Abstract
Sipunculans are unsegmented marine annelids distributed from intertidal to deep sediments, playing important roles in nutrient cycling and ecosystem health. The Sipunculus nudus species complex is exploited as food and bait in several regions, yet reproductive data at the population level remain scarce, especially for Atlantic populations. Understanding morphological correlates of body size and reproduction is essential for sustainable management. Over 17 months, 434 individuals of S. cf. nudus were collected monthly from intertidal sediments in Salvador, Brazil. External (total length, posterior length, width) and internal (number of longitudinal muscle bands, nephridia length, intestinal coils) measurements were taken. Coelomic fluid was examined to determine sex and gamete maturity, and oocytes and spermatocyte clusters were staged and measured. Population structure was analyzed using correlation analyses, DistLM, and dbRDA; reproductive investment via generalized linear models; and sex ratios by chi‐square tests. Environmental data (temperature, salinity, tide, rainfall) were included in redundancy analyses. Posterior length strongly correlated with total length and was identified as a reliable size proxy, while nephridial length distinguished mature from immature individuals. The population showed a near 1:1 sex ratio, with males producing larger spermatocyte clusters relative to size, while female oocyte diameter was size‐independent. Gametes at multiple maturation stages occurred year‐round, indicating continuous reproduction unaffected by environmental variation. These findings reveal morphological indicators of reproductive status and suggest stable, size‐dependent reproductive strategies that may enhance population resilience under harvesting pressure. The study provides key baseline data for managing exploited Sipunculus populations.