The invasive algae Caulerpa racemosa poses a threat to mussel viability and reproduction

The invasive Chlorophyta Caulerpa racemosa raises ecological problems in the Mediterranean Sea both in terms of native algal diversity and toxicity to aquatic organisms. In this paper, we determined the effects of Caulerpa on a sessile, filter-feeding animal, the mussel Mytilus galloprovincialis. By using different histological approaches, we investigated the response of mussel tissues to increasing amounts of Caulerpa. The recorded data showed a multisystem discomfort of M. galloprovincialis to C. racemosa, with structural alterations in the gills, digestive gland, and mantle. Alterations in tissue organization, such as altered thickness of the gill epithelium, appearance of lipofuscin granules in the gills, cellular disorganization of the digestive glands, and hypertrophy of mantle cells are all morphological aspects potentially attributable to the oxidative stress caused by C. racemosa. This mechanism could also underline the most worrisome damage recorded in Caulerpa-exposed specimens, i.e., degeneration of oocytes and altered differentiation of sperm cells with severe gonadal disorganization; these alterations over time could pose a major risk to the survival of adult mussels by also affecting their reproductive fitness, with potential consequences for the continuation of the species.