DESCRIPTION OF THE PHYLUM TUNICATA (LAMARCK 1816)
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PHYLUM TUNICATA LINKS
| Tunicata (too-ne-KA-tuh) is from the Latin word, tunica, the basic Roman attire. The reference is to the outer, tunic-like covering on the sessile sea squirts. Later, Balfour (1881) coined the term Urochorda (the tail chordates), which later became formalized as Urochordata, to stress the relationship with the chordates relative to the older taxonomic designation, Tunicata (Lamarck 1816). |
| INTRODUCTION TO THE TUNICATA The sea squirts or tunicates generally are benthic or sessile marine filter-feeders. As adults they appear almost sponge-like with incurrent siphon, a large filtering pharynx with gill slits, and an excurrent siphon. However, larval forms are shaped like tadpoles with a notochord in their tails. They also have a hollow dorsal nerve chord and pharyngial gill slits, all hallmarks of the “chordate” taxa. However, only the pharyngial gill slits are retained after the animal undergoes metamorphosis from the larval form to the adult. Traditionally, the tunicates are represented by four very different groups of filter-feeders. |
| Ascidiacea are the most common members of this phylum. As adults, they live as sessile sponge-like animals (Figure 1). The the branchial and atrial siphons are close together on the top, away from the point of attachment. The water filters through a large pharyngial basket where the gill slits filter out suspended organic particles and transfer them to the esophagial opening. Once attached, the animal does not move and covers itself with a tunic, by secreting a thick covering of a substance similar to cellulose. Some species are colonial and reproduce by budding. They earn the name sea squirt by suddenly squirting out water and contracting when disturbed. |
| The Thaliacea, also called salps (Figure 2), are free-living planktonic tunicates that are built like a ram jet in that the incurrent siphon and the excurrent siphon are on opposite ends of the barrel-shaped body. The adults swim about in the plankton as predators on the other zooplankton. Sometimes they can achieve astonishing concentrations (up to thousands of animals per cubic meter, Pechenik 2005). |
| The Appendicularia are also called the Larvacea in that they retain the tadpole form throughout their lives (Figure 3). The animals have a relatively large tail, but, rather than swim in the plankton directly, they secrete a gelatinous house with a sophisticated set of filters and openings that allow the water current produced by the beating tail to pass through and provide propulsion. The animal consumes small particles that are filtered out and abandons the home when it is clogged. |
| The Sorbereacea is an ascidian-like animal of the deep ocean. Little is known of them except that they likely have the same life history as ascidians. They do retain a nerve chord as an adult and do not have a perforated pharynx. That is, they have lost their gill slits. Brusca and Brusca (2003) state that the sorbereacea are carnivorous and prey on nematodes and crustaceans. |
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| FIGURE 1. A group of living ascidian tunicates. The sponge-like nature is evident. Note the siphons. Image from: http://www.ucmp.berkeley.edu/chordata/urochordata.html | FIGURE 2. A salp. Note the barrel-like body and the two siphons in opposition. Image from Woods Hole Oceanographic Institute by Laurence Madin | FIGURE 3. Oikopleura, a larvacean, so called because it retains its larval form. The animal (center) secretes a thin mucilaginous “house” around itself with which it filters food particles. Image from NOAA, in the Public Domain |
| The morphological tradition of grouping phyla in the deuterostomes assumes that sessile and simple is primitive and that moving and complex is derived. This, to a large extent is a consequence of our bias as members of the vertebrates [See Deuterostome Description for alternate topologies of the Deuterostome tree]. The traditional view of the deuterostome relationships (e.g. Nielsen 2001 and Brusca and Brusca 2003) is that echinoderms are the most primitive and the chordate groups monophyletic and the most derived. Putnam et al. (2008) examined the amphioxus genome relative to whole genomes of other deuterostomes and found that the tunicates were sisters to the vertebrates, as did Delsuc et al. (2008) who found the cephalochordates were not sisters to the vertebrates, but basal in the whole chordate clade (see Figure 4). If so, the cephalochordate body plan is primitive, suggesting that the basic vertebrate plan also is primitive. Furthermore, the sessile natures of the echinoderms and tunicates are secondary simplifications. We have followed the system of Nielsen (2001) in raising the tunicates to phylum-level status. The relative positions of the classes of the tunicates are equivocal, at best. Tsagkogeorga et al. (2009) show that the tunicate taxonomy likely is more complex than given here. In particular, Ascideacea is polyphyletic forming at least two major groups in different tunicate clades (see Figure 4). Tatian et al. (2011) also explored the relationships of tunicate groups and confirmed the topology of Tsagkogeorga et al. (2009) together with the emergence of the Sorberacea from within the Molgulidae (Ascideacea II in Figure 4). Both analyses confirm that Ascideacea II (including Sorberacea) is monophyletic while the orders that make up Ascideacea I (Figure 4) are all unresolved and may divide into four separate classes. Both the active (larvacean) and sessile (ascidian) forms appeared in the Cambrian. If the amphioxus type is closer to the primitive state, then it follows that the more derived urochordates would have deviated more from the mobile, fish-like body plan. That is, the larvaceans may have evolved through neoteny, which then gave rise to the vertebrates, or the more primitive larvacean form elaborated a sessile adult body plan and also gave rise to the vertebrates. Jeffrey et al. (2004) suggest an affinity between the vertebrates and tunicates by pointing out that tunicates may have an early form of a neural crest that produces pigment cells in the embryo. Either way, the discovery of the close association of the tunicates and vertebrates (e.g. Putnam et al. 2008; Delsuc et al. 2006 and 2008) suggests that we need to rethink what is primitive in the deuterostomes. |
FIGURE 4. Cladogram illustrating the topology of the tunicates (shaded box) within the chordates (Clade C) according to Putnam et al. (2008) and Delsuc et al. (2008). The topology of the tunicate classes follows Tsagkogeorga et al. (2009). According to this cladogram, Ascideacea is polyphyletic. According to Tatian et al. (2011), Sorberacea emerges from within Ascideacea II.
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| By Jack R. Holt and Carlos A. Iudica. Last revised: 04/07/2013 |
