Objective This study aimed to uncover the distribution characteristics and invasion process of Sphaeroma at different infestation stages on mangrove trunks, and to explore the damage mechanisms inflicted on the mangrove species Avicennia marina. The findings provide a scientific basis for developing effective control measures.

Methods The infestation process of Sphaeroma on Avicennia marina was categorized into early, middle, and late stages based on the progression of infestation in Caotou village in 2018. Through sampling and dissection of affected plants, the species, quantity, age distribution, and the number of boreholes of Sphaeroma at various heights of the mangrove's respiratory roots and trunk were quantitatively analyzed.

Results The study revealed that during the early stage of infestation, Sphaeroma were primarily distributed below 50 cm on the trunk, with a concentration within 10～20 cm segment. In the middle stage, the distribution extended below 80 cm, with peak aggregation in the 10～50 cm segment. By the late stage, the distribution reached below 160 cm, yet the highest densities persisted in the 10～50 cm segment. Overall, Sphaeroma retrolaeve was the dominant species within the Avicennia marina. Initially, the number of boreholes exceeded the number of Sphaeroma, but as the infestation progressed, the number of Sphaeroma gradually surpassed the number of boreholes.

Conclusion While Sphaeroma expanded its vertical distribution across different infestation stages, it remained predominantly concentrated within the 10～20 cm region. Early infestation was characterized by high environmental stress, resulting in low isopod survival and a surplus of burrows. Over time, increased Sphaeroma adaptability led to population growth, exacerbating structural damage and accelerating physiological decline in the mangrove trunks.