Megapogon villosus ( 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 : 233-235

Megapogon villosus ( Jenkin, 1908)

( Figs 14 A–F, 15A–D; Table 11 )

Original description. Jenkin 1908, p. 37, pl. XXXVI, figs 115–119.

Type locality. Winter Quarters Bay , Antarctic .

Synonym and citations. Megapogon villosus, Burton 1929, p. 403 ; M. villosus, Burton 1963, p. 93 , 527 (figs 333–334).

Material examined. Lectotype: BMNH-1907.8.6.146 (fragments of one specimen and four slides), National Antarctic Expedition ( HMS Discovery) . Paralectotype: BMNH-1907.8.6.151 (one fragmented specimen and one slide), National Antarctic Expedition ( HMS Discovery) . Paralectotype: BMNH-1907.8.6.153 (one fragmented specimen and one slide), Winter Quarters Bay , National Antarctic Expedition ( HMS Discovery) . Paralectotype: BMNH-1907.8.6.152: one slide, National Antarctic Expedition ( HMS Discovery) .

Morphology. Vase-shaped sponge, without a well-developed oscular fringe. Surface strongly hispid due to a dense mat of very long diactines, which point down towards the base of the sponge ( Fig 14A ). The lectotype is 14.6 mm long, 3.6–7.8 mm wide, and 0.8–1.1 mm thick. Colour beige in ethanol. Aquiferous system seems to be leuconoid with rounded choanocyte chambers scattered in the mesohyl ( Figs 14 B–C).

Skeleton. Cortical skeleton composed of tangentially arranged triactines, and a dense mat of long diactines that cross the surface ( Fig 14D ). Smaller and spiny microdiactines are placed around the inhalant pores ( Fig 14E ). In the choanoskeleton there are chiactines and triactines with their unpaired actines that can project through the surface ( Figs 14 C–F). The atrial skeleton comprises chiactines with their long unpaired actines pointing towards, and often projecting through the cortex ( Figs 14 C–F). The atrial wall is supported by the paired actines of the chiactines, and among them some triactines and microdiactines can be found ( Fig 15A ).

Spicules. Diactines: very long and straight with sharp points. Most of them were broken ( Fig 15B , Table 11 ).

Microdiactines: small and slightly bent diactines. Some smooth, and some with spines. One end lanceolate, and the other hastate ( Figs 14E , 15 C–D). Size: 105.3 ± 15.9 µm length; 4.5 ± 1.1 µm width ( Table 11 ).

Cortical triactines: alate triactines with straight unpaired actines longer than the paired actines. Paired actines nearly straight ( Fig 15B ). Size: unpaired actines 289.6 ± 188.5 µm length, 8.5 ± 2.2 µm width; paired actines 158.5 ± 82.2 µm length, 8.4 ± 2.4 µm width ( Table 11 ).

Chiactines: unpaired actines straight and longer than the paired actines. Apical actine straight, slightly slender, and sharply pointed ( Fig 15D ). Size: unpaired actines 466.8 ± 80.4 µm length, 11.9 ± 2.9 µm width; paired actines 199.0 ± 69.1 µm length, 10.7 ± 1.8 µm width; apical actines 121.3 ± 33.0 µm length, 10.4 ± 2.0 µm width ( Table 11 ).

Distribution and depth. All the specimens included in Jenkin (1908), were taken around Winter Quarters Bay, and most of them from shallow waters.

Molecular identification. Not available.

Remarks. Among the species of Megapogon described from the Antarctic, M. villosus is the only one that presents a strongly hispid surface due to the presence of very long diactines. These diactines were difficult to measure because they were broken but based on the original description ( Jenkin 1908), they can reach up to 1500 µm length, which is as long as in M. crucifer from the Azores.

The four specimens examined here were the same used by Jenkin (1908) to describe the species, and those specimens labelled as cotype are now erected as paralectotype (recommendation 74F of the ICZN).

The oscular region could not be examined, but according to Jenkin (1908), the osculum is at the end of a collar built up by tetractines laying tangentially on the inside, with their unpaired actines pointing downwards and the apical actines projecting into the atrium. On the outside there are also tangentially arranged triactines and a few special diactines placed horizontally ( Jenkin 1908). These horizontal diactines have not been mentioned before in any of the Megapogon species. However, we would need more material where the oscular area is included, to examine this character in greater detail.

Figures

FIGURE 14A–F. Megapogon villosus. A: preserved cotype BMNH-1907.8.6.151. B–C: histological section including cortical layer, choanosome and atrial cavity. D–E: SEM images of cortical layer. F: SEM image of choanosome section, including cortical layer, choanosome and atrial cavity. Abbreviations: At = atrial cavity; Chc = choanocyte chambers; Chi = chiactines; Cor = cortical layer/cortex; Di = diactines; Mic = microdiactines; Os = osculum.

FIGURE 15A–D: Megapogon villosus. A: SEM image of atrial layer. B–D: SEM images of spicules; fragmented diactine and cortical triactines (B), microdiactine (C), chiactine (D). Abbreviations: Chi = chiactines; Ctr = cortical triactines; Di= diactines; Mic = microdiactines; Exa = exhalant aperture; Tri = triactines.