Seasonal morphometry and extractive pressure on polychaetes Hediste diversicolor and Marphysa sanguinea from Southwest Spain

Abstract

This study aimed to investigate the morphometry, length-weight relationships and extractive activity on two commercially significant polychaete species, Hediste diversicolor and Marphysa sanguinea in the Bay of Cadiz (Southwest Spain). Due to measurement challenges with these fragile organisms, jaw length was introduced as a key morphological measure in the length-weight relationships. Specimens were collected monthly throughout 2019, totaling 942 H. diversicolor and 404 M. sanguinea. Eight morphometric variables were measured, including total length, width, right and left jaw lengths, and various weight metrics. Regression models and changepoint analyses were used to assess morphometric relationships and seasonal variations. Extraction rates were estimated through direct observation of shellfish harvesters. Results showed that the inclusion of jaw length improved the accuracy of length-weight relationships, particularly for H. diversicolor. Both species exhibited negative allometric growth patterns, with seasonal variations in length-weight regressions. Significant extraction rates were observed, averaging 0.85 polychaetes per minute per shellfisher. The findings highlight the need for implementing management regulations, such as control of illegal harvesters, to ensure the sustainability of these species. This study provides essential data for informed decision-making in the conservation and management of these species in the region.