Article: Adaptation of test shape for burrowing and locomotion in spatangoid echinoids
In considering ventral shape and longitudinal sectional outline of the test in fourteen living spatanoid species, four types can be recognized: flat, globular, wedge-shaped, and domed, with two intermediate series. Although the basic arrangement of spines is common to all, differences in test profile impart different attitudes to the spines and hence affect their function for burrowing. Three different burrowing mechanisms are recognized, which are the consequence of specific test profiles. Each burrowing mechanism corresponds to a specific burrowing depth and to sediment grain size. The globular urchins burrow by excavating frontal sediment and accumulating it on their backs. This is the consequence of the elevate plastron and the stern-like postero-ventral shape, and is effective for the urchins living deep in sand. Flat urchins live close to the sediment surface and burrow by pushing excavated sediment to the posterior sides of the test. The wedge-shaped type moves through the sediment by thrusting into the sediment in front rather tha excavating it, rocking its anterior end up and down. This is effective in soft, cohesive mud; the-wedge shape is essential for the repeated rocking motion. The overall test profile of each type also accords with a distinctive habitat. The flat profile of shallow-borrowing sand-dwellers keeps them stable against current scouring. The globular profile of deep-burrowing sand-dwellers helps support the burrow wall. The test shape is seeming less constrained than the arrangement of spines, and a wide variety of test shapes enables spatangoids to live in various environments.