Sea anemones (Cnidaria, Anthozoa, Actiniaria) from coral reefs in the southern Gulf of Mexico

Abstract Seven sea anemone species from coral reefs in the southern Gulf of Mexico are taxonomically diagnosed and images from living specimens including external and internal features, and cnidae are provided. Furthermore, the known distribution ranges from another 10 species are extended. No species records of sea anemones have been previously published in the primary scientific literature for coral reefs in the southern Gulf of Mexico and thus, this study represents the first inventory for the local actiniarian fauna.


Methods
Observations and collections of specimens were done at 20 coral reef localities of the SGM during 2009-2011 ( Figure 1). Habitats sampled include sandy patches, seagrass meadows, rocky pavement, coral rubble, and coral patches in several zones of coral Table 1. Distribution of sea anemones found on the coral reefs of SGM in the present study. The symbol "x" indicates localities of previous but not published records, "*" indicates new records for the locality found in the present study, and " †" indicates new records for Mexico. reefs, and depth and habitat characteristics were recorded. Specimens were collected by hand, either by snorkeling or SCUBA diving, using a small shovel, and hammer and chisel. Collected specimens were transferred to the laboratory and maintained in an aquarium to photograph their color in life. Specimens were relaxed using 5% MgSO 4 seawater solution and subsequently fixed in 10% formalin in seawater. Measurements provided for pedal disc, column, oral disc and tentacles were obtained from living and relaxed specimens. Fragments of selected specimens were dehydrated and embedded in paraffin. Histological sections 6-10 µm thick were stained with hematoxylin-eosin  (Estrada et al. 1982) or Ramón and Cajal's Triple Stain (Gabe 1968). For cnidae examination, squash preparations of small amounts of tissue of two specimens from each species (tentacles, actinopharynx, filaments, column, and if present, marginal projections, acrorhagi, acontia and pseudotentacles) were examined using a Nikon Labophot-2 light microscope (1000x oil immersion), photographed and haphazardly measured. Nematocyst terminology follows Mariscal (1974) and Östman (2000).
Specimens were deposited in the Collection of the Gulf of Mexico and Mexican Caribbean Sea (Registration code: YUC-CC-254-11) of the Unidad Multidisciplinaria de Docencia e Investigación en Sisal (UMDI-Sisal) at the Universidad Nacional Autónoma de México (UNAM), and in the American Museum of Natural History (AMNH, accession number 65822). We followed the taxonomic classification and synonymies implemented in Fautin (2013) with modifications from Rodríguez et al. (2012). Taxa are arranged in families in alphabetical order. The diagnosis of each species is based on the features observed in the collected specimens. The synonym list for each species only contains reference to the first citation of the species by a particular name. The number of specimens examined of each species per locality is indicated in the material examined. Figure 1 displays the coral reef localities sampled in this study. Table 1 indicates previous and new records of the species observed and collected at each coral reef locality; Table 2 includes size ranges of length and width of cnidae capsules for each studied species. Material examined. Alacranes reef (22°31'35"N, 89°46'05"W; two specimens), Cayo Arenas reef (22°07'05"N, 91°24'17"W; three specimens), La Gallega reef (19°13'20"N, 96°07'39"W; two specimens), Ingenieros reef (19°08'41"N, 96°05'22"W; two specimens).

Species
Natural history. Anemonia sargassensis inhabits shallow waters of the lagoon reef zone, often above Thalassia testudinum blades, but is also found under stones and coral gravel, between 0.5-2 m. It is often reported on floating Sargassum (Carlgren and Hedgpeth 1952). Asexual propagation by longitudinal fission is common (Carlgren and Hedgpeth 1952) and bifurcated tentacles can occur (Hargitt 1908, 1912, Pax 1924, Corrêa 1964.
Distribution. Western Atlantic, from the northern coast of USA and Caribbean Sea, to the northern coast of Brazil (Carlgren and Hedgpeth 1952, Varela 2002, Zamponi et al. 1998.
