Aplidium knoxi ( Brewin, 1956 )

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 : 1172-1173

Aplidium knoxi ( Brewin, 1956)

( Figures 6e , 8a–c )

Amaroucium knoxi Brewin, 1956: 125–127 , fig. 2D 1, D 2.

Material examined

New records. NEW ZEALAND, Bluff Harbour , Tiwai Point, wharf piles (46.591°S, 168.349°E, 9 m, 21 August 2007, NIWA68091 ; 46.591°S, 168.349°E, 5 m, 8 November 2012, NIWA68167 ) .

Previously recorded. New Zealand, Chatham Islands ( Brewin 1956).

Description

The pink (5R6/10) colony of Aplidium knoxi is approximately 150 mm long, 50 mm wide and 30 mm thick. Common cloacal apertures with thin transparent rims raised 2 mm above the surface occur randomly throughout the colony ( Figure 6e ). Zooids are evenly distributed throughout the colony with no apparent arrangement in systems. The test is soft, gelatinous and invested with numerous test and granular cells, with some incorporated sediment. The zooids are relatively large and robust, the thorax measures 1.3 mm and abdomen 2.2 mm. The thorax has approximately 12 longitudinal muscles, a branchial aperture with 6 low rounded lobes, and a narrow atrial aperture surmounted by a tripartite lappet ( Figure 8a ). There are 14 rows of approximately 10 stigmata per half-row in the branchial sac. The barrel-shaped stomach has 24 folds, some of which are discontinuous. There is a wide intestinal tract ending in a bilobate anus. The post-abdomen is long (6.4 mm) with fine muscles evenly spaced along the entire length. Numerous testis follicles are in 2 irregular rows occupying the posterior half of the post-abdomen with developing ova anterior to the testis follicles ( Figure 8b ). A large convoluted vas deferens runs ¾ of the length of the post-abdomen. Larvae were present in zooids from a colony collected in late spring 2012 (NIWA68167). They have a robust 1.5 mm long trunk with 3 adhesive organs and 3 stout median ampullae. The anterior end of the trunk is covered by an arc of epidermal vesicles each side of the midline ( Figure 8c ).

Remarks

The colony size and colour, and arrangement of zooids, are different from the holotype collected off coastal rocks in the Chatham Islands by Brewin (1956). The morphology of the zooids is, however, the same as in our specimen. The number of folds of the stomach and rows of stigmata are similar, and both have a tripartite atrial languet, bilobate anus and numerous testis follicles in the lower part of the posterior abdomen, posterior to the ovary. Larvae of this species are newly described here. The variation in colony size and arrangement of zooids is most likely to be a function of exposure to high currents. A similar environmental response is common for Pseudodistoma cereum in Fiordland, where encrusting colonies are found in exposed habitats and massive forms are in deep water ( Page et al. 2014).

Figures

Figure 6. In situ images of: (a) Diplosoma velatum (NIWA68092); (b) Polyclinum novaezealandiae (NIWA68097); (c) Synoicum stewartense (NIWA68089); (d) Aplidium benhami (NIWA68101); (e) Aplidium knoxi (NIWA68091); (f) Aplidium phortax (NIWA68109).

Figure 8. Aplidium knoxi (NIWA68091): (a) abdomen and thorax; (b) post-abdomen; (c) larva. Scale bars = 1.0 mm.