Baseline Assessment of Imposex Incidence and Severity in Coastal Gastropods within the Sarangani Bay Protected Seascape (SBPS), Philippines

This study presents the first baseline assessment of imposex incidence and severity in coastal gastropods within the Sarangani Bay Protected Seascape (SBPS), Philippines, while evaluating sediment copper (Cu) and cadmium (Cd) concentrations as supporting environmental variables. A stratified spatial comparative research design was employed across three coastal sites with varying levels of maritime activity. Ninety gastropod specimens were examined for imposex using the Relative Penis Length Index (RPLI) and Vas Deferens Sequence Index (VDSI), while sediment samples were analyzed for Cu and Cd concentrations. Pearson’s correlation analysis was performed using SPSS to determine the relationship between sediment Cu concentrations and imposex indices. Imposex was detected exclusively in Muricidae gastropods (Drupa sp., Drupina grossularia, and Chicoreus torrefactus), with an overall incidence of 10.77%. The highest incidence was observed at Sites 1 and 3 (15.0%), whereas Site 2 exhibited the lowest incidence (4.0%). Overall imposex severity was moderate (RPLI = 39.48%; VDSI = 2.67). Sediment Cu concentrations ranged from 6.53 to 29.56 mg/kg, while Cd remained below the analytical detection limit (<0.03 mg/kg). Both metals were below the National Oceanic and Atmospheric Administration (NOAA) Sediment Quality Guideline thresholds. No significant relationship was found between sediment Cu concentrations and imposex incidence or severity (p > 0.05), whereas Cd could not be statistically evaluated because concentrations remained below the detection limit across all sites. These findings establish the first scientific baseline for imposex within SBPS and support the use of Muricidae gastropods as sentinel bioindicators for long-term coastal biomonitoring. Continued monitoring integrating imposex assessment with sediment quality evaluation is recommended to strengthen evidence-based conservation and sustainable management of the protected seascape.

Imposex; Muricidae; Coastal biomonitoring; Heavy metals; Sarangani Bay Protected Seascape

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