HOLOZOIDAE Berrill, 1950

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 : 1960-1962

Family HOLOZOIDAE Berrill, 1950

Type genus: Holozoa Lesson, 1830 (< Distaplia Della Valle, 1881 nomen conservandum).

Genera now comprising this family once were thought to belong to the family Policitoridae Michaelsen, 1904 ( Monniot C and Monniot F 1991); or to be in a sub-family of the Clavelinidae ( Berrill 1950) characterized by the zooids being clearly divided into thorax and abdomen (lacking a posterior abdomen). Although the larvae resemble those of Clavelinidae in their triradially arranged adhesive organs, these families differ profoundly in their respective vegetative processes, which involve the endodermal epicardial sacs in Holozoidae and the mesodermal terminal ampullae of test vessels in the Clavelinidae . Further, the genus Holozoidae has completely embedded zooids, integrated cloacal systems and a projecting, stalked, holozoid brood pouch formed from a loop of the oviduct projecting from the postero-dorsal corner of the thorax. On the other hand, Clavelinidae usually have only partially embedded zooids, their apertures open separately and although embryos are incubated in the oviduct, which often expands into a brood pouch across the posterior end of the thorax, it is not a loop of the oviduct constricted off from the body wall of the zooid as it is in the Holozoidae ( Kott 1990) . Polycitoridae differ from the present family in their long oesophageal neck, separate six-lobed openings to the exterior, rudimentary common cloacal systems, their method of replication by strobilation of the long abdomen and the lack of conspicuous posterior abdominal vegetative or vascular stolon.

Kott (1990) proposed that the genus Sigillina , including S. mjoebergi (which has a vascular stolon similar to holozoinid genera), should be included in the Holozoidae . However, generally Sigillina (type species S. australis Savigny, 1816 ) appears to have closer affinities with the Pseudodistomidae and lacks many of the characters of the Holozoidae ( Kott 2006) . Only S. mjoebergi has a vegetative stolon similar to that of Holozoidae , yet its larvae are unusually large and have inverted tubular adhesive organs (similar to those of Euherdmaniidae and Pycnoclavellidae ), unlike the large adhesive cones of Holozoidae and Sigillina spp. It appears to be the type of a monotypic family separate from both Holozoidae and Pseudodistomidae (see Vitrumidae , later).

The family Holozoidae contains a diversity of genera, often stalked, but all mainly characterized by the vegetative stolon posterior to the abdomen. This extends into the stalk and generates the buds, which are progressively added to the functioning systems of zooids in the head of the colony. Embryos are incubated in a brood pouch that is separated from the postero-dorsal corner of the thorax by a constriction. In the larvae the three adhesive organs are triradially arranged and precocious buds often are generated from a stolon in the larval trunk. In all genera the zooids are completely embedded and arranged in well-integrated common cloacal systems.

The most speciose genera in the family are Distaplia and Sycozoa with zooids arranged in one or more longitudinal or circular systems. The atrial openings of the individual zooids are wide, sessile openings usually with the upper rim of the opening produced into a wide tongue and exposing most of the branchial sac directly to large common cloacal cavities. The thoraces of the zooids are large and roomy, but there are only four to six rows of stigmata, the stigmata being long, usually with parastigmatic vessels crossing them. Hypsistozoa is a related genus with a complex life history known from two species; one in New Zealand and one on the central eastern Australian coast ( Kott 2007); and another related genus, Neodistoma with a sessile colony and six rather than four rows of stigmata known only from an indigenous south Australian species, N. mammillatum ( Kott 1990) .

Two other related genera in this family, each known from relatively few species, have atrial siphons rather than large sessile openings ( Kott 1990). Their excurrent apertures open either into a common cloacal cavity ( Hypodistoma ), or onto the opposite side of the colony from the incurrent siphons (see the western Australian Polydistoma ).

Also, a deep-water genus, Protoholozoa Kott, 1969 , known originally only from its Antarctic type species, now is represented by another four species (two from New Caledonia, one from the Azores and one from the Cape Basin, at depths from 1000 m to 5000 m; see C. Monniot and F. Monniot (1991)). The abdomina of the species in this genus (with gonads in the relatively short gut loop) and the open atrial aperture are characteristic of this family although the branchial sac is reduced to an open mesh of longitudinal vessels and transverse sinuses as in other abyssal genera .

Only one specimen of Sycozoa is present in this collection. Distaplia is not represented, although it does occasionally occur in deeper water (e.g. Distaplia progressa Monniot C. and F., 1991) and is a frequent and diverse component of the fauna in the shallow tropical waters of the Indo-west Pacific.