Fish and birds both possess circulatory systems designed to transport oxygen and nutrients to their body tissues. However, their anatomy and physiology differ significantly to meet their respective metabolic demands.

Compare and contrast the circulatory systems of a single-circulating bony fish and a double-circulating bird.

Crustacean hyperglycemic hormone (CHH) is a peptide signaling molecule in lobsters. It is synthesized and released by neurosecretory cells in the eyestalks (specifically the X-organ/sinus gland complex) but acts to stimulate glycogen breakdown in target cells of the hepatopancreas, which is located in the main abdominal cavity.

Explain how it is possible for CHH to have an effect on hepatopancreas cells located far from the eyestalks where it is released.

Active swimming in bluefin tuna (Thunnus thynnus) leads to rapid aerobic respiration in their red muscle fibers, producing large amounts of carbon dioxide. This carbon dioxide reduces the affinity of the tuna's haemoglobin for oxygen, facilitating oxygen unloading at the tissues.

Suggest a mechanism by which carbon dioxide reduces the oxygen affinity of haemoglobin in the active tissues of the tuna.