Bellactis lux, Delgado & Larson & Sheridan & Daly, 2023

Delgado, Alonso, Larson, Paul, Sheridan, Nancy & Daly, Marymegan, 2023, Bellactis lux n. sp. (Cnidaria: Anthozoa: Actiniaria: Aiptasiidae), a new sea anemone from the Gulf of Mexico, Zootaxa 5353 (4), pp. 379-392 : 383-387

Bellactis lux , n. sp Delgado, Larson, Sheridan, and Daly, 2023

Base: Well, developed, wide, irregularly shaped ( Figure 1B ), adherent. Basal margin with simple bulges that involve all 3 layers of body wall between mesenterial insertions ( Figure 1B ).

Column: Variable in life, short in preserved specimens ( Figure 1B ). When extended, column diameter is equal to or slightly less than column height. When contracted, column height is less than or equal to column width. Sphincter, weak, alveolar ( Figure 2C ). Body wall thin, with visible mesenterial insertions ( Figure 1B ); column slightly inflated between endo- and exo- coels. Scapus and capitulum distinguishable in life: scapus light pink beige and capitulum being darker, rusty brown. Scapus smooth, except for raised cinclides occurring in rows of 2 or 3 in endo- and exo- coels of first two cycles ( Figure 1C ). A single specimen may have both rows of two and three cinclides. No adhesive structures or adherent material observed on column. No collar or fosse.

Tentacles: Tentacles stout, in cycles of 3–5, arise directly from margin at capitulum and from outer half of oral disc ( Figure 1A ). Tentacle length increases from outer to inner cycles ( Figure 1A ). Tips of innermost tentacles typically fold inward and point to lips ( Figure 1C ). Outer tentacles form bunches in the endo- and exo- coels of last cycles, with two shorter tentacles flanking longer middle one ( Figure 1D ). All tentacles are slightly inflated at the base and taper towards their tips. Tentacles of highly contracted specimens may appear smoothly tapered, without inflated basal portion. Pores at tentacle tip visible as slight divot in contraction. Tentacles translucent, greenish brown with opaque white mottling increasing in density distally; sometimes opaque white spots form annular or sub annular rings of varying thickness ( Figure 1A ). In several specimens, directive tentacles lighter than disc and colored or patterned distinctively from other tentacles.

Oral Disc: Disc slightly wider than the circumference of the body. Disc has a wide tentacle-free area around mouth ( Figure 1A ) but when tentacles are fully inflated and extended, little or none of disc visible. Disc translucent, typically light maroon, with yellow ring around edge in many specimens. Mesenterial insertions visible, typically lighter than oral disc ( Figure 1A ).

Mouth and Actinopharynx: Mouth wide, oval, lips prominent on flat disc, typically cream colored and lighter than background color of the disc ( Figure 1A ). Directives typically but not always marked with cream color. Actinopharynx ribbed, pale, visible through mouth, with two distinct siphonoglyphs.

Internal anatomy: Mesenteries in four cycles; first cycle of 6 pairs perfect ( Figure 2A ). First and second cycles fertile. Retractor muscles on mesenteries of first and second cycle, wide but restricted, becoming weaker on successive cycles. Acontia well developed. Mesogleal marginal sphincter muscle reticulate, weak, short ( Figure 2C ). Strong longitudinal ectodermal muscles in distal column. Basilar muscles strong.

Cnidom: Basitrichs ( Figure 3B, C, E, I, K, N, P ), Microbasic p-amastigophores ( Figure 3 A, D, G, J, L, M, O ), Microbasic b-amastigophore ( Figure 3F ), Spirocyst ( Figure 3H ). Basitrichs found across all tissue types occur in two distinct size ranges in pedal disc. Microbasic p-amastigophores found across all tissues but two size ranges occur in filament tissues. Microbasic b-amastigophores only found in column tissue. Spirocysts only be found in tentacle tissues. See Table 2 for size and distribution.

Etymology: The species epithet “lux” is from the Latin for light, formed as a noun in apposition. This name refers to the common name for the species, the “lightbulb anemone,” given by Dr. Daphne Fautin in response to authors of a popular underwater field guide questioning its identity. The common name highlights the distinctive shape and appearance of the tentacles and has been in use since the early 1990s.

Geographic and bathymetric distribution: Bellactis lux n. sp. has been found throughout the eastern Gulf of Mexico in shallow water reef systems and artificial substrate between 5m – 18m.

Differential diagnosis: Members of Bellactis are distinguishable based on external anatomy and cnidom.

