Polysyncraton globosum Oliveira & Rocha, 2019

Oliveira, Livia M., Hoeksema, Bert W. & Rocha, Rosana M., 2019, Polysyncraton (Ascidiacea, Didemnidae): a re-examination of some specimens and descriptions of three new species, European Journal of Taxonomy 519, pp. 1-25 : 12-15

Polysyncraton globosum Oliveira & Rocha sp. nov.

urn:lsid:zoobank.org:act:1BF4A55D-4A94-4598-8BC0-4A26E33EF5DD

Fig. 7

Didemnum (Polysyncraton) magnetae – Millar 1963: 702 (misidentification, not Hastings, 1931).

Etymology

The name refers to the shape of the colony.

Material examined

Holotype

AUSTRALIA • 1 colony; Bowerbank leg.; NHM 1866.3.21.21, slides NHM 1866.3.21.21, DZUP DID 5.12 .

Paratype

AUSTRALIA • 1 colony; Bowerbank leg.; NHM 1866.3.21.20, slide NHM 1866.3.21.20 .

Description

The species is characterised by its globular colonies, 3.5 cm at maximum length ( Fig. 7a ). The tunic is gelatinous and the colony surface is smooth. The spicules are distributed in low density, mainly on the colony surface. Just one cloacal aperture with a smooth edge was observed in each colony. The globular spicules are 20 to 30 µm in diameter and have numerous thin rays (16 to 18 rays in optical transverse section) ( Fig. 7b ).

The thorax has 12 conspicuous muscle bundles on each side ( Fig. 7c ). The oral siphon has six triangular lobes. The atrial languet is bifurcated but was lost in some thoraces when the zooids were dissected. The lateral thoracic organ is circular, not protruding, located at the level of the 1 st row of stigmata, in the middle of the thorax ( Fig. 7c ). The second gut loop does not overlay the stomach. The testis is divided in six follicles surrounded by two or three coils of the sperm duct ( Fig. 7d ), although the description by Millar (1963) mentioned up to eight follicles. Larvae are elliptical, the trunk 1.0 to 1.3 mm long ( Fig. 7e ). The tail winds about ¾ of the way around the trunk. At the left side of the larva, it is possible to see a wide membrane projecting from the sides of the tail ( Fig. 7f ). The three adhesive papillae with long and thin peduncles are close together and there are eight pairs of rectangular ectodermal ampullae. The sensory vesicle is in the central region of the larval trunk, with the ocellus and estatocyte close together. The larva is not gemmiparous.

Remarks

These Australian specimens, which were deposited at the Natural History Museum in London, have previously been identified as P. magnetae Hastings, 1931 by Millar (1963). The sample NHM 1930.12.17.46 from the Great Barrier Reef, Stat. XXIII, R/V “Magneta”, collected on 12 Mar. 1929 is part of the type series of P. magnetae and was re-examined ( Fig. 8 ). Although also looking globular, the colony is not as solid as in P. globosum sp. nov. and has a high density of spicules in the tunic, with cylindrical rays of a larger diameter and in a smaller number than in P. globulosum ( Fig. 8 ). The atrial languet in P. magnetae is almost imperceptible, short and pointed, and could be missed during dissection, although Hastings (1931) described it as large. Unfortunately, the larva of P. magnetae is not known for comparison with congeneric species. Kott (2001) also realised that Millar’s identification was wrong and suggested that the NHM material belonged to P. meandratum F. Monniot, 1993 . We do not agree with this suggestion because P. meandratum has thin and sheet-like colonies, an atrial languet larger than those observed in P. globosum sp. nov., less male follicles and a larva less than half the size of that of P. globosum sp. nov. (F. Monniot 1993).

Distribution

Type locality: Australia ( Millar 1963).

Figures

Fig. 7. Polysyncraton globosum Oliveira & Rocha sp. nov., Australia (NHM 1866.3.21.21). a. Preserved colony. b. Spicules. c. Thorax (big arrow indicates the lateral thoracic organ and small arrows the muscle fibers present on each side of the thorax). d. Abdomen. e–f. Larva stained with hemalum (the arrows in f indicate the membrane of the tail). Scale bars: b = 10 µm; c–d = 0.1 mm; e–f = 0.2 mm.

Fig. 8. Polysyncraton magnetae Hastings, 1931, type specimen (NHM 1930.12.17.46). a. Preserved colony. b. Cross section of the colony showing cavities found in the median region (arrows). c. Spicules. d. Cross section of a decalcified colony. Scale bars: a = 1 cm; c = 10 µm.