Uptaded checklist, historical overview and illustrated guide to the stygobiont Malacostraca (Arthropoda: Crustacea) species of Yucatan (Mexico)

This study provides an updated checklist and an illustrated guide to the 17 currently known stygobiont Malacostraca species of the state of Yucatan (Yucatan Peninsula, Mexico). The compilation is based on the individuals collected during our cave-diving expeditions (2016–2019), and, has the purpose of expanding previous knowledge on the taxonomy of these subterranean crustaceans. The identification guide contains drawings of the main diagnostic characters of the species as well as a brief introduction of the relevant malacostracan orders. The information is further complemented with a historic account and timeline of the stygobiont Malacostraca species of the Yucatan Peninsula. This is the first study that provides a unified tool for the morphological identification of these highly endemic species.


Introduction
The Yucatan Peninsula (southeastern Mexico) groundwater ecosystems harbour a highly endemic, crustacean-dominated fauna that present a variety of morphological and physiological adaptations to life in subterranean water environments (eg. Álvarez et al. 2008). Currently, 33 stygobiont (subterranean water-restricted) malacostracan species are known from the groundwater ecosystems of the Mexican federal states of the peninsula (eg. Álvarez et al. 2015;Angyal et al. 2020), which belong to six different orders (Decapoda: 14 sp., Amphipoda: 8 sp., Isopoda: 7 sp., Stygiomysida: 2 sp., Mysida: 1 sp., Thermosbaenacea: 1 sp.). All but three of the 33 species are endemic to the Yucatan Peninsula and half of them have bibliographically confirmed distribution records from the state of Yucatan. A timeline and historical data about the description of the groundwater-restricted Malacostraca of the Mexican states of the peninsula can be found in Table 1. The appearance of the species follows a chronological order, starting with the earliest species described and ending with the most recently discovered one.
Discovery of the first stygobiont malacostracan crustaceans in subterranean habitats of the state of Yucatan is dated back to the beginning of the twentieth century, when the Division of Historical Research of Carnegie Institution of Washington invited E. P. Creaser, F. G. Hall and A. S. Pearse to investigate the biology of the aquifers and subterranean ecosystems of Yucatan. In 1932, several "cenotes" (water-filled sinkholes), "aguadas" (shallow water-holes) and "pozos" (karstic wells) were studied (Pearse 1936). Among other findings, this expedition resulted in the description of four subterranean malacostracan species new to science (Creaser, 1936). After a long hiatus, explorations were continued in the 1970's, when J. R. Reddell and his colleagues (Texas Tech University) studied further cenotes and "grutas" (dry solution caves) of Quintana Roo, Campeche and Yucatan states (Reddell, 1977), resulting in the description of, among others, two stygobiont amphipod and decapod species (Hobbs and Hobbs 1976;Holsinger 1977). Cave diving expeditions in cenotes and the associated submerged cave passages of the peninsula began in the 1980's. By the early 2000's, more than ten subterranean malacostracan species had been described from the orders Thermosbaenacea, Stygiomysida, Isopoda and Amphipoda (eg. Bowman and Iliffe 1988;Holsinger 1990;Bowman and Iliffe 1999;Álvarez et al. 2005).
In 2016, the research group "Cenoteando" (www.cenoteando.com), of the Unidad Multidiscipli naria de Docencia e Investigación of the Facultad de Ciencias, Universidad Nacional Autónoma de México in Sisal (UNAM UMDI-Sisal), began their systematic studies of the biodiversity mapping, ecology and taxonomy of the cenote ecosystems in the state of Yucatan. Prior to Cenoteado's work only less than five percent had been zoologically investigated out of the more than 3,000 officially registered cenotes in Yucatan (SDS Yu catan census). The project's goal has been to provide data from cenotes that had never been studied, with expedi-tions leading to the discovery of stygobionts previously unknown in the region and to the description of species new to science (Angyal et al. 2018;Grego et al. 2019). Together with these records and the most recent description of Álvarez et al. (2019) of a new subterranean isopod, the number of stygobiont malacostracan species of the state of Yucatan has elevated to seventeen.
In the absence of a unified taxonomic guide, up to now, the identification of these species was possible only by a thorough knowledge of the original descriptions and other relevant literature on the species' distribution and morphology. In order to facilitate further research of these rare, endemic species, the aim of this paper is to provide an updated checklist accompanied by an illustrated identification guide and a chronological historical account of the stygobiont malacostracan crustaceans of the state of Yucatan.

