溝 - gutter, ditch, sewer

ditch; drain; gutter; trench

oceanic trench; deep-sea trench; deep

ditch; drain; gutter; trench

Japanese night heron (Gorsachius goisagi)

keyway

Chinese pond mussel (Sinanodonta woodiana)

subterranean drain; culvert; conduit

fluting; grooving

bird cherry (Prunus padus)

trench digging machine; trencher

mammary slit (in whales, dolphins and porpoises)

fins (of a fountain pen feed); comb feed