Aplidium meridianum

Aplidium meridianum (Sluiter, 1906)

References: Amaroucium meridianum Sluiter, 1906: 15, fig. 12, pl. 1.

Aplidium meridianum Monniot F. 1978: 4, fig. 1A (type revision); Monniot et al. 2011: 12, fig. 6 (synonymy). Material examined: 22 colonies; net; -38.0164 lat. -54.5054 long. (station 9); 1006 m; 11 August 2012 (Figure 2).

Colonies are dome-shaped, spherical or digitiform supported by small, coarse bases with thin, short rhizoids encrusted with mud and grains of sand. The average size of the colonies is 2.3 cm in diameter and 1.3 cm tall. The tunic is smooth and free of external material. Zooids can be seen through transparency. When alive, the tunic is grayish and the zooids are white (Fig. 2). After conservation in formalin, the tunic turns light gray and the zooids turn pinkish. No regular systems are formed, but some colonies displayed elliptical or longitudinal row patterns. Cloacal apertures are inconspicuous.

Zooids range from five to 12 mm in length. However, some of the post-abdomens can reach a maximum extension of 14 mm. The small atrial aperture is surrounded entirely by a short ring of tunic. It is situated next to the first and second row of stigmata. The atrial languet is wide and short. There are eight to ten thin and long oral tentacles disposed in a circle. The thorax bears from 12 to 18 rows of 12 to 14 stigmata. Six to ten thin longitudinal muscles run through each side of the zooid.

The stomach has six to eight longitudinal well-marked folds of different sizes. Some folds are shorter than others or even incomplete. The gut loop curves dorsal and posterior and widens toward its final part. The bilobed anus ends between the sixth and eighth row of stigmata.

The gonads are situated under the gut loop with no space in between. The testes are arranged in a simple longitudinal row or in clusters. The number of male follicles ranged from seven to 14. The sperm duct ends next to the anus.

Either one or two larvae were found in the atrial cavities of all zooids in an immature state of development. Remarks. This is the northernmost record of the species, previously known from Antarctic and Sub-Antarctic areas.