Megapogon raripilus Jenkin, 1908

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 : 226-228

Megapogon raripilus Jenkin, 1908

( Figs 10 A–F, 11A–D; Table 9)

Original description. Jenkin 1908, p. 38, pl. XXXVI, figs 120–124.

Type locality. Winter Quarters Bay , Antarctic .

Synonyms and citations. Megapogon raripilus, Brøndsted 1931, p. 32 ; M. raripilus, Burton 1963, p. 93 ; not M. raripilus Alvizu et al. 2018, p. 282 ; not M. raripilus Ghiglione et al. 2018, p. 149 .

Material examined. Lectotype: BMNH-1907.8.6.139 (one slide and one specimen), National Antarctic Expedition ( HMS Discovery), collection date 28.11.1902 . Paralectotype: BMNH-1907.8.6.145: one slide, National Antarctic Expedition ( HMS Discovery), collection date 13.9.1902 . Paralectotype: BMNH-1907.8.6.140: four slides and one specimen, National Antarctic Expedition ( HMS Discovery), collection date 17.01.1903, Flagon point, Winter Quarters Bay .

Morphology. Tubular sponge, slightly thicker at the base, with an apical osculum without a well-developed fringe ( Figs 10 A–B). Colour in alcohol is beige-yellowish. Surface hispid due to projecting diactines and trichoxeas. Aquiferous system leuconoid with rounded inhalant cavities and choanocyte chambers, and larger exhalant cavities ( Fig 10C ). The thickness of the sponge wall is 1.7–1.8 mm. The size of the lectotype (BMNH-1907.8.6.139) is 11.43 mm long and 1.1–2.2 mm wide. The size of the paralectotype examined is 9.6 mm long and 1.5 mm wide (BMNH-1907.8.6.140).

Skeleton. Cortical skeleton composed of sagittal tangential triactines, trichoxeas, diactines and microdiactines. Bundles of trichoxeas are arranged radially, penetrating the choanosome but do not reach the atrium ( Figs 10 C–10E). The choanoskeleton is semi-articulated (see remarks), made up by the same type of triactines found in the cortex ( Figs 10 C–E). Small and spined microdiactines are found around the choanocyte chambers and scattered in the atrial skeleton. The atrial skeleton is mainly supported by the paired actines of chiactines with the unpaired actines pointing towards the cortex, and the apical actines crossing through the atrial wall into the atrium ( Fig 10F ). Sagittal triactines are irregularly scattered in the atrial skeleton, and the unpaired actines from those closer to the cortex, can project into the surface ( Figs 10 E–F). The oscular margin is composed of the same cortical diactines and long trichoxeas.

Spicules. Diactines: large, curved towards the distal end and with blunt tip. The proximal end is thinner and sharply pointed ( Fig 11A ). Size: 596.4 ± 92.1 µm length, 20.6 ± 3.6 µm width ( Table 9).

Microdiactines: minute, slightly bent, strongly spined and sharply pointed ( Fig 11B ). Size: 60.4 ± 14.8 µm length, 3.4 ± 1.1 µm width ( Table 9).

Trichoxeas: long and straight. It was not possible to measure the trichoxeas because they were broken or not clearly visible in the slides.

Cortical triactines: sagittal, with the unpaired actines longer than the paired ones. Cortical triactines size: unpaired actines 381.7 ± 98.9 µm length, 13.4 ± 3.5 µm width; paired actines 157.6 ± 21.7 µm length, 10.1 ± 2.4 µm width ( Fig 11C ).

Atrial triactines: “T” shaped with unpaired actines longer than the paired ones. These triactines are longer than the cortical triactines. Size: unpaired actines 556.7 ± 58.5 µm length, 13.4 ± 3.5 µm width; paired actines 221.7 ± 25.2 µm length, 11.2 ± 2.7 µm width ( Fig 10F , Table 9).

Chiactines: unpaired actines straight and longer than the paired actines, which are slightly bent. Apical actines short, straight, slender, and sharply pointed. Size: unpaired actines 555 ± 172.3 µm length, 13.4 ± 1.7 µm width; paired actines 226.3 ± 27.4 µm length, 15 ± 1.4 µm width; apical actines 79.1 ± 12.2 µm length, 10.2 ± 2.9 µm width ( Fig 11D , Table 9).

Oscular tetractines: not measured because they were difficult to find in the spicule preparations and in the sections.

Distribution and depth. M. raripilus was found around Winter Quarters Bay, in shallow waters ( Jenkin 1908). Molecular identification. Not available. Remarks. The three specimens examined here were the same used by Jenkin (1908) to describe the species, and those labelled as cotype should now be considered as paralectotype (recommendation 74F of the ICZN). After re-examining the material of M. raripilus , we found that the choanosomal organization can be unambiguously diagnosed, because in these specimens the choanoskeleton looks like “articulate” due to the layer of triactines present in the choanosome. According to Jenkin (1908), the “dermal cortex” is formed of a thick layer of irregularly scattered triactines. However, an articulated skeleton is defined as “choanoskeleton composed of several rows of similar spicules”, while inarticulate is “choanoskeleton composed only of the unpaired rays of subatrial spicules… without specific spicules of the choanoskeleton” (Boury-Esnault & Rützler 1997). Based on these descriptions, the choanoskeleton of M. raripilus should not be considered articulated, because the triactines present in the choano- some are not well-organized in rows, and they are the same type of triactines found in the cortical skeleton. How- ever, the skeleton is not completely inarticulated neither, and thus we define it as “semi-articulated”. The differences observed in the skeleton organization can be associated with the body wall thickness and the volume of the sponges, as it has been observed by Lanna et al. (2017) in the species Paraleucilla magna where they found that variability in the skeleton organization is mainly influenced by these two morphological traits. According to Jenkin (1908), M. raripilus present a third type of diactines from the body wall, which he de- scribed as small and irregular diactines with a “set-over” often in the middle of their length, and the size is about 350 µm x 20 µm. However, we considered that this kind of spicules is the same as diactines found in the cortical skel- eton, because there is not distinction between sizes, and based on the figure presented by Jenkin, they look similar.

Figures

FIGURE 10A–H. Megapogon raripilus. A: preserved lectotype BMNH-1907.8.6.139. B: preserved of additional material BMNH-1907.8.6.140. C: histological section including cortical layer, choanosome and atrial cavity. D: SEM image of cortical layer. E-F: histological sections including cortical layer, choanosome and atrial cavity. Abbreviations: At = atrial cavity; Atr = atrial triactines; Bun = bundles of trichoxeas; Di = diactines; Chc = choanocyte chambers; Chi = chiactines; Cor = cortical layer/cortex; Ctr = cortical triactines; Exc = exhalant cavity; Inc = inhalant cavity; Os = osculum; Tric = trichoxeas.

FIGURE 11A–D: Spicules of Megapogon raripilus. A: diactines. B: microdiactine. C: cortical triactines. D: chiactines.