In the coral reefs of northern Australia and New Guinea lives a vertebrate that defies the evolutionary boundaries between the ocean and dry land: the epaulette shark (Hemiscyllium ocellatum). When the low tide retreats, this small shark is often trapped in isolated pools where the water loses nearly all its dissolved oxygen and temperatures soar.
Far from suffocating, the animal displays two extreme adaptations:
- Active terrestrial locomotion: It uses its robust pectoral and pelvic fins like articulated limbs. Through muscular undulation identical to that used by the first primitive tetrapods of the Devonian, the shark is able to climb rocks, crawl over dry coral, and cross stretches of dry land to reach the next deep pool.
- Neuronal hypometabolism: To resist extreme anoxia, it abruptly slows its heart rate and selectively shuts down non-essential brain circuits, minimizing the consumption of adenosine triphosphate (ATP). It can withstand up to two full hours in severe hypoxia conditions without suffering neuronal death or tissue reperfusion injury upon reoxygenation.
This brain tolerance to a radical cut in oxygen is now the subject of intense medical research to decipher how to prevent brain damage in patients suffering from strokes or prolonged cardiac arrest.
Source: Physiological and Biochemical Zoology / Integrative and Comparative Biology
