We sampled the gut contents of 709 fishes, representing 61 species and including both cleaner and non-cleaner fishes by opportunistic shore-based or small craft day trips to shallow reef sites in Caribbean waters by the Virgin Islands, and Puerto Rico to compare their consumption of gnathiid isopods, the most common fish ectoparasites

Sampling sites: St Thomas, usvi (18°20′25.3ʺ N, 64°58′36.8ʺ W): "baby", St. John, usvi (18°19’0.2” N, 64°43’20.8” W): "donkey", "jum", "lam", "tektite", La Parguera, Puerto Rico (17°57′18.1ʺ N, 67°03′08.1ʺW): "Enrique", "Mario", "Medialuna".

Fish Collection Methods
A variety of methods and net types were used to collect fish while on snorkel or SCUBA. All fish were caught between sunrise and approximately 11:00 h. This time block was selected to maximize the interface between when diurnal fishes begin to feed and when gnathiids may still be active. Details on sampling for each site can be found in supplementary material (Supplementary Table 1 in Nicholson et al., 2022). We did not account for any effect of site because all sites are similar in composition regarding biodiversity and benthic composition, and all have been confirmed to have substantial gnathiid populations (Artim et al. 2020).

Sample Processing
Gut content samples were collected via lavage (stomach pump, Artim et. al 2017), dissection, or feces collection. Fish that were large enough to have surgical tubing (either 6mm or 3mm outer diameter) safely inserted were gut lavaged. They were then returned to the location on reef where they were caught after recovering in an aerated bin of fresh seawater. Fish that were too small to be lavaged were brought back to the lab and humanely euthanized prior to dissection. However, when possible, fish were placed in aerated tanks for ~24hrs and allowed to defecate. Fecal matter was collected, and the fishes were subsequently returned to the same area reef where they were caught. Samples were screened with a dissecting scope. Since their chitinous exoskeletons are not able to be digested, remnants of gnathiids (specifically the heads, Fig. 1 in Nicholson et al., 2022), can be reliably identified by any of the sample collection methods detailed above.