sentences:  Sea urchins feed mainly on algae, so they are primarily herbivores, but can feed on sea cucumbers and a wide range of invertebrates, such as mussels, polychaetes, sponges, brittle stars, and crinoids, making them omnivores, consumers at a range of trophic levels.Mass mortality of sea urchins was first reported in the 1970s, but diseases in sea urchins had been little studied before the advent of aquaculture.In 1981, bacterial "spotting disease" caused almost complete mortality in juvenile "Pseudocentrotus depressus" and "Hemicentrotus pulcherrimus", both cultivated in Japan; the disease recurred in succeeding years.