02 Hygrosoma hoplacantha (Thomson, 1877)
Hygrosoma hoplacantha belongs to the Family Echinothuriidaie of the Order Echinothurioida. It is a very large sea urchin. Currently, the largest specimen that has been obtained has a diameter of more than 23 centimeters. It is the largest sea urchin in the waters near Taiwan.
Most of the members of Family Echinothuriidaie inhabit silted ocean floor at depths of 100 meters or more. Between the plates of the test is soft connective tissue. Therefore, the test is soft. Due to this characteristic, the diameter of the exoskeleton is able to exceed 10 centimeters and even reach 20 centimeters.
The test of Hygrosoma hoplacantha is dark purple, as are the primary spines. As this species inhabits the soft deep sea floor, the ends of the primary spines on the oral surface often have a specialized white hoof-shaped structure. This serves to increase the contact surface area of the primary spines. This is similar to putting on white riding boots and is a very intriguing evolutionary adaptation.
The distribution of this species is the Indo-Pacific region from Japan to Indonesia, as well as to the western coast of Australia and East Africa. It is mostly found on silted sea floor at 360-2,000 meters in depth. It has been collected and recorded along Taiwan's northeastern cape and the waters off of southwestern Taiwan. Current records of the depths of their habitats are between 500 and 1,650 meters. This is a deep sea species.
