Midwater animals fall into two majorgroups: those that stay in the mesopelagicand those that migrate to the surface atnight. The non-migrators include a few species of small zooplankton, mainly copepods and krill, that filter out
detritus
and the small amount of phytoplanktonthat sinks out of the photic zone. Thefecal pellets of epipelagic copepods andother surface grazers form an importantpart of the detritus eaten by mesopelagicfilter feeders. These pellets sink muchfaster than individual phytoplankton cells,so they stand a better chance of making itto the mesopelagic before being eaten