Trididemnum shawi, Page & Willis & Handley, 2014

Page, M. J., Willis, T. J. & Handley, S. J., 2014, The colonial ascidian fauna of Fiordland, New Zealand, with a description of two new species, Journal of Natural History (J. Nat. Hist.) 48 (27 - 28), pp. 1653-1688 : 1676-1678

Trididemnum shawi sp. nov.

( Figures 12 , 11D )

Type material

Holotype: NIWA 10872 .

Type locality: Fiordland, Crayfish Heights, Thompson Sound (45° 14.279’S, 166° 59.566’E, 10 m, 31 February 2006).

Paratypes: Fiordland, Crayfish Heights, Thompson Sound (45° 14.279’S, 166° 59.566’E, 10 m, 31 February 2006, NIWA 87166 ); The Narrows, Long Sound (46° 03.829’S, 166° 44.16’E, 13 m, 28 February 2009 NIWA 49945 , one colony); Nine Fathom Passage, Dusky Sound (45° 44.237’S, 166° 53.199’E, 16 m, 1 February 2009, NIWA 49960 , one colony); Sunday Cove, Breaksea Sound (45°

35.89S, 166° 44.58’E, 10 m, 18 April 2012, NIWA 68138 , 68139 , 68140 , 68141 and 68142 ).

Etymology

Named after Lance Shaw in recognition of his lifetime passion for conservation in Fiordland.

Description

The colonies of Trididemnum shawi sp. nov. are irregular shaped cushions reaching 150 mm long and 50 mm high. They have large terminal common cloacal apertures (5 mm diameter) located at the proximal end of upright lobes in the colony. The colony is supported by positive hydrostatic pressure and basal test core expanded into the centre of large common cloacal cavities that collapse on removal from water. Colonies are peach coloured (YR 7/8) with characteristic clusters of red pigment cells scattered randomly throughout the test ( Figure 11D ). The texture is gelatinous with zooids regularly packed around the outside edge and sparse spicules concentrated in a layer around zooid branchial apertures. The inner test below the zooid thoraces has no spicules. Developing larvae are present in the test surrounding posterior abdominal cavities in colonies from Breaksea Sound.

The zooids are small, the thoraces measuring 0.45 mm and abdomen 0.8 mm long in contracted specimens. There are reticulated posterior abdominal canals running below a single layer of zooids. The canals connect to a central common cloacal cavity in the anterior half of the colony, the centre supported by an extension of the basal test. The zooid branchial aperture has six sharply pointed lobes, and the atrial aperture is a sessile opening in the centre of the thorax. The branchial sac has three rows of stigmata and a small lateral thoracic organ each side adjacent to the third row of stigmata ( Figure 12A ). The number and shape of stigmata are difficult to determine due to contraction of the thorax. There is a long retractor muscle originating from the base of the thorax. The gut has a smooth globular stomach and the intestine is wide with constrictions occurring between duodenum, posterior stomach and rectum ( Figure 12A ). There is a single large dome-shaped testis follicle with a vas deferens tightly coiled 10–11 times anticlockwise ( Figure 12B ); a large ovum lies on the anterior dorsal side of the testis. Developed larvae in colonies collected in April 2012 from Breaksea Sound (NIWA68138) are large (trunk length 1.1 mm) and have four stout lateral ampullae crowded each side of three slender adhesive papillae ( Figure 12C ). Zooids in these colonies have no testis, suggesting that the reproductive season has ended, but the brooded larvae are still maturing.

Stellate spicules are present in a layer around the zooid thoraces and range in size from <20–75 μm ( Figure 12D ). Small fine spicules with delicate needle-like rays ranging from 15–40 µm in diameter were also observed ( Figure 12E ) and indicate that some dissolution and subsequent calcification occurred during and after preservation.

Remarks

Of nine species of Trididemnum described with sessile atrial openings similar to Trididemnum shawi sp. nov., Trididemnum cyclops Michaelsen, 1921 and T. miniatum Kott, 1977 have only six, and T. nubilum Kott, 1980 seven coils of the vas deferens, compared with 10–11 for T. shawi sp. nov.. Trididemnum paracyclops Kott, 1980 from Australia and the Western Pacific has 10 coils of the vas deferens, but differs markedly in colony (forming thin encrusting colonies), zooid and spicule morphology, a zooid with a long oesophageal neck and larvae with only two adhesive papillae. Trididemnum poma Monniot and Monniot, 2001 from Saipan has a larva with a similar arrangement of adhesive papillae and lateral ampullae, but the larvae are significantly smaller (0.25 mm compared with 1.1 mm) than those of T. shawi sp. nov.. Furthermore, the colonies of T. poma are thin, brittle and encrusting compared with those of T. shawi . Both Trididemnum species recorded from New Zealand, T. sluiteri Brewin, 1958 and T. cerebriforme Hartmeyer, 1913 , differ from T. shawi in possessing atrial siphons. Trididemnum shawi sp. nov. is distinguished from the majority of species in this genus by the presence of a sessile transverse atrial opening, a large number of coils of the vas deferens, cushion-shaped colonies with terminal common cloacal apertures and scattered red pigment granules in the tunic.

Figures

Figure 11. In situ images of: (A) Eudistoma circumvallatum (NIWA 49966); (B) Pseudodistoma cereum (NIWA 49992); (C) Pseudodistoma novaezelandiae (MNP7398) from the Kermadec Islands; (D) Trididemnum shawi sp. nov. (NIWA 10872, holotype); (E) Didemnum jucundum (NIWA 10873); (F) Didemnum lithostrotum (NIWA 49980).

Figure 12. Trididemnum shawi sp. nov. (NIWA 10872, holotype): (A) zooid; (B) testis follicle, (C) larva; (D) scanning electromicrograph of stellate spicules with conical rays; (E) micrograph (×400 magnification) of recrystallised spicule (arrow) with rod-like rays; scale bars: A, C 1.0 mm; B, 0.5 mm.