Holotype: UF Cnidaria 9168, adult individual (35 mm long), Saudi Arabia, island near Jaz’air Sila, (27.651°N, 35.2832°E) (Fig. 1A), 10–30 m depth, fore reef, under rocks, G. Paulay, Seabird McKeon, Daisuke Uyeno coll. (27/ix/2013). Paratypes: UF Cnidaria 9167, adult (31 mm long), same data as holotype. UF Cnidaria 9227, adult (35 mm long), UF Cnidaria 9229, adult (42 mm long), UF Cnidaria 9230, adult (26 mm long) all three from Saudi Arabia, Gulf of Aqaba, Joey’s Shipwreck Bay, (28.1846°N, 34.6381°E) (Fig. 1B), 7–13 m depth, in sand and seagrass bed, collected at night, G. Paulay, Daisuke Uyeno, Casey Zakroff coll. (01/x/2013). UF Cnidaria 9076 (Fig. 2D), adult, Saudi Arabia, Farasan Banks, Atlantis Shoal (18.1917°N, 41.1138°E) (Fig. 1–C), 9–11 m depth, sandy shoal with patch reefs, in sand, collected at night, Arthur Anker, Patrick Norby, Gustav Paulay coll. (07/iii/2013).

Small ceriantharian, up to at least 42 mm long, 4–6 mm wide. With 19–24 translucent marginal tentacles (3–5 mm long in preserved specimens), each with 2–4 brown bands (Fig. 1); tentacle arrangement (1)2.12.12.12.12…; at least 5 pores per tentacle, pores marked by concentration of green fluorescent protein (GFP) (Fig. 1–F); unpaired marginal tentacle present. With 11–15 pale labial tentacles (up to 2 mm long in preserved specimens), tentacle arrangement (0)3.12.31.23.23.12…; unpaired labial tentacle absent. Long actinopharynx extending over 1/3 of total body length, hyposulcus 3–4 mm long, hemisulci distinct; siphonoglyph wide, connected to eight mesenteries; directive mesenteries a little shorter than hyposulcus. Three pairs of protomesenteries (P), P2 and P4 long and P3 short, metamesenteries (M), long, fertile with double mesenteric filament; betamesenteries (B) short, sterile with single mesenteric filament (double in a short part immediately below actinophrarynx) and rather convoluted; acontioids only in mesenteries M3 and M4; see Fig. 2 for schematic arrangement of mesenteries. Cnidome (Fig. 3) of spirocysts, atrichs, microbasic b-mastigophores (three types), microbasic p-mastigophores (two types), and ptychocysts; distributed as shown in Table 1.

Presently known only from the Saudi Arabian Red Sea, from the Gulf of Aqaba to the Farasan Islands in the southern Red Sea. The species was found extended only at night.

The specific name lilith refers to the mythological figure of a female night demon in the vicinity of the Red Sea to ancient Mesopotamia (Saudi Arabia to Iraq).

Column pinkish tan at basal half or along most of its length, becoming clear toward base of tentacles. Marginal tentacles whitish/transparent, with brown and light green bands; extent of banding variable, with a basal brown band commonly developed. Labial tentacles clear to brown, with whitish base and tips. Oral disk with green and white colors.

(UF Cnidaria 9168). Small polyp, 35 mm long, 4 mm in diameter just below the marginal tentacles, 3 mm diameter near aboral end. With 19 marginal tentacles arranged in two pseudocycles, each 4 mm long and 0.5 mm in diameter near base, tentacle arrangement (1)2.12.12.12.12…. With 12 labial tentacles, each ~1 mm long, brown with a white apical tip, directive labial tentacle absent, tentacle arrangement (0)3.12.31.23.12…. Oral disc 0.7 mm wide, actinopharynx 17 mm long, light beige to light brown, siphonoglyph wide and elongate with eight mesenteries attached, hyposulcus 9 mm long. Directive mesenteries shorter than actinopharynx. Protomesenteries as in diagnosis, M-mesenteries (M), long, fertile with a double mesenteric filament; B-mesenteries (B) short, sterile with single mesenteric filament (double in a short part immediately below actinopharynx) and rather convoluted; acontioids only in mesenteries M3 and M4.

Although Fautin et al. (2007) suggested that morphology alone is insufficient to distinguish species of this genus, internal anatomical characters do actually separate all known species (Table 2). While there are cases of cryptic species among tube-dwelling anemones (Stampar et al. 2012), none are yet documented for Arachnanthus.

Arachnanthus lilith has labial tentacles in three pseudocycles, unlike A. australiae, A. oligopodus, and A. sarsi, which all have them in one pseudocycle, while in A. bockii labial tentacles are not clearly organized and may be considered to fall into one or two pseudocycles. The actinopharynx is 2/3 as long as the gastric cavity in A. australiae, less than ½ as long in the other three described species, and a little over ½ as long in A. lilith. The maximum number of the mesenteries attached to the siphonoglyph is especially useful for distinguishing species: A. australiae and A. bockii have 12 each, A. lilith has eight, A. sarsi six, while A. oligopodus has four. The organization of mesenteries, particularly the mesentery P2 and M3, also provides useful characters to separate species (Table 2). Finally, the distribution of acontioids is also quite different in some species, especially in A. lilith where acontioids are present only on mesenteries M3 and M4. These mesenterial characters serve well to differentiate species of Arachnanthus, although how they vary over the ontogeny of each species remains to be studied.

Finally, the present study demonstrates the importance of more detailed investigations using non-standard collecting techniques. Small ceriantharians are rarely collected as they are frequently nocturnal and can be difficult to extract from the sediment as they retract quickly and rapidly. There are few described species of Ceriantharia with small body sizes; however, this may be the result of sampling limitations.