Materials and methods
Samples of malacostracan species were collected in 32 cenotes and submerged caves in the state of Yucatan between 2016 and 2019, during SCUBA cave-diving expeditions in the following municipalities: Abala, Cacalchen, Chochola, Chunchumil, Dzilam de Bravo, Ekmul, Homun, Huhi, Kopoma, Sotuta, Tecoh, Tixkakal, Tixkokob and Uman. Detailed information about the localities and the collected material can be found in Angyal et al. 2020 (https://zookeys.pensoft.net/article/47694/). The collected material was deposited in the Yucatán Colección de Crustáceos of the UNAM UMDI-Sisal, the Colección Nacional de Crustáceos, Instituto de Biología of the UNAM, Mexico City, and in the Collection of Crustaceans of the Hungarian Natural History Museum (HNHM), Budapest. Individuals were examined with a Nikon SM Z800 stereomicroscope. Thermosbaenaceans, stygiomysids, mysids and amphipods were dissected on slides and were observed with compound light microscope. For this, specimens were cooked in 10% KOH solution, rinsed with HCl and washed in distilled water. Cleared exoskeletons were stained with chlorazol black in glycerol, and then dissected in glycerol gelatin using stereomicroscope (Fišer et al. 2009). Photographs of diagnostic characters were made using an OMAX 14 OMP digital USB microscope camera. Line drawings were made after the collected material, based on (i) slide preparations and intact individuals using drawing tubes mounted in a Leica DM 1000 compound microscope and in a Leica M125 stereomicroscope, respectively, (ii) and on photographs. In the case of three isopod species (Haptolana bowmani, Cirolana yucatana and Curassanthura yucatanensis) which we were not able to collect, we relied solely on the illustrations of the original species descriptions. The drawings were edited graphically via computer, to ensure homogeneity of lines and shades. Species identification and creation of the identification guide were performed with the aid of the literature listed in Table 2.

Results
Updated checklist of the stygobiont Malacostraca of the state of Yucatan Table 3 contains an updated checklist of the seventeen stygobiont malacostracan crustacean species that have been described to date from the state of Yucatan (12 species), or else, have bibliographically confirmed distribution records from the state up to september 2020 and were described elsewhere (5 species).
Illustrated guide to the stygobiont Malacostraca species of Yucatan with the introduction of the orders Order: THERMOSBAENACEA Monod, 1927 Figure 1 Antenna 1 biramous, antenna 2 uniramous. Mandibular palp present. Carapace short, forming dorsal brood pouch in females. Thoracic legs reduced, mostly two-branched. Gills lacking. Pleopods reduced or lacking.
Order: STYGIOMYSIDA Caroli, 1937 Figure 3 Body elongated, vermiform. Second thoracopods are enlarged, dactylus and nail bend down to form a subchelate gnathopod. Both male and female pleopods are reduced to comprise a sympod/protopod, a one-segmented endopod and three-segmented exopod.

Stygiomysis cokei Kallmeyer & Carpenter, 1996
Body length up to 15 mm of males and 22 mm of females. Body rather vermiform, carapace housing mouthparts and anterior three pairs of pereopods (Fig. 5A). Telson 1.7-2.0× longer than wide at base; posterior margin with 15-16 spines in five groups (Fig. 5B). Uropod slightly shorter than telson; protopodal process with seven or eight spines along medial and apical margins (Fig. 5C).  The eight thoracic segments are covered by the carapace which is attached only to the first three. First two thoracic segments bear maxilliped, other six pairs of thoracic appendages are biramous perepoods. Ventral brood pouch enclosed by large, flexible oostegites present on females. Fourth pleopod longer than others in males and has a specialized reproductory function. Statocysts on uropods present.

Family: Mysidae Haworth, 1825
Antromysis cenotensis Creaser, 1936 Body length up to 4.5 mm of both gender. Antenna 2 scale is about 4-4.5 times as long as wide, two-segmented (Fig. 7A). Telson nearly as long as wide at base, gradually narrowing posteriorly, armed with robust spine at each corner and one or two short spine in center (Fig. 7B). Inner ramus of uropod with static organ on basal half (Fig. 7C).

Tuluweckelia cernua Holsinger, 1990
Anterior body region bend markedly downward (Fig. 18A). Body length up to 7 mm of males and 10 mm of females. Antenna 1 reaches at least 75% of body length. Gnathopod 2 propod long and relatively narrow (Fig. 18B). Uropod III proportionally long to body size, rami narrow, outer ramus of outer margin with small spines but lacking setae (Fig. 18C).  Order: DECAPODA Latreille,1802 Figure 19 First 3 pairs of thoracopods transformed on maxillipeds. First pair of pereopods usually bears chelae. Gills are usually enclosed by carapace' folds. One pair of uropods is expanded and together with telson form caudal fin.

Typhlatya mitchelli Hobbs & Hobbs, 1976
Body length up to 22 mm of both gender. Rostrum not extending anteriorly beyond eyes (Fig. 21A). Exosceleton sometimes with pigmentation pattern (Fig. 21A). Ratio of carpus/propodus of pereopod II is more than 2.5 (Fig. 21B). Exopod of pereopod V reaching at least distal extremity of basis and often as far as proximal 5 th of merus (Fig. 21C).

