Family DIDEMNIDAE

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Family DIDEMNIDAE

Compiler and date details

P. Kott, Queensland Museum, Brisbane, Queensland, Australia

Introduction

Didemnidae Giard, 1872 are a relatively uniform although very speciose family in the Aplousobranchia. The family is characterised by small zooids divided into thorax (with three or four rows of stigmata), and abdomen (containing a small ovary with only one egg maturing at a time, a dome-shaped to spherical or conical testis sometimes subdivided into circular or grape-like clusters of clavate testis follicles, and a gut loop with its distal end often ventrally flexed). The gut is divided into a vertical oesophagus, smooth stomach, a cylindrical duodenal section, an oval posterior stomach and a large rectum, the latter often divided into a proximal chamber and a narrow distal chamber (see Polysyncraton and Clitella). A pyloric gland extends from the posterior end of the stomach and ends in tubules that encircle the proximal end of the rectum in a more or less conspicuous band. In Atriolum, Didemnum and Trididemnum (with undivided testis) and Polysyncraton and Leptoclinides (with the testis divided into many follicles) the vas deferens, from the central or terminal point of the testis, coils around it in a spiral of increasing diameter and then turns anteriorly from the base, or outer coil, of the spiral to open in the atrial aperture. In Lissoclinum, Clitella and Diplosoma with the testis entire or subdivided) the vas deferens extends in a straight line from its opening from the posterior margin of the testis to the atrial aperture, sometimes in the groove between the two halves when the testis is divided. Body musculature is confined to the thorax, longitudinal bands of muscle occurring in the parietal body wall, narrow transverse bands in the interstigmatal vessels and a pair of broad dorsal pharyngeal longitudinal muscle bands in the wall of the pharynx, one band on each side of the dorsal mid-line. In most species of all genera except Atriolum and Leptoclinides, each dorsal pharyngeal band joins the parietal muscles from the same side of the thorax posterior to the atrial cavity, extends in the body wall around the outside of the posterior end of the thorax or around the oesophageal neck and with the muscles from the opposite side projects away from the ventral margin of the body and into the test as a retractor muscle. The retractor muscle is especially strong in Clitella. Atriolum and Leptoclinides generally have relatively large zooids (up to about 5 mm long in Atriolum), but both dorsal pharyngeal and parietal muscles fade out at the posterior end of the thorax and there is never a retractor muscle.. In Atriolum, Leptoclinides and most species of Trididemnum (which have relatively small zooids), the atrial aperture is on a posteriorly orientated muscular siphon often with five lobes around the rim where it opens into the cloacal cavity. Didemnum, Polysyncraton, Lissoclinum, Clitella and Diplosoma have large sessile atrial openings that often expose the branchial sac directly to the cloacal cavity. The anterior margin of these sessile atrial apertures often are drawn out into a muscular lip which, when inserted into the test around the common cloacal cavity and apertures ensures that the zooid muscles can exercise some control over them. Trididemnum (with coiled vas deferens and undivided testis) is the only genus with three rows of stigmata (all others having four rows). Didemnum and Trididemnum larvae have three rows of stigmata in the larval pharynx, although there are four rows in the larval pharynx of Poysyncraton, Leptoclinides, Atriolum , Lissoclinum and Diplosoma. In most species in the family, the oesophageal buds appear to have similar numbers of stigmata as in the adult zooids. This contributes to accurate assessments of the number of stigmata as the buds are not subject to the same degree of contraction as the adult zooids.

With the exception of Diplosoma and a few species in other genera, minute (seldom more than 0.1 mm diameter) calcareous spicules, synthesised by an ectodermal (lateral) organ of the external thoracic parietal body wall, are in the test (Lafargue & Kniprath 1978; Ballan-Dufrancaise et al. 1995). Sand or other particles are not incorporated into the test in this family, although in one species faecal pellets are included. In a polyphyletic group of species, cyanophyte or Prochloron [Prochlorophyta] obligate symbionts occur in the test or in the cloacal cavities, or non-obligate symbionts occur on the surface of the colony (Kott 1980; Parry & Kott 1988). Such species are, at least partly, autotrophic.

