First record of Xestochironomus Sublette and Wirth, 1972 (Chironomidae: Chironominae) in the Mexican Nearctic with notes on their habitat

Abstract We report the first record of Xestochironomus Sublette and Wirth, 1972 for the Mexican Nearctic. Larvae of Xestochironomus are known from the Neotropics and Nearctic regions. We report them for the Sonora river, NW Mexico, 300 km SW from the closest previous record in the U.S. Habitat data are provided and discussed. Our finding provides supporting evidence for the continuous presence of the genus throughout the Americas, including desert systems.


Introduction
The Chironomidae are widely distributed throughout the world (Ashe et al. 1987. Chironomids are one of the most diverse and ecologically important groups found in freshwater systems. They play key roles in community and ecosystem functioning (Porinchu and MacDonald 2003).
Xestochironomus Sublette andWirth, 1972 substituted Insulanus Sublette, 1967 after a study on adults mostly from the Antillean islands (Sublette and Wirth 1972). The genus includes species with larvae highly specialized on xylophagy, inhabiting lotic environments with little human disruption in the American continent (Borkent 1984). Xestochironomus has mainly been reported for tropical environments in the Neotropicsand in the S and SW United States. (Nearctic region) (Borkent 1984, Hudson et al. 1990, Pinho and Souza 2013. With Mexico being a transition zone between the neotropic and nearctic zoogeographic regions, presence of a member of the genus was expected. It was, however, not reported in Reiss (1982), Spies and Reiss (1996) or Spies et al. (2009), the main sources for data on Chironomidae from Mexico. The genus has only been reported once in Mexico, for the Calakmul Biosphere Reserve in the southern State of Campeche (Contreras-Ramos and Andersen 1999), >2000 km SE from our current record. This is thus the first record for Xestochironomus for the Mexican Nearctic.

Methods
Collection sites are located in the Sonora and Bacanuchi rivers (Fig. 1). Samples were obtained in November 2017 with a D-net (40 cm wide; 0.5 mm mesh) and via vigorous washing of woody debris hand-picked from the collection site. Samples were preserved in 80% ethanol. In the laboratory, individuals were separated from the debris and mounted on microscope slides in Euparal following Saether (1969). Slides were then examined under an optical microscope (Zeiss, model: Primo Star) with Nomarsky phase contrast and 1000x magnification with an immersion oil objective, coupled to a AxioCam ERc 5s camera. Borkent (1984), Epler (2001 and  were used for specimen identification. Habitat variables were obtained during the field collections. Bottom substrate was classified using Wentworth's scale (Cummins 1996). Temperature (°C), pH, dissolved oxygen (mg/l) and conductivity (units) were obtained with a YSI Professional Plus (Xylem Inc) multimeter.  Localities where Xestochironomus larvae were captured in the Sonora River, north-western Mexico.

Identification
Xestochironomus larvae are similar to those of Stenochironomus Kieffer, 1919. However, they can be distinguished by the following attributes (all of which were present in the individuals used for this article): sclerotised mentum concave with well sclerotised teeth (Fig. 2a); mentoventral plates vestigial; anal tubules elongated with 4-5 constrictions (Fig.  2b); cephalic capsule dorsoventrally flattened with a Y-shaped dorsal design (Fig. 2c) and antennal blade extending beyond the apex of the third antennal segment (Fig. 2d). In contrast, Stenochironomus larvae have 10-12 teeth in the mentum, the antennal blade reaches only the apex of the 2nd antennal segment and the anal tubules have, at most, two constrictions.

Environmental variables
Habitat variables for collection sites are presented in Table 1.  1998, Bello-González et al. 2016, Borkent 1984, Hudson et al. 1990, Pinho and Souza 2013, Ruiz-Moreno et al. 2000, Sublette and Sasa 1994, Sublette and Wirth 1972. The nearest record to the one presented here is from approximately 300 km away in the San Francisco Hot Springs area of the Gila River in New Mexico (Borkent 1984). In Table 1. Mexico, the only record is for the Calakmul biosphere reserve in the Yucatan Peninsula (Contreras-Ramos and Andersen 1999), over 2000 km SE of our record.

SIS Altitude (m) Cond (μS/cm) pH DO (mg/l) T (° C) D (cm) V (m/s) S
Following Abell et al. (2008) ecoregion classification, most Xestochironomus records are from humid tropical or subtropical areas. Some of the species in this genus can be very abundant in tropical streams draining rainforests (Ferrington et al. 1993, Grund 2006. These conditions favour woody debris inputs to the channel, which constitute both habitat and a food resource for larvae of Xestochironomus (Epler 2001, Sanseverino and Nessimian 2008). High densities in larval xylophagous chironomids are found under such conditions (Cranston 2008).
The Sonora River basin, located in the Gulf of California climatic province, has a distinctive dry climate (Vidal 2005). Vegetation types dominating this area include shrubs and herbs First record of Xestochironomus Sublette andWirth, 1972 (Chironomidae: ... (Martínez-Yrízar et al. 2009) with Cottonwood (Populus sp.) being an important component of the riparian vegetation. These dry conditions lead to relatively low woody debris inputs to stream channels (Bunn et al. 2006, Davies et al. 1995, Jones 1997, Cushing and Allan 2001. Our record from the Sonora and records from the Gila system nevertheless confirm that larvae of Xestochironomus can occur in streams located in arid regions with little input of woody debris. It is thus possible that the genus has a continuous distribution from the Neotropics to the Nearctic region, with deserts in N Mexico and SW USA not being barriers to their distribution. Most (12/14) captured larvae were collected from sites Bacanuchi and Puente Baviacora, while only one individual was captured in sites Mazocahui and El Gavilan despite all sites having similar sampling efforts. Other than the information related to trophic habits, very little data exists about the overall conditions of the habitat required by Xestochironomus larvae. Since they typically inhabit galleries in submerged wood, their low mobility could render them sensitive to changes in local conditions. Xylophagous larvae usually require good water quality (Borkent 1984, Cranston 2008. Mazocahui and El Gavilan had relatively high conductivity (>1000 μS/cm), values above that threshold are typically associated with pollution (Chapman and Kimstach 1996). Low dissolved oxygen and high temperatures in these sites might also have resulted in little oxygen being available for aquatic fauna. Thus, lower water quality in Mazocahui and El Gavilan might be a cause for Xestochironomus having a relatively lower abundance in these sites.