Remarks. Of the 20 valid species of Anemonia, four species have been recorded in the Gulf of Mexico and Caribbean Sea (Fautin 2013): A. sargassensis, A. melanaster (Verrill, 1901, A. depressa Duchassaing &Michelotti, 1860, andA. elegans Verrill, 1901. The anatomy described for A. sargassensis is conflicting mainly in the presence of directives, siphonoglyphs, and marginal projections (e.g. Hargitt 1908, 1912, Pax 1924, Field 1949, Carlgren and Hedgpeth 1952. Just as in Field (1949), Carlgren and Hedgpeth (1952), and Corrêa (1964), we did not find directives in our specimens but 5-6 siphonoglyphs were present. Although some authors suggest that A. sargassensis and A. melanaster are synonymous (Cairns et al. 1986, Ocaña and den Hartog 2002, Wirtz et al. 2003, further studies are necessary to establish the current taxonomic status of both species. Differences between the other two species of the genus in the region, A. depressa and A. elegans, and A. sargassensis are not clear based on the scarce information available and also require further revision.
Natural history. Anthopleura pallida inhabits the intertidal to shallow subtidal zone attached to coral on sandy shores, at 0.5 m. It is azooxanthellate and it broadcast spawns (Daly and den Hartog 2004).
Distribution. Western Atlantic, from Bermuda (Verrill 1900) to Virgin Islands (Duchassaing and Michelotti 1864). This is the first record for the coast of Mexico; found in Alacranes reef (see Table 1).

Remarks.
Currently there are three valid species of Anthopleura in the Gulf of Mexico and Caribbean Sea: Anthopleura krebsi (Duchassaing & Michelotti, 1860), Anthopleura texaensis (Carlgren and Hedgpeth, 1952), and A. pallida (Daly and den Hartog 2004). Anthopleura pallida is distinguished mainly in column color and shape, and the arrangement of verrucae in rows, only present from the margin to the midcolumn (Daly and den Hartog 2004). However, in A. krebsi and A. texaensis, the column is stout rather than elongate, and the verrucae are arranged in rows along the entire column length, from margin to limbus (Daly and den Hartog 2004). Although we found the marginal sphincter muscle diffuse rather than circumscribed-diffuse, all other features including external and internal anatomy and cnidae fit well with the redescription of A. pallida by Daly and den Hartog (2004).
Diagnosis. Fully expanded oral disc and tentacles to 20-38 mm in diameter. Oral disc 10-22 mm in diameter, smooth, brown-yellowish, brown-reddish or pale olivegreen, sometimes with white or yellowish radial stripes in endocoelic spaces of first two or three tentacular cycles ( Figure 5A, B). Tentacles hexamerously arranged in five cycles (about 96 in number), smooth, simple, conical, moderately long (3-5 mm in length), tapering distally, inner ones longer than outer ones, contractile, olive-green, reddish or pale-orange ( Figure 5A, B), often with white or yellowish spots on oral side and sometimes with purple flashes. Deep fosse ( Figure 5I). Forty-eight endocoelic rounded marginal projections forming acrorhagi ( Figure 5C, I) with holotrichs and basitrichs. Column cylindrical, 12-22 in diameter and 7-15 mm in height, densely covered with rounded vesicles, arranged in 96 longitudinal rows from margin to limbus ( Figure 5C, G). Pedal disc well-developed, 12-19 mm in diameter ( Figure 5C). Column and pedal disc light-brown, orange, reddish, yellowish or olive-green. Mesenteries hexamerously arranged in four cycles (48 pairs in specimens examined): first, second and some mesen- teries of third cycle perfect, others imperfect; same number of mesenteries distally and proximally. All mesenteries fertile (except directives); gonochoric; oocytes and spermatic cysts well-developed in specimens collected in January and May ( Figure 5E). Two pairs of directives each attached to a well-developed siphonoglyph ( Figure 5D). Retractor muscles strong and restricted; parietobasilar muscles well-developed with a relatively long free mesogleal pennon ( Figure 5E). Basilar muscles well-developed ( Figure 5H). Marginal sphincter muscle endodermal, strong and circumscribed ( Figure 5I). Longitudinal muscles of tentacles ectodermal ( Figure 5F). Zooxanthellae present. Cnidom: basitrichs, microbasic b-and p-mastigophores, holotrichs and spirocysts (Figure 5J-T; see Table 2). Natural history. Bunodosoma cavernatum inhabits shallow waters, attached to rocks and coral rubble, in the lagoon zone; between 2-6 m.
