Achramorpha antarctica, Alvizu & Xavier & Rapp, 2019

Alvizu, Adriana, Xavier, Joana R. & Rapp, Hans Tore, 2019, Description of new chiactine-bearing sponges provides insights into the higher classification of Calcaronea (Porifera: Calcarea), Zootaxa 4615 (2), pp. 201-251 : 212-215

Achramorpha antarctica sp. nov.

(Figs 3A–F, 4A–E; Table 4 )

Diagnosis. Achramorpha with a cortical skeleton composed of triactines and long bundles of diactines/trichoxeas which penetrate the choanosome and may reach the atrium. There is only one type of diactines present in this species, which are long, nearly straight, with smooth surface and with both tips sharpened.

Type locality. Weddell Sea, Antarctic.

Synonyms and citations. Achramorpha truncata, Burton 1929, p. 402 ; A. truncata, Barthel et al. 1997, p. 47 .

Material examined. Holotype: NHMD-611894 (one complete specimen), Weddell Sea, between the stations Vestkapp and Halley , 333-338 m depth (73º22.60’ S, 21º 10.60’ W) ( Barthel et al. 1997) . Paratype: BMNH- 1926.10.26.250 (dry material), British Antarctic Expedition 1910–1913, st. nr. 339.

Etymology. Named after the type locality.

Morphology. Tubular sponge with surface hispid due to long diactines and long trichoxeas protruding the surface (Fig 3A). Colour is whitish in ethanol. Consistency fragile. Aquiferous system syconoid with elongated choanocyte chambers with sizes ranging from 429.8 to 599.6 µm length and from 103.0 to 156.7 µm width. The size of the holotype is 24.6 mm long, 8.7 mm wide and 0.46–1.03 mm thick; and the paratype (BMNH-1926.10.26.250) 11.9 mm large, 7.9 mm wide and around 0.8 mm thick.

Skeleton. Inarticulated skeleton composed of diactines, trichoxeas, chiactines and triactines. Sagittal tetractines can be found only in the oscular region (Fig 3B). Cortical skeleton is formed by triactines tangentially positioned, and by long diactines and trichoxeas arranged in bundles (Figs 3C–3E), which penetrate the choanosome and may reach the atrium. Smaller diactines are arranged radially penetrating the choanosome and protruding the surface (Fig 3D). The choanosomal skeleton is composed of the bundles of diactines/trichoxeas (Fig 3E), and of unpaired actines of atrial triactines and chiactines (Fig 3F). Few triactines can also be found in the atrial skeleton (Fig 3F). The atrial skeleton consists mainly of chiactines which are placed in the atrial wall with unpaired actines pointing towards the cortex, paired actines adjacent to the atrial wall, and apical actines straight and projecting towards the atrial cavity (Figs 3B, 3F). The oscular region is the only part of the skeleton with a proper atrial skeleton composed of triactines and tetractines with the unpaired actines pointing downwards. The apical actines from the tetractines are short and slightly bent pointing to the osculum (Fig 3B).

Spicules. Cortical diactines: highly variable in length and thickness. All diactines present smooth surface and are nearly straight with sharp tips ( Figs 4 A–B). Size: 845.6 ± 322.9 µm length, 17.6 ± 5.5 µm width ( Table 4 ).

Atrial/oscular triactines: sagittal. T-shaped triactines with unpaired actines longer than the paired ones, and slender sharp tips. Size: unpaired actine 756.8 ± 138 µm length, 12.7 ± 3.1 µm width; paired actines 282.3 ± 90.1 µm length, 13.1 ± 3.4 µm width ( Fig 4D ; Table 4 ).

Chiactines: unpaired actines longer than the paired actines, with similar width. The apical actine is strait and slender with sharp tip. Size: unpaired actine 599.1 ± 176.3 µm length, 12.9 ± 2.3 µm width; paired actines 230.8 ± 44.5 µm length, 13.1± 1.9 µm width; apical actine 84.9 ± 29.2 µm length, 9.2 ± 1.7 µm width ( Figs 4 E–F; Table 4 ).

Oscular tetractines: measures not available because they were difficult to find intact in the spicule preparations and sections (Fig 3B; Table 4 ).

Distribution and depth. This species has been registered off McMurdo Sound at 256 m depth ( Burton 1929), and in the Weddell Sea, 333–338 m ( Barthel et al. 1997).

Molecular identification. Not available.

Remarks. As it was mentioned above, the specimens analysed here were previously identified as A. truncata , probably because they present external morphology similar to A. truncata . However, after re-examining the holotype of A. truncata and comparing it with the most recently collected specimens, we found morphological differences that confirm that these species are not conspecific. The long bundles of diactines/trichoxeas are fairly noticeable and represent a distinctive character of the species and makes it easy to separate from the most closely related species, A. truncata . Additional morphological differences are: 1) presence of long trichoxeas in A. antarctica sp. nov.; 2) different form and size of the cortical diactines, significantly longer in A. antarctica sp. nov. (845.6 ± 322.9 µm length in A. antarctica sp. nov. vs. 226.5 ± 32.2 µm length in A. truncata ); 3) chiactines, cortical and atrial triactines longer in A. antarctica sp. nov. than in A. truncata .

Figures

FIGURE 4A–F. Spicules of Achramorpha antarctica sp. nov. A: long cortical diactines. B: long and short cortical diactines. C: cortical triactines. D: atrial triactines. E–F: chiactines.

Tables

TABLE 4. Spicule measurements of Achramorpha antarctica sp. nov. (holotype NHMD-611894).

Spicules	Length (µm)				Width (µm)
	Min	Mean	Max	SD	Min	Mean	Max	SD	n
Trichoxeas	280.4	1106.7	2455.1	612.1	1.1	5.1	11.8	2.6	21
Diactines	229.2	845.6	1989.3	322.9	7.4	17.6	33.1	5.5	30
Cortical triactines
Unpaired actines	222.5	318.3	417.5	50.6	7.7	10.0	13.2	1.5	20
Paired actines	147.8	220.0	273.8	30.2	7.4	10.2	13.6	1.6
Atrial triactines
Unpaired actines	525.1	756.8	923.2	138	9.8	12.7	18.4	3.1	10
Paired actines	204.1	282.3	460.7	90.1	11.2	13.1	20.0	3.4
Chiactines
Unpaired actines	299.8	599.1	945.2	176.3	9.3	12.9	17.8	2.3	23
Paired actines	166.0	230.8	292.3	44.5	9.4	13.1	16.8	1.9
Apical actines	32.2	84.9	133.2	29.2	6.6	9.2	13.0	1.7
Oscular tetractines
Unpaired actines	189.1	257.1	325.2	96.2	15.0	16.9	18.8	2.6	3
Paired actines	143.9	168.1	192.4	34.2	15.8	17.4	19.0	21.0
Apical actines	87.6	102.5	117.4	21.0	17.4	18.1	18.8	0.9

TABLE 4. Spicule measurements of Achramorpha antarctica sp. nov. (holotype NHMD-611894).