Aplidium coronum, 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 : 1669-1672

Aplidium coronum sp. nov.

( Figures 10 ; 7D, E )

Type material

Holotype: NIWA 49995

Type locality: Thompson Sound, Crayfish Heights (45° 13.182’S, 166° 58.656’E, 25 m, Vertical wall, coll. M. Page, 30 January 2006).

Paratypes: Thompson Sound , Crayfish Heights (45° 13.182’S, 166° 58.656’E, 25 m, Vertical wall, coll. M. Page, 30 January 2006, NIWA 87165 ) ; Fiordland , south-side Thompson Head (45° 13.772’S, 166° 57.868’E, 5–10 m, 31 January 2006, NIWA 49996 ) ; Fiordland , Nine Fathom Passage Dusky Sound (45° 44.237’S, 166° 53.199’E, 15 m, 29 January 2009, NIWA 49954 ) ; Fiordland , Crooked Arm head (45° 25.12’S, 166° 56.00’E, 20 m, 2 February 2006, NIWA 50002 , one colony) ; Bluff Harbour (46° 35.47’S, 168° 20.96’E, 4–9 m, Tiwai Point wharf piles, 12 February 2011, NIWA 68090 , 17 August 2011, NIWA 68122 ) .

Etymology

From the Latin Coronum: crown.

Description

This species has massive peach (R 6/10) to white ( Figure 7D, E ) lobed colonies of up to 20 cm in diameter and 10 cm high. Colour appears to depend on light exposure. The shaded upper reaches of fiords such as Crooked Arm have only white colonies (NIWA 50002). The zooids are arranged in parallel double rows on each side of large common cloacal canals. The canals branch and radiate out from numerous large (5 mm diameter) terminal common apertures situated at the end of the lobes ( Figure 7D ). The test around the apertures forms raised rims in relaxed colonies. The test is soft, gelatinous and transparent in lighter coloured colonies with no sediment; however, there are small tunic cells measuring 15 μm in maximum diameter scattered throughout the test.

Zooids of this species are 12–15 mm long. The thorax and abdomen are pigmented bright orange and there is a long thin post-abdomen ( Figure 10A ). In relaxed specimens all zooids have six long oral lobes that form a distinctive crown-like pattern, hence the species name coronum . The atrial aperture opens at the second row of stigmata, and can vary in size depending on the degree of relaxation. It can have a long, sometimes trifid languet, occasionally the same length as the thorax ( Figure 10A ). This character is, however, variable among zooids within colonies and between colonies of the same species, and depends entirely on the degree of relaxation. The atrial wall has approximately 10 fine longitudinal muscles and is enlarged to accommodate developing embryos, up to three in the specimens we examined. A network of fine longitudinal and transverse muscles can be seen in the thin body wall of the thorax; the longitudinal muscles coalesce and run the length of the post-abdomen. The branchial sac has 15–17 rows of stigmata with 10 elongate stigmata per half row. The pharynx enters a small barrel-shaped stomach at the distal end of a long oesophagus. The brown stomach has 23 mostly complete folds. There is a wide duodenum with a marked junction to the mid intestine and relatively long oval posterior stomach. The rectum opens in the atrial aperture to a bifid anus adjacent to the 11th row of stigmata.

The post-abdomen is long and narrow with the heart at its distal end. An ovary is located in the centre of the post-abdomen and male follicles are arranged in double rows in the posterior half of the post-abdomen ( Figure 10C ). The non-pigmented larvae are relatively large, measuring 1.4 mm trunk length. There are four thin median ampullae alternating between the three slender adhesive papillae ( Figure 10D ). Numerous vesicles are arranged in a multiple series, forming a crowded mass at the anterior end of the larva and tapering off towards the tail.

Remarks

Aplidium coronum sp. nov. is distinguished from the majority of species worldwide by a combination of numerous stomach folds (more than 15), numerous rows of stigmata (15– 17) and massive conically lobed colonies. Aplidium phortax ( Michaelsen, 1924) , Aplidium adamsi Brewin, 1946 , and Aplidium opacum Kott, 1963 most closely resemble A. coronum in having massive colonies, and similar gastric and branchial characters.

Aplidium opacum Kott, 1963 from South Australia closely resembles A. coronum in zooid size and morphology. It has large fleshy colonies and branching primary cloacal canals, but without the regularly spaced, raised conical lobes and terminal common cloacal apertures described in A. coronum . There are only 12 rows of stigmata compared with 15–17 for A. coronum . Opaque white cells that crowd the siphons and line the zooids in A. opacum , are absent in A. coronum .

The two New Zealand species with fleshy colonies, Aplidium phortax and Aplidium adamsi described by Brewin (1946) differ from the present species by their colony morphology, number of stigmatal rows and preferred habitat. Both have relatively thin colonies that do not form the massive lobes with terminal common cloacal apertures seen in Aplidium coronum . Aplidium phortax colonies reach 20 mm in height above the substratum. There is no tendency to mound formation around common cloacal apertures and zooids have only 12–13 stigmata per half row. Colonies of A. adamsi are up to 35 mm high and form mounds around common cloacal apertures, but these are relatively low (fig. 4, Brewin 1946) compared with the massive, often greater than 100 mm-high conically lobed colonies of A. coronum . Furthermore, A. adamsi and A. phortax occupy the inter-tidal environment under boulders, and shallow sub-tidal artificial structures such as wharf piles, whereas colonies of A. coronum are subtidal, generally occurring below 5 m depth.

The larvae of Aplidium coronum are approximately the same size as Aplidium adamsi according to Brewin (1946) Plate 3 Fig. 1 , although she has given much larger dimensions for A. adamsi in her description. The larvae of both species have a similar arrangement of adhesive papillae and median ampullae. However, Brewin’ s description and drawing of an A. adamsi larva shows no evidence of larval vesicles, whereas A. coronum larvae have obvious multiple layers of larval vesicles.

We have therefore placed Aplidium coronum as a new species distinct from A. adamsi , A. phortax and A. opacum on the basis of differences in colony and zooid morphology, larval structure and habitat preferences.

Figures

Figure 1. Map of Fiordland on the south-west coast of the South Island, New Zealand showing location of sampling stations.

Figure 7. In situ images of: (A) Synoicum occidentalis (NIWA 49972); (B) Synoicum stewartense (NIWA 49984). (C) Aplidium benhami (NIWA 49993,); (D) Aplidium coronum sp. nov. (NIWA 60890); (E) Aplidium coronum sp. nov. (NIWA 49995, holotype); (F) Aplidium phortax (NIWA 9947).

Figure 10. Aplidium coronum sp. nov. (NIWA 49995, holotype): (A) thorax and abdomen; (B) branchial siphon; (C) post-abdomen; (D) larva; scale bars: A, C, 1 mm; B, 0.1 mm; D, 0.5 mm.