A “protective shell” around the larval cocoon of Cephalodiscus densus Andersson, 1907 (Graptolithoidea, Hemichordata)

Abstract  Within the framework of the Italian XVII PNRA expedition (austral summer 2001–2002), several colonies of the Graptolithoidea (=Pterobranchia+Graptolithina) species, Cephalodiscus densus Andersson and C. hodgsoni Ridewood, were collected by trawl from Tethys Bay (Terra Nova Bay, Ross Sea). These organisms were maintained in aquaria for 2 weeks and their behaviour observed. The dissection of some colonies of C. densus allowed the reconstruction of the eggs fate and the documentation of the formation of a protective shell around the cocoons of larvae settled within the parental colony encasement. The shell is produced by cementing size-selected debris, either of organic or inorganic origin, accumulated inside the encasement. This is the first documentation of such a structure within living Hemichordata and it helps to reinterpret some of the resting structures within zooidal tubes or thecae of particularly well-preserved fossil graptolites.