Family: Paleomonidae Rafinesque, 1815
Creaseria morleyi (Creaser, 1936) Body length up to 42 mm of both gender. Rostrum with dorsal and ventral teeth (Fig. 23A). Telson with two-two short spines on distal half, apex with strong spines on both side and some long, fine setae in center (Fig. 23B).

Discussion
One third of the stygobiont Malacostraca fauna of the Yucatan Peninsula has been discovered in the last 20 years, showing an increasing interest of biodi versity surveys  in underwater ecosystems in this region. Half of the currently known species were described from the state of Quintana Roo, partly due to the intensive diving explorations of the Nohoch Nah Chich and Sac Actun submerged cave systems conducted since the late eighties (Álvarez et al. 2015). The diversity of the stygofauna of the anchialine caves of Cozumel Island (Quintana Roo) is remarkable: nine malacostracan species have been discovered in the island in the last three decades (e.g. Bowman 1987;Mejía-Ortíz et al. 2017;Ortiz and Winfield, 2015). Only twelve of the stygobiont malacostracan species of the peninsula have been described from the state of Yucatan, but five additional species have distribution data from this state (Angyal et al. 2020). New discoveries of recent years indicate the need of further subterranean biological explorations to gain deeper knowledge on the species richness and distribution of these cryptic habitats of Yucatan (e.g. Angyal et al. 2018;Álvarez et al. 2019).
The taxonomic status of five species of the presented checklist has been changed since the original descriptions. Creaseriella anops was originally described as Cirolana anops Creaser, 1936. In a thorough taxonomic revision of cirolanid isopods from Mexico, Rioja (1953) considered necessary to establish a new monotypic genus: Creaseriella Rioja, 1953, of which Creaseriella anops (Creaser, 1936) was the type species.
The cirolanid isopod Cirolana yunca was originally described as Haptolana yunca Botosaneanu & Iliffe, 2000. More recently, however, the species has been reallocated to the widely distributed genus Cirolana by Bruce (2008).
There are four monotypic, highly endemic genera (Tuluweckelia Holsinger, 1990;Creaseriella Rioja, 1953;Yucatalana Botosaeanu & Iliffe, 1999;Creaseria Holthuis, 1950) known from subterranean waters of Yucatan. The other eight genera presented in the updated checklist have more than one species and have a wider distribution range, as follows.
Apart from Tulumella unidens, there are two other species of the genus Tulumella Bowman & Iliffe, 1988, described from anchialine caves of the Bahamas (Yager 1988).
Currently, the genus Stygiomysis Caroli, 1937 contains five species from the Dominican Republic, the Caicos Island, Cuba, Jamaica and Italy (Bowman et al. 1984). There are two described species of the genus within the Yucatan Peninsula.
Further species of the genus Antromysis Creaser, 1936 are known from Suriname, The Bahamas, Jamaica, Costa Rica and Cuba. The second species discovered of the genus Antromysis in Mexico was Antromysis reddelli Bowman 1977 from Cueva de las Maravillas in Oaxaca (Bowman 1977).
The genus Curassanthura Kensley, 1981 consists five species. The four previously known species are from Caribbean and North Atlantic islands. C. yucatanensis is the first Curassanthura species to be described from a continental environment (Poore 2009;Álvarez et al. 2019).
The isopod Cirolana Leach, 1818 is a widely distributed genus with more than 200 species worldwide.
Currently, there are five more valid species of the genus Haptolana Bowman, 1966, known from subterranean waters of Belize, Cuba, Somalia, and Western Australia. H. bowmani is the only species reported from Mexico.
The genus Mayaweckelia Holsinger, 1977 consists of three species. Validity of the third species, M. yucatanensis Holsinger, 1977, known from a single locality in Campeche is questionable, as the author points out that "the original description was based on what appear to be submature specimens, therefore raising the strong possibility that the differences noted between M. yucatanensis and M. cenoticola are due primarly to age" (Holsinger 1990).
The currently known seventeen valid species of the genus Typhlatya are characterized by a disjunct distribution around the Caribbean (Yucatan Peninsula, Cuba, Honduras, Bermuda, The Bahamas and the United States of America), in Europe (France and Spain) and the Galapagos and Ascension Islands, which makes this genus an interesting group to test biogeographical hypotheses (Álvarez et al. 2005;Espinasa et al. 2019;Chávez-Solís et al. 2020). Currently, there are four described species of this genus within the Yucatan Peninsula.

Conclusions
Subterranean habitats of Yucatan are characterized by a remarkable diversity of highly adapted, narrowly distributed crustacean species. Seventeen out of the 33 groundwaterrestricted malacostracan species of the peninsula are currently known from waterfilled sinkoles (cenotes) and the associated submerged cave passages of the state of Yucatan. The scope of this paper was to present an updated checklist and the first unified identifiaction guide for the morphological determination of these stygobiont crustaceans. We hope this study will motivate future researches to focus on the taxonomy of the highly endemic stygofauna of the groundwater ecosystems of the Yucatan Peninsula.