although there are a few benthic species, most are gelatinous zooplankton and form a considerable proportion of the entire planktonic biomass worldwide. a few species, such as the sea gooseberry, native to the north sea, have reached such high populations that they clog fishermen's nets, while of other species only a few examples are known. the fragile makeup of ctenophores makes research into their way of life extremely difficult. for this reason their lifespan is not known, but ctenophores begin to reproduce at an early age.