Distribution. Western Atlantic, from North Caroline to Barbados; along the Caribbean Sea and Gulf of Mexico (Carlgren and Hedgpeth 1952);and Caroline Islands, Micronesia (Bosc 1802). This is the first record for the coast of Mexico; found in the VRS (see Table 1).
Remarks. Currently four valid species of Bunodosoma have been reported in the Gulf of Mexico and Caribbean Sea (González-Muñoz et al. 2012, Fautin 2013): B. cavernatum, B. granuliferum (Le Sueur, 1817, B. kuekenthali Pax, 1910, andB. sphaerulatum Duerden, 1902. Bunodosoma cavernatum differs from B. granuliferum because it lacks the distinct chromatic pattern of the column with alternating pale and dark longitudinal bands but also based on molecular evidence (reviewed in González-Muñoz et al. 2012). Our specimens show that the circumscribed marginal sphincter muscle tends to split in two parts as suggested by Carlgren (1952) (Figure 5I). The distinction between B. sphaerulatum and B. kuekenthali and their Caribbean congeners are not clear based on the information available.
Distribution. Western Atlantic, from Bermuda to Barbados, and along the Caribbean Sea (Belém et al. 1996, Daly andden Hartog 2004). This is the first record for the coast of Mexico; found in the VRS (see Table 1).
Diagnosis. Fully expanded oral disc and tentacles to 18-22 mm in diameter. Oral disc smooth, 3-5 mm in diameter, beige and translucent ( Figure 7B). Tentacles hexamerously arranged in 3-4 cycles (about 24-52 in number), moderately long (about 5-8 mm length), tapering distally, inner ones longer than outer ones, contractile, gray or beige, translucent, with tips whitish or yellowish and scattered bluish dots along the entire length ( Figure 7B, C). Column short, smooth, 3-6 mm in diameter and 6-10 mm in height, bright-brown with faint stripes corresponding to mesenterial insertions. Column distally with 4-6 outgrowths (pseudotentacles). Pseudotentacles branched, ending in globular-shaped vesicles with batteries of macro-and micro-basic p-amastigophores and basitrichs; bluish with gray or brown circle in center ( Figure 7A-C). Pedal disc welldeveloped, circular, 3-7 mm in diameter, light brown or beige, translucent ( Figure 7C). Mesenteries hexamerously arranged in 2-3 cycles (12-24 pairs in specimens examined): first cycle perfect and sterile, others imperfect and fertile; more mesenteries proximally than distally (two and three cycles, respectively). Hermaphroditic ( Figure 7G). Two pairs of directives each attached to a well-developed siphonoglyph ( Figure 7D). Retractor muscles diffuse, strong; parietobasilar muscles with short and thick mesogleal pennon ( Figure  7E, F). Basilar muscles relatively poorly developed ( Figure 7H). Marginal sphincter muscle absent. Ectodermal longitudinal muscles in distal column. Longitudinal muscles of tentacles ectodermal ( Figure 7I). Zooxanthellae present ( Figure 7F). Cnidom: basitrichs, macrobasic and microbasic p-amastigophores, and spirocysts (Figure 7J-V; see Table 2). Natural history. Lebrunia coralligens inhabits narrow fissures of live coral with only the end of the pseudotentacles visible, between 3-6 m. During the day, the tentacles remain contracted and the pseudotentacles fully expanded allowing the zooxanthellae (particularly abundant in this area) to capture sunlight; at night the situation is the opposite, allowing tentacles to capture food (Sebens and DeRiemer 1977).

