Population Structure and Growth of the Threatened Pen Shell, Pinna rudis (Linnaeus, 1758) in a Western Mediterranean Marine Protected Area

Coastal ecosystems are being extensively degraded by human activities. Benthic, slow-growing and long-lived species are highly vulnerable to these impacts. Marine protected areas may avoid biodiversity losses through habitat protection. The pen shell Pinna rudis is a protected species, but scarce data are available on its ecology and biology. The present study is a comprehensive ecological study encompassing several unknown aspects of the growth and inner record in relation to habitat types, density and size distribution. During the summers of 2011, 2012 and 2013, a total of 418 strip transects were conducted by scuba diving in the Marine Protected Area of Cabrera National Park (39.14 degrees N, 2.96 degrees E). Samples were conducted across different habitats and depths, exploring 152,146.35 m(2) in total. A large range of sizes and ages were recorded within the park with densities ranging from 0 to 6.89 ind./ 100 m(2). Most pen shells were patchily distributed and concentrated mainly in caves. Two hotspots represented the highest densities ever recorded worldwide, showing a potential link to high larval accumulation and settlement. The population size structure showed a unimodal distribution with shell width ranging from 6.2 to 25.0 cm, with an average shell width of 16.0 +/- 3.4 cm. The absolute growth was asymptotic, with a maximum age of 28-31 years and length of 45 cm. This study on the biology and ecology of a well-established population of Pinna rudis in the Western Mediterranean could set a baseline for the conservation of this species in other areas.