The Subphylum Urochordata, or Tunicata, is considered traditionally to be the sister taxon of the Cephalochordata-plus-Vertebrata clade, and includes three Classes: ascidians, thaliaceans and larvaceans (or appendicularians). Recent molecular data, however, have suggested that urochordata may be teh sister group of vertebrata (Delsuc et al., 2006).
Urochordate larvae possess the defining chordate features of notochord, dorsal hollow nerve cord, gill slits, and a muscular, post-anal tail. In ascidians, an elaborate metamorphosis transforms the free-swimming larva into a sessile adult, and erases most of the chordate features as the notochord is reabsorbed and the CNS is dramatically restructured. It is possible that selective pressures leading to rapid metamorphosis in ascidians have obscured or compromised some developmental mechanisms present in stem chordates.
Among epipelagic larvaceans, the cosmopolitan species Oikopleura dioica often occurs as the most abundant. Adults have a trunk and a tail, which together are about 3mm long. The trunk secretes a mucilaginous, cellulose-containing, disposable house with feeding filters that collect picoplankton and colloidal carbon from water drawn into the house by the beating of the muscular, notochord-stiffened tail. A single adult can filter 20 liters and build and discard more than a dozen houses a day. Furthermore, they are often very numerous, with blooms of 25,000 to 57,000 individuals per cubic meter of seawater. Larvaceans bypass the microbial food web by efficiently converting picophytoplankton directly into biomass immediately available to predatory fish and to fast-sinking carbon in their discarded houses that are transported from the surface to the ocean depths. This has a significant effect on the global carbon cycle; for example, in Bermuda, larvaceans contribute to at least 8% of the vertical transport of carbon to ocean depths. Rising levels of atmospheric carbon dioxide and the greenhouse effect is thought to play a role in the warming of the Earth, and this warming is impacting the spatial distribution of plant and animal populations.
Oikopleura dioica, as its name suggests, is dioecious, with separate male and female individuals distinguished by gonad appearance. Animals can be cultured continuously in the lab by single-pair or mass matings. In Oikopleura dioica, development from zygote to juvenile is rapid, lasting only 15 hours at 12C, and a full generation requires less than two weeks. Larvaceans are transparent from egg to adult. Oikopleura dioica develops rapidly into a ball of cells, and at 4 hours after fertilization begins to show an indentation that separates trunk from tail. An hour later, the notochord is delineated. In the hours that follow, organs such as the brain are defined, and the mouth and body lumens open. By 15 hours, embryonic development is complete and the animal makes its first house. Individual cells of the adult brain, its nerves, and spinal cord are all clearly visible through the body wall. These characteristics, plus year-round reproduction, large clutches of up to 500 eggs per female, and laboratory culture, make Oikopleura dioica suitable for the study of embryogenesis.
Last updated on March 24, 2006.
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