On organ development in larvae of Diplosoma migrans (Menker & Ax, 1976) (Tunicata, Ascidiacea, Didemnidae)

Development in the colonial ascidian genus Diplosoma is unusual in producing two zooids already during the free-swimming larval period. The development of D. listerianum had been investigated in two studies. The present study reports embryonic development of a second species D. migrans. In D. migrans the pharynx of the oozooid develops from two separate primordia. The intestinal tract develops as well from two distinct primordia that fuse and subsequently connect to the prospective pharynx. The pharynx of the blastozooid develops from the paired epicardia of the oozooid. In the oozooid the primordia of the peribranchial sacs often derive from the endodermal wall of the prospective pharynx, but occasionally develop from thickenings of the ectodermal epidermis. No atrial siphon is formed; instead, the body wall retracts and the gill slits open directly to the exterior. In the oozooid the rectum develops from the initial prospective intestine, while the remaining intestine develops from a diverticulum of the stomach. In the blastozooid the main part of the intestinal tract develops from the initial prospective intestine, whereas the rectum develops as a diverticulum of the initial intestine. Due to this developmental mode, the intestinal tracts of oozooid and blastozooid are transitorily connected at the respective border of rectum and proximal part of posterior intestinal tract. The paired epicardia of the two zooids derive from the epicardial outgrowths of the pharyngeal primordia of the oozooid that are also the primordia of the heart and pericardia. Budding in the larva and the adult is highly similar, in particular concerning the development of a new thorax and the heart from paired epicardial Anlagen and the origin of the distal part of the posterior intestine. Discrepancies in the descriptions of the embryonic development of D. listerianum and differences to the development in D. migrans are discussed. The development of the pericardium-heart-complex from epithelially organized tissue constitutes the plesiomorphic condition within Didemnidae and Aplousobranchiata. The development from mesenchymatic cell masses is a secondarily derived simplification.