A new and rare epigean asellid from western Mexico Caecidotea contrerasbalderasi sp. nov. (Isopoda: Asellidae) with a new suture pattern in pleopod IV for the Mexican epigean species

Abstract A new epigean asellid isopod, Caecidotea contrerasbalderasi sp. nov. is described from Nayarit, northwestern Mexico. This new species has morphological characters belonging to two different types of environments, both depigmentation and anophthalmia, indicators of organisms adapted to live in constant darkness as cavernicolous species. Despite showing characteristics of hypogean species, the new taxon does not show elongation of appendages, such as pereiopods, antennae or uropods. Its pleopod IV exopod lacks false sutures. This is a new pattern type “C”, and differs from that observed in types A and B. The description of this new species brings a new total of 101 species and three subspecies, for the genus Caecidotea in North America all morphologically poorly differentiated and with a restricted distribution.

In Mexico, the genus Caecidotea Packard, 1871 comprises 16 species that have been recorded in a wide variety of freshwater ecosystems, both lentic and lotic.The information about their distribution and habitat is provided in Tab. 1.
The species Caecidotea contrerasbalderasi sp.nov.represents the first record for the genus in the Mexican state of Nayarit.The updated number of species in the genus Caecidotea in North America is 101: 74 from hypogean habitats and 27 from surface waters, including three subspecies, C. bicrenata whitei Lewis and Bowman, 1981, C. racovitzai australis (Williams, 1970), C. r. racovitzai (Williams, 1970) (García-Vázquez et al., 2021), with morphological differences of taxonomic importance that guarantee their reassignment to a full species rank (García-Vázquez, unpubl.data).

MATERIAL AND METHODS
During an ecological survey near the municipality of Xalisco, Nayarit, Mexico (Fig. 1), nine specimens of an unidentified epigean isopod in the genus Caecidotea were hand collected from a crystalline shallow pond supplied by upwelling spring water, with a bottom of mud and gravel.The pond lacked aquatic vegetation with a temperature of 24-26 °C.The collected specimens were preserved in 70% ethanol and were subsequently analyzed at the Laboratorio de Entomología y Artrópodos of the Facultad de Ciencias Biológicas, Universidad Autónoma de Nuevo León (FCB-UANL).After morphological examination, the specimens were recognized to belong to a new species of Caecidotea that is hereby described and compared with other congeneric epigean and hypogean species.The specimens were measured and drawn using a stereomicroscope Olympus-SZX12 equipped with a camera lucida, then dissected and mounted in Entellan® in permanent slides; the limbs were illustrated using a Carl Zeiss compound microscope with camera lucida.Anatomical details of the male appendage follow the nomenclature of García-Vázquez et al. (2019).
The specimens were deposited in the Colección Carcinológica (FCB-UANL) and the National Museum of Natural History, Smithsonian Institution, Washington DC (USNM).
Antennula not overreaching protopodal article 3 of antenna; flagellum with 7 articles, aesthetascs in formula 1-1-1-1; articles tapering distally, first article 1.5 and 2.6 times longer than second and third segments; 1-3 longer than flagellum.First article 1.5 and 2.6 times longer than second and third articles.
Antenna, flagellum broken in all specimens except a paratype female in which it reaches posterior margin of pleotelson, with 4 protopodal articles, and flagellum of 19 articles.
Pleopod IV (Fig. 4F), protopod small bearing a basal fringe of small setae plus 2 long simple setae; exopod type B after Lewis and Bowman (1981), ovate, with single transverse suture on median margin; protopod distal article length 1.2 × proximal article length.
Etymology.The present species is named in honor of Dr. Salvador Contreras-Balderas, as a posthumous tribute to his important and successful career as a taxonomist of fishes and crustaceans, as well as Professor of Systematics and Evolution in the Facultad de Ciencias Biológicas of the Universidad Autónoma de Nuevo León.
Habitat.The species was collected in a crystalline, freshwater shallow pond at 20 cm deep (temperature 24-26 o C) under submerged rocks and between the decaying organic matter (willow leaves), with muddy and gravelly bottom.
Distribution.Only known from the type locality.
Remarks.Morphologically, the eyes in the type series of Caecidotea contrerasbalderasi sp.nov.are absent, as well as body pigmentation, features usually associated with a subterranean lifestyle (Rudy et al., 2018).In contrast, this species does not present elongation of appendages such as pereiopods, antennae or uropods.A similar situation is found in some Eastern Australian phreatoicidean isopods belonging to the genus Crenoicus and in the New Zealand genus Notamphisopus, in which epigean species lack functional eyes and pigmentation (Fenwick et al., 2009;Wilson and Ho, 1996).

DISCUSSION
Caecidotea contrerasbalderasi sp.nov. shares with its congeneric Mexican hypogean species a pleopod IV type B (sensu Lewis and Bowman, 1981; see Tab. 2, Fig. 4F).However, the species can be easily separated from the rest of their epigean congeners by the presence of a pleotelson with a caudomedial lobe emarginate and the absence of eyes.Another character of taxonomic importance is the shape of the endopod apex of pleopod II (Fleming, 1973); in subterranean species such as Caecidotea chiapas Bowman, 1975, Caecidotea vomeroi Argano, 1977, and Caecidotea zullini Argano, 1977, the endopod apex presents an abrupt elongation, while in Caecidotea contrerasbalderasi sp.nov.and Caecidotea pasquinii (Argano, 1972) (subterranean species) the apex does not present this abrupt elongation.This is more similar to that of epigean species with the presence of a caudal process, and the cannula is short and with a mesial process (see García-Vázquez et al., 2021, p. 46, Fig. 1).For this reason, a dual epigean/hypogean lifestyle cannot be discounted in Caecidotea contrerasbalderasi sp.nov.

Figure 5 .
Figure 5. Suture patterns of pleopod IV: A, Pattern A, with 2 false sutures and apical incision; B, Pattern B, with single sigmoid false suture; C, Pattern C, without sutures.Figures A and B taken and redrawn from Lewis and Bowman (1981: 6, fig 7).

Table 2 .
Pleopod IV suture pattern for the Caecidotea species of Mexico (data compiled from the literature). 7A