Family Plexauridae

Thesea nivea Deichmann, 1936

Thesea nivea Deichmann, 1936:122, Plate 7 fig. 7; Plate 11 figs. 16-18.

Remarks. This species branches more or less in one plane with slender branches that bend upward, sometimes at an abrupt 90-degree angle. In life, the colony is reddish violet, with dark red to purple polyps that often have white centers. Under magnification, the tissue of the polyps and coenenchyme is violet, but colorless sclerites are visible on the surface of the stems and at the base and neck of the expanded polyps. When preserved in alcohol the coenenchyme color is a gray, creamy white or light brown. The conical calyces are scattered around the branches and have a visible eight-lobed star. The branches are 3-4 mm in diameter and tips on some branches are slightly inflated. This species may be fouled with epibionts such as ascidians and barnacles, and often has multiple galls per colony that are occupied by invertebrates such as the spionid worm Polydora sp. and the crab Pilumnus floridanus (pers. obs.).
Sclerites are colorless and consist of large, warty, acute spindles and small double heads, and some spiny or warty curved rods. Curved or arched rods extend into the lower neck of the anthocodia, and are present as distinct points in the crown, but rarely extend into the tentacles. The spindles range from 0.3 – 0.5 mm; double heads, 0.1- 0.2 mm; and the curved rods, 0.3 – 0.6 mm.
Some colonies have a bushy appearance that departs from the typical one-plane flat growth form, with slightly shorter and more crooked branchlets. These colonies also have thicker branches up to 5 mm in diameter, but the sclerite assemblage remains consistent with the more slender colonies.

Atlantic distribution: North Carolina to South Carolina, 24-49 m; Dry Tortugas, 71 m; St. Lucia, 371 m; Guadalupe, 358 m (type) (Deichmann, 1936; NMNH collections; SERTC collection).

Figure 4. Thesea nivea sclerites (USNM 16836): a) curved rod from anthocodia; b-e) coenenchymal sclerites; b) double head; e, c) double cones; d-f) spindles. Scale bar = 50 µm.

Note: In addition to the identified specimens examined, one Thesea specimen was collected and examined but unidentified. The specimen is approximately 63 cm in height with a lateral branching growth form. All stems appear to be 1-2 mm in width and gently curving upward from the main stem. The calices are moderately protruding and domelike with retractile polyps. The most obvious character about this specimen is the sclerites, which range from 0.1 mm to over 1 mm in length. The largest of the sclerites are bluntly elongate to globular deposits with two distinct sides. The outer side displays undulating, irregular humps which are quite smooth with the exception of the clusters of low, fine spines at the apex of each hump. The opposing side, which tends to face the axis, displays a very coarse texture and crowded, fine tubercules. The smaller sclerites tend to be concentrated toward the calices and are warty, elongate spindles with acute tips. All sclerites are coral red with the exception of the yellow opercular rods. Although several of the Thesea species in Deichmann (1936) are described as having large, ridged or undulating outer sclerites, no mention is made of sclerites with such a contrasting texture.

Figure 1. Colony of Thesea nivea with expanded polyps, approximately 15 cm length (live specimen, S2695).

Figure 2. Contractile (left) and expanded (right) polyps of Thesea nivea (live specimen, S2695).


Figure 3. Calyces of Thesea nivea (live specimen, S2695).