Didemnum jucundum Kott 2001

Page, M. J., 2018, Colonial ascidians from the Foveaux Strait region of New Zealand, Journal of Natural History (J. Nat. Hist.) 52 (17 - 20), pp. 1157-1180 : 1158-1161

Didemnum jucundum Kott 2001

( Figures 2a,b , 3a–d )

? Trididemnum spiculatum Kott, 1972b , 16.

Didemnum jucundum Kott, 2001 , 197–199, fig. 94, 167D, pl. 10G, H. Material examined

New records. NEW ZEALAND: Bluff Harbour , Tiwai Point wharf piles (46.592°S, 168.352°E, 20 August 2007, 1 colony, NIWA68108 ; 1 February 2011, 4 colonies, NIWA87152 ; 7 February 2011, 8 colonies, NIWA87155 ) .

Previously recorded. New Zealand; Fiordland ( Page et al. 2014); Western Australia; Victoria ( Kott 2001); Kangaroo Island, South Australia ( Kott 1972b).

Description

Living colonies from Bluff vary in colour from black to red-ochre (5R3/10), whereas colonies from Fiordland are always black ( Figure 2a,b ). However, the morphology of spicules, test, colony and zooids is the same between the two regions. Large stellate spicules ranging from 45 to 75 µm in diameter with 13–15 conical rays in optical transverse section ( Figure 3a ) are sparsely distributed in the test. The colonies form extensive encrusting sheets to 100 mm in maximum diameter, often overgrowing solitary ascidians and bivalves. Colour is restricted to a superficial spicule-free layer of test containing bladder cells. A surface layer of test below with few spicules overlies shallow thoracic sub-dermal canals. Test surrounds the zooid thoraces and occasional ramparts support the fine surface layer. Common cloacal apertures 3 mm in diameter are regularly spaced, or located at the apex of lobes formed over encrusting organisms. Zooids are not arranged in obvious systems. White stellate branchial apertures are clearly visible on the colony surface. The zooid abdomens are embedded in a basal test layer, sparsely invested with spicules. Below is a layer with numerous test cells and no spicules.

Zooids are relatively large, each with a long branchial siphon and 6 low pointed lobes. The thorax is 0.7 mm long, there is a long oesophageal neck of equal length, and a large wide abdomen 1.1 mm in length ( Figure 3b ). The atrial aperture is sessile and has no atrial lip. The branchial sac has 4 rows of approximately 8 stigmata per half-row. An oval-shaped lateral thoracic organ lies on each side of the body wall level with the third transverse vessel. A narrow retractor process extends from the anterior end of the oesophageal neck for 1 mm. The stomach is short and globular extending to a wide intestine that curves horizontally across the top of the stomach on the right-hand side, and turns sharply upward to a wide rectum. There is a single large beehive-shaped testis follicle with 11–12 coils of the vas deferens ( Figure 3c ). A large egg is in the gut loop adjacent to the stomach, slightly posterior to the testis follicle.

Large larvae (1 mm trunk length) are incubated in the basal test of colonies collected in February 2011 (NIWA87155). They have 4 stout lateral ampullae on each side adjacent to 3 adhesive organs on long stalks ( Figure 3d ).

Remarks

Didemnum jucundum Kott, 2001 is common in Bluff Harbour on wharf piles. Closely spaced colonies can vary from red-tan to black. The holotype of D. jucundum from Western Australia is a sooty black colour like colonies from Fiordland ( Page et al. 2014). However, Kott (2001) also noted tan coloured patches on a colony from South Australia. Both colour variants in Bluff have the same characters as the Australian populations, with a spicule-free layer on the surface of the test, zooids with a long branchial siphon and oesophagus, numerous (11–12) coils of the vas deferens and a large larva with 4 lateral ampullae each side of the adhesive organs. Didemnum via Kott, 2001 has 11 coils of the vas deferens, similar brown sheet-like colonies and relatively large embryos with four pairs of lateral ampullae. However, the spicules of this species are different than those of D. jucundum , having only 7 to 9 long tapered rays in optical transverse section.

Figures

Figure 2. In situ images of: (a) Didemnum jucundum (NIWA87152);(b) Didemnum jucundum (NIWA87153) a red-ochre colour variant;(c) Didemnum marineae sp.nov.(NIWA68169);(d, e) Lissoclinum violaceum sp.nov. (NIWA87177, NIWA68100 (holotype), respectively); (f) Lissoclinum notti (NIWA87176).

Figure 3. Didemnum jucundum: (a) spicule (NIWA87152); (b) zooid; (c) testis; (d) larva. Scale bars: a = 20 µm; b = 1.0 mm; c = 0.5 mm; d = 1.0 mm.