Function of the gizzard in Bryozoa

The gizzard, a modified region of the stomach cardia incorporating chitinized teeth or plates surrounded by circular muscle, has been reported in 16 bryozoan genera (one Cheilostomata, 12 Ctenostomata, three Cyclostomata) and has independently evolved at least six times. Its occurrence is reviewed in the light of revised taxonomy of stolonate ctenostomes. The ability to crush diatom frustules and to separate valves intact has been investigated in bryozoans with (Bowerbankia spp., Amathia pruvoti Calvet) and without [Electra pilosa (L.) and Flustrellidra hispida (Fabricius)] a gizzard. The gizzard-bearers have significantly greater ability to break the frustules, except for Amathia pruvoti in comparison with Flustrellidra hispida. F. hispida and Electra pilosa have good ability to open frustules of some diatoms, even to the extent of equalling the percentage broken by the gizzard of Bowerbankia. These findings have been related to the size of the lophophore and mouth in bryozoans, and to the size of the diatoms. Notwithstanding the recently recognized profusion of nanoplanktonic diatoms, a majority of the common species are large, often too large to be ingested by the characteristically small bryozoans that possess a gizzard. This paradox is discussed in the context of gut function, habitat, and possible specialization by the bryozoans to exploit different food resources.