RESEARCH ARTICLE Comparison of absolute and relative growth patterns among five Pinna nobilis populations along the Tunisian coastline: an information theory approach Lotfi Rabaoui Æ Sabiha Tlig Zouari Æ Stelios Katsanevakis Æ Oum Kalthoum Ben Hassine Received: 19 February 2007 / Accepted: 11 April 2007 / Published online: 15 May 2007 Ó Springer-Verlag 2007 Abstract The variability in absolute and relative growth of Pinna nobilis along the Tunisian coastline was investi- gated. Five populations of P. nobilis were sampled, three from northern and two from eastern Tunisia. The specimens were aged and ten morphometric characters were measured on each individual. To test if differences existed in absolute and relative growth patterns among the different popula- tions an information theory approach was followed. For absolute growth, von Bertalanffy, Gompertz, the logistic and the power models were fitted in combination with three assumptions regarding inter-population differences in absolute growth patterns: no differences, differences among all five populations or just between northern and eastern populations. The assumption of common absolute growth parameters among all five populations had the greatest support by the data, whereas the assumption of different growth patterns among all five populations had no support. Von Bertalanffy growth model and the power model were both equally supported by the data (while Gompertz had considerably less support and the logistic model had no support), and thus it may not be definitely concluded whe- ther P. nobilis grows asymptotically or not. The P. nobilis populations of the Tunisian coastline had a slow growth and up to an age of ~ 9 years their shells were smaller than from all other reported populations in the Mediterranean. For relative growth, apart from the classical allometric model Y = aX b , relating the size of a part of a body Y to another reference dimension X, more complicated models were used in combination with the three abovementioned assumptions regarding inter-population differences. Those models, of the form logY = f (logX), either assumed breakpoints in the relative growth trajectories or non-linearities. For most morphometric characters, the classical allometric model had no support by the data and more complicated models were necessary. In most cases, different relative growth either among all five populations or between the northern and eastern population groups was supported by the data. Further investigation is needed to relate the morphological differences observed among different populations of P. nobilis to environmental factors. Introduction The fan mussel Pinna nobilis is endemic to the Mediter- ranean Sea. It is one of the largest bivalves of the world, attaining lengths up to 120 cm (Zavodnik et al. 1991). It is long lived, living up to 20 years according to Butler et al. (1993), whereas in Thermaikos Gulf (Greece) an age of 27 years has been reported (Galinou-Mitsoudi et al. 2006). It has very variable recruitment (Butler et al. 1993), and occurs at depths between 0.5 and 60 m, mostly in soft- bottom areas overgrown by meadows of the seagrasses Posidonia oceanica, Cymodocea nodosa, Zostera marina or Zostera noltii (Zavodnik et al. 1991) but also in bare sandy bottoms (Katsanevakis 2006a, 2007a). The population of P. nobilis has been greatly reduced during the past few decades as a result of recreational and Communicated by O. Kinne. L. Rabaoui  S. Tlig Zouari  O. K. Ben Hassine Research Unit of Biology, Ecology and Parasitology of Aquatic Organisms, Department of Biology, Faculty of Sciences of Tunis, University campus, El Manar, 2092 Tunis, Tunisia S. Katsanevakis (&) Department of Zoology-Marine Biology, Faculty of Biology, University of Athens, Panepistimioupolis 15784 Athens, Greece e-mail: stelios@katsanevakis.com 123 Mar Biol (2007) 152:537–548 DOI 10.1007/s00227-007-0707-z