Saccocalyx microhexactin, Gong, Lin, Li, Xinzheng & Qiu, Jian-Wen, 2015

Gong, Lin, Li, Xinzheng & Qiu, Jian-Wen, 2015, Two new species of Hexactinellida (Porifera) from the South China Sea, Zootaxa 4034 (1), pp. 182-192 : 188

Saccocalyx microhexactin sp. nov.

( Figures 3–4 )

Material examined. Holotype: MBM179994, South China Sea (17°33.95' N, 117°45.67' E), 5 July 2013, 3542 m depth, hard rock cliff above the mouth of an extinct volcano.

Description. Body is globular with several rows of radially arranged protrusions surrounding a central atrial cavity. The color is white. It looks like a beautiful flower with a long stalk growing on the cliff of a seamount ( Fig. 3 A). Central atrial cavity is large, with a 100 mm diameter at the upper margin. Roughly 20 suboscula, 10–25 mm diameter open into the central atrial cavity. Radially arranged digitiform protrusions are up to 15 mm long and 10 mm in diameter. A lateral osculum with 5–10 mm diameter is present at the apex of each protrusion. In situ, the body is swollen, with protrusions pointing outward, like a blooming snow lotus herb. After being transported to deck, body with protrusions collapsed. Sponge attached to rock with a long tubular peduncle, 10 mm in diameter, 1 mm thick wall, and at least 250 mm in length.

Choanosomal skeleton solid ( Fig. 4 O), with some spicules separated from skeleton only after digestion by concentrated nitric acid overnight. Spicules mainly contain hexactins and diactins. Pinular hexactins with rays having sparsely distributed short spines are present in dermalia and gastralia ( Fig. 4 P), whereas diactins are present in the choanosome. Spirodiscohexasters are numerous and present in the whole body. Drepanocomes occur near dermal and gastral surfaces.

Spicules. Megascleres consist of diactins and hexactins. Diactins are smooth, 2120.3–3557.5/5.3–15.1 µm, usually with four tubercles in middle ( Fig. 4 H). Dermalia and atrialia are similar in size and shape. Dermalia or atrialia ( Fig 4 A) pinular rays ( Fig. 4 I) are 245.8–450.6 µm long, tangential rays are 173.6–246.8 µm long, proximal rays are 188.2–585.8 µm long. Choanosomal hexactins (( Fig. 4 O) are relatively less than diactins, they have short spines in terminal, rays are 126.5–355.8 µm. The skeleton of the peduncle ( Fig. 4 N) is composed of diactines fused to each other by numerous synapticulae.

Microscleres consist of spirodiscohexasters, drepanocomes, plumicomes and microhexactins. Drepanocomes are in two types. Drepanocomes I ( Fig. 4 B) 254.4–318.6 µm in diameter, with 6 clusters of 8 hook-like secondary rays ( Fig. 4 J). Drepanocomes II ( Fig. 4 C) 123.0–137.5 µm in diameter, with 6 clusters ( Fig. 4 K) of 4–6 hook-like secondary rays. Structure of drepanocomes easy to destroy during digestion, and hard to isolate completed ones. Spirodiscohexasters ( Fig. 4 D) 112.4–149.8 µm in diameter, formed by 6 spirally twisted clusters of roughly 8–12 terminal rays ending in discs ( Fig. 4 L) with approximately 18 marginal teeth. Plumicomes ( Fig. 4 E) have six shield-like primary termination, one on the top and one on the bottom, the other four are in the middle area. A single sheild ( Fig. 4 M) have approximately 70 marginal sigma-like secondary rays. Each sigma-like secondary rays has prominent teeth on the inside curve. The diameter of Plumicomes is 35.8–58.5. Microhexactins ( Fig. 4 F–G) with rays covered short and minute spines, rays are 21.4–70.3 µm long.

Etymology. “ mikros ”, Greek, small; “ hex ”, Greek, six; “ aktis ”, Greek, ray, light beam. Microhexactin is a kind of sponge microscleres. The specific name refers to the presence of microhexactins in the new species.

Remarks. Only two species of Saccocalyx are known in the world: Saccocalyx pedunculatus Schulze, 1896 and Saccocalyx careyi (Reiswig, 1999) . The main difference between the two species is that all pinular rays of S. careyi are clavate, while only part of the pinular rays of S. pedunculatus are clavate. However, since these spicules have a big variety in size and shape, and their ranges overlap, S. careyi was considered a doubtful species and could be a junior synonym of S. pedunculatus (Tabachnick, 2002) . Because there are no described microsclere structure details available for S. pedunculatus , we cannot tell what other differences between both species there may be. But our specimen has two distinctive characteristics which are different from the two known species: (a) presence of drepanocomes II, (b) presence of microhexactins. Both types of microscleres have not been found in the other two species of Saccocalyx . The new species has all pinular rays clavate as reported for S. careyi . However, there are other differences: S. microhexactin sp. nov. has less terminal discs in spirodiscohexasters, and more marginal sigma-like secondary rays in plumicomes than S. careyi .

Figures

FIGURE 3. Saccocalyx microhexactin sp. nov. A, photograph showing the specimen in its natural habitat; B – D, external morphology on deck.

FIGURE 4. Saccocalyx microhexactin sp. nov. A – N, SEM images of spicules. A, dermal or atrial hexactin; B, drepanocome I; C, drepanocome II; D, spirodiscohexaster; E, plumicome; F, microhexactin I; G, microhexactin II; H, detail of tubercles in the middle of a diactine; I, detail of the pinular ray of a hexactin; J, detail of the hook-like secondary ray of a drepanocome I; K, detail of the middle part of a drepanocome II; L, detail of the tooth disc of a spirodiscohexaster; M, detail of the whorl of a plumicome; N, spicules of the peduncle; O – Q, LM images of spicule and skeleton; O, choanosomal hexactin; P, tangential view of choanosomal structure; Q, transversal view of choanosomal structure.