A cryptic species of the nemertean egg predator <i>Carcinonemertes conanobrieni</i> (Simpson et al., 2017) detected using a barcoding approach infects the Caribbean spiny lobster <i>Panulirus argus</i> (Latreille, 1804) in the southwestern Caribbean

A recently discovered nemertean egg predator, <i>Carcinonemertes conanobrieni</i>, inhabiting <i>Panulirus argus</i> egg masses poses a potential threat to this ecologically and commercially relevant lobster. This study assessed the prevalence of <i>C. conanobrieni</i> in the southwestern Caribbean Sea; Costa Rica and Panama. Brooding females of <i>P. argus</i> were collected by fishermen near Punta Uva beach, Costa Rica (<i>n</i> = 17), and Guna Yala, Panama (<i>n</i> = 19) and examined for the presence of <i>C. conanobrieni</i>. Prevalence of <i>C. conanobrieni</i> in brooding lobsters, determined as the presence/absence of adults, juveniles, encysted juvenile worms, or carcinonemertid egg masses was 47.06% and 31.58% in Costa Rica and Panama, respectively. Moreover, when indirect evidence (empty capsules and/or dead embryos presumably attacked/consumed by worms) of the presence of <i>C. conanobrieni</i> in brooding lobsters is considered in addition to direct evidence, prevalence of <i>C. conanobrieni</i> in brooding lobsters was 64.71% and 47.37% in Costa Rica and Panama, respectively. The observations suggest that this parasitic worm completes its life cycle locally in the southwestern Caribbean. Notably, a Maximum Likelihood phylogenetic analysis based on a fragment of the mitochondrial <i>cox1</i> gene clustered two specimens collected in Costa Rica together with four other specimens previously collected in Saint Kitts into a single fully supported monophyletic clade that segregated from a second clade containing six specimens of <i>C. conanobrieni</i> collected in Colombia, Florida, and Saint Kitts. The barcoding analysis suggests that there is an undescribed species of <i>Carcinonemertes</i>, anatomically like <i>C. conanobrieni</i>, infecting <i>P. argus</i> in Costa Rica.