Ciallusiidae Huus, 1936

Kott, Patricia, 2008, Ciallusiidae (Ascidiacea, Tunicata), a monotypic family from deeper waters of the tropical Indo-West Pacific, Zootaxa 1742, pp. 47-52 : 47-49

Family Ciallusiidae Huus, 1936 .

Type genus: Ciallusia Van Name, 1918 (< Pterygascidia Sluiter, 1904 )

This monotypic family presently is known to contain only its type genus (a junior synonym of Pterygascidia Sluiter, 1904 ) which also is monotypic. The genus is characterised by a solitary habit, finely tapering languets constituting the dorsal lamina, absence of epicardial sacs, rows of short parallel muscle bands, a more or less straight gut, a thin-walled stomach lined internally with numerous fine parallel folds, a tight network of fine, crowded testis tubules spread over the gut wall, an ovarian tube closely associated with the gut wall, and a long branchial sac with numerous internal longitudinal vessels and many rows of regular oval perforations in the pharyngeal wall. Ciliated epithelium has not been detected lining these perforations.

From the outset, there has been a problem in assessing the phylogeny of the genus. Initially Sluiter (1904) thought it possibly related to the Corellidae on the basis of its lack of ciliated epithelium lining the pharyngeal perforations. However, this may not be of profound phylogenetic significance, such loss occurring generally in taxa in all subordes adapted to deep waters. Further, the gut loop is on the left rather than the right as it is in the Corellidae . Millar (1963) proposed a relationship with Cionidae based on the presence of two channels of a test vessel in the stalk similar to those that occur in the stalk of juvenile Ciona and in the stalks of the Perophoridae (despite the fact that Millar’s argument regarding the similarity of the cionid and perophorid stalks implies a relationship between Perophoridae and Cionidae that is not established). Tokioka (1967 and 1971a, b) also assigned the genus (as Ciallusia Van Name, 1918 ) to the Cionidae . Monniot (C. and F. 1991), discussing a specimen they assigned to a new species in the genus Pterygascidia , also proposed a relationship with the Cionidae (albeit their new species may not belong to this genus; see below). However the apparent lack of epicardial sacs in all of these taxa argues against their cionid (aplousobranch) affinity. A more acceptable hypothesis regarding the affinities of Pterygascidia (again as Ciallusia ), first suggested by Huus (1936) and supported by Kott (1985), is based on the presence of dorsal languets which, although they occur in most aplousobranchs (including Ciona ), also occur in some phlebobranchs ( Agneziidae ). Further, although it occurs in Ciona , a large branchial sac with numerous internal longitudinal vessels also occurs in most solitary Phlebobranchia. Millar (1963) observed that the short, muscle bands, straight gut loop and gonads associated with the gut loop occur in certain Phlebobranchia ( Ascidia spp.) as well as in Pterygascidia and could indicate a phylogenetic relationship. However, these characters could just as easily be convergent; and the rows of short transverse muscle fibres and the straight gut loop may be specific rather than generic or familial characters. The presence of a gastrointestinal gland (reported by Millar 1963) is also a character that occurs in all ascidian suborders and appears not to have a phylogenetic significance at higher taxon levels ( Berrill 1950).

Pterygascidia inversa C. and F. Monniot, 1991 from New Caledonia is said to have a long, narrow stalk from the anterior end of the head and the body is said to hang from the top of this stalk. It is unlikely that the body is other than held aloft by the stalk (as commonly occurs in stalked species of all taxa) and the stalk does not appear to have any convergent or phylogenetic relationship with the stalk in P. mirabilis which is short, thick and cylindrical and arises at the posterior end of the body. Further, the New Caledonian species has an open mesh of relatively few internal longitudinal vessels crossing transverse sinuses; and a short gut loop across the posterior end of the left side of the body while P. mirabilis has an entire branchial wall perforated by regular rows of numerous stigmata (albeit they lack ciliated epithelium) and a straight gut extending from the posterior end of the pharynx to the base of the excurrent aperture. Also, in P. inversa gonads are in the gut loop and consist of a sac-like ovary and branched testis follicles associated with it while P. m i r a b i l i s has a long tubular ovary and a unique tight sleeve of small testis follicles around the proximal half of the intestine and over the stomach wall.

Pharyngodictyon Herdman, 1886 , a deepwater colonial species resembles the present genus only in its separately opening apertures and lack of ciliated epithelium. It has its gonads in a posterior abdomen and also lacks stigmata. Kott (1969 and 1992, respectively) proposed its affinity with Euherdmaniidae or Ritterellidae , related families with separately opening apertures and gonads in a posterior abdomen. Other stalked species with separate zooids such as Rhopalaea tenuis Sluiter, 1904 , Euherdmania dentatosiphonis ( Millar, 1975) and many Clavelina spp. are distinguished by many significant characters but resemble one another and the present species in their solitary habit and their similar recurved branchial siphons (see Kott 1990 and 1992) These recurved incurrent siphons, that have the effect of turning the aperture toward the substrate, appear to be a convergent character found throughout the Class.

The presence of dorsal languets supports the hypothesis of a relationship with ancestors of the family Agneziidae before the loss of the entire longitudinal vessels, reduction in the size of the pharynx (reduced number and length of rows of stigmata) and the development of spiral stigmata. The absence of ciliated epithelium lining pharyngeal perforations is very likely a secondary adaptation associated with the deep-water habit of this genus and the small crowded testis tubules forming a tight casing around the gut may characterise the family.