Origin and function of tunicate hemocytes

Tunicates, or urochordates, are a subphylum of the phylum Chordata, sharing
with other members of the phylum: (1) a permanent or temporary notochord,
in the form of a dorsal rod; (2) a central nervous system, in the form of a dorsal
tube; (3) a pharynx provided with gill slits or pharyngeal pouches, and a ventral
gland secreting iodoproteins (endostyle or thyroid); and (4) a muscular tail.
Tunicates are considered the sister group of vertebrates,1 forming with the
latter the clade Olfactoria. Recently, it has been proposed to classify them as a
phylum within the superphylum Chordata.2 They are traditionally subdivided in
three classes: (1) Ascidiacea (benthic and sessile), (2) Thaliacea, and (3) Larvacea
or Appendicularia (pelagic).
Ascidians have a free-swimming, tadpole-like larva, an adult sac-like body
with two siphons that allow water flux, and a large branchial basket, provided
with a ventral endostyle that secretes the mucous net required for filtration. They
comprise two orders: Enterogona (including the suborders Phlebobranchia and
Aplousobranchia) and Pleurogona (with the suborder Stolidobranchia).3 Thaliaceans
include three orders: the colonial Pyrosomida, and the solitary/colonial
Doliolida and Salpida. They have a barrel-like adult body, and, with the exception
of Doliolida, are devoid of larval stages.4,5 Larvaceans or appendicularians
resemble the ascidian larvae and use the tail to create the water current for filtration;
filters are included in the gelatinous house secreted by the animals themselves.
4 Most of the recent authors consider larvaceans as a sister group of the
other tunicates, and thaliaceans as a sister group of Enterogona5–10 (Fig. 2.1).
Ascidians include about 2300 species, and most of the information on tunicate
hemocytes comes from studies on this group of organisms. This review
will, then, focus mainly on ascidian hemocytes and will discuss their role in
immunity. Where possible, information on circulating cells of pelagic tunicates
will be added.