Didemnum marineae, Page, 2018

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 : 1162-1164

Didemnum marineae sp. nov.

( Figures 2c , 4a–e )

Type material

Holotype. NIWA68103 .

Type locality. NEW ZEALAND: Bluff Harbour , Tiwai Point wharf piles (46.592°S, 168.352°E, 7 m, 20 August 2007, NIWA68103 , 1 colony) .

Paratypes. Bluff Harbour, Tiwai Point wharf piles (46.591°S, 168.348°E, 9 m, 22 August 2011, NIWA87156 , 3 colonies; 46.591°S. 168.349°E, 6 m, 8 November 2012, NIWA68169 , 2 colonies; 46.591°S, 168.349°E, 7 m, 8 November 2012) .

Etymology

Named after the late Marina Senatorski who contributed to this paper by translating the work of Michaelsen (1924) into English.

Description

The bright orange colonies (5YR7/10) of this species have lobate sheets of up to 150 mm in greatest diameter. They often overgrow sponges and hydrozoans on vertical surfaces such as wharf piles. When encrusting flat surfaces, colonies have numerous regularly spaced raised lobes with terminal raised common cloacal apertures, approximately 1 mm in diameter. Sub-dermal canals are obvious, radiating out from common cloacal apertures along the lobes. On vertical substrates, the lobes can form drooping tendrils several centimetres long ( Figure 2c ). There is no apparent arrangement of zooids into systems. The colony is cream-coloured in formalin and thickness varies from between 1.5 mm when encrusting to 6 mm in diameter across drooping lobes. Spicule-filled papillae are present between regularly spaced stellate 6-lobed branchial apertures, giving the colony test surface a microscopic orange-peel appearance ( Figure 4a ). Spicules are densely crowded throughout the test to form a layer 0.1 mm thick overlaying extensive primary thoracic canals lined with zooids. A sheath of test with a single layer of spicules encapsulates each zooid, and a plug of spicules is adjacent to each lateral thoracic organ. The spicules are small and stellate, ranging from 15 to 25 µm in diameter, with 5–7 conical pentangular rays in optical cross section, sharply truncated at their tips ( Figure 4b ). A fine layer of test and spicules encapsulates zooid thoraces that open directly into the thoracic canals. Zooid abdomens are embedded in a 0.75 mm thick layer of test.

The zooids are small and fragile, the thorax measuring 0.6 mm, and the abdomen approximately equal in length. They have a short, wide branchial siphon surrounded by 6 pronounced triangular lobes ( Figure 4c ). The atrial aperture is large and sessile without an anterior tongue. A thin retractor muscle extends from halfway down the dorsal side of the oesophageal neck. The branchial sac has 4 rows of 5–6 stigmata per half-row, and small lateral thoracic organs project from the parietal wall of the thorax between the third and fourth rows of stigmata. The abdomen is ventrally flexed under the thorax, and the stomach is smooth and oval. The intestine is short and wide with the distal part folded into a secondary loop. A large undivided conical testis with 6–7 coils of the vas deferens is adjacent to the mid-intestine. Large developing yellow eggs (in preservative) were present in colonies collected in November, spring 2012 (NIWA68169, Figure 4d ). Mature larvae were found incubating in the basal test of colonies collected from Bluff Harbour in November 2012 (NIWA 68169). The larval trunk is 0.60 mm long, with the tail wound two-thirds of the way around the body. Four ectodermal ampullae are along each side of 3 anterio-median adhesive organs, and an otolith and ocellus are present ( Figure 4e ).

Remarks

This species is distinguished by small stellate spicules from 15 to 25 µm in size with 5 conical, blunt tips in optical transection crowded throughout the test. Bright fluorescent orange colonies further characterise Didemnum marineae sp. nov. from other Didemnum species described in New Zealand. Of the 8 species of Didemnum in New Zealand ( Kott et al. 2009), Didemnum tuberatum ( Nott 1892) most closely resembles D.marineae sp. nov. with similar stellate branchial apertures and papillae on the test surface. However, from Nott’ s description ( Nott 1892), the minute papillae of D. tuberatum occur in a superficial spicule-free layer, whereas the papillae in D. marineae sp. nov. are spiculefilled and the test not layered. The colonies of each species also differ markedly in colour, and although the spicules of both are stellate, D. tuberatum spicules are larger and have a greater number of rays. Also, the zooids of D. tuberatum have usually only 5– 6 coils of the vas deferens compared to 7 in D. marineae sp. nov.

A species from South Africa, Didemnum milliari Monniot, Monniot, Griffiths & Schleyer, 2001 , is also encrusting and possesses small stellate spicules, but they are irregularly distributed throughout small white encrusting colonies and do not have the spiculefilled surface papillae characteristic of D. marineae sp. nov. The zooids of D. marineae sp. nov. and D. milliari are of equal size, but D. millari zooids have large circular lateral thoracic organs, the larvae are smaller and the test morphology differs too greatly for these two species to be the same.

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 4. Didemnum marineae sp. nov. (NIWA68103, holotype): (a) surface papillae and stellate branchial apertures; (b) spicules; (c); thorax; (d) abdomen showing conical testis and granulated developing egg; (e) larva (NIWA68169). Scale bars: a = 0.1 mm; b = 20 µm; c, d = 1.0 mm; e = 0.5 mm.