DIAZONIDAE Seeliger, 1906

Kott, Patricia, 2009, Taxonomic revision of Ascidiacea (Tunicata) from the upper continental slope off north-western Australia, Journal of Natural History 43 (31 - 32), pp. 1947-1986 : 1952-1953

Family DIAZONIDAE Seeliger, 1906

Type genus: Diazona Savigny, 1816 .

This family is distinguished from other aplousobranch ascidians by its entire longitudinal branchial vessels ( Kott 1990) and the usual position of the gonads enclosed in the vertical gut loop at the posterior end of the long abdomen (although they are in a posterior abdomen in the Antarctic Tylobranchion ). The monotypic temperate Australian Pseudodiazona , previously thought a genus of the Diazonidae , appears to be a genus of the Protopolyclinidae with its gonads in a posterior abdomen ( Kott 2008b). Syndiazona Tokioka, 1955 is known to be a synonym of Diazona (see Monniot and Monniot 2001). With the exception only of some solitary Rhopalaea , species in this family are colonial and the only known method of replication is by abdominal strobilation. Some Rhopalaea spp. are known to regenerate thoraces from senescent or severed abdomina ( Kott 1990; Vasquez and Young 1996) but although this has added to the speculation that budding occurs in this genus ( Millar 1975; Monniot F and Monniot C 2001), this has not been confirmed.

Superficially, diazonid species are similar to those of the Clavelinidae . The thoracic test of both taxa sometimes is thin, delicate and translucent, both containing taxa with translucent to transparent terminal heads on vertical colony lobes with the remainder of the test often seen as a firm stalk, in both families some species have zooids that project separately from the surface of the colony and neither family has sand or other foreign particles embedded in the test. In other respects the zooids of Diazonidae are more like those of Polycitoridae , both having six-lobed apertures (while the apertures of Clavelinidae are smooth-rimmed), both regenerate body organs from epicardial endoderm (rather than budding from mesodermal terminal ampullae as in the Clavelinidae ) and both lack the long, branched vascular stolon of the Clavelinidae . Nevertheless, in polycitorid species the whole test usually is firm and often contains sand and other inclusions and the zooids always are completely embedded. Polycitoridae also differ from diazonids in their relatively short thoraces, long oesophageal necks and smaller stomachs (without the long, crowded parallel striations of Diazonidae in the stomach lining). Polycitoridae occasionally have a long posterior abdominal stolon but more often it is relatively short, as in the Diazonidae .

Diazona and Rhopalaea are the most speciose genera in the Diazonidae , although only one or two species in each are particularly well known. Colonies of the former genus have partially or completely embedded zooids, two longitudinal muscle bands on the abdomen and often-conspicuous transverse thoracic muscles that appear to branch from the longitudinal muscles. In the (usually solitary) zooids of Rhopalaea spp. , muscles are confined to the thorax and do not extend onto the abdomen. In contracted specimens the disposition of the muscles often is difficult to determine and many species in these genera have been wrongly assigned. In fact, Rhopalaea circula Monniot F. and C., 2001, its zooids joined by stolons and R. desme Monniot F. and C., 2003 with zooids joined basally in common test, are known to be colonial (although the mechanism of replication is not known). Rhopalaea tenuis ( Sluiter, 1904) zooids occur in similar bunches (albeit they are said to be isolated) and the species appears to be related or even conspecific with R. desme and/or R. circula . These species all have muscles on the abdomen and may belong to the genus Diazona rather than Rhopalaea . Also wrongly assigned to Rhopalaea , R. piru Monniot C. and F., 1987 is found ( Kott 1990, 2006) to be a junior synonym of Diazona chinensis (Tokioka, 1955) . Excluding those species thought to be wrongly assigned, Rhopalaea is known from two species from the Mediterranean and one each from Norway, the western Atlantic and the eastern Pacific and only one, R. crassa , from the Indo-west Pacific ( Tokioka 1971; Kott 1990; Monniot F and Monniot C 2001). Among the synonyms proposed for the latter species are those (such as R. fusca Herdman, 1880 , R. perlucida Monniot C., 1997 and R. respiciens Monniot C., 1991 ) distinguished from one another only by apparent differences in the relative length and shape of parts of the body and in the musculature, both directly affected by growth and/or contraction. These differences include variations in the consistency of the test that often occur with growth and in the numbers and the thickness of longitudinal muscle bands as they are drawn together by contraction of their oblique connecting branches ( Kott 1990, 2003, 2006).

Diazona species diversity in the tropics is greater than is known for Rhopalaea . The genus is known from at least nine species in waters of the western Pacific ( Monniot C 1987; Monniot C and Monniot F 1991; Monniot F and Monniot C 1996, 2003). The new records of the genus in the present collection are the first from Australian waters. Only one species is reported from European waters and two from North America ( Van Name 1945).