Didemnid colonies are sometimes cushion or sheet-like, often thin (encrusting the substrate); or they may be more substantial flasks or massive arborescent structures, or stalked heads or complex, 3-dimensional trabeculae with a zooid-free internal test core and the zooids themselves in a layer at the surface. Zooids are arranged around a simple common cloacal cavity or along each side of long, branching two or three-dimensional systems of canals; and the colony may consist of a single system served by a single common cloacal aperture, or there may be many. This family is unusual in that the zooids are always short, and the greater part of thicker colonies consists of zooid-free basal or central test, in which embryos are incubated. Usually living colonies are kept inflated by positive pressure in the excurrent water as it passes through large cloacal cavities, behind or around the zooids. Calcareous spicules embedded in the test can also be crowded together to form a supportive internal skeleton for large branching colonies.

Replication in the Didemnidae is by oesophageal budding involving the epicardial sacs. It is a modification of transverse strobilation in other aplousobranchs (Berrill 1950). Colonies of several species of the didemnid-Prochloron symbioses are known to subdivide or lobulate (Ryland 1990). This is associated with colonies moving apart to space themselves, although movements also can occur independently of lobulation (Cowan 1981; Birkeland et al. 1981).

In the Didemnidae, eggs (fertilised through an otherwise vestigial oviduct: Burighel & Martinucci 1994) usually rupture directly from the abdomen and move into the base or centre of the colony where they are incubated. They are liberated as tailed larvae into the cloacal cavity or directly to the exterior through the surface of the colony. The genus Atriolum and a few Leptoclinides species are the only exceptions, with embryos being brooded in a thoracic brood pouch as in some Holozoidae.

The larvae are relatively uniform, with lateral ampullae each side of three adhesive organs in the anterior median vertical line. A blastozooid, as well as the oozooid, occurs in the larvae of a few species of Didemnum, Lissoclinum and Diplosoma, and in most species of Polysyncraton, but never in Leptoclinides, Atriolum, or Trididemnum. Polysyncraton and some Didemnum species have up to eight pairs of lateral ampullae, although usually there are not more than six on each side in other taxa. The monotypic genus Clitella has highly modified adhesive organs forming a convoluted band of adhesive cells on frontal lobes of the large larval trunk and prolific budding takes place in the larvae.

The Didemnidae have long been regarded as the most highly evolved of the Aplousobranchia. Certainly colony organisation and zooid size reduction and simplification are more marked in this family than in others. Nevertheless, these developments do not imply a direct linear relationship with other aplousobranchs. Compelling evidence for an earlier origin (from a common ancestor) for Didemnidae than for most other aplousobranch families exists in the presence in Leptoclinides of high concentrations of vanadium (Hawkins et al. 1983). This element is present in similar high concentrations in the more primitive families of Aplousobranchia and Phlebobranchia. Unless the Didemnidae are polyphyletic, the presence of vanadium in Leptoclinides suggests an origin for the Didemnidae directly from a diazonid ancestor, and implies that the evolution of complex cloacal systems parallels their evolution in other Aplousobranchia.

All genera of the Didemnidae are well represented in the tropical Indo-west Pacific and in Australian temperate and tropical waters, although their diversity is much reduced in the Antarctic and Subantarctic.

The basis for the study of the Didemnidae in Australian waters are the reports on the collections made by European expeditions of 19th century, namely, HMS Challenger (Herdman 1886) and the Dutch Siboga Expedition (Sluiter 1909). An account of existing Australian collections (by Kott 1962), did not advance the understanding of the family to any great extent; although later studies (Kott 1980, 1981, 1982) elucidated the taxonomy of the large group of didemnid-algal symbioses. A major revision of the family in Australian waters by Kott (2001), followed by supplementary works (Kott 2002, 2004a-c, 2005) has resulted in the documentation of 255 species, of which 190 have been described since 1960. Of the species known to occur in Australian waters, 103 are indigenous temperate species recorded from the southern half of the Australian continent and 82 are tropical species with a range in the wider Indo-West Pacific tropical region. Another 70 species are known only from Australian tropical waters but, in due course, may be found to be part of that wider tropical fauna.