Esteban-Vázquez, B.L., De los Palos-Peña, M., Solís- Marín, F.A., & Laguarda-Figueras, A. (2021). Morphology of endoskeleton and spination in the sea star Midgardia xandaros (Brisingida: Brisingidae) from the Gulf of Mexico. Revista de Biología Tropical, 69(S1), 404-422. DOI 10.15517/rbt. v69iSuppl.1.46381 DOI 10.15517/rbt.v69iSuppl.1.46381 Morphology of endoskeleton and spination in the sea star Midgardia xandaros (Brisingida: Brisingidae) from the Gulf of Mexico Brenda Lizbeth Esteban-Vázquez1* Magdalena De los Palos-Peña2 Francisco Alonso Solís-Marín3 Alfredo Laguarda-Figueras3 1. Facultad de Ciencias, Universidad Nacional Autónoma de México, Ciudad de México, México; brendaestebanv@ gmail.com (*Correspondence). 2. Posgrado en Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Ciudad de México, México; [email protected] 3. Colección Nacional de Equinodermos “Dra. Ma. E. Caso Muñoz”, Laboratorio de Sistemática y Ecología de Equinodermos, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Ciudad de México, México; [email protected], [email protected] Received 15-VII-2020. Corrected 10-IX-2020. Accepted 26-X-2020. ABSTRACT Introduction. The deep-sea asteroid species of Brisingida have a nearly global distribution but have remained poorly understood due to their deep bathymetric distributions and fragile skeletons. Objective. To describe the external and internal morphology of Midgardia xandaros including the skeletal arrangement, through multifocal and SEM techniques. Methods. We examined a total of 21 specimens, including 27 arm fragments, from the Gulf of Mexico and Honduras. Two specimens were dissected. Results. Detailed descriptions of pedicellariae, abactinal, intercostal, inferomarginal, adambulacral, ambulacral, odontophore, and oral ossicles, and their spines are provided, emphasizing the articulations and muscle attachments. C-shaped valves pedicellariae and small pedicellariae valves with shorter denticulation areas were recognized. Conclusions. The morphological descrip- tion of M. xandaros is expanded, providing the most extensive description of abactinal, first adambulacral, first and subsequent inferomarginal ossicles, abactinal spines, and C-shaped, crossed pedicellariae, as well as the distal arm plates, for a brisingid species using SEM to date. Key words: ossicles; oral frame; pedicellariae; spines; deep-sea. Brisingids are deep-sea asteroids that have remained poorly understood due to their deep a body shape similar to ophiuroids due to their bathymetric distributions and fragile skeletons. small, circular disk, which is clearly differenti- They were initially considered as one family, ated from their six to twenty, long and slender but after comparison of all the genera from arms. They also resemble crinoids when they the Atlantic waters, two well-defined families raise their long, spined arms into the water col- were designated: Brisingidae Sars, 1875 and umn for suspension feeding (Clark & Downey, Freyellidae Downey, 1986 (Downey, 1986; 1992; Mah & Blake, 2012; Gale, Mah, Hamel Clark & Mah, 2001; Gale et al., 2014). & Mercier, 2014). The family Brisingidae is defined by lack- The species of Brisingida Fisher, 1928 ing bare interradial plates on the disk, and have a nearly global distribution but have having arms constricted at their base whose 404 Revista de Biología Tropical, ISSN electrónico: 2215-2075 Vol. 69(S1): 404-422, March 2021 (Published Mar. 10, 2021) abactinal plates form costae, and includes the photographs of brisingid pedicellariae, describ- genus Midgardia Downey, 1972 (Downey, ing them in great detail and making morpho- 1986; Zhang, Wang, Zhou, & Zhang, 2019). logical comparisons with Labidiaster annulatus This genus is distinguished by having 11 to 13 Sladen, 1889 and Stylaterias forreri (deLo- deciduous arms, constricted at their base, and riol, 1887). Also, Mah (1999) in his taxonom- acute interradial arcs; bare interradial plates ic work of Brisingaster robillardi (deLoriol, absent on the disk and numerous, remarkable 1883), showed SEM photographs of pedicel- costae made up of imbricate plates (Downey, lariae for this species and Novodinia helenae 1986). It includes only one species, Midgardia Rowe (1989). Finally, Vickery and McClintock xandaros Downey, 1972, whose type locality (2000) described the tube-feet of brisingids as is Veracruz, Mexico (19°2’36” N & 95°27’30” semi-flat-tipped and non-suckered, proposing W, 457.2 m), and has been reported for the their morphology as a taxonomic character at Gulf of Mexico and Honduras (366 to 475 an ordinal level. m) (Downey, 1972; Pequegnat, Gallaway & Recently, the phylogenetic studies of Gale Pequegnat, 1990; Clark & Downey, 1992; (2011) and Fau and Villier (2019) provided