BackgroundThe Caribbean spiny lobsterPanulirus argusis heavily fished throughout its Greater Caribbean and Gulf of Mexico distribution, suggesting a heightened susceptibility to a fisheries collapse. In 2017, a nemertean worm,Carcinonemertes conanobrieniwas described from ovigerous females ofP. argusin Florida, USA. A year later, the presence of the same egg predator was recorded along the southern Caribbean coast (Colombia). The effect of this egg predator on the reproductive performance, including fecundity, embryo mortality, and reproductive output, of its host is unknown. This study tested whetherC. conanobrieniaffects embryo mortality, fecundity, and reproductive output in brooding females ofP. argus.ResultsArtisan fishers caught 90 ovigerous lobsters near Pueblo Viejo, Magdalena, Colombia. Each ovigerous female was examined for the presence/absence of the egg predator. Lobster egg mortality (%), fecundity (nº eggs female−1), and reproductive output (%) were estimated. Prevalence ofC. conanobrieniin the studied population was 87.78%. The mean intensity ofC. conanobrieni(all life stages) in the population was 11.68 (± 1.98) egg predators per brood mass sample. Infected females brooding late-stage embryos exhibited lower fecundity, lower reproductive performance values, and higher embryo mortality compared to infected females brooding early-stage embryos. Embryo stage and worm infection level negatively impacted fecundity and reproductive output. Worm infection level and the number of adult nemertean worms also negatively affected embryo mortality.ConclusionsThese results demonstrate an adverse effect ofC. conanobrienion the reproductive performance ofP. argus. The interactive impact of this egg predator, natural stressors, and anthropogenic stressors on individualP. argusreproductive performance could facilitate losses at large-scale fisheries levels.Supplementary InformationThe online version contains supplementary material available at 10.1186/s40850-023-00165-w.