stressed by the treatment conditions presented.
Scanning electron microscopy (SEM) imaging revealed no visual differences in the exoskeleton structure between the various treatment conditions either in samples taken from the raptorial appendage or from the less specialized carapace. Total thickness of the cuticle (corrected for body size) as measured from the SEM images was also not significantly different between the different treatments or exposure periods. To further study exoskeleton construction and mineralization the weight percentage of calcium and magnesium were measured using energy-dispersive X-ray spectroscopy (EDX). After 6 months of treatment, samples from both the carapace and the raptorial appendage showed a higher percentage of Ca as compared samples taken after 3 months of treatment. However, because this increase was consistent across all of the treatment conditions, including those animals that were maintained at the ambient oceanic conditions noted when they were collected, it would appear that this result is not significant in terms of the experimental parameters. There were however significant differences in the % Mg between treatments. The % Mg in the samples from animals maintained at reduced pH was significantly higher than that observed in animals maintained under ambient conditions, or those exposed to both reduced pH and increased temperature. Furthermore, this increase in Mg was observed only in samples collected from the raptorial appendages, and not in samples from the carapace. Since changes in mineral composition could potentially affect the hardness of the raptorial appendage strike surface, deVries et al. also mechanically tested the hardness and stiffness of cuticle samples from the carapace and the strike surface of the raptorial appendage across the different experimental groups. Despite the change in % Mg in the animals exposed to reduced pH, no significant changes in hardness or stiffness were observed.
Overall it seems that mantis shrimp are well equipped to adapt to the types of climate induced changes expected within their ecosystems in the future. A lucky situation for them indeed! One only hopes their prey species are as fortunate.
Because different species adapt in different ways it is as important to understand species that adapt well as it is to understand species that fail to adapt successfully. Both contribute to the overall picture in different ways. Climate change at this point is inevitable at least in the near term, and it is critical that we understand as much as possible about the way it will impact our ecosystems.