A new species of Arachnanthus from the Red Sea (Cnidaria, Ceriantharia)

Abstract A new species of the genus Arachnanthus (Cnidaria: Ceriantharia), Arachnanthus lilith Stampar & El Didi, sp. n., is described. This species is widely distributed in the Red Sea, and recorded from 2–30 m depths. Arachnanthus lilith Stampar & El Didi, sp. n. is the fifth species of the genus and the first recorded from the Red Sea. The number of labial tentacle pseudocycles, arrangement of mesenteries, and distribution of acontioids allow the differentiation of the new species from other species of the genus.


Introduction
While tube anemones are common objects for underwater photographers and are widely exhibited in aquaria, they remain undersampled in most regions of the world, and the diversity and distribution of species remains poorly documented (Stampar et al. 2016). This is especially true for species that are difficult to observe and collect, because of nocturnal habits, small body size, or deeply extended burrows. The small, nocturnal tube anemones of the family Arachnactidae are a case in point (den Hartog 1977;Stampar et al. 2012Stampar et al. , 2015a. This family is comprised of two benthic genera, Arachnanthus Carlgren, 1912and Isarachnanthus Carlgren, 1924(Stampar et al. 2016, although other genera have been proposed based only on larval forms (Molodtsova 2004). However, larval genera are not currently linked to those of adults and therefore their status remains unclear (Stampar et al. 2015a). Carlgren (1912) established Arachnanthus for A. sarsi (which he described from the North Sea) together with Cerianthus oligopodus Cerfontaine, 1891 from the Mediterranean. Carlgren (1924Carlgren ( , 1937 later described A. bockii Carlgren, 1924 from Fiji andA. australiae Carlgren, 1937 from Australia. Since these studies, the genus has received little attention, with Picton and Manuel's (1985) study and redescription of A. sarsi being the most substantive. Here a fifth species of Arachnanthus is described, the first known from Red Sea.

Materials and methods
Specimens were collected by hand at three sites across the Red Sea, from the Gulf of Aqaba to the Farasan Islands, in Saudi Arabia (Fig. 1). Collected polyps were preserved in 10 % buffered seawater formaldehyde solution, and later transferred to 75 % ethanol. The holotype and five paratypes are deposited in the Invertebrate Collections of the Florida Museum of Natural History, University of Florida (UF Cnidaria).
The anatomical study of polyps and cnidome were based on characters defined by previous authors (Carlgren 1912;den Hartog 1977;Stampar et al. 2012Stampar et al. , 2015b. Six specimens were opened along the ventral side (opposite the siphonoglyph), using surgical scalpels, for anatomical study.

Class
Distribution. 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. Etymology. 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).
Live color. 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.
Comparison with other members of the genus. Although Fautin et al. (2007) suggested that morphology alone is insufficient to distinguish species of this genus, Table 2. Comparison of anatomical features of species of Arachnanthus (after Carlgren 1912b;Carlgren 1924;Carlgren 1937;Picton and Manuel 1985; this study).

A. australiae A. bockii A. oligopodus A. sarsi A. lilith sp. n. Marginal tentacles
Up to 40 Up to 30~20 Up to 35 Up to 24

Arrangement of labial tentacles
(0)1.11.11.11.11 (0)1.11.11.11.11(?) (0)1.11.11.11.11 (0)1.11.11.11.11 (0)3.12.31.23.23.12  (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 mesenter-ies, 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.

Length of actinopharynx~2
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.