The inner tentacles of B. caeruleus are short, whereas the outermost tentacles are slightly extended and distally inflated. In contrast, the tentacles of B. ilkalyseae are stout but tapering and their outer-most tentacles form bunches, whilst those of B. lux are stout and distally inflated, with the outer-most tentacles from bunches of three in endo- and exo- coels of last cycles, were two short tentacles flank a longer middle tentacle. It is unknown if the outer tentacles of B. caeruleus also form bunches. The microbasic p-amastigophores in the acontia of B. lux (63.1–82.6 µm: Table 2 ) are similar in size to those reported for B. ilkalyseae (66.4–88.6 µm: Grajales and Rodriguez 2014), whereas those of B. caeruleus are smaller (60–73 µm: Ocaña et al. 2015). The column of B. lux n. sp. has cinclides in groups of two or three, whereas in B. ilkalyseae and B. caeruleus the cinclides are in groups of three or four. In B. caeruleus , the cinclides are relatively more proximal on the column, occurring closer to the limbus than in B. Ilkalyseae or B. lux n. sp.

Remarks: Two specimens of B. lux were maintained by N. Sheridan in an aquarium for about two years, until their unfortunate demise from a predator gorilla crab ( Xanthidae ). Neither specimen was observed to reproduce sexually or asexually in captivity. The very limited evidence of sexual biology suggests that it is a gonochoric, seasonally reproducing species: two specimens collected August 19, 2010, were fertile with only spermaries present; specimens collected October 30, 2019, were infertile. This suggests release of gametes in late summer or early fall, but more specimens are required to exclude the possibility of a more complex life history in terms of sexual development and sex allocation. Specimens in the wild may live close to one another in the same crevices, suggestive of asexual reproduction.

In addition to endosymbionts, ectosymbionts are also common in Aiptasiidae and can be facultative and or obligate relationship with the host Aiptasiid. Both B. annulata and L. lucida can host up to six species of crustations each (see Brooker et al. 2019) and B. ilkalyseae has been noted to associate with a snapping shrimp species, Alpheus aff. Armatus ( Wirtz, Melo, and Grave 2009) . Here we report an observation of B. lux associating with P. yucatanicus ( Figure 5 ). The extent of the relationship is unknown, but this report offers a baseline understanding of symbiotic relationships of this species and contextualizes a potential shared derived trait for this genus. There are no reports of Bellactis caeruleus hosting any ectosymbionts, however there has been no attempt to observe this species across its distribution.

Figures

FIGURE 1. External anatomy of specimen Bellactis lux, n. sp. A. Top view of living animal, showing oral disc and mouth. B. Side view of living animal, showing tentacles, column, and limbus. Pedal disc is retracted into the proximal column. C. Lateral view of living animal, showing raised cinclides in mid-column. Tentacles inflated and pointed inwards. The scale bar is 5mm. D. Lateral view of preserved animal, showing outer tentacles forming bunches in the endo- and exo- coels of last cycles of two short tentacles flanking a longer middle tentacle. Scale bar is 10mm.

FIGURE 2. Histology of Bellactis lux, n. sp. A. Cross section through column just below actinopharynx, showing cycles of mesenteries (Roman numerals). The scale bar 2mm. B. Close up of mesenteries, proximal to view in A, showing one mesentery from each of two pairs in the first cycle and later cycles between them. The scale bar 200µm. C. Longitudinal section through distal column. Sphincter muscle is very weak, consisting of a single band of alveolae within the mesoglea. The mesoglea is not thicker in the sphincter region than proximally. Photosymbionts are small, red-stained inclusions in the distal column endoderm. The scale bar 250µm. D. Cross section through a tentacle showing the ectodermal longitudinal musculature (LM), mesoglea (M) and dense dinoflagellates (Df) in tentacle endoderm.The scale bar 250µm. E. Longitudinal section through the column, showing an aconitum (Ac) protruding through a cinclide. Note the slight difference between thickness of ectoderm and mesoglea in distal column (top of image) and mid column, near cinclide.

FIGURE 3. Cnidae of Bellactis lux, n. sp. See Table 2 for size information. All capsules are oriented so that their apex is on the right hand of the page. Tissue from which capsules were taken indicated. Basitrichs (B, C, E, I, K, N, P), Microbasic p-amastigophores (A, D, G, J, L, M, O), Microbasic b-amastigophore (F), Spirocyst (H).

FIGURE 5. In situ images without scale bars of B. lux associated with P. yucatanticus taken by N. Sheridan (A) and H. Thoricht (B).