Pinna nobilis -noble pen shell-
This species who can reach 1 m in length is the largest Mediterranean mollusks.
Among the bioindicators of a well preserved environment, the first place is for that endemic species  which is present all around Posidonia meadows. 
This species  very  sensible to pollution, as disappeared, to day, in numerous Mediterranean coastal  stations. In Tunisia,  therefor, it remains  frequent, specially in the northern littoral and in the neighbourhood of Kerkena islands where it is specially abundant.

Endemic = an endemic species is a species living only in a  well determinate geographical area.

Photo 1: young specimen  (27cm) - la Galite islands.

 
Photo 2:  Kerkena  islands : adults (mean length 45 cm) covered by epiphytic organisms
1= an ascidian-  2= the Indo - pacific  oyster Pinctada radiata. - 3= the  green para-tropical alga Caulerpa prolifera- 4= the  green para-tropical  alga  Halimeda tuna.

some other species bioindicators  of the state of "good health" of the sea  -part 1
part 2
part 3
photos by J. Zaouali