Aplidium sp.

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 : 1175-1176

Aplidium sp.

( Figures 9a , 10a–c )

Material examined

NEW ZEALAND, Stewart Island , Bob’ s Point (46.854°S, 168.126°E, 10 m, 21 August 2007, NIWA68113 ) .

Description

The grey (5R8/10) colony forms a large (250 mm maximum diameter) irregularly folded investing sheet up to 10 mm thick. The zooids are in depressed crater-like areas of test. They form mostly circular to oval systems around a central common cloacal aperture ( Figure 9a ). The test is tough, stiff and heavily invested with sand throughout. The zooids are 8 mm long, and the post-abdomen is 2/3 the length of the body ( Figure 10a ). The thorax measures approximately 2.5 mm, and has 17 fine longitudinal muscles on each side. The fine muscles coalesce into a single band on each side of the mid-ventral line of the abdomen, and run the entire length of the post-abdomen. The branchial aperture has 6 low rounded lobes, and the atrial aperture is surmounted by a plain stout lappet. The lappet is anterior to and separated from the atrial aperture. On less mature zooids the atrial aperture appears as a short siphon, described by Millar (1982, fig. 13d) for Aplidium scabellum ( Michaelsen 1924) . The branchial sac has 12 rows of approximately 8 stigmata per half-row. The stomach is located halfway down the abdomen and has 4–5 distinct folds. The intestine is narrow at its proximal end, widening anteriorly to the gut loop, with the rectum opening to a bilobate anus adjacent to the ninth row of stigmata ( Figure 10b ).

A large oval ovary is in the posterior half of the posterior abdomen, immediately anterior to approximately 20 testis follicles arranged in a single row ( Figure 10a,c ). The zooids had no larvae.

Remarks

There are several species of sand-invested Aplidium in New Zealand; A. cottrelli ( Brewin 1957) , A. gilvum ( Millar 1982) and A. pseudoradiatum ( Millar 1982) , which all have 10–13 rows of stigmata and 4–5 stomach folds, but differ from the present species in their stalked or flat-topped cushion-like colony morphology, and in zooids lacking a separated atrial aperture and lappet. Aplidium scabellum ( Michaelsen 1924) has similar zooids, and its colonies can also have a similar sand-encrusted test (2 specimens described by Millar (1982) from station C957 on the north-east coast of the South Island). However, A. scabellum zooids have 20–22 rows of stigmata in the branchial sac ( Michaelsen 1924; Kott 1963; Millar 1982), compared to 12 rows in the present specimen. The Aplidium species described here is most likely related to an unpublished species first described by Stocker (1985) as Aplidium sp. 6 (Sandy flat Aplidium ) collected from Northland, New Zealand. This species has similar characters to Aplidium sp. with 12–15 rows of stigmata and a sand-encrusted test thrown into folds, with zooids in circular systems in depressions around a common cloacal aperture ( Stocker 1985). Additional colonies with larvae are, however, needed to further resolve the taxonomic placement of Aplidium sp.

Figures

Figure 9. In situ images of (a) Aplidium sp. (NIWA68113); (b) Pseudodistoma cereum (NIWA68110).

Figure 10. Aplidium sp. (NIWA68113): (a) zooid; (b) thorax and abdomen; (c) post-abdomen showing course of vas deferens and location of ovary. Scale bars = 1.0 mm.