**Arely Martínez-Arce 1,\*, Alberto De Jesús-Navarrete <sup>1</sup> and Francesca Leasi 2,\***


Received: 6 February 2020; Accepted: 15 March 2020; Published: 19 March 2020

**Abstract:** Nematode biodiversity is mostly unknown; while about 20,000 nematode species have been described, estimates for species diversity range from 0.1 to 100 million. The study of nematode diversity, like that of meiofaunal organisms in general, has been mostly based on morphology-based taxonomy, a time-consuming and costly task that requires well-trained specialists. This work represents the first study on the taxonomy of Mexican nematodes that integrates morphological and molecular data. We added eleven new records to the Mexican Caribbean nematode species list: Anticomidae sp.1, *Catanema* sp.1, *Enoploides gryphus*, *Eurystomina* sp.1, *Haliplectus bickneri*, *Metachromadora* sp.1, *Odontophora bermudensis*, *Oncholaimus* sp.1, *Onyx litorale*, *Proplatycoma fleurdelis*, and *Pontonema* cf. *simile*. We improved the COI database with 57 new sequences from 20 morphotypes. All COI sequences obtained in this work are new entries for the international genetic databases GenBank and BOLD. Among the studied sites, we report the most extensive species record (12 species) at Cozumel. DNA barcoding and species delineation methods supported the occurrence of 20 evolutionary independent entities and confirmed the high taxonomic resolution of the COI gene. Different approaches provided consistent results: ABGD and mPTP methods disentangled 20 entities, whereas Barcode Index Numbers (BINs) recovered 22 genetic species. Results support DNA barcoding being an efficient, fast, and low-cost method to integrate into morphological observations in order to address taxonomical shortfalls in meiofaunal organisms.

**Keywords:** ABGD; BINs; DNA barcoding; meiofauna; mPTP
