Cold seeps occur over fissures on the seafloor caused by tectonic activity. This is the remains of seafloor hydrothermal vents. A hydrothermal vent is a fissure on the seafloor from which geothermally heated water issues. Some scientists interpreted these as requiring plate tectonic processes, such as seafloor spreading. The initial herbivorous mesozooplankton were probably larvae of benthic (seafloor) animals. Bathydraconids guard eggs on the seafloor of shallow waters. Dugongs sift through the seafloor in search of seagrasses. Dinoflagellate theca can sink rapidly to the seafloor in marine snow. Shallow geohazards are those occurring at less than below the seafloor. In the 1960s, geologists discovered and began to propose mechanisms for seafloor spreading. The corals do not rely on hydrocarbons seeping out of the seafloor. Corals probably competed mainly with red and coralline algae for space on the seafloor. Evidence of this motion can be found in paleomagnetic striping on the seafloor. Siliceous ooze is present along the seafloor in these subpolar regions. Shrimp trawling can capture shrimp in huge volumes by dragging a net along the seafloor.