Meeting Abstract

70.2  Sunday, Jan. 6  Enigmatic transitory epidermis in planktonic larval development of a hoplonemertean Paranemertes peregrina and its significance in evolution of development in nemerteans (Phylum Nemertea; Lophotrochozoa) MASLAKOVA, S. A.*; VON D�HREN, J.; U of Washington; Freie Universit�t Berlin maslak@u.washington.edu

Nemerteans are non-segmented marine worms related to the trochophore-bearing phyla such as Mollusca and Annelida. Nemertean development can be direct or indirect. The nemertean clade Pilidiophora is characterized by indirect development with a unique transparent planktonic pilidium larva inside which the juvenile develops from several independent rudiments. Pilidial development culminates in rapid metamorphosis as hatching juvenile devours the larva. The so-called �direct� development is by far less dramatic: a juvenile-like, opaque and uniformly ciliated planuliform planktonic larva develops without a drastic metamorphosis, or development can be entirely encapsulated. Recent data on nemertean development and phylogeny suggest that direct development is ancestral to nemerteans and that pilidial development is derived. However, it is unclear when and how this novel larval type (the pilidium) evolved during nemertean diversification. What little we know about nemertean direct development suggests that its diversity is greatly underappreciated. Palaeonemertean planulifrom larvae are modified trochophores with a hidden prototroch derived from the classical spiralian trochoblast cell-lineage, whereas the little-studied hoplonemertean planulifrom larvae are very different and possess a mysterious transitory larval epidermis. We describe for the first time presence of such epidermis in larval development of a hoplonemertean Paranemertes peregrina and illustrate its gradual replacement by the definitive epidermis using immunohistochemistry and confocal microscopy. We discuss the possible function of larval epidermis and its evolutionary significance.