**4. Discussion**

This paper was derived from a workshop on marine meiofaunal organisms of Costa Rica with a focus on Gastrotricha (and Kinorhyncha) held in January 2019 at the CIMAR (University of Costa Rica) (Supplementary Material Figure S10). During the event, intended for undergraduate and graduate students, general information on the phylum was provided in the classroom and the main techniques regarding sampling, extraction, and observation were illustrated. Techniques were put in practice by some of the students who participated in subsequent 15-day field work. The effectiveness of an early versions of the taxonomic keys was determined by the pupils based on direct observation of living, relaxed specimens (some of the students) and/or based on photographs of freshly sampled specimens (all the students). The proposed version of the keys benefited from the insightful comments which emerged during the testing. At the end of the training period, all the students were able to correctly identify at the genus level the gastrotrich involved in the testing. Based on this outcome, we are confident in the work's usefulness to many others.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1424-2818/11/7/117/s1, Figure S1: Sampling of marine gastrotrichs of the littoral zone, Figure S2: Sampling of marine gastrotrichs of the sublittoral zone, Figure S3: Sampling freshwater gastrotrichs, Figure S4: Processing of freshwater sample for *in vivo* studies, Figure S5: Extraction of interstitial fauna, Figure S6: Extraction of interstitial fauna, Figure S7: Mounting of the specimens, Figure S8: Mounting of the specimens, Figure S9: Morphological analysis and documentation, Figure S10: Instructors (background) and students (foreground) participating at the work-shop on marine meiofaunal organisms of Costa Rica with focus on Gastrotricha.

**Author Contributions:** Conceptualization, M.A.T.; Data curation, M.D.; Funding acquisition, M.A.T. and J.A.S.-C.; Investigation, J.A.S.-C., O.A.S.-B., J.D.B. and M.D.; Supervision, M.A.T.; Writing—original draft, M.A.T.; Writing—review and editing, J.A.S.-C., O.A.S.-B., G.C.-D., N.G.-O., J.D.B., M.C.-D. and M.D.Z.

**Funding:** The research was funded in part by a Universidad de Costa Rica (project 808-B8-184 Marine meiofauna of Costa Rica) grant to J.A.S.-C. The publishing cost was met by a MIUR grant "Finanziamento annuale individuale delle attività base di ricerca" to M.A.T.

**Acknowledgments:** The first draft of this paper was prepared during the workshop on marine meiofaunal organisms of Costa Rica with a focus on Gastrotricha and Kinorhyncha, held in January 2019 at the CIMAR (University of Costa Rica). We thank the director of CIMAR for supporting the event.

**Conflicts of Interest:** The authors declare no conflict of interest.