Remarks.
Currently there are two valid species of Lebrunia, both of them distributed in the Western Atlantic (Fautin 2013). They differ in the branched pseudotentacles: those of L. danae are long and dark-brown whereas those of L. coralligens are shorter, bright bluish-gray, with rounded ends (González-Muñoz et al. 2012).
Natural history. Actinoporus elegans inhabits sandy bottoms, at 1-2 m; the column is burrowed in the sand but the pedal disc is strongly attached to rocks. When disturbed, it contracts the oral disc suddenly, completely burrowing the entire body.
Distribution. Western Atlantic, from the northern coast of Brazil to Guadeloupe, Jamaica, and Curaçao (Corrêa 1973), and Cape Verde Islands (Wirtz 2009). This is the first record for the coast of Mexico; found in the VRS.

Remarks.
Currently there are two valid species of Actinoporus: A. elegans and A. elongatus Carlgren, 1900(Fautin 2013. Actinoporus elongatus is reported for India, Mozambique and Australia (Carlgren 1900, Menon 1927, Clayton and Collins 1992, and it lacks the longitudinal rows of vesicles in the distal column of A. elegans (Carlgren 1900, Corrêa 1973. Additional color patterns observed for A. elegans in coral reefs off the coast of Venezuela include tentacles and oral disc almost completely white with dark-brown stripes, or completely bright orange (unpublished data). or brown ( Figure 9B). Mesenteries hexamerously arranged in four cycles; same number of mesenteries proximally and distally (to 48 pairs in specimens examined): first cycle perfect, others imperfect; third and fourth cycles poorly developed, without filaments or acontia. Gametogenic tissue not observed in specimens examined. Two pairs of directives each attached to a well-developed siphonoglyph ( Figure 9E). Retractor muscles weak and diffuse; parietobasilar muscles poorly developed ( Figure 9E, F). Basilar muscles poorly developed ( Figure 9H). Marginal sphincter muscle mesogleal, strong, transversally stratified ( Figure 9G). Longitudinal muscles of tentacles ectodermal. Acontia numerous, bright orange ( Figure 9C), with basitrichs. Zooxanthellae present. Cnidom: basitrichs, microbasic p-mastigophores, and spirocysts (Figure 9J-Q; see Table 2). Natural history. Calliactis tricolor typically dwells on the shells of living hermit crabs often carrying more than one individual ( Figure 9D), between 10-30 m. This peculiar symbiotic relationship has been widely studied (reviewed in Gusmão and Daly 2010).
Distribution. Western Atlantic, from the northern coast of USA to the northern coast of Brazil, along the Caribbean Sea and Gulf of Mexico Hedgpeth 1952, Zamponi et al. 1998). This is the first record for the coast of Mexico; found in Serpientes and Alacranes reefs.
Remarks. Of the 18 valid species currently considered as valid of Calliactis, only two have been reported in the Gulf of Mexico and Caribbean Sea (Fautin 2013): Calliactis polypus (Forsskål, 1775) and C. tricolor. These two species differ in the color of the cinclides, white in C. polypus and dark-red in C. tricolor (Gusmão 2010). In addition, C. tricolor is distributed almost exclusively in the western Atlantic whereas C. polypus has a wide distribution range, being found in the Red Sea, Hawaii, French Polynesia, Australia, South Africa, East Africa, Maldives, Cape Verde Islands, Japan, Galapagos, and Louisiana (Gusmão 2010, Fautin 2013. García, M.S. Manuela Muhlia, M.S. Fernando Lazcano (PCMyL, UNAM) helped in the field, and Dr. Anastazia Banaszak helped with the English version of this paper. Comments of Dr. Lee van Ofwegen and one anonymous referee improved this manuscript.