Lime-twig glands: a unique invention of an Antarctic entoproct

Specialized glands that release formed secretions of a complex structure are known from several invertebrate phyla. A novel type of such an extrusive organ has been detected in the newly described Antarctic entoproct Loxosomella brochobola Emschermann, 1993 and is reported here. The specialized extrusive organs known from other invertebrates are generally unicellular, but these entoproctan glands are multicellular organs. The structured secretion of these glands is an extracellular product homologous to the body cuticle and is discharged in long sticky, hollow threads. In evolutionary convergence to the glutinant spirocysts of the Anthozoa, these threads are assumed - like set out single lime-twigs - to trap larger prey organisms inaccessible to the ciliary feeding current of the entoproct. Specialized glands of this kind have not been known previously in Entoprocta. This 'invention' by a nanoplankton feeder must be seen as a specific adaptation to life in an environment that is poor in nanoplankton. L. brochobola was found exclusively on the inner, abfrontal surface of the tube-shaped, calcareous colonies of the bryozoon Porella malouinensis and shares this microhabitat only with some smaller predators, such as the hydrozoan Halecium sp.; no other ciliary feeders are present.