Yellow Boring Sponge

Cliona paucispina

DIAGNOSIS.—Brown incrustations, penetrating rock crevices, limited excavating efficiency. Densely arranged, slender, bent tylostyles, with inconspicuous, frequently subterminal heads, 340.7 × 6.2 μm (mean length × width). Spirasters with thin shafts, average 4–5 bends, 28.8 × 1.3 μm (mean length × width of shaft), and low spines, 1.6 μm (mean length). Spiraster spines on shaft characteristically reduced. Abundance of intracellular zooxanthellae.

DESCRIPTION.—This sponge forms irregular, patchy, thin incrustations (Figure 8a) on concave surfaces of rock. Judging from the small extent of penetration, it is not a very effective burrower. The erosion pattern on the limestone surface suggests, however, that not only existing crevices are penetrated but that active excavation occurs. The sponge crusts are about 0.5–2.5 mm thick and 3 × 3 mm-10 × 35 mm in horizontal extension. Maximum area coverage observed was 7.5 cm2.

Ectosome: The color at the smooth surface of the incrustations is chestnut brown to grayish brown in life, with small yellowish areas around the oscula. The color in alcohol is drab. The oscula are slightly elevated and open to 6 mm in diameter. No observations on open ostia have been made. Zooxanthellae arc present in the tissue in large numbers. They are intracellular and measure 10 μm, like in Cliona caribbaea. There are also some of the large cells with inclusion described below.

Choanosome: It is light brown to yellowish brown, turning to grayish brown in alcohol. The tissue penetrates and fills small irregular spaces in the substratum (Figure 8b). Larger spaces are only lined with tissue. Its horizontal extension does not exceed the extension of the incrustations on the surface. Depth of penetration reaches 12 mm at the most. Zooxanthellae are still present here. Large cells, elongate fusiform or irregular in outline, which measure 30 × 10 μm–40 × 24 μm, are very conspicuous. They are packed with weakly staining spherical inclusions of 1.5–2.4 μm. The nucleus of 3 μm is provided with a nucleolus. Choanocyte-chambers measure 30 μm in diameter.

Excavations: Little can be added to the observations above. The substratum surface below the incrustations is eroded and existing spaces are enlarged. There is no chamber formation, nor distinct tunnels. All observations were made on very porous or poorly cemented substrata which can easily be broken down. Future findings may demonstrate that this sponge can produce more distinctive burrowing patterns in compact substrata.

Spicules: A series of vertical tylostyles at the surface is also present in this species, although this structure can be obscured in contracted specimens. The spicules are densely packed throughout the tissue; a small number of spirasters is interspersed among the tylostyles. Some tylostyles are organized in rows which branch and brush out toward the surface The tylostyles (Figures 9a,b,e, 10a) are long and slender. They are usually bent in the upper third or fourth of their length, behind the head. The heads are not well set off, narrow, and elongate. Most of them are mucronate or subterminal; many are reduced. The spirasters (Figures 9a–e, 10b) are delicate, with slender shafts and small spines. The number of spines is frequently reduced on the shaft, but small spines are always present at the ends of the spicule. Some shafts are straight or irregularly bent, and W-shaped forms are not uncommon.

Spicule dimensions (in μm): Tylostyles, length × width: 124.6–471.7 × 3.0–8.9 (340.7 × 6.2). Neck width: 2.2–7.4 (5.5). Head length × width: 5.2–22.2 × 4.1–12.6 (12.8 × 8.9). Spirasters, length × width (shaft only): 12.8–43.2 × 0.8–3.0 (28.8 × 1.3). Spine length × width (at base): 0.5–3.2 × 0.5–1.9 (1.6 × 0.8). Number of spines: 6–25 (14.9). Number of bends: 0–7 (4.4)

DISTRIBUTION.—Bermuda.