Abstract
Polychaetes play key ecological and cultural roles in Lombok Island, Indonesia, particularly during the annual Bau Nyale spawning event. Despite numerous studies on species identification, biometric information for swarming polychaetes remains limited. This study provides the first morphometric and length–weight relationship (LWR) baseline for polychaete assemblages collected at Tanjung Aan during the 2025 Bau Nyale season. A total of 1,341 individuals representing nine families were recorded, dominated by Eunicidae (432 indv.) and Amphinomidae (343 indv.). Length–weight frequency distributions showed that most families occupied medium-length and low-weight classes, with apparent size-structure variation across families. LWR analysis of six dominant families consistently showed negative allometry (b< 3), indicating that weight did not increase proportionally with length. This trend suggests that swarming polychaetes allocate metabolic energy toward somatic elongation and reproductive investment rather than biomass accumulation during the spawning period. Variation in b-values and R² among families reflects ecological niche specialization, trophic strategies, and differences in reproductive timing. These findings address a significant research gap in tropical polychaete biometrics and offer essential baseline data for assessing environmental and anthropogenic impacts on Bau Nyale populations. Management implications include integrating annual biometric and LWR monitoring into Bau Nyale assessments, establishing harvest controls to prevent overexploitation of dominant families, and prioritizing habitat protection at Tanjung Aan. Incorporating biometric indicators into coastal management policies will help sustain both cultural traditions and long-term ecological resilience of polychaete populations in Lombok.