The condition index: an effective eco-physiological indicator in the invasive cockle Fulvia fragilis.
This study is the first on condition index monitoring for Fulvia fragilis, a nonindigenous and invasive species in Tunisia. It is a simultaneous hermaphroditic cockle with a diffuse gonad. Condition indices (total and those relating to the visceral mass (gonad and digestive gland) and the rest of the flesh) were monitored monthly in two northern Tunisian sites (Bay of Tunis and lagoon of Bizerte). In favourable conditions, as found in the Bay of Tunis, the condition index reflects the progress of gametogenesis with synchronous increases in maturation and spawning periods. In the Bay of Tunis population, the strong correlation between the total condition index (CI) and partial spawning stage reflects low gamete emission. This index can be used as an eco-physiological index, since it reflects well the deterioration of the cockle metabolism in polluted environments, such as Bizerte lagoon. In addition, seasonal and synchronous development of the visceral mass condition index (CIVM) and the remaining tissues condition index (CIR) indicates simultaneous gonadic and somatic development in favourable conditions (nutrient availability and temperatures) in this non indigenous cockle.