It would be the endocrine system.
<span>Mark may be getting lower amounts of nutrients and vitamins, namely omega-3s and omega-6s. This would directly affect the brain and normal body functions because the body needs vitamins/nutrients to thrive. This could affect the chemical balance in his brain, resulting in depression/schizophrenia/other issues, including eating disorders. Cutting down on fats might also result in overeating carbs and protein, which leads to an overbalance of nutrients -- and a host of other health issues.Not getting enough fat could also affect bone growth.</span>
They find shells that other crabs have left behind and use them
During normal breathing, the brain is stimulated to breath with increasing acidity as a result of CO2 concentration from basic metabolic processes. The brain is quite selfish and only really wants to maintain it's pH which should be at a range of 7.3-7.45, and will not tolerate any decrease.
In patients who have a chronic respiratory disorder with things like COPD. The brain has become accustomed to excessive acidic content, and is now stimulated by the Hypoxic drive or by low oxygen content.
