Megapogon crispatus 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 : 238-241

Megapogon crispatus Jenkin, 1908

( Figs 17 A–H; Table 13 )

Original description. Jenkin 1908, p. 41, pl. XXVII and XXXVIII, figs 131–136.

Type locality. Winter Quarters Bay , Antarctic .

Synonym and citations. Megapogon crispatus, Brønsted 1931, p. 32 ; M. crispatus, Burton 1963, p. 93 .

Material examined. Holotype: BMNH-1907.8.6.131, (two specimens and slides). Winter Quarters, Antarctic, National Antarctic Expedition ( HMS Discovery).

Morphology. Vase-shaped without fringe but with a well-developed oscular collar. Colour whitish in ethanol. Surface villose due to long diactines that cross the surface ( Fig 17 A–B). Size of the fragment 2.4–3.1 mm long, 1.5–1.8 mm wide and 0.1 mm thick. Aquiferous system is leuconoid with spherical choanocyte chambers of similar size than inhalant cavities ( Fig 17C ).

Skeleton. Skeleton inarticulated and composed of diactines, microdiactines, triactines and chiactines. Tetractines are found only in the oscular area ( Figs 17B, 17D ). Cortical skeleton composed of tangential triactines ( Fig 17E ) and of large projecting diactines which are bent towards the proximal end ( Figs 17 B–E, 17G). Two types of microdiactines with spines are placed irregularly on the surface ( Fig 17E ). These microdiactines are also present amongst the atrial spicules, and around the choanocyte chambers ( Figs 17F ). Chiactines and few triactines form the atrial skeleton, with their unpaired actines pointing to the surface, and the paired actines giving support to the atrial wall. Tri- and tetractines of the same size, laid tangentially with the unpaired actines pointing downwards, are forming the oscular collar ( Fig 17D ). The oscular fringe is mainly composed of slightly shorter diactines, and by diactines similar to those found in the cortical skeleton but longer and thinner. It was not possible to measure the spicules from the oscular region, because they were broken or not easily visible in the sections.

Spicules. Diactines: large and curved towards the distal end which has a knob. Proximal end hastate ( Figs 17 A–B, 17G). Size: 513.8 ± 168.2 µm length, 15 ± 4 µm width ( Table 13 ).

Microdiactines I: small, straight and with spines. Size: 23.2 ± 4.0 µm length, 1.1 ± 0.2 µm width ( Fig 17F ; Table 13 ).

Microdiactines II: small, curved, hastate and strongly spined diactines. Size: 33.6 ± 9.4 µm length, 3.1 ± 1.3 µm width ( Figs 17 E–17F, 17H; Table 13 ).

Cortical triactines: almost regular with straight and sharply pointed actines. Size: unpaired actines 149.4 ± 42.7 µm length, 9.4 ± 2.2 µm width; paired actines 149.1 ± 40.8 µm length, 9.0 ± 2.0 µm width ( Fig 17E ; Table 13 ).

Atrial triactines: alate with straight and sharply pointed actines. Measurements not available (see Table 13 ).

Chiactines: Size: unpaired actines 289.0 ± 74.5 µm length, 9.7 ± 2.1 µm width; paired actines 142.4 ± 28.7 µm length, 11.1 ± 1.6 µm width; apical actines 69.4 ± 13.3 µm length, 8.0 ± 1.0 µm width ( Fig 17C ; Table 13 ).

Distribution and depth. The species has been found in shallow waters, in two different localities around the Antarctic; stations Winter Quarters ( Jenkin 1908) and Gauss-station (Brønsted 1931).

Molecular identification. Not available.

Remarks. M. crispatus has a characteristic surface due to the curved diactines, which also present a distinctive shape with a knob on the distal end. This character seems to be constant because both specimens examined present the same type of diactines in the cortex. Also, the presence of a second type of microdiactines with spines is also characteristic of the species and has not been reported in other Megapogon spp. Jenkin (1908) also described the spicules found in the oscular region: two categories of diactines which are similar to the cortical diactines I and II; and two size categories of tetractines. However, we could not measure the spicules of the oscular region because they were difficult to find in the slides and sections.

Figures

FIGURE 17A–H. Megapogon crispatus. A: preserved holotype BMNH-1907.8.6.131. B: histological section including the oscular region. C: histological section including cortical layer, choanosome and atrial cavity. D: histological section including cortical layer and oscular region. E: SEM image of cortical layer. F: histological section of a choanosome chamber. G–H: LM and SEM images of diactines, showing details of the distal ends. Abbreviations: Apa = apical actines of tetractines; At = atrial cavity; Chc = choanocyte chambers; Chi = chiactines; Cor = cortical layer/cortex; Ctr = cortical triactines; Di = diactines; MicI = microdiactines I; MicII = microdiactines II; Inc = inhalant cavity; Tet = tetractines.