Durán-González, Laguarda-Figueras, Solís- well-described ossicles of brisingids, using Marín, Buitrón-Sánchez & Torres-Vega, 2005; SEM techniques, and updating the skeletal Solís-Marín et al., 2013). terminology for asteroids in general. Likewise, In the 19th century, asteroid ossicles start- Fau and Villier (2018) described the first ambu- ed to be described as part of taxonomic studies lacral ossicles of B. robillardi. However, they (e.g. Agassiz, 1877; Viguier, 1879; Ludwig, showed isolated ossicles from different brisin- 1897), providing the foundations and terminol- gid species; therefore, the complete description ogy for future skeleton morphological works. of the skeleton of a brisingid species is still In particular, Sars (1875) described Hymeno- needed. Here, we describe the external and discus coronata (= Brisinga coronata), giv- internal morphology of Midgardia xandaros ing details and illustrations of the skeleton, Downey, 1972, including the skeletal arrange- pedicellariae, spines, muscles, nervous and ment, through multifocal and SEM techniques, digestive systems, as well as notes about its also providing additional notes about the gen- regenerative capacity and ontogeny. Until now, eral morphology of a brisingid. Sars (1875) work has been the most extensive description for a brisingid species. MATERIALS AND METHODS Subsequent taxonomic studies about bris- ingids have largely been based on external Study area: The Gulf of Mexico is a morphology (e. g. Fisher, 1917; Fisher, 1918; semi-enclosed ocean basin located between Fisher, 1928; Baranova, 1957; Tortonese, 1958; Cape Sable, Florida, United States and Cabo Downey, 1972; Downey, 1973; Downey, 1986; Catoche, Quintana Roo, Mexico. It has a Clark & Downey, 1992; McKnight, 2006; Bena- shoreline extension of 5 696 km and a mean vides-Serrato, Borrero-Pérez, & Díaz-Sánchez, water depth of 1 615 m (Fautin et al., 2010; 2011; Mah, 2016; Zhang et al., 2019; Zhang, Turner & Rabalais, 2019). It also receives a Zhou, Xiao, & Wang, 2020). Meanwhile, Spen- vast sediment discharge from the Mississippi cer and Wright (1966), Blake (1987) and Gale Delta, and other rivers such as Coatzacoalcos (1987) mainly provided insights into the skele- and Papaloapan, making the deep-sea floor ter- tal morphology of brisingids at an ordinal level, rigenous and muddy. In contrast, the platforms emphasizing the arrangement of the ossicles of of Florida and Yucatan Peninsula have carbon- the oral frame. ate sediments (Armstrong-Altrin et al., 2015; Regarding pedicellariae morpholo- Ramos-Vázquez, Armstrong-Altrin, Rosales- gy, Emson and Young (1994) provided the Hoz, Machain-Castillo & Carranza-Edwards, first Scanning Electron Microscopy (SEM) 2017; Ward & Tunnell, 2017). The deepest Revista de Biología Tropical, ISSN electrónico: 2215-2075, Vol. 69(S1): 404-422, March 2021 (Published Mar. 10, 2021) 405 area of the Gulf of Mexico is the Sigsbee Abys- Microscopía y Fotografía de la Biodiversidad sal Plain which ranges from 3 750 to 4 400 m I, IB, UNAM, and the SEM Academic Service, (Fautin et al., 2010; Turner & Rabalais, 2019). ICML, UNAM. The water circulation is mainly given by The description of ossicles, spines and the Loop Current that originates at the Yucatan pedicellariae follows the terminology proposed Channel, which connects the Gulf of Mexico by Chia and Amerongen (1975), Clark and with the Caribbean Sea and is a semi-enclosed Downey (1992), Emson and Young (1994), basin located between 9° N and 22° N latitude Gale (2011), Fau and Villier (2018), and Fau and 60° W and 89° W longitude. The Caribbean and Villier (2019) (Appendix 1). deep-sea has many small canyons, seamounts, and table mounts, such as the Explorer Bank, RESULTS located off Honduras (Miloslavich et al., 2010; Palanisamy, Becker, Meyssignac, Henry, & Systematics Cazenave, 2012; Ward & Tunnell, 2017). Midgardia xandaros Downey, 1972 Midgardia xandaros Downey, 1972: 422-425; Specimen examination: A total of 21 Downey, 1973: 99; Downey, 1986: 19-20; specimens and 27 arm fragments deposited at Clark & Downey, 1992: 470-471. the National Echinoderm Collection “Dra. Ma. Elena Caso Muñoz” (ICML-UNAM) and the Description: The 21 specimens and 27 National Museum of Natural History, Smith- arm fragments have the following range in sonian Institution (USNM), and previously dimensions: r = 13-19.3 mm, DD = 29-38.2 collected in the Gulf of Mexico and Honduras, mm, and DH = 5-8.9 mm. Due to the lack of were examined through a stereoscopic micro- complete specimens, R was not measured, scope (Olympus SZX7). except for the holotype (R = 680 mm). The following measurements were taken Disk small, round, flat. The abactinal for each specimen: minor radius (r), disk diam- membrane of the disk is thin