sentences:  The urchin's growth rate is slow as it is limited both by the amount of organic material that can be extracted in this way and also by the lack of oxygen in the sediment, especially at night.To overcome this limitation, adult urchins often emerge onto the seabed at night and burrow down into the sediment again next day while juveniles remain permanently buried.During feeding, the large urchins disturb the substrate and increase its water content while decreasing its density.